Aerospace and Defense

QZSS’s Indoor Messaging System

Japan’s Indoor MEssaging System (IMES) promises to provide “seamless” indoor and outdoor positioning with no extra hardware for a GPS-enabled phone, receiver, or other portable device. But what is it?

Japan’s Indoor MEssaging System (IMES) promises to provide “seamless” indoor and outdoor positioning with no extra hardware for a GPS-enabled phone, receiver, or other portable device. But what is it?

IMES is a lesser-known part of the regional Quasi-Zenith Satellite System (QZSS) being developed by the Japan Aerospace Exploration Agency (JAXA). The QZSS program will place three satellites in high-altitude orbits transmitting ranging signals to improve navigation performance in areas of Japan that prove difficult with GPS alone. The first launch of a QZSS satellite is scheduled for 2010.

Five of six QZSS signals transmitted from these satellites will use the same signal structures, frequencies, spreading code families, and data message formats as GPS and GPS satellite-based augmentation system (SBAS) signals.

An annex to the interface specification for QZSS (IS-QZSS) sets forth the IMES signal design. (See reference in Additional Resources section at the end of this article.)

Although QZSS satellite signals will only be available in a western Pacific Ocean region centered over Japan, IMES is a separate terrestrial element based on an open specification that could be implemented anywhere.

According to the annex, the IMES signal is designed by JAXA to contribute to “the development of QZSS-ready receivers as well as satellite positioning applications by realizing the seamless positioning environment.” However, IMES signal transmitters are not part of the QZSS component; rather, the IMES specification is intended to aid third-party vendors in further development and installation of the system.

Primarily, IMES is designed to provide accurate positioning indoors where reception of GPS and other GNSS signals is blocked or unreliable. By being able to operate indoors using GPS-capable equipment (by upgrading a receiver firmware with the IMES navigation bit decoding algorithms), the system is meant to pick up where GPS fails, in a “seamless” way.

IMES is really competing with assisted-GPS or GNSS (AGPS/AGNSS), which has been used with some success — although substantial inaccuracy — for indoor positioning. In particular, its advocates advance IMES’s utility for enhanced 911 (E911) automatic location of emergency calls from mobile phone users.

At the recent International Symposium on GPS/GNSS in Tokyo, a number of technical presentations and exhibitors discussed the implementation of IMES technology by JAXA, GNSS Technologies (GNSST), and other companies. Hitachi and GNSST had receivers on their exhibit stands and graphics explaining how IMES worked.

For someone with a GPS background, these graphics were somewhat confusing. It looked as though IMES was a system of GPS retransmitters mounted to ceilings — something that wouldn’t help much at all for indoor navigation.

Further investigation, however, revealed that these transmitters, although they operate in the GPS L1 band, are in fact transmitting a completely separate signal. The IMES signal simply gives the location of the nearest transmitter, which an appropriately configured receiver can then take to be “its” position.

At this point, of course, the alarm bells go off. Transmitting in L1? To GPS users, that’s tantamount to jamming. So, is IMES the perfect answer to the “seamless” ubiquitous positioning problem – or is it a dangerous jamming threat to the integrity of GPS?

Perhaps the answer lies somewhere in between. . . . With that thought in mind, this article briefly describes the IMES program and raises several key issues about its practical use in combination with GNSSs.

The IMES Signal
In June 2008, JAXA released version 1.0 of the IS-QZSS document, which includes the IMES specification. The RF characteristics of IMES are the same as the L1 C/A code for GPS and QZSS.

Transmitted at the GPS L1 center frequency (1575.42 MHz), IMES has a bandwidth of 2.046 MHz or more including the main lobe. Like the L1 C/A code, the IMES signal is right-hand circularly polarized and BPSK-modulated with a pseudorandom noise (PRN) code.

In the current interface specification for GPS (IS-GPS-200D) the U.S. government has approved allocation of the Gold (or PRN) codes 173 to 182 for use by other GNSS applications such as IMES. The received power level at an IMES-capable receiver is specified to fall in the range between -158.5 dBW to – 94 dBW. Word structure, bit rate, and modulation are the same as L1 C/A code.

IMES Messages
IS-QZSS defines four different types of messages, as shown in Table 1 (above, right). Two messages give the location of the transmitter. Message type 0 gives the latitude (23 bits) and longitude (24 bits) in WGS-84. However, height or altitude appears as a building floor number (8 bits). Message 1 provides latitude (24 bits), longitude (25 bits), altitude (12 bits), and floor number (9 bits — with units of 0.5 floor).

Messages 3 and 4 simply send an identifier, which, according to GNSST, can then be used to address a location in a database corresponding to that ID. Messages 3 and 4 also transmit a “BD” bit, which is a border or boundary indicator, set whenever the transmitter is the one “nearest” the outdoors or a GPS-accessible area. (As yet undefined, Message type 2 is reserved for later development.)

IMES can be used by a suitably modified stand-alone GPS receiver but is primarily intended for use with a GPS-enabled mobile device. Notably, messages 3 and 4 rely on the device to be able to access a database via a network.

The Additional Resources section includes a number of articles, particularly those by D. Manandhar et alia, that describe IMES, its applications, and test results to date in greater detail.

What’s Good about IMES?
What IMES delivers that other indoor location systems does not is reliability and accuracy. When receiving an IMES signal, the receiver has a strong idea of where it is to within tens of meters. The accuracy is better than AGPS, especially in height, and receivers are not required to have high sensitivity.

The big selling point for IMES in Japan is its ability to add a lot of value to simple positioning with additional data, such as maps and route guidance. For example, locations extracted from the databases used with messages 3 and 4 can be accompanied by location-based service (LBS) information.

In fact, the location may not even be returned at all. The sorts of applications used by Hitachi and GNSST to promote or, in Australian slang, spruik IMES include indoor navigation, finding products in a store, geofencing of children, location-specific instructions in case of emergency, asset management, and so forth.

What’s Not So Good about It?
The technology is in its infancy; so, the list of problems that we will raise here looks large, but each issue needs to be dealt with if IMES is in fact going to succeed AGPS.

1. Infrastructure. For IMES to work, the transmitters need to be very densely located in all indoor spaces where location is required, at separations of 20–30 meters. This represents a massive investment in infrastructure. However, unlike AGPS, this investment does not necessarily need to be made by the telecom companies alone or even at all. Also, if the IMES infrastructure is incomplete, AGPS may still be required to fill the gaps.

2. Jamming. IMES should only affect a small area around the transmitter. However, within this area, IMES will completely jam GPS.

The QZSS specification suggests that the IMES power levels are high enough to jam GPS outdoors. A maximum transmitted signal strength of -94 dBW is about 65 dB stronger than an unobstructed GPS signal received outdoors. The receiver will generally receive the signal at a much lower level; the minimum is specified to be -158.5 dBW, but jamming is still likely to occur nearby to a transmitter. Indoors, where the direct GPS signals are strongly attenuated, the situation would be worse.

The Gold codes are designed so that a receiver should be able to detect a GPS signal that is about 21dB weaker than the strongest GPS signal. This is known as the cross-correlation margin. Advanced signal-processing techniques, such as those developed by Eamonn Glennon at the University of New South Wales, can increase this margin substantially, but even with indoor GPS, cross-correlation margins of 65 dB have not even been considered.

Other systems that have been assigned codes in the IS-GPS-200D, such as SBAS and QZSS itself, transmit at levels that do not exceed the cross-correlation margin. In practice, in Japan the levels will not be as high as those in the specification. GNSST suggest that transmitters will be limited to -100 dBW (or -70 dBm), still significantly stronger than GPS.

Initial GNSST experiments suggest that this transmitted signal strength only has a noticeable effect on a GPS receiver within one meter (with the stronger level of the specification affecting receivers out to three meters’ distance). These affected regions seem small, and independent tests should be used to confirm the GNSST results. If they are valid, IMES will only jam a relatively small area.

The system model of IMES is also not “backward compatible,” meaning that if a user has an AGPS device that has happily worked for years, once IMES is installed, that device can no longer use AGPS.

3. Seamlessness. Interestingly, in the papers presented about IMES, the word “seamless” appears prominently. The idea is that the same equipment can use GPS outdoors and IMES indoors, “seamlessly.” Some papers even claim that the seamlessness has been proven by experiment.

This isn’t quite the case, however.

Modified receivers and mobile phones have been shown to work indoors with IMES signals and outdoors with GPS, but as yet none of the published experiments appear to showing a receiver happily using IMES and transitioning without interruption to GPS outside, or vice versa. The transition from GPS to IMES should be “seamless” — that is, the receiver holds on to GPS until it is jammed or blocked indoors, by which time it has picked up IMES (as long as it is looking for it).

Going the other way is more problematic. IMES can be used until it is too weak to be received, but then the receiver should be able acquire four GPS satellites. If the equipment is being used in an environment broadcasting Messages 3 or 4, then it will have been warned by the BD bit to start looking for GPS signals. In a Message 1 or 2 environment, no such warning will have been given.

Because IMES requires the receiver to receive one IMES signal at a time, a “handover” between IMES transmitters must take place. So, while receiving from one transmitter, the receiver must be searching for others, not knowing which of the PRNs is best to search for.

The near-far phenomenon means that the current IMES transmitter will jam others until they are within the cross-correlation margin. Consequently, once a transmitter’s power has been set, maximum and minimum distances emerge for the placement of other transmitters.

These transmitters must use different PRNs or else they will spoof each other. This means that the IMES transmitters must be laid out based on a code-reuse pattern similar to the frequency-reuse patterns employed by cellular networks, with the added complication of operating in three dimensions, i.e., different floors of a building by definition have different IMES transmitters.

Returning to the question of GPS/IMES transitions, although a huge effect is not likely, this cross-acquisition between GPS and IMES will use more energy than for one system alone, if for no other reason than it increases the PRN search space by the 10 codes used for IMES.

4. “You are the ground segment.” The IMES transmitters, like GPS satellites, tell the receiver where they are. However, in the case of GPS satellites, a secure, centralized ground segment infrastructure is used to create the transmitted messages.

In the case of IMES, the lat/long/height/floor that is transmitted must come from somewhere else. Each of the locations must be surveyed in. The accuracy of this survey need not be great, because the resolution of the transmitted messages is 1–3 meters and, in any case, that position applies for regions of tens of meters.

However, the surveyed position of an IMES transmitter still needs to be accurate to, say, meters in WGS-84. Given the enormous numbers of transmitters needed for this system, a huge scope for error arises in a) the measurement of the position, b) the data entry of the position, and c) the installation of the equipment (a transmitter with a perfectly well entered position being installed in the wrong place).

5. “They are the ground segment.” Messages 3 and 4 don’t tell you your position directly. That information comes from a database. The business model put forward by GNSST suggests that the database may be owned by, “for instance, department store, underground mall and etc.”

What that means is that users may not get the position they’re looking for, just what the department store or underground mall wants them to receive. As with casinos in Las Vegas, navigating one’s way out of the shop as soon as possible may not be in the store owner’s best interest.

6. Security. Tens or hundreds of thousands of these devices are needed for IMES to operate properly. They will have to be installed in public areas such as shopping centers and railway stations, and private areas such as shops. Their installation needs to be cheap because of the sheer numbers involved.

However, if an IMES transmitter can be installed relatively simply, it is also likely to be easy to remove. A stolen transmitter, or one simply bought from the manufacturer, could cause havoc. Driving through a city with a high-powered IMES transmitter on the dashboard could cause all GPS navigators nearby to either cease working, or worse, if they are IMES-enabled, to think they were in the location of the stolen device (i.e., inside a building). It makes GPS jammers (or IMES spoofers) readily available to any small-time thief.

7. Frequency allocation and regulation. Ultimately, IMES will only ever operate in countries where the IMES signal has been sanctioned in the L1 band. This may in some cases require a change in the law before it can operate. In Australia, for instance, any transmission in the L1 band must not have an intention to jam GPS, otherwise it is considered a criminal act. IMES works by deliberately jamming GPS (or at least AGPS) in indoor environments.

Although IMES transmitters are not pseudolites, it would make sense if they were regulated in a similar way. Recently, the European Conference of Postal and Telecommunications Unions (CEPT) has taken the lead in trying to come up with a global standard for pseudolite regulation. The relevant committee is yet to report (possibly early in 2009).

In summary, it appears that before IMES can make significant inroads into indoor positioning, a series of hurdles must first be overcome. IMES is not a magic bullet for the current hot problem of ubiquitous or “seamless” positioning. It’s just another option to add to the list, with its own set of strengths and weaknesses.

Additional Resources
Japan Aerospace Exploration Agency, Quasi-Zenith Satellite System Navigation Service Interface Specification for QZSS (IS-QZSS) V1.0, June 17, 2008 (available on-line at <>)

NAVSTAR Global Positioning System Interface Specification IS-GPS-200, Revision D, IRN-200D-001, Navstar GPS Space Segment/Navigation User Interfaces, GPS Wing, Space and Missile Systems Center, Los Angeles AFB, California, USA, March 7, 2006

Glennon, E., and R. Bryant, A. Dempster, and P. Mumford, “Post Correlation CWI and Cross Correlation Mitigation Using Delayed PIC,” Proceedings of ION GNSS 2007, Fort Worth, Texas, US, September 26-28, 2007

Manandhar, D., and K. Okano, M. Ishii, M. Asako, H. Torimoto, S. Kogure, and H. Maeda, “Signal Definition of QZSS IMES and Its Analysis,” Proceedings of ION GNSS 2008, Savannah, Georgia, USA, September 16–19, 2008

Manandhar, D., and K. Okano, M. Ishii, M. Asako, H. Torimoto, S. Kogure, and H. Maeda, “Development of Ultimate Seamless Positioning System for Global Cellular Phone Platform Based on QZSS IMES,” Proceedings of ION GNSS 2008, Savannah, Georgia, USA, September 16–19, 2008

Manandhar, D., and S/ Kawaguchi, M. Uchida, M. Ishii, and H. Torimoto, “IMES for Mobile Users: Social Implementation and Experiments based on Existing Cellular Phones for Seamless Positioning”, Proceedings of International Symposium on GPS/GNSS, Tokyo, November 11–14, 2008

Martin, S., and H. Kuhlen, and T. Abt, “Interference and Regulatory Aspects of GNSS Pseudolites,” Journal of Global Positioning Systems, vol. 6, no. 2, pp. 98-107, 2007

January 13, 2009

Critical Infrastructure: The United States and GPS

So, President Obama wants to spend some money on infrastructure, eh? Well, here’s an idea: send some of it GPS’s way.

Infrastructure isn’t just concrete and rebar. We can also build highways to the stars and — pardon the clichés — bridges to the future rather than bridges to nowhere.

And talk about bang for the buck. The billion dollars or so that the United States spends on GPS each year produces many tens of billions of dollars in products and services.

So, President Obama wants to spend some money on infrastructure, eh? Well, here’s an idea: send some of it GPS’s way.

Infrastructure isn’t just concrete and rebar. We can also build highways to the stars and — pardon the clichés — bridges to the future rather than bridges to nowhere.

And talk about bang for the buck. The billion dollars or so that the United States spends on GPS each year produces many tens of billions of dollars in products and services.

Of course, a big chunk of that GPS market is outside of this country. But after our recent lamentable contribution to global financial troubles, perhaps its time to remind the world about the unprecedented U.S. generosity in creating an entirely new public utility and making it available everywhere.

Not only that, but U.S. policy forced other GNSS providers to be generous, too. As the would-be Galileo public-private partnership discovered, you can’t compete with free.

Anyway, back to Obama and infrastructure.

The Global Positioning System has many unusual, novel, perhaps even unique features. But the one that relates to the current topic is that GPS is both a critical infrastructure in itself — notably its ground control and space segments and the pervasive, strategic installation of high-performance receivers  — and a contributor to other critical infrastructures, such as communications networks or transportation.

That should earn GPS double the attention, if not twice the budget.

But there’s more. GPS not only allows us to do things that we couldn’t do before; it allows us to do them more efficiently — greater productivity at less cost, whether surveying forest boundaries or guiding a thousand airplanes at once.

And though those efficiencies may reduce the job opportunities at individual enterprises, they stimulate a far greater amount of job creation overall — design and engineering, manufacturing, professional fieldwork — most of it high-skilled and higher-paying than the positions that were lost.

The United States really hasn’t had an industrial policy since just before and during World War II, when the Roosevelt administration converted much of the nation’s jobless into public employees (Works Progress Administration, Civilian Conservation Corps), its manufacturing sector into an armament assembly line, and gasoline and foodstuffs into ration coupons.

After that, we saw occasional, isolated initiatives — the interstate highway system, the lunar missions of the 1960s, SEMATECH — large-scale infrastructure and technology programs that could have served as potential components of an industrial policy, if one had existed.

GPS can help thread the new infrastructure efforts together, and expand the role that it already plays.  Many commercial GPS manufacturers are looking forward to the opportunities that building or restoring highways, bridges, and (imagine!) maybe even railroads will bring.

But the United States is still running the GPS program as though we had all the time in the world. Well, no offense to those atomic clocks on board the GPS satellites (another first of its kind), but the world is quickly catching up with us in matters of GNSS. And, if we take a close look at the world’s four GNSS program schedules, over the next few years just about every other GNSS system is going to pass GPS by in terms of signal availability, modernity, and diversity.

The United States risks seeing its GPS brand decline amid the growing choices in the GNSS marketplace.

It’s time that the GPS leadership, civil and military, revisited its prevailing philosophy and began launching for scheduled capability, rather than as needed to sustain an aging constellation.

And, while they’re at it, they should take another look at the size of the constellation. Every other GNSS system is committed to a true 30 satellite/30 slot configuration. If the advent of the biggest infrastructure investment in American history isn’t the right time to do the same with GPS, when is?

As American poet Edwin Markham asked on behalf of the man with the hoe gazing at the ground, “Give back the upward looking and the light/ Rebuild in it the music and the dream”


Thales Alenia Space Italia Wins Two Galileo Receiver Development Contracts

The European Space Agency (ESA) has awarded Thales Alenia Space Italia (TAS-Italia) two contracts for development of Galileo ground station receiver equipment.

One contract is for Galileo In-Orbit Validation Element (GIOVE) phase A/B ground station receivers capable of tracking the multiplex binary offset carrier (MBOC) signal that is common to both the Galileo Open Service (OS) and the new GPS L1 civil signal, which will be transmitted beginning with the GPS III generation of satellites.

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By glen
January 11, 2009

Former U.S. Space Commerce Official Joins ITT

Edward M. Morris

Edward M. Morris, formerly executive director for the U.S. Office of Space Commercialization, Department of Commerce, has joined ITT Space Systems Division (SSD) as executive director of strategic business development.

In his new position, Morris will be responsible for strategic program and business development of GPS navigation systems and additional space-related capabilities.

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By glen
January 9, 2009

Gates Backs Lynn for Key Defense Post

William J. Lynn III

(Updated Jan.26) President Barack Obama’s nomination of William J. Lynn III, a senior vice-president at Raytheon Corporation, for deputy secretary of defense and his granting Lynn a waiver from the new administration’s own rules on former lobbyists has provoked considerable criticism from some quarters.

As the number two official in the Department of Defense (DoD), Lynn would report directly to Robert Gates, the current secretary of defense who has continued in that position in the new administration, the only holdover from ex-President Bush’s cabinet. Gates has come out strongly in support of Lynn, saying that he requested the waiver from the president.

Among other responsibilities, the deputy secretary serves as the co-chair of
the Space-Based Positioning, Navigation, and Timing (PNT) Executive
Committee (ExCom). Lynn would succeed Gordon England, who has paid a lot of attention to GPS during his term in office and enhanced the role of the PNT ExCom as an arbiter and advocate for the GPS program throughout the federal government.

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By glen
November 25, 2008

GPS 21st Century Milestones (2001-2008)

(Back to GPS Focus page)

December 1. Deputy Secretary of Defense Paul Wolfowitz expresses resistance to Galileo in a letter to European defense ministers.
December 1. Russia’s system rebuilding project begins with the launch of a modernized GLONASS satellite prototype (GLONASS-M)
November 25. The U.S. Coast Guard moves from Transportation to the newly established Department of Homeland Security.

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By Inside GNSS
November 14, 2008

GPS Wing Reaches GPS III IBR Milestone

The GPS Wing has completed an integrated baseline review of the GPS IIIA program, the first major milestone for the $1.4 billion development and production contract for which Lockheed Martin serves as the prime contractor.

The IIIA contract, awarded earlier this year, provides for development and production of the first two GPS IIIA satellites with an initial launch set for 2014. The IBR paves the way for the establishment of an integrated cost, schedule, and technical baseline for the program.

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By glen
October 27, 2008

Galileo’s New Era

A paraphrase of Julius Caesar may be in order here: “Galileo est omnis divisa in partes tres.” Because, as with any global navigation satellite system, Galileo as a whole is divided into three main parts: the political, the infrastructural, and the user application marketplace.

A paraphrase of Julius Caesar may be in order here: “Galileo est omnis divisa in partes tres.” Because, as with any global navigation satellite system, Galileo as a whole is divided into three main parts: the political, the infrastructural, and the user application marketplace.

Developments over the past year — and past month — have markedly rearranged the progress and prospects of all three. Political concerns are subsiding (though not disappearing), building the space and ground segment under a public procurement is finally and irrevocably under way, and the locus of concern is shifting to matters involving market development and industry’s impatience to begin designing, manufacturing and selling Galileo-capable user equipment.

Until final approval of the program’s €3.40-billion by the Transport Council and Parliament last spring, the political realm — encompassing such issues as policy, funding, institutional relations — had dominated the 15-year-old conversation on Europe’s GNSS initiative.

The dynamics of a 27-nation association stretching from the North Sea to the Aegean ensure that political issues will remain prominent, including such matters as a Galileo cooperation agreement still to be completed between the European Space Agency (ESA) and the European Commission (EC). But replacement of the public-private partnership (PPP) approach, final budget authorization, and passage of a new regulation on Galileo has resolved most of the roadblocks and removed much of the doubt surrounding the program.

FOC Procurement Begins
Last July’s launch of the procurement process to complete a fully operational capability (FOC) Galileo has the program moving ahead. Although running slightly behind schedule, the September 19 announcement of candidates selected to compete as prime contractors for Galileo’s six infrastructure work packages has advanced the process to the rather novel “dialog” phase, with final contracts expected to be signed early next year.

In a September 19 announcement, the European Commission (EC) and the European Space Agency (ESA) said they had chosen the candidates out of 21 applicants in the initial phase of the procurement procedure.

Following an invitation issued July 1, interested parties submitted a “Request to participate.” Candidates were shortlisted on the basis of pre-defined selection and exclusion criteria. Marco Falcone now serves as the system and operations manager for ESA’s Galileo Project Office.

Candidates were named in six areas of work or “work packages” — System Support: ThalesAleniaSpace, Italy, and Logica (Netherlands); Ground Mission System: ThalesAleniaSpace (France) and Logica (United Kingdom); Ground Control System: Astrium (UK) and G-Nav grouping represented by Lockheed Martin IS&S (United Kingdom); Space Segment (Satellites): Astrium (Germany) and OHB System (Germany); Launch Services: Arianespace (France); Operations: Nav-up grouping represented by Inmarsat (UK), and DLR (German Aerospace Center) and Telespazio (Italy).

EU officials say the next step of the procurement procedure — a dialog phase — will take place over the next 15 to 30 weeks. ESA will manage the process as the delegated procurement agent, in close coordination with the EC as contracting authority.

As the first step, candidates will be required to deliver a first iteration of their proposal in relation to the work package(s) for which they will bid. The initial dialog meetings will proceed on the basis of the contents of these preliminary proposals.

After those discussions, ESA will provide to candidates a detailed set of documents, including technical management and contractual requirements, in response to which the companies will submit “refined proposals” with updated tender documentation and more precise information.

At a certain point, ESA will close the dialog phase for a given work package and invite the candidates to submit their “best and final offers,” with a final selection, award, and signature following within eight weeks, according to the nominal timeline established by the EC and ESA.

If the European agencies can stick to their schedule, that would place the final award of the FOC contracts sometime between early March and late May 2009. However, the nominal contracting timeline, which called for a seven-week selection phase to choose the short list of companies, ran nearly a month late. Under the current schedule for the public acquisition of Galileo’s ground and space infrastructure, four in-orbit validation (IOV) spacecraft will be launched beginning in 2010 with the full 30-satellite constellation in place by 2013.

Cancellation of the overall contract to build the four in-orbit validation (IOV) satellites and renegotiation of terms with subcontractors has pushed the scheduled launch of the first IOV spacecraft into 2010.

GIOVE-B Returns to Service
After being off-line for a couple of weeks, apparently due to space radiation, Galileo’s GIOVE-B satellite began transmitting again on September 25. According to ESA spokespersons, the spacecraft stopped operating between September 9 and September 24, entering automatic shutdown mode in order to protect delicate circuitry.

Although generally a rare event, cosmic rays can affect satellites adversely. Part of the IOV satellites’ mission is to test and qualify the electronics systems design in preparation for construction of the FOC Galileo spacecraft.

EGNOS Nears Completion
New financial commitments will support an operational status next spring for the European Geostationary Navigation Overlay Service (EGNOS) — a satellite-based augmentation system that was once known as GNSS 1 in the European satnav program.

Paul Verhoef, head of the rapidly expanding Galileo unit in the EC Directorate-General for Energy and Transport (DG-TREN), says that certification for aviation use of the system should occur by the end of next year. The EC is in the process of selecting an EGNOS service provider who will operate the system through 2013. October 15 was the deadline for companies to respond to a call for tenders to provide EGNOS service.

About 70 percent of the flights in Europe today are made with aircraft equipped with GPS/RAIM-capable receivers, said Francisco Salabert of Eurocontrol’s GNSS policy office, referring to the equipment’s use of receiver autonomous integrity monitoring.

More Interest Brings New Frustrations
Breaking the long-standing political and infrastructural logjam — with the likelihood of a significant availability of additional modernized signals within a couple of years — has stimulated renewed interest and activity among equipment manufacturers and would-be Galileo/GNSS service providers — what’s known in Europe as the “downstream industry.”

Just in time to intersect with the FOC procurement process, the EC appears to have sorted out its position on the participation of companies from non-European companies in the Galileo FOC procurement. (Canada, as an associate member of ESA, represents an exception to the EU-only provision.)

Except for components incorporating classified information and technologies, the prime contractors for the six work packages — which must have their headquarters in nations directly sponsoring Galileo — will be able to purchase goods and services from foreign companies.

Aside from that, another consideration that may limit foreign companies are the so-called ITAR restrictions — International Traffic in Arms Regulations — that in the case of U.S. firms requires approval from the State Department to export certain products or technical information.

In an interview with Inside GNSS, Verhoef referred to this as a matter of “security of supply” with potential delays caused by the ITAR rules. “All things being equal,” Verhoef said, “if one company has an ITAR risk associated, I’d go with the other company.”

Verhoef’s comment prompted a follow-up question: what are the specifics of the EC/ESA relationship in which ESA is the Galileo design authority and is also charged with conducting the acquisition process?

“ESA makes the judgment on the technical side,” Verhoef said. “We come in on policy-related questions” that may arise.

The EC’s DG-TREN is building up its capacity to participate in this process, bringing over about 30 staff members from the European GNSS Supervisory Authority (GSA), the agency that had been charged with supervising the industry consortium and contract once envisioned under the PPP model. Eventually, these and other staff members will comprise a third unit under Verhoef’s direction.

Not-So-Merrily Downstream
Resolution of issues in the other two areas leads finally to the increasingly controversial matter of commercial development of user equipment and services. Chief among the related concerns is the completion and release of a final Galileo open service (OS) interface control document (ICD) and the terms under which companies can use the ICD to build and sell equipment commercially.

In a panel discussion on the subject at the September 16 meeting of the Civil GPS Service Interface Committee (CGSIC), Verhoef joined Ed Morris, director of the U.S. Department of Commerce’s Office of Space Commercialization; Mike Swiek, executive director of the U.S. GPS Industry Council, and John Pottle, director of marketing for Spirent Communication’s GNSS simulator business.

Verhoef and Morris cochair a working group established under the 2004 GPS/Galileo cooperation agreement signed by the United States and the European Union. That agreement calls for a “non-discriminatory approach,” which allows U.S. and European companies to build GPS- and Galileo-capable equipment.

However, although the GPS ICD and updates are available without restraints on commercial development, the draft ICD for Galileo’s open service, released in 2006 and updated earlier this year, prohibits use of the Galileo ICD’s specifications for commercial purposes. Although the draft specs can be used for research and development, only companies that have developed products under ESA or EC contracts can sell their equipment — and then only to agencies and companies involved with the Galileo program.

That may change for simulator manufacturers in the near future. In a plenary speech at the U.S. ION GNSS 2008 conference in Savannah, Georgia, Verhoef said, “The U.S. Government [with all due lobbying from U.S. industry] has recently made a proposal to release the sales of Galileo OS simulators developed under ESA contracts for testing purposes. In the coming weeks I will get a response to Ed Morris, but let me say that we are considering it very seriously in the spirit of boosting the market.”

That could be accomplished, Verhoef subsequently told Inside GNSS, by changing “the legal text on the first page of the ICD” exclude simulators from the constraints on commercial developments.

Not by Simulators Alone
When it comes to Galileo receiver development, however, the situation is more problematical for companies. Morris said that a second meeting of the US/EU working group on trade issue in July considered proposals for enabling equipment manufacturers to develop and build GPS/Galileo receivers on a level playing field.

Verhoef described the EC’s stance on the subject as “rather conservative,” and proceeding on a “step-by-step basis.” But uncertainty about the timing and elements of a final decision on commercial use of a final ICD is frustrating the downstream industry, which says it cannot size the risk of investing in Galileo user equipment development without knowing all the full dimensions of potential costs.

In the CGSIC panel discussion, Swiek cited unresolved issues involving intellectual property rights (IPR), commercial licenses, user fees, simulator availability, and the fear of a “closed club of Galileo-favored companies.

Although some technical issues remain to be worked out, based on the results of tests conducted using the experimental GIOVE-A and –B spacecraft now in orbit, the main obstacle to releasing a commercially usable ICD is the possibility that the EC may try to put a PPP in place to operate Galileo once the system infrastructure has been built.

“If you’re talking about a system that a future operator needs to opportunity to make money on,” Verhoef said, “it would be silly to rule out some of these options,” such as licenses, taxes on Galileo equipment, and user fees.

Verhoef added that “next year around this time we will be able to provide a lot more product and stability in the deliverables,” referring to the Galileo ICD and policies on commercial development.


GNSS Watch Dog: A GPS Anomalous Event Monitor

BPSK signal magnitude spectrum

The Avionics Engineering Center at Ohio University has developed a real-time, continuously operating GPS Anomalous Event Monitor (GAEM), designed primarily to assist with ground-based and space-based augmentation systems (GBAS and SBAS).

The Avionics Engineering Center at Ohio University has developed a real-time, continuously operating GPS Anomalous Event Monitor (GAEM), designed primarily to assist with ground-based and space-based augmentation systems (GBAS and SBAS).

The GAEM uses high-performance GPS receivers to flag abnormal events originating in the space segment (i.e., satellite anomalies) or the operational environment, such as interference and jamming, multipath, and ionospheric and tropospheric effects.

Upon the detection of erroneous behavior, the monitor records live RF GPS data in the form of intermediate frequency (IF) samples. The RF data are then processed to inspect the spectrum, signal quality, acquisition, and tracking results, together with other aspects that might have an impact on GPS ranging performance.

This article describes the requirements, design, and operation of the GAEM and then presents a series of examples to demonstrate its capabilities.
(To view the many figures and graphs referenced in this article, please download the PDF version using the link, above)

GAEM: The Need and the Benefit
Currently, GAEM hardware is installed at three facilities: the Memphis International Airport in Memphis, Tennessee; the FAA William J. Hughes Technical Center in Atlantic City, New Jersey; and one the Gordon K. Bush Airport of Ohio University, in Albany, Ohio.

The GPS Anomalous Event Monitor is able to perform the following functions:

  • operate continuously and automatically, under remote control
  • detect and flag a large variety of GPS anomalous events in real-time
  • automatically create reports to present all the processing results and to integrate multiple data/information sources that are related to the events
  • differentiate causes of the anomalies
  • separate anomalies from normal operations
  • provide an evaluation of the impact on GPS performance
  • provide timely information on detected events and available reports, with a message broadcast to the public or an interested community.

In addition to operators of GBAS and SBAS facilities, these capabilities are useful to specialized users of GPS as well as researchers.

With the RF data and the analysis reports, researchers can investigate any captured events in postprocessing. These postprocessing results not only identify the nature of the event, but also help to determine the origin of it. Furthermore, the consequences of such events can often be directly observed from the reports.

A thorough study on the origin and consequence of past anomalous events is essential for dealing with future anomalies — to predict their occurrence and to minimize the damage. The GAEM also serves as a data archive for those who are interested in evaluating the overall GPS performance at particular locations using historical data.

GPS users can benefit from the estimation of positioning performance in a local environment. Safety-related applications, such as aircraft landing navigation with GBAS and SBAS, have stringent requirements for accuracy, integrity, continuity, and availability.

In general, integrity and accuracy tend to attract more attention, and both can be monitored with the GAEM. Although SBAS already provides certain monitoring functions for GPS integrity, it cannot oversee performance changes in a local operational environment. Users who rely on GPS and SBAS for business purposes need to be able to monitor GPS availability for obvious reasons.

Another example: For GPS positioning in mining and construction operations, a local GPS monitor like the GAEM will be especially helpful. These users demand an extremely accurate positioning capability with a certain level of continuity and availability guarantee for cost reasons.

The GAEM can be most helpful when a sudden change —either positive or negative — occurs in GPS performance occurs.

For example, when new GPS satellites, new frequencies (e.g., L5), or a new signal structure on an existing frequency (e.g., L2C, L1C) are added into the system, they are considered positive changes. The GAEM can monitor and process the new signals with corresponding updates in the RF front-end.

Significant events in a GNSS independent from GPS, such as GLONASS and Galileo, may also have unpredicted effects on GPS. For example, a non-GPS satellite that shares the same spectrum as GPS has the potential of producing in-band interference. Although such interference should be well controlled in principle, a GAEM recording live RF data will definitely help to verify that. On the other hand, the GAEM can also be extended to monitor multiple GNSSs as well as their augmentation systems.

GPS performance at any location depends on the local atmosphere, including troposphere and ionosphere, and the operational environment. Local weather forecasts can predict dramatic tropospheric events, whereas ionospheric effects are often correlated with space weather, for example, the well-known solar activity. Operational environment changes are mostly responsible for local RF interference, multipath, or signal blockage. Data from the GAEM can help to determine whether a local GPS anomaly is due to atmospheric effects or the operational environment.

GAEM Design & Operation
The current architecture of the GAEM consists of three major real-time subsystems — an anomaly detection system, a data collection server, and a remote control, configuration, and monitoring system — with a separate postprocessing network.

The anomaly detection subsystem monitors the status flags and raw measurements output by two commercially available high-quality GPS receivers. If either receiver detects an anomaly, the subsystem generates a trigger.

A few categories of anomalous events are currently being monitored in the system; loss of code lock, loss of phase lock, automatic gain control (AGC) flag, jamming flag, phase distortions, pseudorange steps. The subsystem can also receive triggers from an external source, for example, a local area augmentation (LAAS) ground facility (LGF) or a separate detection box specified by the user.

The data collection server maintains a past history of GPS IF data in a buffer of specified duration, currently 20 seconds in the Ohio set-up. When a trigger is received, the contents of the buffer are written to disk up to a specified end point. The post-trigger duration is currently 5 seconds. Remote clients can access the files generated in this way via the Internet or Intranet, using the tile transfer protocol (FTP) or secure file transfer protocol (SFTP).

The remote control, configuration, and monitoring subsystem (Figure 1) is comprised of file servers, remote console/desktop applications running within the anomaly detectors/GAEM server, an uninterruptible power system (UPS), a power control server, and a camera.

The power server enables independent power-recycling — the capability to switch the power off or on — for each component of the system. This is mainly to enable worst-case recovery, should a component become non-responsive or experience other abnormal behavior.

A webcam enables monitoring of the audio-visual environment of the installed site by authorized clients. The camera surveys physical changes in the environment surrounding the installation that could cause multipath, blockage, or equipment failures. Figure 2 shows the Ohio installation.

The postprocessing network connects one or multiple GAEMs to a super user (a dedicated computer that acts as a remote controller), a few post processing computers, a file server and clients. A diagram of this processing network is shown in Figure 3.

The network can be constructed via Internet or Intranet. The super user acts as the remote controller of the computers and the file server. This user is responsible for managing the file server and sending specific commands to the computers, such as the distributing of computer tasks and software updates. The goal of the postprocessing network is to prepare “quick look” and “detailed look” results that are accessible through printed reports that are generated automatically or a graphical user interface (GUI). Figure 4 shows a step-by-step procedure illustrating how the postprocessing network functions, and Figure 5 shows the cover pages of both quick-look and detailed-look reports.

The quick-look results include overall signal quality measures and system information:
1) system information (location, time of event, antenna type etc.)
2) overall signal quality, approximate CNR (carrier to noise ratio) estimations of all satellites, comparison of visible satellites before and after the trigger sets, signal spectrum
3) output of both reference GPS receivers
4) system log message that reflects the reason and time of trigger, elevation and azimuth angles of the triggering satellite
5) related notice advisories to NAVSTAR users (NANUs), from US Coast Guard <>.

Certain types of events can be easily observed from the quick-look report, such as in-band interference or equipment outage. The detailed-look results come from tracking the visible satellites in the RF data with a research software radio receiver.

The following detailed measurements are available at double precision and a 100 Hz update rate:
1) accurate CNR estimates
2) Doppler and accumulated Doppler
3) carrier phase
4) code-minus-carrier (CMC)
5) navigation data (GPS time decoded from the signal, etc.)

All of these measurements are independently observed and collected using two software receivers; one receiver uses block processing techniques and the other simulates receiver tracking loops. A user can cross-compare different satellites and find out what could have been wrong with the target satellite.

The block processing receiver is robust and resistant to signal interruptions, which is an ideal choice for anomaly analysis. The software tracking loops provide high-rate receiver measurements, which are especially useful because regular GPS receivers often temporarily lose their outputs during an anomaly.

Application Examples: Anomalies or Normal Operations?
When a change in GPS performance or signal is detected, the first thing to determine is whether an anomaly is causing the change or that it just reflects a result of normal operations. A normal operation can be scheduled maintenance, in which case it can be verified with GPS authorities, or it can be a normal reaction by satellites to certain incidents.

The nature and origin of a normal operation that affects GPS performance needs to be determined in order to avoid false alarms, because regular GPS receivers may not anticipate them. As an advanced feature, the GAEM will be able to predict the duration of such events and consequently forecast any future changes related to it.

On the other hand, if the detected change does not stem from a normal operation, it should be considered an anomalous event. Such events can happen in the space segment or in the user segment, that is, they may arise in the satellites, from the signal propagation path, or from the local environment.

The following discussion shows a few examples of the GAEM handling both normal operations and anomalous events. The postprocessing results shown in this section are all included in the reports that the GAEM automatically generated for each of the events.

Satellite Maintenance
Scheduled and predicted satellite maintenance shouldn’t be included in the anomaly category. However, the GPS receivers will detect off-nominal behavior in the signal because of it. Without external information, the anomaly detection system would be triggered and record this event. However, the GAEM is able differentiate an expected maintenance by retrieving information from the websites of GPS authority.

On Aug. 17, 2007, the GAEM detected an event on a GPS satellite, space vehicle number (SVN) 52/PRN31. The system log message shows that at GPS time 224741.0 both reference receivers simultaneously lost lock on the satellite’s signal. As part of the quick-look report, the outputs of both receivers recorded for one hour can be found in Figure 6. The graphs on the left represent data derived from the output of Receiver 1, while the graphs on the right side of the figure represent data from Receiver 2.

Viewed from top to bottom, the graphs show sequentially differenced pseudorange measurements, sequentially differenced carrier phase measurements, code-minus-carrier measurements, carrier-to-noise ratio (C/N0), and receiver lock time, respectively.

All the measurements from both receivers appeared to be normal, until the moment both receivers lost lock. The quick-look report also showed that the signal strength appeared to be regular, and spectrum data reveals no noticeable interference.

Even though the GPS receivers didn’t continue to track this satellite, a 25-second long RF data file was recorded and processed with the software radio receiver. Acquisition and tracking measurements from the software radio receiver also seemed normal, except that the health bits in navigation data were all set to one, which indicate that the satellite should not be used, according to the Interface Control Document GPS-200 (ICD-GPS-200).

A NANU message related to this event, citing the satellite’s pseudorandom noise (PRN) number and time, is also included in the quick-look report:

NANU NUMBER: 2007090
NANU DTG: 142132Z AUG 2007
REF NANU DTG: 101333Z AUG 2007
SVN: 52
PRN: 31
(14 AUG 2007) BEGINNING 1424 ZULU UNTIL JDAY 226 (14 AUG 2007)

The NANU message matches the health bits, and it becomes obvious that the loss of lock was due to scheduled maintenance.

Although this example may seem trivial since both the GPS NANU and health bits can clearly identify the cause of the event, such an event can prove more interesting if, for instance, the GPS NANU and health bits are not synchronized.

On April 10, 2007, a satellite maintenance issue arose on SVN54/PRN 18. A Federal Aviation Administration (FAA) report of the event, referenced in the Additional Resources section at the end of this article, indicates that a NANU message forecast scheduled maintenance of this satellite some time between 13:30 GMT on April 10 and 1:30 GMT on April 11.

The actual maintenance work initialized at approximately 15:53 GMT on April 10, while the satellite health bit was still set to “healthy,” apparently by mistake. Large range errors occurred before the health bit was corrected at 17:04 GMT. Having RF data, GPS receiver output, and the NANU all included in one report would have greatly helped users to understand what had happened.

Non-Standard Code
Although seemingly “abnormal,” the second example here is not an anomaly either. As documented in ICD-GPS-200, in order to protect users from receiving and utilizing erroneous satellite signals, GPS satellites switch off regular broadcast of C/A code and P/Y code and transmit the non-standard C/A code (NSC) and non-standard Y code (NSY) in case of “unhealthy” conditions on the spacecraft.

For example, non-standard code will be received when a malfunction occurs in the satellite reference frequency generation system, a certain data failure in satellite memory, or scheduled maintenance. Therefore, the transmission of non-standard code is considered a normal operation in itself, even though it may reflect a glitch in the satellite.

The NSC consists of alternating 0s and 1s transmitted at the C/A code chipping rate, which is designed to have negligible effect on tracking other healthy GPS satellites. In many cases over the years, however, the transmission of NSC could not be predicted, and it caused an unexpected change in the performance of user equipment. So, in effect, such an anomaly affects GPS availability and integrity.

Multiple events have been observed, and as an example, we will analyze the one recorded on November 28, 2006 at 12:38 p.m. EST, when SVN38/PRN 8 was switched to NSC mode.

Figure 7 displays a screen shot of the GAEM GUI that demonstrates quick-look outputs from this event. The upper left graph illustrates estimated CNR as a function of time for 26 seconds for all visible 32 PRNs. In that graph, red corresponds to strong satellites and deep blue corresponds to unavailable satellites. A red vertical line is used to indicate the approximate time of the trigger.

The two graphs located on the lower left-hand corner of Figure 7 show the average signal strength of all 32 PRNs before the trigger and after the trigger, which is used to identify change of available satellites due to the possible anomaly. The upper graph on the right illustrates the signal spectrum in a blue line, estimated using one millisecond of signal at the end of the data. For reference and comparison, the red line in this graph represents the signal spectrum averaged over 100 milliseconds at the beginning of the data before the system was triggered.

The middle right-hand graph provides a three-dimensional time-frequency plot showing the change of signal spectrum over time, and the lower right graph is a top view of that same time-frequency plot.

Figure 8 is a screen shot of the GUI displaying the detailed-look process, which shows the tracking results for PRN8. The upper, middle, and lower graphs on the left correspond to graphs of the C/N0, accumulated Doppler, and the differential of integrated Doppler measurements, respectively. The graphs on the right show the carrier phase tracking residual, code phase tracking residual in form of code minus carrier and navigation data bits, respectively.

If the data bits have been decoded correctly, the GPS time of the beginning of the observation window is also shown. In the detailed-look process GUI as shown in Figure 9, a user can simultaneously zoom in on the same part of the run-time in all graphs. The quick-look and detailed-look results presented in the GUIs are contained in the corresponding printed postprocessing reports.

As can be seen from Figure 7, the C/N0 estimation of PRN 8 shows loss of signal around the 19th second into the file. Figure 8 shows that the tracking of PRN 8 seems to be in a normal mode up until the moment when it becomes absent.

A zoomed-in view around the 19th second is shown in Figure 9 and provides more signal-tracking details. More interestingly, the GPS time was decoded from all the navigation data bits recorded before the loss of PRN 8, from which the occurrence of this event can be precisely synchronized to GPS time.

The signal spectrum and time-frequency plot in Figure 7 contain irregular components that are not standard in a regular GPS signal spectrum. Two spikes are located at approximately 500 kHz and -500 kHz at a sustained level for six seconds until the end of the observation window.

Recall that the NSC is a BPSK (Binary Phase Shift Keying) signal with alternating 0s and 1s at a chipping rate of Rc = 1.023 Mbps. This signal has a magnitude spectrum given by the formula found at the top of this article (above, right), which consists of a series of impulses under the envelope of a sinc wave with a two-MHz null-to-null bandwidth. Within the two-MHz bandwidth, two major impulses are located at +/- Rc/2, which are responsible for the signal spectrum observed in Figure 7. A further study shows that during the six-second outage, NSC can be acquired and tracked in the recorded RF data. As a result, we can differentiate non-standard code broadcasts from truly anomalous GPS outages.

Ionospheric Effects
The effect of the ionosphere on GPS signal transmission has received considerable attention ever since the early days of the Global Positioning System. Monitoring ionospheric effects such as scintillation serves to ensure the integrity of safety-of-life systems as well as everyday users.

Since the GAEM system started continuous operation, hundreds of events have been collected at the Ohio installation alone. We believe that a significant number of events collected in late 2006 could be attributed to ionospheric effects.

The year 2006 is not supposed to be a peak year for sunspot activity according to the 11-year solar cycle. However, on most of the days from November 30 to December 17, 2006, sunspot activities were reported with magnetic classes of BG, BD or BGD, while the sunspot activities at a normal day is usually labeled as Class A or B. (For a discussion of the U.S. Air Force and National Oceanic and Atmospheric Administration classifications, see the reference in Additional Resources.) The sunspot activities identified as BG, BD, or BGD are considered potentially problematic for radio transmission.

Figure 10 shows a screen shot of the quick-look output GUI for an event collected on December 6, 2006, at 12:56 p.m. EST. Apparently PRNs 4, 8, 11, 17, and 28 could be acquired. Only PRN 11, however, can be considered a strong signal, and the power level of PRN 4 would make it difficult to track.

Fortunately, the GAEM block-processing software receiver does not easily lose lock, and the tracking results of PRN 4 are demonstrated in Figure 11. The CNR measurement fluctuates around 33 dB-Hz and can be as low as 30 dB-Hz. Such effects are sometimes referred to as amplitude scintillations.

Having a total of five visible satellites with only one strong signal can result in degraded positioning accuracy as well as system integrity. Furthermore, because the CNR of a GPS signal largely depends on individual antenna patterns. Some antennas could receive fewer than four satellites in this situation. When that happens, ionospheric effects also affect the continuity and availability of GPS and SBAS positioning. (At the time this data was collected, GAEM used a choke ring antenna. Now it connects to a pinwheel + multipath limiting antenna or MLA.)

The GAEM is able to associate the observed the results with space weather information and determine the cause of such events. More importantly, knowing the cause, it can forecast the occurrence of the same type of events.

Our final example deals with the solution of an obscure problem involving a series of events observed on a single satellite.

The Ohio GAEM system was installed on the Ohio University campus before it was moved into the OU airport in 2008. From June to September 2007, 14 events were recorded for PRN 21. Although no NANU message related to these events could be located, both reference GPS receivers reported loss of lock during all of the events. However, receiver outputs did not contain enough information to explain why loss of lock happened at these particular moments.

The software radio receiver can provide high resolution and update rate of signal tracking results. CNR measured with the software receiver during one of these events is shown in Figure 12, in the upper left figure. It carries a clear sinusoidal variation with a peak-to-peak magnitude of approximately two decibels.

Code-minus-carrier (CMC) calculations are also a helpful diagnostic measurement. In the GPS receivers used in GAEM, code phase is measured using pulse aperture correlators and filtered by loop filters and carrier smoothing. The approach has minimized the effects of error sources such as multipath but is less effective as an error indicator.

The code phase measured with the software radio receiver uses standard correlator spacing and is unfiltered. Although these raw measurements contain much more noise, they can help in analyzing error sources.

In the middle right graph of Figure 12, the CMC also displays a sinusoidal pattern at the same oscillatory frequency as CNR. The appearance of this pattern in both CMC and CNR suggests the possibility of multipath in the signal, which could not have been recognized otherwise.

Although this initial investigation indicated the nature of the anomalies, their exact cause remained unknown. However, the phenomena were found to be geometry-dependent. The satellite azimuth and elevation angles recorded in the system log message for each of the events are reflected in the azimuth-elevation plot in Figure 13.

The azimuth angles measured in 11 events were concentrated within the range of 170 to 200 degrees, while the remaining three had an azimuth of approximately 310 degrees. The elevation angles, on the contrary, spread out in the range of 35 to 80 degrees. Notice that the mask elevation angle of the receivers had to be set to 35 degrees in a suspected multipath environment.

The consistency of azimuth angle measurements further supports the assumption of multipath. These measurements also make it possible to identify the source of multipath.

Figure 14 contains an aerial photo that provides a bird’s eye view of the antenna and surrounding environment. The red circle marks the building where the antenna is installed. To the southeast of this building is a white dome, seen in the lower right-hand corner, which is the Ohio University convocational center. The curved roof of the convocational center forms many reflecting surfaces at various attitudes.

Because of the roof’s curved surface, signals from a wide range of elevation angles and a smaller range of azimuth angles can be reflected off of it and received by the antenna. The red arrows illustrate a possible path for the signal reflection, which comes from a satellite with the azimuth angle of approximately 170 degrees. As further indication that the convocational center dome was the culprit, these multipath events no longer occurred after the Ohio GAEM installation moved to the OU airport, shown in Figure 2.

What’s the Plan?
The GAEM has been further developed to enable it to process SBAS signals. In the future, this system will also be able to monitor other GNSS signals, including the following functions:
1) intelligent analysis: enable the GAEM to act like an analyst and give a definitive answer
2) customized monitoring: capture events that would affect a user-specified performance property
3) additional data sources: troposphere status from local weather input, ionosphere status for space weather input, and multipath environment from on site camera
4) networking multiple locations: to easily identify whether an event occurred in a local area or can be attributed to the space segment
5) wide band data collection: increase the monitoring bandwidth from 2.2 MHz to 24 MHz, the full GPS bandwidth, at L1, L2, and L5 frequencies
6) hardware acceleration of the postprocessing: data samples using a field-programmable gate array (FPGA) to greatly reduce the calculation time..

This research was supported by the Federal Aviation Administration Aviation Research Cooperative Agreement 98-G-002.

Additional Resources

Doherty, P. and S. H. Delay, C. E. Valladares, and J. A. Klobuchar, “Ionospheric Scintillation Effects in Equatorial and Auroral Regions,” Proceedings of ION GPS 2000, Salt Lake City, Utah, p. 662-671, September 2000

Federal Aviation Administration, “Global Positioning System (GPS) Standard Positioning Service (SPS) Performance Analysis Report #58,” July 31, 2007

GPS JPO (Joint Program Office), ICD-GPS-200 (GPS Interface Control Document), prepared by ARINC Research, April 2000

USAF (U.S. Air Force) and NOAA (National Oceanic and Atmospheric Administration), Sunspot data, available: <>

Zhu, Z., and S. Gunawardena, M. Uijt de Haag and F. van Graas, “ Advanced GPS Performance Monitor,” Proceedings of ION GNSS 2007, Forth Worth, Texas, September 2007

Zhu, Z., and S. Gunawardena, M. Uijt de Haag and F. van Graas, “Satellite Anomaly and Interference Detection Using the GPS Anomalous Event Monitor,” Proceedings of ION 63rd Annual Meeting, Cambridge, Massachusetts, April 2007