In the beginning, there was just the Global Positioning System. But it provided an astounding start to the world of GNSS. 

Since the United States declared full operational capability (FOC) for GPS in 1995, two major things have occurred: 

In the beginning, there was just the Global Positioning System. But it provided an astounding start to the world of GNSS. 

Since the United States declared full operational capability (FOC) for GPS in 1995, two major things have occurred: 

• a phenomenal uptake of GPS technology — more than a billion receivers estimated to be in use worldwide — and
• the emergence of three other GNSS systems, two regional systems, and several space-based augmentations.

Today, we have well more than 60 operational GNSS satellites in orbit from several systems transmitting a variety of signals on multiple frequencies. Within five years, given the announced plans of GNSS system operators, the number of satellites will reach 90-plus — with even more signals and frequencies. 

All of which represents good news and maybe some not-such-good news for GNSS product designers, service providers, and end users. Because even as this trend increases access to the basic GNSS resource — signals in space — and the prospect of interoperability that produces a synergistic effect from using multiple systems, it brings new challenges for the global GNSS community.  

For example, recent studies have indicated that too many GNSS systems operating in the same band may create more problems by raising the RF noise floor for receivers than it solves by increasing the accuracy and availability of positioning services. 

That raises fundamental questions about ensuring GNSS compatibility, which practically means the prevention of harmful interference with use of individual services or signals. In many ways, GNSS compatibility is primarily a matter of dealing with the laws of physics.

With these issues in mind, we turned to Dr. Anthony Pratt, a leading British consultant and GNSS signal expert, to explore the challenges and opportunities presented by multiple GNSSes. 

Pratt is a consultant to the UK government in the development of the Galileo satellite system and a member of the European Commission Signal Task Force that developed the multiplexed binary offset carrier (MBOC) signal enshrined in the 2004 EU-US Agreement on GPS Galileo Cooperation. He currently is senior consultant with the GPS Telematics Group at QinetiQ Ltd.. and manages the navigation course at TU Delft Spacetech Master’s Program. 

IGM: How do overlapping signals conflict with one another?

PRATT: Every navigation signal leaks into adjacent signals in the same band. This is an inevitable consequence of the choice of spreading symbol and transmission bandwidth of a GNSS system. We can see an example of this in the L1 band just by observing the power spectral densities (PSDs) of the actual and proposed signals from GPS, Galileo, and Compass II. 

A single-sided plot [see accompanying figure] of the spectrum out to 20.46 MHz on either side of the L1 band center (1575.42 MHz) shows a very busy RF environment. We can readily observe that individual transmissions have spectra which substantially overlap one another. The more important question is to what extent is the overlap tolerable. From a technical perspective, the leakage of one signal into another can be easily determined through spectral separation coefficients (SSC) and the power levels of the individual signals upon reception in a GNSS receiver. 

By appropriate computation, we can readily determine that spectra (PSDs) where the sidelobes of one fall inside the main lobe of another signal have lower SSC values, whereas signals where the mainlobes overlap generally have higher SSC values. This is the case for Galileo PRS signals, which are clearly overlapped by the main lobe of the Compass II QPSK 2 spectra, and to a lesser extent, for the influence of Compass signals on the U.S. military M-code signal. 

IGM: Is such interference troublesome?

PRATT: Ostensibly, on a worldwide basis such interference is regulated by the International Telecommunication Union (ITU), which requires that users of the radionavigation satellite system (RNSS) bands, amongst others, do not cause harmful interference to one another. 

The ITU concentrates on regulating the mutual interference between two parties (bilateral), but there appears to be no regulation of the overall noise (multilateral) in the L1 band resulting from the combination of all constellations (as is the case in the L5 band). Perhaps, this might be a role for the UN International Committee on GNSS (ICG). 

Clearly, mutual interference causes a degradation in service levels. Some might be insignificant; in other cases, it might be significant, especially if the system performance margin is small initially. How serious this is, will depend upon the use of the system and the intended performance envelope. 

IGM: How about the issue of a rise in the overall “noise floor” caused by the increasing numbers of GNSS signals operating in the same RF bands?

PRATT: We have known for some time that the combined effect of the C/A code transmissions from the GPS constellation raises the noise floor, above thermal levels, of a receiver adapted to process C/A-code signals. This is not of much concern to a receiver with a poor noise figure in which the noise from the receiver masks the constellation effects. 

However, the best C/A-code receivers have their ultimate measurement accuracy limited by, inter alia, the cross-correlation noise of C/A code from satellites other than the one being measured. In reality, the effect is not serious. 

In the early days of the EU/US Agreement, the effect of two constellations both transmitting the same spreading symbol (MBOC) did raise the noise level for MBOC receivers, but not by much. Since then, several additional GNSS system providers have been persuaded to adopt the same modulation format. Ultimately five satellite constellations may all be broadcasting the MBOC spectrum. 

Great for signal choice! However, some of the proposals are for significantly increased power flux density than, for example, Galileo transmissions. This has the potential to significantly increase the background noise level for MBOC receivers. A combined calculation for five global GNSS constellations with reasonable assumptions suggest that the noise floor for MBOC receivers will be raised by up to five decibels above thermal levels (-204dBW/Hz). 

An increase as large as 5dB is certainly of concern because this implies a background noise level of -199dBW/Hz. Good quality receivers should have better noise figures than 5dB; so, their performance will be constrained by such increases. 

IGM: What does all this mean for the prospect of GNSS interoperability?

PRATT: Evidently, the EU/US negotiation focused, in part, on interoperability of the civilian signals in the L1 band. But interoperability does not have to cover every aspect of a system design. We might argue that as long as the civil signals from two different systems can be received with no significant receiver biases on a single receiver, this is a sufficient step forward. At least, having the same transmission frequency and PSD could be considered as necessary attributes. 

Ideally, compatibility should extend to the use of common geodetic reference and time systems. This does not mean that the realizations of such systems need to be common however. 

An alternative definition might encompass the concept of interchangeability, which suggest a wider mix of common attributes, potentially including common code families, common navigation data formats (and orbital models), and common geodetic and time reference systems and their realizations. 

Use of common code families can provide a definite benefit — where other system attributes are also common. This holds the potential to reduce mutual interference levels by one decibel or more. However, agreement on this issue is hard to grasp. Unfortunately, the performance of one code family mixed with those from another is no better than if each code set were chosen from a random code without any optimization. The present situation whereby each GNSS system provider selects their own code family is the least good of all the choices from the perspective of best overall performance.

The post Multiple GNSS: Compatibility & Interoperability appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

In a remarkable change of momentum, the Federal Communications Commission (FCC) issued a public notice yesterday (September 13, 2011) stating that “additional targeted testing” is needed in the matter of LightSquared Subsidiary LLC’s interference to GPS.

In a remarkable change of momentum, the Federal Communications Commission (FCC) issued a public notice yesterday (September 13, 2011) stating that “additional targeted testing” is needed in the matter of LightSquared Subsidiary LLC’s interference to GPS.

The FCC’s notice came out a day before the Center for Public Integrity (CPI) published an article chronicling LightSquared’s high-level contacts with presidential aides that cited the company’s and its chief backer Harbinger Capital’s fundraising for Democratic causes and President Obama. Quoting e-mails (heavily edited by the White House) and other records obtained under the Freedom of Information Act (FOIA) by CPI’s iWatch News, today’s article tracked a series of communications with administration technology advisors. These included an offsite April 28, 2011, meeting described in White House e-mails as the “Lightsquared interference meeting,” in which names of the people attending were blacked out in the records released under FOIA.

In addition to the CPI report, an August 26 meeting brokered by NTIA between LightSquared and concerned federal officials representing public GPS users brokered by NTIA may have been the turning point in what had been a fairly overt backing of the LightSquared initiative by FCC Chairman Julius Genachowski and the White House.

Although the FCC’s notice, issued jointly by the chiefs of the agency’s International Bureau and Office of Engineering and Technology, did not elaborate on the required testing, a footnote in the document cites a September 9, 2011, letter to the deputy secretaries of defense and transportation from Lawrence Strickling, NTIA administrator and assistant secretary for communications and information at the Department of Commerce.

That letter outlines a plan that would evaluate the effects of LightSquared terrestrial transmissions in the lower LightSquared band (1526–1536 MHz) on cellular and personal/general navigation GPS receivers by November 30. A second phase of testing would evaluate proposed mitigation plans for high-precision and timing receivers. The latter tests would not begin until LightSquared develops a filtering solution for such devices.

The requirement for additional testing marks the latest development in a controversy that blew up quickly beginning with an FCC invitation for comment issued last November about LightSquared’s plans to offer a wholesale wireless broadband service using a network of base stations transmitting in the 1526-1559 MHz band adjacent to GPS L1.

FCC’s International Bureau issued an order and authorization on January 26, 2011, allowing LightSquared to proceed with its plans on the condition that GPS interference concerns were satisfactorily resolved. That led to an expedited series of tests conducted by a Technical Working Group (TWG), which found widespread effects on many GPS receivers caused by transmissions in the upper 10-megahertz band where LightSquared initially planned to begin broadcasting.

Subsequently, LightSquared proposed to begin its transmissions in the lower 10-megahertz band, reduce the power of transmissions at its terrestrial stations (known in FCC vernacular as ancillary terrestrial components or ATCs), and help the GPS industry find ways to mitigate the effects on high-precision receivers.

Strickling’s letter formally requested that the Executive Steering Group of the National Executive Committee for Space-Based Positioning, Navigation, and Timing (PNT) work with LightSquared “to develop as expeditiously as possible a joint testing plan. . . .” Deputy Secretary of Defense Bill Lynn and Deputy Secretary of Transportation John Porcari co-chair the PNT executive committee.

Strickling noted that direct discussions between LightSquared and the Federal Aviation Administration (FAA) and NASA regarding ways to mitigate the effects of the lower 10-megahertz transmissions on, respectively, aviation and space-based receivers meant that those types of user equipment did not need to be included in the high-precision test plan for the time being.

The NTIA administrator also noted that solutions involving the filtering out of LightSquared signals, including a PCTEL antenna identified by LightSquared as a possible solution for timing receivers, might also worsen the overall performance of high-precision GPS equipment.

Despite the growing evidence of LightSquared’s adverse effects on GPS, as detailed in Strickling’s letter, the FCC remained generally optimistic about a mutually satisfactory outcome to the controversy.

In their public notice yesterday, the FCC chiefs concluded, “We appreciate the extensive resources devoted by all of the participants in the technical working group and by the federal agencies and departments. We are satisfied that the process that was established through the conditional authorization, as well as the consultation process that exists between the FCC and NTIA, is meeting our objective to fully understand the potential for harmful interference and develop solutions before LightSquared is permitted to deploy service.

“We strongly encourage all parties to work in good faith and expeditiously towards a solution that serves our dual goals of facilitating the introduction of new wireless broadband services while protecting GPS against harmful interference.”

The post FCC Calls for More Testing of Lightsquared Interference to GPS; White House Ties Revealed 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

1. HERE, KITTY KITTY
Santa Cruz, California USA
√ First, get the cougar on the treadmill . . . that’s what UC Santa Cruz researchers did to measure baseline behavior and design a tool that tells what puma concolor do every minute. Their super-advanced CARNIVORE collar uses GNSS and a 3-axis accelerometer to create a 24-hour diary of the wild life.

1. HERE, KITTY KITTY
Santa Cruz, California USA
√ First, get the cougar on the treadmill . . . that’s what UC Santa Cruz researchers did to measure baseline behavior and design a tool that tells what puma concolor do every minute. Their super-advanced CARNIVORE collar uses GNSS and a 3-axis accelerometer to create a 24-hour diary of the wild life.

2. TOXICS TEST
Uxbridge, London,UK
√ 300,000 suspected toxic waste sites in Europe make testing your own dirt a good idea. British entrepreneur Ed Bell invented the briefcase-sized Safe Soil Tester for do-it-yourselfers. Marine bacteria identify carcinogenic PAHs onsite within minutes and map it with Galileo satellites. EUREKA, a European Commission program, funded his R&D.

3. NOT THAT SOYUZ
Evry-Courcouronnes, France and Altai Republic, Russia
√ In a reassuring August 28 release, Arianespace said Galileo’s first launch on October 20 is just fine. The Soyuz-ST rocket scheduled to carry the satellite trio does not use the suspect third-stage motor as did the model that disappeared on its way to the space station, causing a “thunderous explosion” over Siberia.

4. FIRST OF MANY
Tokyo, Japan and Jeju Island, Korea
√ Japan’s first Quasi-Zenith satellite’s positioning signals are A-OK as of July 14. GNSS experts will talk about making the most of it and the 100 other space vehicles expected over Asian skies in the next decade at a November 1–2 workshop in Jeju, Korea, part of Multi-GNSS Asia (MGA).

5. SPACEKEEPING
Out in Space
√ The National Research Council’s sobering August report reminds us that space debris threatens satellite navigation and more. But GNSS has thrown its share of orbiting detritus into the mix. Aerospace Corporation records show that GPS and GLONASS rocket parts have reentered the airspace over Argentina, Saudi Arabia, South Africa, Uruguay, Thailand, and Kansas, to the occupants’ great surprise.

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