ESA renewed its New Year’s tradition, briefing the news media on January 15 at the European Space Agency headquarters in Paris, with the agency’s new director general, Johann-Dietrich Wörner, presiding for the first time..  

The contrast with his predecessor, the well-liked Jean-Jacques Dordain, is clear but not stark. Wörner is a bit more whimsical than Dordain, but not too much. His presentation features a few more cartoons, he makes a few more jokes, and there is a picture of the Star Trek (Next Generation) crew to make us chuckle.  

ESA renewed its New Year’s tradition, briefing the news media on January 15 at the European Space Agency headquarters in Paris, with the agency’s new director general, Johann-Dietrich Wörner, presiding for the first time..  

The contrast with his predecessor, the well-liked Jean-Jacques Dordain, is clear but not stark. Wörner is a bit more whimsical than Dordain, but not too much. His presentation features a few more cartoons, he makes a few more jokes, and there is a picture of the Star Trek (Next Generation) crew to make us chuckle.  

Captain Jean-Luc Picard was, after all, a Frenchman, Wörner explains, and he was portrayed by a British actor, thus confirming Europe’s long heritage as a continent of space pioneers. Certainly something to think about.  

Wörner brings new ideas and his own energy. “A German is not just a Frenchman who speaks German,” he insists. 

Cooperation is a key word for the new ESA boss. Even his idea for a “Moon Village,” he explains, is really mostly about creating a cooperative space, “where people can come together, work, and exchange ideas.”  

By the way, Wörner says he was glad to hear Jerzy Buzek call his Moon Village crazy, “because sometimes you need a crazy idea.” It makes you think, it raises new questions.  

Going a step further, Wörner says he sees ESA as a broker, a mediator, an enabler of global cooperation.  

“We work with the U.S., Russia and China, and with many others, nations that have different objectives, competencies, and cultures. We know how to do this because ESA is already a diverse organization itself, with different Member States, with different objectives, specialties and cultures.”  

Woerner displays an image of the most recent space crew preparing to fly to the International Space Station, composed of ESA astronaut Tim Peake, NASA astronaut Tim Kopra, and cosmonaut commander Yuri Malenchenko.  

“Isn’t it extraordinary,” says Woerner, “that we have here a U.S. astronaut, a Russian cosmonaut, and a European astronaut, working together as a team at a time of global crisis?”  

Civil or Military, Again  
The civil nature of Galileo was always and continues to be cited as one of its main reasons for being. Europe had to build it, the story goes, because the existing systems, GPS and GLONASS, were run and controlled by national military agencies.  

Asked whether it matters today that GPS is run by the Pentagon, Wörner replies, “I don’t care if GPS is military. It’s an open signal and it works. On our side we have a civilian Galileo program, and that’s good too.”  

He insists, “Galileo is not against GPS.” This is not about competition — except on the technical level — because, yes, everyone wants their system to be the best, and just to illustrate this we are already talking about the next generation of Galileo.”  

For his part, redundancy is an important asset, Wörner says. “It is much better to have two systems than to have one.”  

He confirms one thing the EC crowd is saying about Galileo: initial services are on the way. The next Galileo launch will not take place until October, “but for the first time we will see four satellites — Galileos 13, 14, 15 and 16 — going up at the same time, on board the Ariane-5 launcher.” It promises to be a spectacular event.  

Free Galileo?  
Wörner wishes everything didn’t have to come down to euros. He wonders aloud, “If Albert Einstein had lived in our time, when all we wait for is the next iPhone, would anyone have agreed to pay him to develop his theory of relativity?”  

Space is not expensive, he argues, but, we ask, should Europeans really be paying billions, footing the entire bill themselves, for a system that will benefit the whole world?  

“Does it always have to be about return on investment?” Wörner counters. “Did the Americans ask if they should pay the cost of GPS? No European should be worried about paying for Galileo users in South America.”  

Wörner provides some further insight into his thinking about a different question on the availability of free data. Wörner reminds his audience that he is still a professor and that he has been in situations where colleagues have urged him to keep his own research results under wraps rather than publishing too quickly and possibly giving his competition a leg up.  

“If you are afraid to publish data because you think someone else might be able to use it faster than you can your self,” he says, “then you have a problem.”  

Finally, journalists being the troublemakers that they are, one of them asks, “Who’s really in charge, the European Commission or ESA?” Wörner shakes his head and answers, “It’s like a marriage. It’s really an equal partnership. Neither one is the boss . . . but don’t tell my wife I said that!”

The post New ESA Chief Briefs Media, Hails Galileo Progress appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

Okay, if I had wanted to pander to GNSS fans, I might have called this, “Making Great Greater.”

But there are only so many superlatives that can be lathered on this remarkable technology before simple praise turns into hagiography.

So, it’s time once more to give a little love to those unsung heroes of GNSS: the augmentations and regional navigation satellite systems.

In my comments, I usually try to avoid beating a drum for the contents of the current issue of our magazine. But the serendipitous convergence of articles in the following pages argues for an exception.

And, here it is.

GAGAN is the fourth satellite-based augmentation system (SBAS) to go live — after the U.S. Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS), and Japan’s MTSAT Satellite Augmentation System (MSAS). In this issue, authors from the Indian Space Research Organization (ISRO) and the Airports Authority of India — the primary instigators and benefactors of GAGAN — celebrate their accomplishment and explain how it is “redefining” navigation in the Indian region.

Another ISRO engineering team, against the backdrop of the Indian Regional Navigation Satellite System (IRNSS), describe their efforts to map the time delays in a navigation payload to improve the ranging accuracy of satellites.

Finally, this issue’s Working Papers column presents some novel ideas aimed at improving the performance of timing systems in future generations of SBAS

Similar ground- and satellite-based GNSS augmentations are also under development in Russia and China. Moreover, the emergence of a multiverse of GNSS, with four independent yet interoperable systems, can itself be considered an augmentation, adding diverse signals, frequencies, and sources to an aggregate capability.

Those, of course, are just the governmental contributions. Commercial services, also mainly satellite-based, bring even higher levels of accuracy and resilience for demanding applications around the world.

Casting the definition of augmentation even more widely, we also encounter the products of the International GNSS Service (IGS) and regional, real-time geodetic networks, as well as augmented GNSS-specific techniques as real-time kinematic (RTK), assisted GNSS, and precise point positioning.

By extension, of course, we can add the other PNT technologies that are making GNSS more robust, accurate, flexible, and/or durable: such things as inertial navigation, chip-scale atomic clocks, WiFi and other RF resources and techniques, camera vision and optical sensors, magnetic anomaly navigation, and so on.

Rather than highlighting the inadequacies and vulnerabilities of GNSS, however, these wide-ranging efforts to augment, supplement, and improve on it actually underline the centrality of space-based systems for modern PNT capabilities.

Rarely, if ever, do we hear advocates and experts with such systems assert the primacy of those technologies or propose that they could take the place of the global, generally accessible, affordable, and unprecedented accuracy provided by GNSS.

The backers of enhanced Loran (eLoran) are not seeking to bring back a World War II era system as a stand-alone multi-modal rival to GNSS. They are seeking a backstop, a basic capability to carry PNT users — particularly in the precise time and synchronization domain — through periods of operational conditions or in environments in which GNSS cannot provide the level of assurance and performance that certain critical applications require.

Even with all of these useful enhancements and endeavors — singly or together — public officials can’t duck their obligation to sustain, modernize, and defend the core GNSS systems, even as they work to ensure overall PNT availability through robust and cost-effective augmentation.

The post Making Good Better appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

1996 soccer game in the Midwest, (Rick Dikeman image)

Nouméa ground station after the flood

A pencil and a coffee cup show the size of NASA’s teeny tiny PhoneSat

Bonus Hotspot: Naro Tartaruga AUV

Pacific lamprey spawning (photo by Jeremy Monroe, Fresh Waters Illustrated)

“Return of the Bucentaurn to the Molo on Ascension Day”, by (Giovanni Antonio Canal) Canaletto

The U.S. Naval Observatory Alternate Master Clock at 2nd Space Operations Squadron, Schriever AFB in Colorado. This photo was taken in January, 2006 during the addition of a leap second. The USNO master clocks control GPS timing. They are accurate to within one second every 20 million years (Satellites are so picky! Humans, on the other hand, just want to know if we’re too late for lunch) USAF photo by A1C Jason Ridder.

Detail of Compass/ BeiDou2 system diagram

Hotspot 6: Beluga A300 600ST

Catching thieves in California, Galileo satellites test Einstein, Russian space agency remodel, and 911 training for operators who can’t read maps.

Catching thieves in California, Galileo satellites test Einstein, Russian space agency remodel, and 911 training for operators who can’t read maps.

1. TO CATCH A THIEF
Arcadia, California USA
√ Porch package delivery bandits have met their match. Police northeast of Los Angeles partnered with citizens and local businesses to bait certain alluring packages with GPS tracking devices that notify police when they have been stolen. Police Sgt. Brett Bourgeous says they have arrested numerous suspects who fell for the decoys. Even some real stolen packages were recovered and returned to their intended recipients.

• January 10, 2016 Pasadena Star-News: Arcadia theft suspect arrested with use of GPS ‘bait package’
• December 21, 2015 Inside Edition: Police Department Puts GPS in ‘Bait’ Packages to Track Doorstep Thieves
• December 20, 2015 CBS Los Angeles: Police Department Puts GPS in ‘Bait’ Packages to Track Doorstep Thieves

2. TESTING EINSTEIN
Germany, France, and MEO
√ German and French physicists are repurposing two Galileo satellites to test Einstein’s general theory of relativity. The European Space Agency’s Galileo V and VI satellites were launched into “very eccentric” orbits in 2014, due to a technical glitch. For the next year, researchers will see if rubidium atomic clocks on the satellites tick more slowly — in microseconds — the closer they are to Earth. This will test gravitational redshift or the gravitational time relation, a “very basic prediction of general relativity,” says Sven Hermann, an experimental physicist at the University of Bremen. Eccentric orbits and Einstein? Seems like a happy match.

• November 9, 2015 ESA website: Galileo Satellites Set For Year-Long Einstein Experiment
• December 21, 2015 PRI: How a mislaunched satellite might help us test Einstein’s theory of general relativity
• December 4, 2015 Science Friday: Errant Satellites Provide Test Case for General Relativity

3. REMODELING
Moscow, Russia
√ President Vladimir Putin dissolved the Russian Federal Space Agency, effective January 1, after numerous scandals and mishandled launches. Its functions will merge with those of a state-owned, centrally managed company — United Rocket and Space Corporation established in 2013 — to create a “unified command structure and reduce redundant capabilities.” But the more things change, etc. The new all-in-one state corporation responsible for the whole space sector, soup to nuts, will still be known as Roscosmos.

• January 14, 2016 TASS (Russian News Agency): Russia decreases number of satellites in its 2025 plan due to budget cuts
• December 30, 2015 InterFAX: Daily Headline News for December 30, 2015
• December 28, 2015 TASS (Russian News Agency): Russian space agency gets replaced by state corporation — Kremlin

4. 911 AND GPS
Nationwide, U.S.A
√ Where did that old mill used to be, anyway? Reliance on GPS is starting to cause worry among emergency dispatch agencies and their operators. With the new workforce unfamiliar with map-reading, and no long-term memory of landmarks due to satellite navigation, dispatchers can be at a loss to locate a frantic cell phone caller who is in a locally known but hard-to-map area — or if systems are down. Training is starting up in some areas to address the issue. In September of last year, the police commissioner in Staten Island ordered dispatchers to undergo retraining after a 911 caller claimed the operator she reached had no knowledge of the Staten Island Railway.

• January 12, 2016 Officer.com: GPS and the 9-1-1 Operator
• September 29, 2015 Staten Island Live: 911 call takers, dispatchers ordered to learn more about Staten Island
• APCO Minimum Training Standards for Public Safety Communicators

The post GNSS Hotspots | January 2016 appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

GAGAN — India’s SBAS
Redefining Navigation over the Indian Region
Recognizing the potential benefits of satellite-based augmentation systems, India took the early initiative to create its own SBAS called GAGAN — GPS Aided Geo Augmented Navigation.

GAGAN — India’s SBAS
Redefining Navigation over the Indian Region
Recognizing the potential benefits of satellite-based augmentation systems, India took the early initiative to create its own SBAS called GAGAN — GPS Aided Geo Augmented Navigation.

GNSS Satellite-Based Augmentation Systems
A Potential New Time Keeping System for Future Generations
This article describes development and testing of a novel time-keeping system that could provide increased time-synchronization performance for future upgrades of both the onboard and ground segments of SBAS systems.

Measuring Navigation Payload Absolute Delay
In Radiation Mode
Satellite navigation signals transmitted on different carrier frequencies are imperfectly synchronized due to different hardware paths corresponding to each signal. An engineering team from the Indian Space Research Organization presents a laboratory method to measure the total navigation payload delay in satellite transmissions.

The post SBAS appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

The events brought out all of the relevant voices and served to illustrate not only the disposition of materiel and troops but also their intent and even the level of morale. 

Europe’s space community rang in the New Year with two of its brightest annual fixtures: the European Union (EU) Space Policy conference in Brussels and the European Space Agency (ESA) media briefing in Paris.

The events brought out all of the relevant voices and served to illustrate not only the disposition of materiel and troops but also their intent and even the level of morale. 

Insights from ESA briefing can be found in a news article in the “360 Degrees” section here. Here we will focus on the broad range of personalities and sentiments expressed at the Brussels event.

Elżbieta Bieńkowska, the EU Commissioner for Internal Market, Industry, Entrepreneurship, and SMEs (DG-GROW), i.e., the “Space Commissioner,” opened the Space Policy conference with good news about Galileo. After a troubled 2014, she said, 2015 turned out much better. 

“In 2015, we did indeed launch, as we said we would the last time we were here, six new Galileo satellites, doubling the number of satellites now into orbit.”

The European Commission (EC) will continue to build Galileo in 2016, striving for a speedy deployment schedule, including another batch of satellites that will enable the launch of initial services before the end of the year. 

“This is a challenge, but I count on ESA to make it happen,” Bieńkowska said.

So, if it doesn’t happen, we all know whose fault it is.

Galileo, Bieńkowska said, is not just an infrastructure; the EC is working to ensure that citizens get the full benefits, by promoting greater private and public uptake, with space data and apps expected to proliferate into many fields. And the commission will also work to identify and exploit synergies with the defense community.

All of which fits in nicely with Bieńkowska’s new main priority in the space arena — the EC’s 2016 European Space Strategy initiative. Although details are still fuzzy, the general idea seems to be to come up with “an ambitious strategy” to promote innovation and competitiveness in this key sector, to be adopted in the fall of 2016. Of course, it will encompass all of the EU’s space initiatives, not just Galileo.

“We will not develop the Strategy in isolation,” Bieńkowska said. “The coming months will feature a broad consultation process, and you are all invited to participate, starting today.”

And Bieńkowska repeated her call for improved governance, something we found surprising when she said it last year, coming, as it did, on the heels of a major governance breakthrough for the EC-ESA-GSA triangle.

This time she elaborated.

“Governance is not an end in itself; it is about how it allows us to deliver benefits.” 

By governance, we understand, she probably means cooperation among all the governing bodies, including national agencies, and, again, not just with respect to the Galileo program. “We must recognize the increasing role of the EU in space strategy,” Bieńkowska concluded.

Many themes and topics were covered at the Brussels event, and some recurring threads emerged, including, of all things, the importance of culture.

But first, a bit more on the nuts and bolts of Galileo. 

Matthias Petschke, director of EU Satellite Navigation Programs at DG-GROW said the “recovered” satellites 5 and 6, the ones launched into anomalous orbits in 2014, will ultimately serve a useful purpose, perhaps in relation to the Galileo search and rescue (SAR) service. “We are not writing them off,” he assured his audience.

He also reported good progress on the Galileo ground segment and repeated Bieńkowska’s announcement of initial services this year, including the Public Regulated Service (PRS). The much-anticipated Commercial Service (CS), however, is not likely to be ready, Petschke conceded, and indeed the details of how it will work have yet to be finalized.

We do know one thing relating to CS, he said. “On the user side, we see a big demand for authentication, for trusted PNT, and this will be a key element for new pay-as-you-go services like road charging.”

Culture Counts
“Culture,” with a capital C, and a diversity of it, are defining characteristics of the European character. Proof: although English was the working language at Space Policy event, a fair number chose to address the conference in the more poetic language of Molière, slightly fewer in Italian or German.

The Europeans remain rightly interested in who they are, and in being who they are, particularly in relation to the United States. Here are just some of the things people were saying about Europe, who and what it is, and who it isn’t:

Monika Hohlmeier, chair of the European Parliament’s Sky & Space Intergroup, said, “In America, they act first, then see what happens. Here in Europe, we think first and then talk a lot before we do anything.”

François Auque, head of Space Systems at Airbus Defence and Space, said, “Traditionally, culturally, we are terrified of failure. But today we are realizing that failure is acceptable.”

Michel de Rosen, president and CEO of Eutelsat, said, “Europe must lead, but Europe must also learn. The U.S. Department of Defense knows sovereignty is not the enemy of private sector, including private satellite operators. Europe needs to learn this too.”

Marco Fuchs, CEO of Galileo satellite builder OHB, urged, “We need to dare to start new projects. We shouldn’t be talking about just using Galileo. We need to be looking towards the next big project.”

Member of European Parliament Cora van Nieuwenhuizen also referenced, in her own way, Europe’s fear of failure, and its result: “We have a problem in Europe with venture capital. We put a lot of investment into start-ups — but as much as we do, they end up in Silicon Valley anyway.”

Maroš Šefčovič, European Commission vice-president, spoke about daring to look ahead: “I just heard something about US legislators who are discussing passing new laws to cover space mining, understanding that one day it will be cheaper to bring raw materials home from space than to bring them up from bottom of the sea. That’s looking ahead and this is the kind of visionary approach we need in Europe.”

Jean-Yves Le Gall, president of France’s CNES, drew the conference’s attention to the unexpected passing, just a day earlier, of the British musical artist David Bowie, who’s portrayal of an astronaut in his classic song ‘Space Oddity’ inspired millions. The mention of Bowie drew an unusual and heart-felt round of applause from the room. The Europeans do, after all, know good culture when they see it (or hear it).

Finally, Lowri Evans, the European Commission’s new Director-General of DG-GROW, being very much in touch with who and what she is, called out “Welsh!” when Parliament Vice-President Antonio Tajani imprudently referred to her as “English.” Talk about a “faux pas!”

Explain That?
Europe is a diverse place, and so it can be misleading to pretend that there is a single unitary European culture at all. But we might generalize, and many did at the Brussels conference, about the effectiveness of the communication practices among the European space community.

Thierry Mandon, France’s Minister of State for Higher Education and Research, said, “In space, Europe is world class, not just in engineering and technologies but also in the work of our institutions. But we need to tell our European success stories, like Galileo and Copernicus, better than we are doing now. We need to encourage our students, turn opinion, and sell our space programs.”

Jerzy Buzek, distinguished chair of the European Parliament’s ITRE Committee, also stressed to need to communicate more effectively: “We need to explain space to our public opinion. If we need more money, we need to say why.”

Buzek saluted Johann-Dietrich Wörner, the new Director General of the European Space Agency (ESA), referring to one of Wörner’s more forward-thinking ideas. “Your ‘Moon Village’ idea seems crazy,” Buzek said, ‘but maybe we need a crazy idea.” Fifty years ago, he recalled, the United States turned a crazy idea — sending a man to the moon — into half a century of technological excellence and space achievements.

To which sentiment, Pascale Ehrenfreund, chair of DLR’s Executive Board, jumped in. “The ‘Moon Village’ is not a crazy idea but a long-term road map, leading to a far goal with many defined milestones.” 

Ehrenfreund, by the way, is the person who replaced Wörner as DLR’s Executive Board chair when he went off to head up ESA.

Turning again to culture and communication, Ehrenfreund continued, “Solutions that work in the U.S. don’t necessarily work here.” She said Europe shouldn’t be afraid to talk about its successes, including the Rosetta mission, and she lamented, as had van Nieuwenhuizen, the crippling lack of venture capital in Europe.

Finally, Pierre Delsaux, EC Deputy Director-General of DG-GROW, supported the notion, now becoming more popular by the minute, of better communication. “All of us here are convinced of the importance of space, but what we need to do is convince everyone else. We all have a duty to explain.”

GSA Head Speaks
Taking a welcome break between sessions, we sat down for a chat with Carlo des Dorides, executive director of European GNSS Agency (GSA), the organization tasked with delivering Galileo services, to see if he could explain a few things.

The Galileo open signal will deliver plenty of new benefits to users everywhere in the world. So, how do you explain to the European taxpayer that only he/she is paying for it? It is true, des Dorides said, that the new precision and increased availability resulting from a whole new constellation of navigation satellites will certainly create new economic opportunities for everyone. “It’s like building a highway,” he said, “We can all use it, and we all need it to create new value.”

As for what Europe gets that no one else gets, he pointed to the PRS, with its more robust and secure signal. “The PRS is an EU 28–only service – with some exceptions. We are not talking only about defense but also institutional users — for example, banks, the insurance industry, national authorities and civil protection.”

The Commission is currently undertaking highly confidential negotiations for third-party access to the PRS, including with the US. And we know that manufacturers are moving forward with the preparation of PRS receivers.

Is the 2016 date for initial services achievable? After all, we’ve seen too-optimistic timetables for Galileo before. Des Dorides said the timetable is realistic. In October, he said, if all goes according to plan, the first tests of initial services will be undertaken. 

“This will be a very important opportunity,” he said, “not just for the system itself but as a signal to the market. Receiver manufacturers need this signal to be able to release their new chipsets.”

What kind of performance will we see? 

“With a constellation of 10 operational satellites to start with, we don’t expect to see the final performance levels,” said des Dorides, “but we will immediately see superior continuity and availability with the initial service.”

Des Dorides said initial PRS pilot projects are planned, and the SAR service will also be launched on a limited basis. Just as Petschke acknowledged, the CS, des Dorides said, is not ready, but it is coming along and will feature high accuracy and authentication.

The world as a whole certainly is benefiting from the development and deployment of new global satellite navigation systems. The current scenario sees at least four major systems delivering worldwide coverage within the next 10 years, and most future receivers being able to use all of the signals at any given time.

So, is this really the most efficient way to run world navigation, with four overlapping systems, each with its own complete hierarchy, from political to technical to operational control? Wouldn’t it make more sense to consolidate at some point and save a little money?

“I personally think this is the way to go,” said des Dorides. “The current view of many, however, is to see the diversity of the systems as valuable in itself, reducing vulnerability. And then, these are also high-strategic-value assets; so, political convergence is needed first.”

“But we do need a vision,” des Dorides added, “and it is my own view that we need to bring the systems closer together.”

Big Data and Dual Use
Back on the Space Policy conference program, we saw some serious discussions about data and defense taking center stage.

Carlos Suarez, executive vice-president of Indra Sistemas, a large, multinational Spanish information technology and defense systems company, said, “Big data, combining all data that is geo-located, even from social media, presents a huge opportunity. But,” he adds, “U.S. industry is always ahead of us — and I mean companies like Google and Amazon, not just the classic aerospace giants.”

Philippe Brunet, the EC’s director for Aerospace, Maritime and Defense Industries at DG-GROW, responded to a question from the audience on the wisdom of releasing Copernicus-generated Earth observation data for free, and then seeing companies like Google using it to create exciting new apps and services. Google then offers such services to its users for free, essentially undercutting any potential opportunities for European (or any other) small- and medium-sized enterprises (SMEs) to use this data to develop and sell their own apps.

Brunet said the policy of open and free data is fundamental, and ultimately benefits everyone. “There are lots of SMEs in Europe making money off of free U.S. LandSat data,” he pointed out. “That’s U.S.-generated data, and European companies are not paying for it. Copernicus gives us raw data, but the added value, where the money is made, is in the algorithms, and that opportunity is there for everyone.”

On the subject of dual use, Brunet said, “We in Europe now feel the threats all along our borders and within our borders and the EU is reassessing these threats.” 

Next June, he said, the EU’s new global strategy on security and foreign policy will be unveiled, with DG-GROW coordinating and with input from the European External Action Service (EEAS). Space will be an important element in this new strategy, Brunet confirmed.

The fact is, “defense” was, not so long ago, virtually a taboo subject within the European Commission, where, Brunet said, “the concept of civil programs and ownership is fundamental.”

But the line between civil and military is becoming more and more blurred, he argued. “Raw data produced by satellites is neither civilian nor military. It is the way that the data is shaped, based on need, that makes it civilian or military, the way it is used to cover specific needs.” 

He pointed to the example of weather satellites that serve average citizens every day but also provide crucial information for military operations.

“Galileo,” Brunet continued, “is a civilian system under civilian control, but the PRS, for example, will be suitable for applications where robustness and reliability are needed. It will be up to governments to decide who can use it and what for.”

Jorge Domecq, chief executive of the European Defense Agency, expressed concern in simple terms. “In Europe, the European Commission, ESA, we have never had a comprehensive dual-use approach, and this is damaging to cooperation with our partners.”

Jean-Pierre Serra, vice-president for defense and security, Airbus Defense and Space, put it more simply still. “Given budget restrictions, we must use space assets for both.”

Other notable speakers took the stage in Brussels, lending words of wisdom and strong encouragement of a more political nature, including Vice-President Antonio Tajani, but we fear we may already have tarried too long in the European capital.

The post Galileo Themes, Threads and Visions appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

Working Papers explore the technical and scientific themes that underpin GNSS programs and applications. This regular column is coordinated by Prof. Dr.-Ing. Günter Hein, head of Europe’s Galileo Operations and Evolution.

With the support of the European Space Agency (ESA), a European team designed a frequency- and time-transfer process and validated its performance in a complex navigation test bed. This two-way time-transfer technology took advantage of the following:

Working Papers explore the technical and scientific themes that underpin GNSS programs and applications. This regular column is coordinated by Prof. Dr.-Ing. Günter Hein, head of Europe’s Galileo Operations and Evolution.

With the support of the European Space Agency (ESA), a European team designed a frequency- and time-transfer process and validated its performance in a complex navigation test bed. This two-way time-transfer technology took advantage of the following:

• the existing GMSSS (Ground Mission Segment to Space Segment) uplink tracking, telemetry, and control (TT&C) ground station of the Galileo system
• an ad hoc design of the geostationary satellite (GEO) navigation payload which included the possibility of eliminating the onboard atomic clock reference through the use of an onboard oven-controlled crystal oscillator (OCXO) whose fine frequency tuning is controlled from a long onboard-to-ground delay locked loop (DLL),  
• the tracking accuracy provided by enhanced correction capability provided by using a downlink Galileo navigation signal generation unit (NSGU) based on the latest generation of Galileo signals, which employ a binary offset carrier (BOC) modulation. 

As discussed in this article, the measured clock synchronization performance can be summarized by the following metrics: 325 picoseconds (1σ) standard deviation of one pulse per second (1PPS), onboard-to-ground error (time transfer) and an Allan Deviation better than 10^-12 over an averaging time of 180 seconds (frequency transfer). This performance would enable an order of magnitude improvement in accuracy of the current European Geostationary Navigation Overlay Service (EGNOS) specification for the time offset error between the system’s GEO time scale (GEO Time) and the EGNOS Network Time (ENT) at 3 σ.

Current and Future SBAS Architectures
A long, on-going technical discussion has taken place about what the next generation of satellite-based augmentation systems (SBAS) should be in light of the evolution of the new GNSS constellation systems and the maintenance cost of the complex ground synchronization layer of first-generation SBAS systems. 

The original shortcut of using the transparent approach portrayed in Figure 1 was conceived as a very ergonomic solution because the payload impact was very low in terms of power and weight. (Both EGNOS and the U.S. Wide Area Augmentation System, or WAAS, are currently based on this approach). In this context, “transparent” means that the downlink SBAS signal is not generated on board the satellite and the payload segment will only amplify and filter, with up- and down-conversion at the RF front end of the payload, the ground-generated SBAS signal. 

This operating mode shown in Figure 1 was usually conceived as an add-on, piggy-back solution, to be used on a standard GEO communication payload This approach was certainly viewed as a good trade-off back in the early and mid-1990s when those systems were conceived, because it minimized the SBAS payload cost of being hosted on a standard telecommunication satellite.

In addition to simply broadcasting wide area corrections, a GEO satellite capable of providing a ranging service can clearly bring added value to the SBAS system performance as implemented, for example, in the WAAS system. Also, the accuracy of the corrections could benefit from the use of new SBAS architectures in which the GEO Time of a set of GEO SBAS payloads could be closely controlled and synchronized.

With the purely transparent approach, a major difficulty in using the SBAS ranging signals, or defining synchronous payload times, arises from various problems. These include a large group-delay wandering of the fully analog onboard signal path and difficulty maintaining the code/carrier coherence due to the onboard L-band downlink and uplink synthesizers. (In the absence of any clock reference synchronization, these are de facto asynchronous with respect to the reference clock in the navigation land Earth stations, or NLES, that serve as ground transmitters for the current EGNOS system). 

The issue is further exacerbated considering that the existing GEO SBAS systems typically use more than one satellite to cover the regional area, thereby making it challenging to define a very accurate common synchronous time for GEO space vehicles (SVs). (EGNOS for Europe and North Africa coverage originally planned the use of three GEO SVs and currently is using only two of them) 

A certain level of mitigation of the aforementioned issues could be achieved using the “regenerative” approach portrayed in Figure 2. Here, “regenerative” means that the carrier reference clock is regenerated as a quasi-synchronous replica of the ground clock, allowing the code-carrier coherency losses to be reduced.

With this system architecture, an onboard 10.23-megahertz reference clock could be extracted by means of an add-on navigation uplink MISsion RECeiver (MISREC) responsible for synchronizing the incoming symbol carrier through its phase locked loop (PLL). This would allow, to a certain extent, a sort of feed-forward, one-way carrier synchronization whose performance should depend on a mandatory controlled de-embedding of the contribution of the GEO-estimated Doppler from the recovered carrier reference. 

Even in this case, the synchronization performances were expected to be much lower than a real two-way frequency transfer. Moreover, the large group delay variation due to the wandering in time (ageing) and temperature of the downlink and uplink analog electronics did not have any way to be controlled by design (as in a generative approach which allows separating the uplink and downlink contribution). Nor could this be controlled by onboard monitoring, given the completely analog payload structure that does not allow digital monitoring and compensating of the group delay (GD) variations. 

Most relevant is the fact that both of the aforementioned solutions (transparent and regenerative) have issues related to the integrity of the navigation message received by the GEO payload and broadcasted to the user segment. The currently available strategy for SBAS transparent (or regenerative) approaches is to perform an integrity check of the navigation message uplinked from the NLES with the same message being received by the ground control stations on the downlink. However, the navigation message integrity check being performed on the ground implies that the corresponding counteraction of broadcasting a specific alert message, in the case of round trip errors, is not instantaneous (resulting in limitation of the time-to-alarm actuation). 

Ultimately, there is no possible way of preventing the broadcasting of a misleading message with the SBAS systems presented in Figures 1 and 2 except by switching off the SBAS payload via a remote telecommunication link from the ground, which ultimately leads to a loss of availability for the augmentation service. A brute-force solution for guaranteeing synchronized and accurate broadcasting of ranging signals by a GEO, high Earth orbit (HEO), or inclined geosynchronous orbit (IGSO) SV is to imitate a standard navigation MEO SV reusing all the existing middle Earth orbit (MEO) payload hardware as shown in Figure 3. 

This approach has the inconveniences of poorly using the potential associated with the currently available time and frequency transfer technology, increasing the payload hardware cost due to the need for an onboard atomic clock in the clock monitoring and control unit (CMCU) with the associated redundancy scheme (as described in the article by D. Felbach et alia listed in Additional Resources), and reducing the navigation payload mean time to failure (MTTF) due to the limitations of the atomic clock in maintaining its frequency stability and accuracy for an extended number of years.

New Approaches to Implementing SBAS Payload Architectures
An initial innovative effort to find an optimized solution to these issues, at least for possibly using GEO or HEO signals of a generative payload for ranging and increased availability, came from the Japanese Quasi-Zenith Satellite System (QZSS). The QZSS space segment consists of three SVs placed in periodic highly elliptical orbit. The perigee altitude is about 32,000 kilometers and the apogee altitude, about 40,000 kilometers. All QZSS satellites will pass over the same ground track. The QZSS system was designed so that at least one SV out of three available would always be present near zenith over Japan. 

Given its orbit, each satellite appears almost overhead most of the time (i.e., more than 12 hours a day with an elevation above 70 degrees). This gives rise to the term “Quasi-Zenith.” As for controlling the group delay wandering and the code/carrier coherency on board, despite the intrinsic advantage of the regenerative payload approach vis-a-vis the transparent one, neither of the first two architectures (transparent and regenerative) presents the advantages of the generative approach. 

However, using a generative payload with an atomic clock on board (as shown in Figure 3) is not efficient in terms of recurrent cost, weight, and power consumption compared to the existing transparent implementation. Additionally, its MTTF would always be limited compared to the analog transparent payload counterpart, or to a generative approach, with an OXCO on board.

Therefore, current design/implementation trends for regional augmentation satellites sought to take advantage of the fact that:

• a GEO SV has constant visibility from a ground control station, optimally placed in the regional coverage area, and
• a GEO SV Doppler dynamic is quite limited (also compared to the HEO of the QZSS system) reducing the Doppler shift effect in the onboard clock’s control loop.

This led authorities (such as Japan) to fly a quite stable OCXO controlled from a ground station on their satellites, with a very accurate, long-loop (onboard to ground) frequency and time transfer process.

For these reasons Space Engineering decided to explore the possibility of implementing an improved version (with respect to that implemented in the QZSS system) of the generative SBAS payload design with an OCXO on board (Figure 4). In this system, the OCXO frequency and phase alignment to the ground reference atomic clock is precisely controlled from the ground as in the Japanese QZSS system. Therefore, a real navigation-payload proof of concept has been designed, taking into consideration the compatibility with — and actually enhanced with respect to — the Galileo MEO payload as regards the uplink and downlink signaling.

With respect to an NSGU, the EGNOS Regenerative Payload (ERP) only needs to be augmented with on-board SBAS signal generation (Galileo-plus-SBAS dual-frequency downlink signaling), in addition to the OCXO synchronization control module. Additional MISREC firmware modification should also be ported from the ERP design to enable the ERP system payload implementation based on the reuse of Galileo full operational capability (FOC) space-qualified mission receivers.

ERP System Innovative Aspects Compared to QZSS 
The proposed architecture for ERP time keeping is quite innovative even with respect to the only existing worldwide navigation application where an OCXO was first flown in a SV suitable for navigation signal broadcasting, i.e., the QZSS.

The innovative aspects of the ERP (which is the short name of the ESA contract financing this study) stem from the following considerations regarding the QZSS implementation design choices:

• The QZSS system actually modified the uplink signaling providing an uplink CDMA signal (equivalent to the GMSSS uplink signal used in ERP), with the carrier frequency being Doppler-compensated on the ground, i.e., with a time variable frequency offset profile steered by the uplink TT&C modulator. (For details, see the article by M. Fukui et alia in Additional Resources.) For the QZSS-equivalent MISREC onboard receiver, the net effect would be that of receiving a signal with almost null Doppler.
• The QZSS master control station (MCS) would keep two replicas of the QZSS time reference — one being the equivalent of the ENT, that is, the QZSS reference time steered to GPS system time. The other replica would always remain tied to the QZSS time reference, but be time phase–shifted in anticipation of the long delay locked loop–estimated uplink delay time. This implies that the QZSS equivalent MISREC on-board receiver would demodulate a CDMA ranging signal with almost zero delay (i.e., that it would already be aligned with the QZSS time reference). However, the two aforementioned QZSS design implementation choices have the following drawbacks: 
• The QZSS approach implies that for each augmentation SV a separate transmitted signal replica should be handled by the MCS, which would bring increased costs for the necessary digital and RF hardware as well as the expense of calibration maintenance. (EGNOS initially planned the use of three GEO SVs.) The costs would be avoided by using a single TT&C modulator with different spreading codes as proposed for the ERP system. 
• The uplink TT&C station could not be a standard TT&C because of the accurate frequency and time-steering agility required for the QZSS system. For the ERP implementation, the same GMSSS control station used for Galileo MEO uplink message control could be fully reused.  
• The accuracy of the QZSS Doppler compensation, and therefore of the on-board 1PPS synchronization, will also depend on the analog performance of the uplink and its capability to preserve and accurately steer the Doppler profile estimated and injected from the ground control loop. (For the same class of OCXO device, in the ERP system the clock control precision is determined only by the accuracy of the digital algorithm as implemented.) 
• The accuracy of the onboard 1PPS phase alignment with respect to that on the ground would depend on the accuracy of the phase-delay technology used on the ground segment for finely controlling the uplink delay.
• Finally, nulling the uplink Doppler would become vastly more complicated using the uplink carrier-phase ranging measurement to increase the accuracy of the time-keeping system. This would occur because in the multi-frequency uplink signaling mode — even in the most complex triple-carrier ambiguity resolution approaches — the carrier ambiguity resolution algorithms reveal convergence issues when applied to propagation channels with zero or near-zero Doppler shift. (This factor has not yet been addressed and solved in the technical literature or applications).

Although the Japanese should be given the credit of having first pioneered the concept of flying a navigation reference OCXO, these limitations of QZSS drove the ERP system architecture design toward a different and improved solution.

Validation Test Bed for ERP Time-Keeping
Figure 5 provides a schematic representation of the ERP test bed architecture. All the ground and payload equipment is represented with real hardware devices operating in real time so that any test configuration could be operated for an indefinite period of time. The test-bed architecture can also validate an SBAS user segment with third-party, real-time receivers capturing signals in space (SIS) and mixing them with virtual GEO RF signals propagated by the test bed.

This real-time test bed is suitable for assessing any frequency and time transfer performance, including emulation of transparent, regenerative, or generative SBAS payloads. It has this capability thanks to a single, fully reconfigurable COTS software defined radio (SDR) hardware platform. 

The ERP test bed is also conceived to be interfaced with the ESA Support Platform for EGNOS Evolutions & Demonstrations (SPEED) test bed where the new or current correction algorithms could be executed and validated in real time. These could then be virtually uplinked and broadcast employing the ERP test bed with the generative approach so as to characterize the ERP test bed’s performance improvements for the user segment operating any third-party user SBAS receivers.

The overall test bed shown in the accompanying photo is quite compact, due to the reduced form factor of the COTS hardware platforms (all three-unit racks, i.e., 10 by 16 centimeters). The test bed can implement the entire digital and RF processing and synthesis of all the segments involved in an SBAS system of arbitrary complexity, in a six-unit 19-inch sub-rack (colored segment of left side of Figure 6). 

The downlink ground controller timing receiver (TRX) is a triple-channel, dual-carrier GNSS receiver in a two-unit (2U), 19-inch assembling case built from the same SDR board described earlier and a dual-carrier, antenna-ready GNSS RF front end down converter. Two custom hardware board designs have also been developed, manufactured, and successfully tested. The first is the CMCU board hosting the onboard OCXO and its fine frequency controller. The second board is a custom 6U 19-inch backplane board designed to mechanically and electrically connect all the test bed boards within the 6U sub-rack. Accompanying photos (see inset photo, above right) show the ground controller time receiver and its various components.

Performance Achieved by ERP Time Synchronization
The equipment designed for the ERP test bed uses state-of-the-art hardware and real-time digital signal processing. Just to give a measured figure of merit of the overall accuracy (hardware plus channel emulator and demodulator algorithms) of the ERP test bed, we have compared the downlink GNSS ground receiver pseudorange (PR) with the reference trajectory of the EGNOS PRN 120 SV expressed in floating point. This reference trajectory was, of course, controlling the channel emulator during the test execution. Figure 7 shows the layout of the test bed for this calibration test. This configuration is synchronous and basically noiseless so as to achieve the highest-accuracy boundary performance. 

In this test, the interaction of the test bed orbital propagator (OP) with the real-time hardware — composed of the NSGU, frequency up-converter unit (FUU), downlink channel emulator (DCE), and downlink test receiver (TRX) — was portrayed and quantified in terms of synchronization and calculus accuracy. The TRX code-phase delta pseudorange is estimated at 10 hertz and quantized at 64 bits (fixed point). The Test-bed ConTroller (TCT) reads these data directly from the TRX hardware receiver, and compares the logged data with the EGNOS PRN120 SV trajectory log expressed in 64-bit IEEE-754 float-ing-point format at 10 hertz. 

This test also calibrated the optimal index of the orbital propagator (OP) controlling the channel emulator during the test. The optimal index was used to compare the 10 downlink pseudorange TRX readings per second used by the downlink compensation algorithm with the OP reference trajectory, producing 100 controlling values per second (and thus identify the actual downlink channel emulator actuation index) needed to accurately model the channel emulator hardware. 

In Figure 8 the blue curve represents the error for the optimal, selected OP index because, overall, it minimizes the peak-to-peak TRX hardware pseudorange error with respect to the floating point OP SV trajectory represented in IEEE 64-bit standard numerical format. Therefore, comparing the TRX hardware pseudoranges to the floating point EGNOS PRN 120 OP trajectory and elaborating the first statistical moments of the blue curve we could obtain 15 picoseconds of standard deviation error (as reported in Table 1). 

For what concerns the real payload operations, the ERP clock control algorithm is derived from a long ground-to-onboard DLL loop based on the onboard uplink pseudorange estimation. Such estimations are compared with the ground-estimated GEO position and Doppler. The time difference of the uplink-estimated pseudoranges and those of the ground-based orbit determination (OD) estimates is filtered through a second-order proportional integrative (PI) loop and is used to control the onboard OCXO frequency through a 16-bit, serial digital/analog converter. 

If there is no downlink compensation, the error between the real GEO trajectory and the OD ground estimates will cause the onboard 1PPS tied to the OCXO and the ground 1PPS tied to the atomic clock to drift with respect to each other. Figure 9 presents the 1PPS board-to-ground phase error for a tested set of initial position and velocity errors in GEO orbit determination as measured by a time interval counter (TIC) when no downlink compensation is applied. 

Therefore, the operative conditions of the new time-keeping system were tested in two different scenarios: single-frequency and dual-frequency downlink compensation. The single-frequency downlink compensation is used for low-cost systems and by itself, without additional prediction aiding, can fully recover only the OD errors with respect to the actual SV trajectory.

To make it completely operative, this approach needs to be augmented with a prediction of the uplink and downlink ionosphere delays. However, because the uplink signal is transmitted in the C-band while the downlink uses the L-band, an expected residual theoretical error occurs due to the different delays experienced by the various carrier signaling frequency bands. (Ka-band was also a possible alternative design choice although this choice would have increased the GEO Doppler shift dynamic. For this reason we selected the C-band.) 

Assuming in addition to this factor a 15 percent mismatch in knowledge of the maximum ionosphere delay on both frequency bands and the daily ionosphere solar activity, the error between the actual and predicted ionosphere delays on both uplink and downlink frequency bands would generate a theoretical error as the one presented in Figure 10. In such conditions the ERP test bed measured the 1PPS phase error at 45 dB-Hz of carrier-to-noise (C/N₀) ratio (applied on both uplink and downlink) is the one reported in Figure 11. As the reader may see, the measured results are absolutely in line with the theoretical expectation of Figure 10 overall for the ionosphere daily solar activity and night inactivity period.

As shown in Figure 11, partial knowledge of the ionosphere (the 15 percent mismatches) clearly results in a slightly varying bias error on the two onboard and ground 1PPS signals in the case of single-frequency downlink compensation only. Figure 12 instead reports the frequency stability in the single-frequency compensation mode when the OCXO is under the control of the ground-to-onboard DLL. 

The metric selected for frequency stability was the OCXO Allan Deviation (ADEV). In this mode, the ERP performance curve was compared to the ADEV of the rubidium atomic reference. It is also evident that frequency stability is not affected by the mismatch in predictions of the ionosphere delay, even in single-frequency downlink compensation mode, because the stability figure of merit actually converges to the same as the ADEV floor of the ground atomic clock.

Instead, when using the dual-frequency downlink compensation mode, the ionosphere delay could be completely solved and removed as a bias in the OCXO DLL control. Figure 13 shows the measured Allan Deviation (ADEV) in dual-frequency downlink mode. To accomplish this goal the ERP time-keeping system uses the Public Regulated Service (PRS) components of the Galileo L1 and E6 dual-frequency signaling system. The choice of using the Galileo PRS signals was due to the sensitivity of the ancillary message information needed to control the OCXO from the ground, so that the signal authentication protection layer of the Galileo PRS signals was invoked. 

Using the frequency-independent and frequency-dependent biases on the downlink enables correction of the onboard-to-ground loop phase error. In turn, this makes it possible for the ERP ground control station to fully control the onboard OCXO, achieving the measured 1PPS onboard-to-ground phase error performance, at 45dB-Hz C/N₀ on both uplink and downlink paths, as reported in Figure 14. 

As is evident from Figure 14, the dual-frequency downlink-compensation mode shows no bias error because both orbit determination errors and ionosphere biases are completely removed from the ground-to-onboard long DLL loop controlling the onboard OCXO. The performance of the measured 1PPS ground-to-onboard phase alignment can be expressed in terms of the minimum, maximum, and standard deviation error of the measured curve of Figure 14, as reported in Table 2.

Therefore, assuming a convergence threshold represented by an ADEV better than 10-12, the test results for the frequency-transfer process of the ERP time-keeping system, as reported in Figure 12 and Figure 13, show that the dual-frequency downlink mode needs an averaging time of 180 seconds to achieve the same stability as the atomic frequency standard on the ground. In comparison, the single-frequency downlink mode requires an averaging time of only 90 seconds. So, despite the absolutely superior performance of the dual-frequency downlink mode, which totally removes the ionosphere bias from the OCXO clock control loop, the single-frequency downlink mode exhibits a significantly shorter frequency-transfer convergence time.

Finally, the measured clock synchronization performances can be summarized by the following figures of merit: 

• 325 picoseconds (1σ) standard deviation of 1 PPS onboard-to-ground error for the TIC 1PPS phase error log as a function of the test time, and  
• an Allan Deviation better than 10^-12 in an averaging time of 180 seconds under the following operative conditions: uplink and downlink C/N₀ = 45dB-Hz, downlink dual-frequency ionosphere compensation, initial orbit determination errors of five meters (range) and 100 mm/sec (speed), and no GEO ephemeris correction for a two-day continuous test time (i.e., with a large OD accumulated error over the test time). 

The user segment ranging error in the ERP test bed is measured by comparing in real time the additional NSGU L1 channel tied to the ground station 1 PPS generated by the atomic clock signals and processed by a downlink channel emulator. This comparison assumes the same signal-in-space (SIS) propagation impairments of the signal generated on board the SV and its L1 SIS replica processed with the same DCE configuration generated by the on-board NSGU tied to the on-board CMCU 1 PPS and OCXO clock signals. 

Hence, this user ranging test quantifies how the clock synchronization performance, measured by the TIC (to derive the ground-to-onboard 1PPS phase error), would ultimately affect the user segment ranging information. It does this by comparing the two L1 Pseudoranges recorded by the same downlink TRX ground receiver tied to the ground atomic clock.

The 1σ ranging standard deviation error on the pseudorange received by the user segment, measured in the dual-frequency downlink mode without any SBAS correction being applied, was only 576 picoseconds, showing a very small delta error of 251 picoseconds (576 ps – 325 ps) due to the residual error from the OCXO clock reference synchronization spreading through the downlink NSGU and up-converter synthesizers. 

Conclusions
The ERP time-keeping system designed in the frame of the European GNSS Evolution Program (EGEP) has been validated in terms of performance for the EGNOS PRN120 SV study case. This validation included several configurations with all real-time hardware in the loop and with uplink Galileo TT&C and downlink Galileo SIS–compatible modulations. 

The ERP time-keeping system is an improvement and optimization of the QZSS early concept and allows control of a large set of navigation and communication GEO payloads with a single ground control station. This will also be possible with an IGSO satellite for polar region augmentation. The ERP system uses standard CDMA uplink TT&C without — as in the QZSS system — the need for nulling the SV-to-ground channel delay and Doppler profile.

With the QZSS system, having more GEO satellites to control requires more specialized and separate modems. In contrast, regardless of the number of satellites, the ERP system only needs a single TT&C modulator equipped with the various PRN sequences in addition to differentiated and uplink-pointed ground antennas. This represents a great savings in hardware cost and complexity and the possibility of reusing the existing GMSSS uplink ground stations. 

As discussed in this article, the ERP synchronization performance is very good: dual-frequency downlink compensation with orbit determination errors of five meters and 100 mm/sec with an uplink and downlink C/N₀ level of 45dB-Hz can synchronize with a 1PPS standard error deviation below 0.325 nanosecond over two days continuous test time and without any update of the OD’s GEO ephemeris estimates. 

We have also identified several areas where additional improvements could be made to gain additional tens of picoseconds of accuracy or in case it would be needed, to maintain the same synchronization performance of the ERP Test Bed when implementing the prototype flight model payload (such as GEO-3 Payload) with real space-qualified components. 

Assuming that the ground atomic clock time of the ERP test bed represents the ENT (EGNOS Network Time), we have shown that the generative EGNOS payload performance (GEO Time – ENT) is equivalent to the TIC-measured on-board (GEO Time) to ground 1 PPS (ENT) phase error performance of 0.325 nanoseconds (1σ), i.e., below one nanosecond. Compared to ESA’s EGNOS System Requirement Document specification — GEO Time/ENT synchronization ≤10 nanoseconds (3σ), this performance represents at least one order of magnitude improvement in the GEO Time synchronization. 

In contrast with the current EGNOS transparent implementation where the GEO Time/ENT synchronization ≤10 nanoseconds (3σ) must be considered, after SBAS ranging corrections are applied to the user receiver, as anticipated and demonstrated through testing, an order-of-magnitude improved GEO time synchronization performance of the ERP time-keeping system is achieved. This improvement is independent from that of the corrections applied by a user SBAS receiver and, therefore, independent from the accuracy of the correction being broadcasted by the SBAS system. Moreover, after 180 seconds of averaging time, the onboard OCXO frequency stability is indistinguishable from the that of the ground-based atomic clock.

Finally, an easier and more agile upgrade of the atomic ground reference clock technology (following the improvements of such technology in the upcoming decades) could be made on ground leaving the payload segment untouched during service operations.

Acknowledgments
The work described in this article was financed under ESA Contract No. 22704/09/NL/IA, “Development of Breadboard for Regenerative Payload for Regional Navigation Services to Support EGNOS Evolutions” (ERP) from the European GNSS Evolution Program (EGEP 12 Contract). The views expressed in this article are solely the opinions of the authors and do not necessarily reflect those of the European Space Agency.

Additional Resources
[1] Digimimic s.r.l., Dual Carrier GNSS RF Front End, DM9100
[2] Digimimic s.r.l., 1.1-1.6 GHz Triple RF Up Converter, DM9300
[3] Felbach, D., and D. Heimbuerger, P. Herre, and P. Rastetter, “GALILEO Payload 10.23 MHZ Master Clock Generation with a Clock Monitoring and Control Unit,” Proceedings of the 2003 IEEE International Frequency Control Symposium and PDA Exhibition Jointly with the 17th European Frequency and Time Forum, May 2003
[4] Fukui, M., and M. Iwasaki, T. Iwata, and T. Matsuzawa, T., “Hardware Experiment of RESSOX with Ground Station Equipment,” Proceedings of the 40th Annual Precise Time and Time Interval (PTTI) Meeting, December 23, 2013
[5] Global Positioning System Wide Area Augmentation System (WAAS) Performance Standard, 1st Edition, October 31, 2008
[6] Office of National Space Policy, Cabinet Office, Government of Japan, “Quasi-Zenith Satellite System,” presentation to the Scientific and Technical Subcommittee of the UN Committee on the Peaceful Uses of Outer Space, 2013
[7] Space Technology, Galileo Timing Receiver
[8] Space Technology, S4MDM1.5 Wideband Programmable SW Radio IF-BB Modem Board (Datasheet)

The post GNSS Satellite-Based Augmentation Systems appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

Q: What is Doppler collision and is it a problem in GNSS?

A: Doppler collision is a physical effect in code-division multiple access (CDMA) systems where code measurement errors are observed due to cross-correlation effects. Doppler collision may occur when the Doppler frequency between signals from two different transmitters is smaller than the code lock loop bandwidth.

Q: What is Doppler collision and is it a problem in GNSS?

A: Doppler collision is a physical effect in code-division multiple access (CDMA) systems where code measurement errors are observed due to cross-correlation effects. Doppler collision may occur when the Doppler frequency between signals from two different transmitters is smaller than the code lock loop bandwidth.

In GPS, GLONASS, and Galileo, Doppler collision is rarely a problem because MEO satellites have equal Doppler frequencies for only very short periods. However, in systems where ground-based or geostationary transmitters are used for ranging the impact of this effect cannot be ignored.

The Problem
To understand how Doppler collisions can affect a CDMA signal let us consider the GPS C/A code for example. The 1023 chip GPS C/A pseudorandom noise (PRN) codes have ideal auto correlation peaks of +1023 and auto- and cross-correlation values of +63, –1, or –65. This results in a 24-decibel-protection level between the primary auto-correlation peak and any cross-correlation peak.

Most of the time additional protection is provided by the fact that each received signal has its own Doppler frequency offset as the result of each tracked satellite having a different range rate with respect to the receiver. However, if the difference between the Doppler frequencies from two different satellites is small enough, the cross-correlation peaks may interfere with the primary peaks and cause tracking errors. The tracking errors are similar to multipath in that a secondary correlation peak is distorting the main peak.

How small does the relative Doppler have to be for a risk of Doppler collision to appear? If relative Doppler is larger than the receiver code loop noise bandwidth (typically one hertz), then the tracking error is filtered and no Doppler collision effect is observed. In the case of GPS, although many instances over a given day occur in which the relative Doppler between two GPS satellites is small, the duration of each occurrence is very short because the satellites’ Doppler shifts are changing rapidly. However, for geostationary or ground-based transmitters, the relative Doppler remains small for significant periods and the effect can be severe.

Figure 1 (see inset photo, above right, for figures) shows the Doppler frequencies of two U.S. Wide Area Augmentation System (WAAS) satellites as observed over a two-and-a-half-day period at Calgary, Canada. Geostationary satellites are not perfectly stationary, and both WAAS satellites have slowly changing range rates. PRN 135 has a 12-hour period while PRN 138 has a more significant 24-hour component. As a result two periods occur each day when the relative Doppler between the two satellites is less than one hertz. Each of these periods lasts on the order of two hours.

Of course, a Doppler collision does not necessarily occur just because two satellites are transmitting signals with the same Doppler frequency. In addition, a maximum cross-correlation must occur. For this to happen, the receiver must be located where the relative delay between the two PRN codes corresponds to a peak (either +63 or –65 for GPS C/A) in the cross-correlation function of the two signals involved.

Geographically, for two geostationary transmitters such as WAAS 135 and 138, the cross-correlation amplitude creates a hyperbolic interference pattern that in low- and mid-latitudes is approximated by north-south running stripes where cross-correlation occurs. The width of each cross-correlation zone on the Earth’s surface is on the order of hundreds of meters as a result of the projection of the 293-meter-long C/A code chips onto the surface.

Figure 2 (see photo at the top of this article) shows the cross-correlation of PRNs 135 and 138 projected onto a 25-kilometer east-west by 5-kilometer north-south area centered at our lab at the University of Calgary (51 N, 114 W).

In addition to being located where a cross-correlation can be observed, the data bits must also be correlated for a Doppler collision to occur. In general, the magnitude of the cross-correlation peak will change as the data bits on each signal vary. If the relative Doppler of two GPS satellites was less than one hertz for a period of a few minutes, for example, at least the magnitude of the cross-correlation would change with every data bit transition that was not common between the two signals.

Unfortunately, navigation messages are often correlated across satellites within a GNSS. In the case of WAAS, the transmitted data bits are synchronized, which results in a narrow strip between the coverage areas of two WAAS satellites where navigation bits are highly correlated. As a result, when the relative Doppler is less than one hertz and a user is located at a cross-correlation peak, the same distorted correlation peak will be repeatedly observed and tracked, resulting in a Doppler collision.

To demonstrate the effect, we observed real signal samples in several locations around the University of Calgary (on our lab roof and also in an open field to minimize multipath) and also generated multipath free samples using a GPS/SBAS simulator. A software receiver was used to process the data.

Figure 3 presents plots of correlation peaks acquired from simulated data. The distortion in the peak cannot be attributed to multipath because the signal was generated in a simulator with the multipath-error function disabled. It is important to note that the distortion of the correlation triangle for one satellite is reflected as a mirror image in the correlation triangle of the second satellite.

Similar results were obtained with live data. Figure 4 shows the same effect in real data collected from the roof of our lab while Figure 5 shows live data collected in a soccer field approximately 400 meters to the west.

Both figures show the computed and observed correlation peaks, including a secondary trough to the right on PRN 135 (and on the left on PRN 138). The green line is the theoretical correlation peak while the red and purples lines show the correlation triangles generated from live samples.

The distortion is more observable, and more symmetric, in Figure 5 even though the theoretical trough is further from the main peak at this location. Note that the antenna in Figure 4 is in a moderate multipath environment (a roof) while the data shown in Figure 5 was collected in an open field. The code delay on the x-axes is not absolute, as there is a different receiver clock offset at each location.

These kinds of correlation triangle distortions can result in large multipath-like ranging errors. The paper by L. Lestarquit and O. Nouvel, listed in the Additional Resources section near the end of this article, shows that cross-correlation can lead to up to 9 meters of ranging error for a receiver with standard correlator spacing, and the error can be even higher, up to 18 meters, if the cross-correlation peak has a transition from –65 to +63.

Solutions
The simplest solution to the Doppler collision problem is to simply not use geostationary satellites for ranging. A second simple approach is to know when and where Doppler collisions will occur and either remove or de-weight measurements during collision epochs.

Finally, because the error is similar to multipath, many of the standard multipath reduction receiver designs — for example, narrow correlator spacing — will also reduce the effects of Doppler collision.

Impact on IRNSS and BeiDou
Not very many applications use SBAS signals for ranging. However, BeiDou uses and IRNSS plans to use both geostationary and geo-synchronous satellites for ranging signals. The effect of Doppler collision on code measurement error, in addition to stationary multipath error, will be significant.

The impact will depend on the codes used and on receiver design. BeiDou B1 uses 2046-chip truncated codes; so, unlike GPS, where only two values of cross-correlation peak (other than –1) occur 25 percent of the time, in BeiDou many possible cross-correlation values exist. The maximum peaks occur less than one percent of the time, although the others are frequent enough that Doppler collisions will occur. For IRNSS, the open service will use a binary phase-shift keying (BPSK) modulation similar to GPS; so, again depending on the codes used, the effect could be significant.

An additional problem is that, because Doppler collision results in a multipath-like bias in the code measurement that can last for minutes to hours, using code measurements from these geostationary satellites to estimate the corresponding carrier phase ambiguities is risky. Obviously not many carrier-phase measuring receivers use standard correlator spacing, but this effect will have to be considered.

Further Reading
For a detailed introduction to GPS C/A codes and their cross correlation properties, refer to:
Van Dierendonck, A. J., and G. A. McGraw, and R. J. Erlandson “Cross-Correlation of C/A Codes in GPS/WAAS Receivers,” in Proceedings of the ION GPS-99, September 14–17, 1999, Nashville, Tennessee USA, pp. 581–590

For an introduction to Doppler Collision and its effects on GNSS satellite tracking, refer to:
Balaei, A. T., and D. M. Akos, “Cross Correlation Impacts and Observations in GNSS receivers,” Navigation: Journal of The Institute of Navigation, vol.58, no.4, Winter 2011, pp. 323–333

Lestarquit, L., and M. Malicorne, M. Bousquet, and V. Calmettes “Correction Algorithm For SBAS C/A Code Interference,” in Proceedings of the ION GPS/GNSS 2003, September 9–12, 2003, Portland Oregon USA, pp. 1345–1354

For further results of the effects of Doppler collisions, refer to:
Lestarquit, L., and O. Nouvel “Determining and Measuring the True Impact of C/A code Cross-Correlation on Tracking,” in Proceedings of the IEEE/ION PLANS 2012, April 23–26, 2012, Myrtle Beach, South Carolina, pp. 877–885

The software receiver used in this work is described in (and included with):
Gleason, S., and M. Quigley and P. Abbeel, “A GPS Software Receiver,” Ch 5 of GNSS Applications and Methods, S. Gleason and D. Gebre-Egziabher, eds., Artech, Boston, 2009

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The GPS Aided Geo Augmented Navigation (GAGAN) system was developed by the Indian Space Research Organization (ISRO), together with Airports Authority of India (AAI), to deploy and certify an operational satellite-based augmentation system (SBAS). The system’s service area covers the Indian Flight Information Region (FIR), with the capability of expanding to neighboring FIRs. 

GAGAN provides a civil aeronautical navigation signal consistent with International Civil Aviation Organization (ICAO) Standards and Recommended Practices (SARPs) as established by the Global Navigation Satellite System (GNSS) Panel. The GAGAN system provides non-precision approach (NPA) service accurate to within the radius of 1/10th of a nautical mile (required navigation performance or RNP-0.1) over the Indian FIR as well as precision approach) service of APV-1.0 (APproach with Vertical guidance) over the Indian landmass on nominal days.

The system is interoperable with other international SBAS systems such as the U.S. Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS), and the Japanese MTSAT Satellite Augmentation System (MSAS), and provides seamless air navigation across regional boundaries.

GAGAN Elements
The GAGAN system consists of the following elements for the effective implementation of SBAS over India. (See Figure 1.)

1. Indian Reference Station (INRES) — at 15 locations across India
2. Indian Master Control Center (INMCC) —two at Bangalore
3. Indian Land Uplink Station (INLUS) —three stations, two at Bangalore and one at New Delhi
4. Geostationary satellites (GSAT8/GSAT10) in orbit and one on-orbit spare in GSAT-15 launched on November 10, 2015
5. A data communication subsystem —two optical fiber communication (OFC) circuits and two very small aperture terminal (VSAT) circuits.

The following sections describe each element in greater detail.

INRES – Indian Reference Station. The INRES stations collect measurement data and broadcast message from all the GPS and GEO satellites in view and forward them to the INMCC for further processing. The 15 INRES stations are established at Ahmedabad, Bangalore, Jammu, Guwahati, Kolkata, New Delhi, Port-Blair, Trivandrum, Jaisalmer, Goa, Porbandar, Gaya, Dibrugarh, Nagpur and Bhubaneshwar. 

INMCC – Indian Master Control Center. The data collected by each INRES across the country are transmitted to INMCC in real time (every second) and processed for the generation of correction and integrity parameters,in the form of SBAS messages. The generated SBAS messages are sent to INLUS for further processing. 

INLUS – Indian Land Uplink Station. The INLUS receives the SBAS messages from INMCC, formats them for GPS compatibility and uplinks the SBAS messages to GEO Stationary satellite for broadcast to the user community. The SBAS messages contain information that allows SBAS receivers to remove errors in the GPS position solution, thereby allowing for a significant increase in location accuracy with reliability. Along with the corrections, the confidence parameters (integrity) are also computed and provided to the users as messages. The messages are up linked in C-band to GSAT-8/GSAT-10 GEO satellite through Indian Land Uplink Station (INLUS) which are down linked in L1 & L5 band to the users. The broadcast messages are used by SBAS compatible receivers which compute its position while applying corrections over GPS signals.

GEO Satellite. ISRO is responsible for providing GEO Satellites (SPACE segment) to the GAGAN program. Three GEO satellites GSAT-8 (see accompanying photo, above right), GSAT-10, and GSAT-15 carry the GAGAN payload. GSAT-8 (located at 55 degrees East) and GSAT-10 (83 deg East) are already transmitting GAGAN SIS (Signal in Space) with PRN127and 128. The GSAT-15 carrying the GAGAN payload is stationed at 93.5 degrees East and has been allocated PRN132.

Figure 2 provides a schematic overview of the GAGAN final operational phase (FOP) configuration. Figure 3 shows the coverage area of GAGAN signals.

Ionospheric Model Development
The ionospheric behavior over the low-latitude Indian regions is not quiet and is characterized by such features as scintillation, plasma depletion, large-scale energy density gradients, and so forth. To ensure optimal performance of GAGAN, ionospheric irregularities must be detected and the statistical confidence bounds adjusted accordingly. 

Need for a Region-Specific Model. To meet the required GAGAN performance, development and implementation of the best suitable ionosphere model were needed, a model that would be applicable to the Indian region. Moreover, preferentially the selected ionosphere-model algorithm should not call for changes in the existing SBAS message structure because any changes in the ICAO Minimum Operational Performance Requirements (MOPS) for GPS/WAAS airborne equipments-DO-229 would require agreement among all member states. Certifying GAGAN system with MOPS changes would have been a very time-consuming process. 

In constructing the model, grid ionosphere vertical error (GIVE) confidences must bind the errors not only at the ionosphere grid points but also for all interpolated regions between the grid points. Moreover, the error bound must be valid for both the nominal and the disturbed ionosphere. In order to capture large-scale features of the equatorial ionosphere anomaly, the model must provide an approach to overcome the inadequacy of the thin shell model (of electrons) over equatorial regions and capture the complex three-dimensional nature of the equatorial ionosphere.

Features of the Region-Specific Model Required for Indian SBAS. The region specific ionosphere model must have the following characteristics:

• implementable
• able to demonstrate integrity  
• provide adequate availability for the precision user 
• backward compatibility 
• reduce modeling errors and support precision approach service in the Indian region  
• define the equatorial ionosphere with sufficient accuracy and integrity 
• upon safety certification, be able to support operationally useful availability of vertical guidance in the service area over the landmass of the Indian FIR.

ISRO GIVE Model – Multi Layer Data Fusion (IGM-MLDF). In order to model the vertical movement of the ionosphere, the IGM-MLDF Model is designed to capture the ionosphere variability at two different ionospheric electron shell heights and finally provide a value for the user at a 350-kilometer shell height using a weighted average method. The model ensures that the broadcast GIVEs have a sufficiently high level of integrity so that the user ionosphere vertical errors (UIVEs) computed by user receivers will bound their vertical ionosphere errors with a very high probability. The GIVE guarantees the integrity of UIVEs, not only at the grid point but at all points of the four grid cells surrounding the grid point. The algorithm provides the delay and the confidence values for the user, resulting in improved accuracy and availability. 

The algorithms were analyzed and reviewed, and performance of each of these was examined based on exhaustive studies carried out using INRES and total electron count (TEC) data over Indian region (Figure 4). Several nominal and storm (disturbed ionosphere) days were selected and the performance of the candidate algorithms was tested on these days. 

A down-selected IGM-MLDF algorithm was further improved through extensive review by expert teams aiding in a series of performance enhancements followed by an extensive analysis to tune parameters in the algorithm and determine the achievable accuracy, availability, continuity, and integrity of GAGAN service. The key function of the algorithm is to compute the delays and confidences (fit error) at various shell layers and combine them at 350 kilometers, which does not require any MOPS changes.

This model is incorporated into the main GAGAN operational software by M/S Raytheon to compute and provide ionospheric corrections and integrity factors needed to meet the APV requirements.

Performance Evaluation
Regular performance evaluation is a key activity to estimate a system’s utility, effectiveness, and suitability to meet the desired requirements. 

Figures 5 through 9 provide a few snapshots of performance monitoring of GAGAN.

Signal-In-Space Validation through Dynamic Test
To obtain a preliminary assessment of GAGAN performance, a dynamic flight test was conducted between Hyderabad and Bangalore (Figure 10) using a GAGAN receiver on board an ISRO aircraft. Figure 11 shows the configuration of the GPS and SBAS antennas on the fuselage of the aircraft.

The GAGAN receiver data was analyzed to evaluate the position accuracy of the aircraft in kinematic environment at every one-second epoch and to verify the GAGAN SIS performance with respect to the required level of service. Based on the differential GPS (DGPS) and GAGAN data, the accompanying plots show the position accuracy along the flight path (Figure 12) in East (Figure 13), North (Figure 14) and Up (Figure 15) components. The horizontal protection limit (HPL) and vertical Protection Limit (VPL) were computed from the GAGAN data recorded during the flight test at every epoch.

The dynamic flight test results revealed the following:

1. The accuracy of GAGAN along East with reference to the truth (DGPS East reference) is less than 7.6 meters, 100 percent of the time, which is within the GAGAN performance requirement for all the sorties.
2. The accuracy of GAGAN along North with reference to the truth (DGPS North reference) is less than 7.6 meters, 100 percent of the time, which is within the GAGAN performance requirement for all the sorties.
3. The accuracy of GAGAN along Up with reference to the truth (DGPS Up reference) is less than 7.6 meters.
4. The position standard deviation of latitude, longitude, and altitude were found to be less than 4 meters, which indicates that the position accuracies of the GAGAN are well within the 7.6-meter requirement.

GAGAN Certification
The Director General of Civil Aviation (DGCA) formed a Technical Review Team (TRT) to examine specific safety-related artifacts and hazard records and to provide recommendations for resolving any observed issues. AAI had engaged the MITRE Corporation to support the certification effort. A two-step certification decision was adopted for GAGAN implementation. The TRT reviewed the integrity-related artifacts and hazard records and recommended the findings to the certifying authority (DGCA) for certification.

A Hazard Review Board analyzed and reviewed the effect of software and hardware changes recommended by the TRT on system performance, and cleared GAGAN for certification. The system and facility certification process was completed with joint inspection and review of all ground-based installations and documentation, as well as the system safety and environmental management processes. A team comprising members from DGCA, AAI, and ISRO conducted the inspection.

Initially, the DGCA certified GAGAN for en route operations (RNP 0.1) on December 30, 2013, and subsequently on April 21, 2015, for precision approach services (APV 1). APV1-certified GAGAN signals are being broadcast since May 19, 2015. 

Benefits to Indian Aviation
By adopting GAGAN for aviation, flight delays, diversions, and cancellations (DDC) will be minimized, while reducing controlled flight into terrain (CFIT) incidents by 75 percent. In addition, India’s new SBAS enables direct flight paths and reduction of separation minima, which will ease the workload for pilots and controllers.

Further, GAGAN facilitates enhanced oceanic air traffic control in areas where a ground-based navigation system is unavailable. Also, shorter direct flight paths enabled by GAGAN lead to time and fuel saving. Consequently, decrease of emission of greenhouse gases due to shortened flight paths results in reduction of air pollution. At the same time, flexible flight paths enabled by SBAS makes route deviations due to adverse weather uncomplicated and also enables reduction of aircraft-related sound effects in noise-sensitive areas. GAGAN is an enabler for low cost regional airports and will usher in greater connectivity to India’s fast-growing aviation market.

Although primarily intended for civil aviation, the GAGAN signal can be used by a wide range of civilian and non-aviation users. Across the Indian subcontinent, GAGAN will provide benefits beyond aviation to many other user segments, such as intelligent transportation, maritime, highways, railways, surveying, geodesy, security agencies, telecom industry, location-based services for consumers, and so forth.

Conclusion
We note that India is the fourth country in the world to implement an SBAS system for civil air navigation after WAAS, EGNOS, and MSAS. India has become the third country in the world to have APV-1 precision approach capabilities, and the first SBAS system in the world to serve the equatorial region. The GAGAN system is interoperable with other SBAS systems and will offer seamless navigation in the region bridging the gap between Europe and Japan.

Acknowledgment
This article is made possible through the support and contributions from ISRO, AAI, DGCA, Raytheon engineers, TRT review committee members, SIS Validation test team, MITRE, and other industry partners who have worked for realizing the GAGAN system.

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In satellite navigation, the user receiver finds its position by measuring its distance to satellites and knowledge of the satellite position. The distance is measured by ranging, i.e., finding the delay of the signal from the transmitter to the receiver. The delay will comprise of payload hardware delay and the geometric range delay. Hence, the payload delay of the signal from generation to radiation is very important and needs to be transmitted in navigation data. 

In satellite navigation, the user receiver finds its position by measuring its distance to satellites and knowledge of the satellite position. The distance is measured by ranging, i.e., finding the delay of the signal from the transmitter to the receiver. The delay will comprise of payload hardware delay and the geometric range delay. Hence, the payload delay of the signal from generation to radiation is very important and needs to be transmitted in navigation data. 

Figure 1 (see inset photo, above right, for figures and tables) presents a typical navigation payload diagram for multiple frequency signal transmission. Corrections for the bias components of the group delay differential are provided to the user segment in the navigation message using parameters designated as (T_GD) and inter-signal delay correction (ISC) as described in the Indian Regional Navigation Satellite System Standard Positioning Service Signal in Space Interface Control Document (IRNSS SPS SIS ICD). User receivers employ these parameters to calculate a position and timing solution.

The navigation signals transmitted on each carrier frequency are imperfectly synchronized due to the different hardware paths corresponding to each signal. Each satellite’s navigation message contains parameters describing the timing bias. A GNSS receiver uses these parameters to compute the clock correction for each observation. 

Dual-frequency receivers directly employ such corrections as explained in the article by A. Tetewsky et alia listed in the Additional Resources section near the end of this article. However, a single- frequency receiver has to use the computed offset, which must be adjusted to account for the differential group delay between the principal signal and another frequency signal. These delays, known as T_GD, result from hardware differences in the onboard signal paths and will vary between satellites. 

Channel Delay Parameters
Three different delay parameters arise in a satellite payload: fixed/bias group delay, differential group delay, and group delay uncertainty in bias and differential values as explained in the article by P. Majithiya et alia (Additional Resources). The fixed delay or hardware group delay is a bias term included in clock correction parameters transmitted in the navigation data and is therefore accounted for in the user computation of system time.

This fixed group delay is the time taken for the signal to travel from the common clock through the baseband signal generator, modulator, up-converter, transmitter, and filter to emerge from the satellite antenna. The group delay uncertainty shows the variability in the path delay due to operational environment and other factors. The effective uncertainty of the group delay is typically less than three nanoseconds.

Differential group delay is the delay difference between two different navigation signals. It consists of random plus bias components. The mean differential is defined as the bias component and will be either positive or negative. For a given navigation payload redundancy configuration, the absolute value of the mean differential delay should not exceed a few nanoseconds. The random variation about the mean is typically less than three nanoseconds. 

Corrections for the bias components of the group delay differential are transmitted in the navigation message as T_GD and ISCs. The measurement inaccuracy and variation in the total end-to-end delay of the payload up to the antenna phase center will directly affect the pseudorange accuracy and ultimately user equivalent range error (UERE). 

Navigation Payload Absolute Delay Measurement
The total navigation payload delay can be measured in two ways: addition of integrated payload (without antenna) delay and estimated antenna delay, and total payload (including antenna) delay in radiation mode.

The first method involves two steps. In first,the integrated payload (without antenna) delay is measured. In the second step, the antenna delay is estimated analytically. However, errors in estimation may arise due to variety of reasons. The integrated payload delay measurement technique is described in the following section and is based on details given in the article by P. Patidar et alia (Additional Resources).

Integrated Payload Delay Measurement. The fundamental operation in navigation signal processing is the correlation. The received navigation signal is correlated with a reference code, and the location of the correlation peak provides the estimate of signal delay. A typical navigation receiver does this for many satellites over multiple correlating channels and tracks them in real time to provide continuous positioning. It gets timing reference from the time stamped navigation signal from satellites.

The same principle of correlation is used for delay measurement, although in non–real time. In the offline delay measurement, the RF signal is digitized and recorded in memory. This stored signal can be processed immediately after reception or later. Hence, it is non–real-time or near–real-time operation. 

To obtain a measurement of the absolute delay, laboratory equipment representing the satellite payload is connected directly to a measurement test setup. The payload is usually driven by an atomic frequency standard (AFS). The measurement setup is driven by a separate AFS with one order of magnitude better performance. Because highly stable atomic frequency standards are used, no signal dynamics are involved and continuous signal tracking is not required. Figure 2 shows a schematic of the offline delay measurement setup.

The following subsections explain the three main elements of this measurement technique.

Signal Capture. A signal digitizer and recorder digitizes the RF signal from the payload and stores it in memory. The record duration should be at least one code period. Direct RF sampling at very high sampling frequency (10Gsps/40Gsps) is employed to get better resolution and accuracy. A high-speed digital phosphor oscilloscope is used for this purpose. 

A computer runs a software routine in MATLAB that commands and acquires data from the oscilloscope. Data recording is triggered by the code epoch from the signal generator unit. Of the payload, which marks the beginning of the codes.

Post Processing. The captured signal in the oscilloscope is transferred through an Ethernet link to the computer for processing. We sampled the recorded signal directly at RF, which needs to be downconverted to baseband before correlation. In the post-processing software, which is implemented in MATLAB, the captured signal is down-converted and filtered prior to correlation. 

A reference signal is generated based on the same pseudorandom noise (PN) codes that were used in the payload signal generator. The baseband recorded signal and reference signals are correlated in the frequency domain using Fast Fourier Transform (FFT). The frequency domain version of recorded and reference signals are multiplied and an inverse FFT operation provides the time domain correlation function. 

The time domain correlation function is subjected to an acquisition test in order to eliminate false peak detection, after which the peak magnitude and index of correlation function are determined. The index of the correlation peak provides the delay in the samples, which is converted to a time delay by multiplication of the sample duration. To obtain a raw delay measurement, this estimated delay is adjusted for the software processing delay due to filtering involved in the algorithm.

Setup Calibration. Additional digital and RF cables are used in establishing the measurement setup. The code epoch generated in payload signal generator is transported to the digitizing oscilloscope using a digital interface such as a low-voltage differential signaling (LVDS) cable. This cable actually delays recording of signals in the oscilloscope. Hence, its delay has to be added in raw delay values.

Similarly, many additional RF elements such as cables, couplers, attenuators, splitters, and so on are incorporated into the RF path before the oscilloscope digitizes the signal. These RF elements provide additional delay to navigation signals. Therefore, their delay has to be subtracted from raw delay values. The RF elements are also calibrated by means of the previously mentioned offline delay measurement technique, using stimulus from a standard source. (commercial signal generator). The raw delay values are adjusted for digital and RF calibration to determine the absolute hardware delay.

This measurement technique was validated by measuring the delay of calibrated cables. RF cables with calibrated delays of 1, 2, 5, and 10 nanoseconds were compared with offline delay measurements and achieved an accuracy of 0.1 nanosecond. The accuracy is one order better than the typical payload delay–uncertainty specification (three nanoseconds). 

The repeatability of this measurement has been verified by various measurement setups involving the mating and demating of cables. The measurements were repeatable within the measurement uncertainty (0.05 nanosecond, 2-sigma). Our proposed method does not need to implement a specific early-minus-late tracking loop in the navigation receiver, as there are no signal dynamics involved in the laboratory test. Hence, the results obtained here are independent of any tracking loop implementation by the receiver.

Antenna Hardware Delay Measurement/Estimation. Most GNSS satellites have helix array antennas with a common shared aperture for transmission of multi-frequency signals. The antenna will have different hardware for different frequency signals. In conventional methods, the antenna delay is measured for different frequency signals by estimating the path length of signal input to the reference surface and from the reference surface to the phase center of the antenna. This kind of delay estimation is based on calculation/analysis and may not represent the actual performance of antenna hardware. 

Both the measurement steps (integrated payload measurement and antenna delay measurement) will have a separate measurement setup comprised of many RF and digital elements. Measurement carried out at each step will have its associated errors.

Furthermore, the test setup has to be calibrated and at every step residual calibration errors will be generated due to setup limitation and human errors. These measurement and calibration errors will accumulate at each step and result in an overall delay uncertainty. To avoid this error accumulation, we propose use of the single-shot measurement procedure described in the next section. 

Total Navigation Payload Delay in Radiation Mode
We subjected the integrated spacecraft — along with all the antennas — to a radiation mode test. During this test the total payload delay can be measured as per the test setup shown in Figure 3. The payload-generated navigation signal of a particular frequency band is radiated from the respective phase center of a transmit antenna. The spacecraft is mounted at the estimated phase center of that particular signal. 

The range from point of transmission to the point of reception is well defined. The signal travels fixed distance in the free space and received by the antenna at its phase center. The received signal is given to offline delay measurement setup. The total measured delay can be expressed as:

τ_total = τ_payload + τ_fs + τ_RxAnt + τ_setup

The total measured delay (τ_total) consists of payload delay (τ_payload), free space delay (τ_fs) between transmit antenna phase center to receive antenna phase center, receive antenna delay (τ_RxAnt) and measurement setup delay (τ_setup). The free space range and receive antenna delay are calibrated values available beforehand.

The measurement setup delay is measured using the procedure mentioned in previous section. After applying all delay values/test results in the preceding equation we can determine the absolute payload delay from ranging signal generation up to the antenna phase center for each frequency and navigation signal. 

The delay measurement procedure can be repeated for different antenna off-axis angles. This will give the payload delay variation pattern over the antenna off-axis boresite angle. The antenna phase center variation can be derived from the measured delay pattern.

Experiment Results
An experiment was carried out to validate proposed total payload delay measurement technique in radiation mode. The experimental test setup is shown in Figure 4 (see photo at the top of this article). It consists of a navigation signal simulator (representing the navigation payload) operating in S-band at 2492.028 MHz, a transmit patch antenna, an identical receive patch antenna, and the delay measurement setup described earlier.

The simulator with transmit antenna serves as the analog to a navigation payload. The phase centers of the receive and transmit (both are identical) antennas are known to be on the surface of these antennas.

The simulator output was connected to the transmit antenna. Transmit and receive antennas are kept at a 100-centimeter distance (i.e., the distance between transmit antenna phase center to the receive antenna phase center). The receive antenna output is connected to the delay measurement setup. The total delay in radiation mode was measured as 485.32 nanoseconds.

The measured total delay in the radiation mode will consist of payload delay (i.e., simulator + transmit antenna), free-space range delay, receive antenna delay, and measurement setup delay. Range and measurement setup delays are known whereas the receive antenna delay is unknown. 

In this setup, two identical S-band patch antennas are used to transmit and receive signals. The identical antennas will have the same delay. To measure the antenna delay, the delay measurement was repeated in cable mode, i.e., connecting the payload output directly to the delay measurement setup using a bullet adaptor (without antenna). The delay of this adaptor is negligible. 

The difference in the delay in radiation mode from that of the delay in cable mode consists of two antenna delays and a range delay. The common delays, including epoch cable delay, will be eliminated in differencing. The receive antenna delay can be easily calculated as shown in Table 1. Table 2 provides a sum-mary of radiation-mode payload delay measurement results. The accuracy of this measurement is 0.1 nanosecond.

Conclusion
The proposed technique measures the total payload hardware delay up to the antenna phase center in a single-shot measurement. The correctness of the final measured delay was cross checked by directly measuring the chip transition delay on a modulated signal at L-band with respect to the code epoch using a high-resolution oscilloscope.

This will eliminate the errors accumulated due to multiple measurements and calibration cycles. It will also avoid the analytical estimation of the antenna delay. ISRO will use the delay measurements obtained in radiation mode to derive payload hardware delay parameters that are broadcasted in the navigation message to correct the satellite clock offsets. The improved measurement accuracy of this delay will, in turn, result in improved position and timing accuracy to the users.

Moreover, the proposed technique can be applied to measure the antenna delay variation pattern over off-axis bore-site angles. The antenna phase center variation can be derived from the measured delay pattern.

Acknowledgements
The authors are grateful to Shri. Tapan Misra, Director, Space Applications Centre (SAC), Ahmedabad and Shri D K Das, Deputy Director, SATCOM & Navigation Payload Area, SAC, for providing overall guidance and encouragement to carry out this work. Authors also would like to thank the SAC antenna team. 

Additional Resources
[1] Indian Space Research Organization, “Indian Regional Navigation Satellite System Standard Positioning Service Signal in Space Interface Control Document (IRNSS SPS SIS ICD),” Version 1.0, June 2014
[2] Majithiya, P., and K. Khatri, and J. K. Hota, “IRNSS System: Correction Parameters for Timing Group Delays,” Inside GNSS, January/February 2011
[3] Patidar, P., and D. Upadhyay and P. Majithiya, “Absolute Hardware Delay Measurement of Navigation Payloads,” ASI-SBN Conference 2013, Bangalore, India
[4] Tetewsky, A., and J. Ross, A. Soltz, N. Vaughn, J. Anszperger, C. O’Brien, D. Graham, D. Craig, and J. Lozow, “Making Sense of Inter-Signal Corrections: Accounting for GPS Satellite Calibration Parameters in Legacy and Modernized Ionosphere Corrections Algorithms,” Inside GNSS, July/August, 2009

The post Measuring Navigation Payload Absolute Delay appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

Small GPS jammers, particularly the “personal privacy devices” readily available on the Internet, pose one of the greatest risks to the nation’s critical infrastructure, according to a now-public Homeland Security assessment. 

The devices, also called PPDs, rank among the three most likely causes of GPS disruption, according to researchers from the Department of Homeland Security’s Homeland (DHS) Infrastructure Threat and Risk Analysis Center (HITRAC). Of those three, however, only the scenario incorporating multiple PPDs was identified as being both among most likely to happen and the most potentially damaging to the operation of industrial infrastructure.

If you’ve not heard this perspective previously, it may be because it is scattered across the 200+ pages of a limited-circulation report called National Risk Estimate: Risks to U.S. Critical Infrastructure from Global Positioning System Disruptions. The report was prepared in 2011 at the request of the National Executive Committee for Space-Based Positioning, Navigation, and Timing (PNT ExCom) and released the following year on an “official use only” basis — that is, strictly within the federal government. 

Although a very brief, process-focused fact sheet about the study was made public in 2013, it was not until last year that a redacted version of the full report was obtained and released by Government Attic, a Freedom of Information Act organization. Despite its aging, the report’s content remains remarkably relevant and timely.

Expert Insights
To conduct the study, HITRAC convened panels of GPS and infrastructure experts to rank the likelihood and potential impact of eight types of GPS disruption in the United States. (See accompanying sidebar (at the end of this article), “National Risk Estimate — Disruption Scenarios”). The study also weighed the likelihood and impact of a wide range of other events, such as solar flares, hacker attacks, the sudden loss of GPS satellites to old age, and even the intentional, malicious manipulation of other international PNT systems. These events, while potentially devastating, were considered unlikely, however, and not covered deeply — at least not in the released portion of the report.

Brandon Wales, then-director of DHS HITRAC summarized some of the findings in November 2011. He told the National Space-Based PNT Advisory Board that spoofing was judged to be of higher consequence than jamming because of the length of time it might take to discover signal tampering. Even so, he said in his charts, jamming was far less technically challenging and therefore seen as more likely to occur.

Of the eight scenarios HITRAC looked at, two potentially involved PPDs. Scenario B looked at the impact of a single low-power jammer while Scenario D comprised multiple low-power jammers on the ground. These jammers were described as both stationary and mobile, with some only intermittently active. Between them they caused sporadic tracking and acquisition disruptions across a metropolitan area.

The experts agreed that the likelihood of interference from multiple PPDs was high “based on the increase in commercially available jammers, the ease of acquiring them (such as through the Internet), and their falling cost,” wrote the researchers. Documented examples of such interference supported the conclusion, in particular an incident at Newark Liberty International Airport (EWR) four years ago where hard-to-find PPDs interfered with operations.

“During a 127-day period in 2011, there were 127 events of (radio-frequency interference) at EWR attributable to PPDs,” the report said. Another study found as many as five events per day, possibly from PPDs. Aviation receivers, said the researchers, suffered “unintended, collateral damage.” 

Anecdotal evidence from pilot forums, the authors added, indicated “that low-level flight above certain stretches of roadways (such as along I-95 and I-35 near certain convenience stops) typically results in loss of GPS satellite tracking in small aircraft. PPDs are a suspected cause of the disruptions.”

“The aviation experience seems to indicate a higher prevalence of PPDs in the United States,” said the researchers, “as well as a larger jamming radius for common cigarette lighter styles than previously assumed.” 

Moreover, some of the panelists said they fully expected the problem to get worse. 

“The (subject matter expert) from the (Federal Aviation Administration) noted that in the near term, possibly within the next 12 to 24 months, this sort of scenario could become the most frequently occurring because of the increasing numbers of mobile jammers and our current lack of mitigation options,” wrote the authors in 2011. 

Not only have the laws regarding PPDs not changed since the panels met — they are still legal to buy and own, although not to use — but new developments may drive demand for the devices even higher.

The use of PPDs, also called pocket jammers, has largely been associated with workers trying to avoid minute-by-minute oversight of their company vehicles. Whether its a delivery person stressed by demands for more productivity, a lunch-time Romeo (or Juliet) hiding a tryst, or a trucker hoping to avoid restrictions, many of the examples of the use of privacy jammers are, anecdotally, linked to commercial activity. Criminals are also suspected of using the jammers to thwart the tracking of stolen vehicles and generally undermine GPS-based surveillance. 

But market forces arising since the report was finalized may be conspiring to drive up demand in the general population. For example, mandatory road-usage fees, determined with the help of GPS, are being suggested as a way to address declines in gas tax revenues caused by a shift to higher-mileage cars and electric vehicles. Experience with efforts to spoof electronic toll collection systems in some European nations suggests that these are credible concerns.

Insurance companies are also increasingly incorporating options for car monitoring, which can include location tracking, into their rate setting models. Although such tracking is currently voluntary and advertised as a way to lower rates, it could be used to raise the rates of those whose driving patterns are seen as more risky — perhaps someone on the overnight shift who does most of their driving at night. Eventually the consensual aspect of such monitoring may be replaced by mandatory requirements if refusal to be tracked comes to be seen as a warning sign of a risky driver, an expert told the Washington Post. 

“When such programs become more common, opting out could serve as a “red flag” to insurance companies, according to Renee Stephens, vice president of U.S. auto quality for J.D. Power and Associates.

The prospect of new fees and higher insurance premiums may drive more people to seek out and PPDs. The panelists anticipated such an increase in privacy concerns and even postulated a possible public backlash against GPS. They suggested a study of the factors motivating people to disrupt GPS and how prevalent it might become. 

Dire Consequences 
Having looked at the likelihood of different kinds of GPS disruptions, the study authors then assessed the impact of such interference. The greater the chance of a type of disruption occurring, and the higher its potential impact, the higher its overall risk.

To better understand what could happen if GPS signals were degraded or there were signal outages, DHS looked closely at how GPS is integrated into 4 of the 16 infrastructure sectors deemed critical to the nation by the agency. These four sectors — communications, emergency services, transportation (all types) and energy — were picked because GPS PNT is used to support or fulfill their core missions.

While the operations of all four sectors could be seriously undermined by at least two of the eight scenarios, Scenario D — the one incorporating two or more personal privacy devices — was the only one of the eight that made the high-impact list for every single sector.

Transportation. The transportation sector was divided into air and surface/marine modes for analysis. For aviation, the impact of PPDs would most likely be seen as isolated instances of GPS signal degradation, with problems continuing for more than a month, most of the experts agreed. This, however, would be more of a nuisance and a capacity issue because the nation’s air traffic control system has layers of redundancy. 

If pilots and air traffic controllers come to see GPS as unreliable, however, it could seriously undermine efficiently and capacity over time, the experts said. And if the problem is not dealt with by the time the new NextGen air traffic control system is implemented, the overall problem would become serious. The nation cannot absorb the nation’s projected growth in air traffic without NextGen, and NextGen depends on GPS. 

The panelists could not agree on the extent of the impact of GPS interference and spoofing on maritime and surface transportation. Some suggested it would be isolated degradation while others believed there could be widespread adverse effects and even outages. Maritime services would become less efficient as they shift to conventional methods of navigation, but overall marine and land transportation would be fairly resilient. 

Problems could arise, however, where modes of transportation meet. For example, the unloading of shipping containers at a port for the next leg of delivery was recently halted for hours when a driver with a pocket jammer drove into the cargo trans-shipment area and the cranes lost their GPS lock. 

Energy. The energy sector “depends on GPS for providing electrical power system reliability and grid efficiency, synchronizing services among power networks, and finding malfunctions within transmission networks,” according to the researchers. GPS is a key component of wide area power distribution monitoring systems, phase monitoring units, and disturbance monitoring equipment.

Operators use phasor measurement units (PMUs) that rely on the precise, ubiquitous timing information in the GPS signal for extremely accurate time stamping, which is correlated with sampled voltage and current inputs. “Collecting and collating these measurements,” explained the authors, “provides powerful techniques for monitoring and modeling power networks.” 

As with transportation the panelists were divided on how long PPD-triggered problems would last and whether or not they would be more isolated. The electrical grid also would likely take a hit to its overall efficiency as synchronization can be lost if a jamming incident lasts longer than 15 seconds. Energy exploration, which increasingly uses GPS to synchronize seismic monitors, could also be effected. 

Communications. Communications infrastructure, of which there are many types including wireless, cable, satellite and broadcasting, use timing signals derived from GPS-disciplined oscillators (GPSDOs) — that is, clocks that maintain their accuracy through continuous reference to a GPS time source. 

But communications firms have long factored in national disasters and accidental disruptions and, as a result, are generally prepared for problems. If a timing system loses lock on the GPS signal, it goes into holdover mode, relying on its internal clock to slow degradation of timing accuracy. The duration and level of performance of the system depends on the quality of the non-GPS timing source.

The dependence of other sectors on efficient and reliable communications, however, makes this sector particularly important, and disruptions of communications infrastructure could have far wider consequences than is the case for other sectors, according to the DHS report.

Emergency Services. Emergency Services appears to be the sector most vulnerable to even short-term GPS disruptions. First responders use GPS to navigate to incidents and, as with the overall communications sector, they stay in touch with each other over networks that often rely on GPS-disciplined oscillators.

“If a first responder‘s radio network architecture pivots around GPS Timing, there is no readily available backup if the GPS component is compromised,” says the DHS report. “While dispatchers may still be able to communicate with individual first responder units, there could be debilitating effects on radio signals or untimely delays in communications voice radio systems using simulcast technology.”

Falling back on older technology could create chaos, the researchers said, if, for example, an entire department had to rely on one communications channel. 

Without GPS E911 services also would be compromised and computer-aided dispatch systems would be hampered, making it harder to locate accidents and stolen vehicle and dispatching fire, medical, and police. “While this Sector has not reached the point of total dependency on GPS services,” wrote the researchers, “the use of GPS improves the ability of the sector to perform damage mitigation and assist in timely rescue response.”

Not So Rosy Future
The particular vulnerability of the emergency services sector is probably captured best when the study looks ahead 20 years to how trends will strengthen or undermine its operations. When DHS researchers described the best case for future first responders during a GPS disruption, they deemed it a “learning experience” nicknamed “As Good As It Gets.”

Unfortunately for emergency personnel in 2016, that best case is still a good ways off. It assumes the United States has put a backup for GPS in place — a long-debated proposition that has yet to come to pass. Though the PNT ExCom put its stamp of approval on the ground-based eLoran system, which would be a completely independent alternative for timing, no federal money has been allocated as yet for its creation or support.

So what name did DHS give the no-backup future for emergency services? That depends on how completely first responders come to rely on satellite navigation. If they have not utterly lost their pre-GPS chops for locating and then navigating to those in need, the future was deemed a “Should Have Known Better” scenario. If dependence on GPS grows and no alternatives emerge, said DHS, a disruption will be the preparedness equivalent of bringing a “Knife to a Gun Fight.”

As for the other sectors, researchers said signal diversity would greatly improve the future prospects of the energy sector, which could otherwise be facing intermittent outages and energy shortages. To support this approach, the report says, DHS could encourage GPS receiver manufacturers “to make multi-system/multi-frequency receivers.” 

Fortunately receiver manufacturers, if they haven’t already developed multi-GNSS chip sets, are chomping at the bit to do just that. Unfortunately the availability of reliable, usable signals from other constellations is unclear. The only non-GPS constellation completed so far has been the Russian GLONASS system, which has suffered some technical problems. The other global constellations — Europe’s Galileo and China’s BeiDou — will come fully online soon enough, but questions remain about the permissibility of using their signals in the United States for official purposes such as supporting E911. 

The Europeans applied to the Federal Communications Commission for approval more than a year ago but are still waiting for an answer. Bureaucratic foot dragging on the part of the United States has now raised doubts about American access to PRS, Galileo’s encrypted, jam-resistant signal — a service that could prove useful for countering problems like PPDs.

The trend appears similar for both the transportation and communications sectors. Without government action the sectors will be drawn to GPS because it is reliable and free, becoming increasingly vulnerable as their dependence on satellite navigation grows. 

The needed government action, underscored by the panelists and the HITRAC team, is deployment of a backup for GPS. It is the key difference, according to the report, between a smooth-running future and a dystopian outcome for all four sectors. 

This is a rather surprising assessment to find in a years-old DHS report given that DHS has yet to fulfill its 11-year-old mandate to help develop a GPS backup. In fact the Coast Guard, which is part of DHS, continued dismantling the infrastructure essential to eLoran until 2014, when it was finally ordered to stop by Congress. 

“Unfortunately,” wrote the researchers in what may prove to be their most prescient forecast, “it may take a major GPS disruption to prompt investment in these types of initiatives.”  

SIDEBAR: National Risk Estimate — Disruption Scenarios 

Scenario A: A stationary interference source is causing continuous unintentional disruption. Ground receivers within a 30-kilometer ground-to-ground (GTG) radius are affected, and airborne receivers within radio line-of-sight (radio LOS) are affected. 
Scenario B: Continuous jamming disruption from a single low-power, stationary jammer. GPS receiver tracking is affected within a 500-meter GTG radius and a 20-kilometer radio LOS radius. GPS receiver acquisition is affected within an 800-meter GTG radius and a 30-kilometer radio LOS radius. 
Scenario C: Continuous jamming disruption from a single high-power, stationary jammer (e.g., mounted on a tall building or hilltop). GPS receiver tracking is affected within a three-kilometer GTG radius and a 230-kilometer radio LOS radius. GPS receiver acquisition is affected within a four-kilometer GTG radius and a 350-kilometer radio LOS radius.
Scenario D: Jamming disruption from multiple low-power jammers on the ground. The jammers are stationary and mobile, with some continuous and others intermittently active. Pockets of intermittent tracking and acquisition disruption occur across the metropolitan area. 
Scenario E: Continent-scale natural disruption caused by a severe geomagnetic storm (G4 or higher). Tracking threshold of GPS is reduced significantly. 
Scenario F: Continuous pinpoint spoofing attack against a single target receiver. The spoofer walks off the time and position reported by the target receiver without raising alarms. 
Scenario G: Sophisticated, coordinated, continuous pinpoint spoofing attacks against multiple target receivers (one spoofer per targeted receiver). Each spoofer independently walks off the time and position reported by its target receiver without raising alarms. 
Scenario H: Continuous attack whereby a strategically placed high-power transmitter generates GPS-like spoofing signals after an initial interval (several minutes) of jamming. Receivers within a three-kilometer GTG radius and a 230-kilometer radio LOS radius report a confident timing and position fix, but the timing is wrong by up to hundreds of microseconds and the position fix is wrong by up to tens of kilometers.

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