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The System: Glitches and Vulnerabilities

October 1, 2009  - By

A range of unrelated events in September show that GPS, the world’s preeminent GNSS, remains a work in progress.

The first in a series of deviations from normal GPS signal broadcasts during September was noted by researches at the University of New Brunswick, among others around the globe, who found that normal signals from the L1 and L2 transmitters on the GPS satellite PRN01/SVN49 were unavailable for more than two hours on the morning of September 4.

The satellite did not transmit useful signals on L1 and L2 from about 12:00 to 14:11 UTC, as reported by International GNSS Service stations in Europe. The L5 test signal continued to be tracked by some receivers but not others.

One possible explanation for the inability to track PRN01 is that the satellite rejected an upload and automatically went into non-standard mode, resulting in GPS receivers being unable to track the L1 and L2 signals. In other words, the L1/L2 transmitters were still on but transmitting a non-standard signal.

“It is not known for sure what actually happened with the satellite, but perhaps it is related to the ongoing issues with the signal reflections on the satellite and that the GPS Wing was conducting further tests,” said Richard Langley, GPS World’s Innovation editor and professor at the University of New Brunswick. “Luckily, the problem was short lived.” As to why some receivers continued to track the L5 signal but others did not, Langley speculates that some receivers may need to acquire and track the L1 signal before they can track the L5 test signal.

HDOP Warning. On September 10, the U.S. Coast Guard Navigation Center (USCG NavCen) issued a high dilution of precision (DOP) warning for certain locations in the U.S., Asia, and Oceania, reporting that GPS users might experience a temporary degradation in GPS reception in parts of the southwest and central United States from 13:02 UTC to 13:23 UTC on September 11.

“The warning is based on a best-four satellite scenario: what the DOPs would be if we only used the best four satellites (the combination providing the lowest DOP value) of all the satellites in view at a particular location,” said Langley.

“However, most civil receivers these days track eight or 10 or all satellites in view. I contacted the Coast Guard about this, and they did another analysis and confirmed DOP spikes for all-in-view users too. Prompted by that, I did my own analyses and found that with PRN31 out of action for the delta-V and PRN01 not yet declared healthy, only five satellites above 5 degrees elevation angle (and almost colinear in the sky) will be visible at the stated locations and times, resulting in GDOP spikes approaching 100!

“So, in this case, the warning is for all users in the affected areas, not just receivers with only four channels.”

Although a window stretching from 00:30 to 15:00 UTC had been allocated for the PRN31 delta-V maneuver, prompting the high DOP alert, the GPS Wing avoided any problem to users by delaying the start of the operation until 01:27 UTC and completing it in little more than one hour. The satellite was back on line by 02:37 UTC.

Sat Moves. After 22:00 UTC September 12, system operators began transitioning satellite SVN25 (PRN25) into the broadcast almanac for all satellites. Meanwhile, they moved satellite SVN24 (PRN24) out of the almanac.

The current GPS operation control system (OCS), known as AEP, cannot handle 32 satellites. However, the recent move gave rise to speculation that the maximum number of operable satellites has now been reduced from 31 to 30, for some reason. Apparently, the military cannot allow more than 30 space vehicles to be in active service at any one time. So when a new SV is activated, one must be deactivated. SVN24 will be placed in caretaker status, ready to be brought back on line should the situation change or the 30 SV limit be overcome.

Recent pronouncements by GPS Wing personnel on the benefits of the next operating system, OCX, have stated that it will be able to handle many more satellites, as many as 60. This figure now appears in doubt.

Russian Vision. Grigory Stupak and Mark Shmulevich reported Russia’s plans to restore a full GLONASS constellation of 30 space vehicles, laying out a road map leading to full interoperability with GPS. They envisaged a world orbited by 117 navigation satellites, with GLONASS operating alongside GPS, Galileo, and China’s COMPASS, supported by a further 29 augmentation satellites. That would certainly mitigate many of the vulnerabilities of GNSS due to propagation effects — but not those from interference in the frequency bands they will all share.

Solutions Sought to GNSS Vulnerabilities

Baska conference report by David Last

The second conference on GNSS Vulnerabilities and Solutions, September 2–5 in Baska, Croatia, focused on GNSS vulnerability to space weather, unintentional interference, jamming, and multipath propagation.

The conference was a joint venture by the Royal Institute of Navigation, London, and Nottingham University’s Institute of Engineering Surveying and Space Geodesy. Sixty-four delegates, mostly European, came from 21 countries.

Nearly half the papers focused on space weather and ionospheric and tropospheric propagation, taking in long-term and short-term solar effects, scintillation, signal attenuation, tropospheric delay variations, meteorological influences, and even gravity waves. The approach of the physicists was: Understand these things and maybe you can mitigate your vulnerability to them.

GNSS vulnerability can threaten safety-critical and mission-critical systems, including navigation in the air, maritime automatic identification systems, and the transportation of nuclear waste and other dangerous materials on land. Mitigations include EGNOS (the European WAAS) and GBAS (ground-based augmentation systems.)

Road Tolling. An unexpectedly hot topic was the enthusiasm of European governments to deploy road-user charging schemes based largely on GNSS technology. Some say road pricing is a rare and novel case of GNSS users who are hostile to the technology and seeking to exploit its vulnerability to the maximum. To enforce charges through the legal system may require levels of integrity approaching those of aircraft instrument-approach systems.

Suggestions for jamming defenses came mostly from Germany: Ulrich Engel and Angelika Hirrle proposed exciting new mathematical techniques to help separate GNSS signals from noise and interference, while Michael Felux sought refuge in low-cost inertial systems.

Hank Skalski of the U.S. Department of Transportation laid out U.S. government plans to detect and track down sources of GPS jamming. The SETS (Space Event Tracking System) will deploy aircraft, vans, fixed-base units, and trained technicians.

See Last’s report on low-cost jammers in criminal employ in Expert Advice, October 2009.

Smartpath Approved

The U.S. Federal Aviation Administration (FAA) has certified Honeywell’s Smartpath precision-landing system for airport installations. As this magazine went to press, neither the FAA nor the Department of Transportation had issued an official release, but industry contacts were notified in mid-September.

The ground-based augmentation system provides aircraft with precise navigation data for CAT I approaches and landings, enabling closely spaced parallel and curved path approaches to increase airport capacity. It asserts improved navigation accuracy over instrument landing systems (ILS), using differential GPS and broadcasting both pseudorange corrections for each satellite in view as well as approach path information in a digital broadcast.

According to Honeywell, most current-production Airbus and Boeing aircraft now carry GBAS avionics or offer it as an option. Future Smarpath upgrades include the ability for CAT III approaches.

Arctic Passage Traversed by Merchant Ships

Two German merchant ships traversed the Northeast Passage from South Korea, leaving in late July, to Siberia, and plan to continue their journey to Rotterdam in the Netherlands.

A sea lane traditionally blocked by heavy ice floes or solid sheet ice, this route has opened because of to global warming. In 2007, Arve Dimmen, director of maritime safety for Norway’s Coastal Administration, told the U.S. National Space-Based Positioning, Navigation, and Timing Advisory Board that disappearing ice across the Arctic poses potential threats: 25 percent of undiscovered oil resources lie in that region, and the route could now be used by supertankers and large container ships, as it is more economical and less time-consuming.

Precision navigation faces more challenges north of the Artic Circle, from atmospheric affects in polar regions and the low elevation of SBAS satellites at those latitudes. A June 2009 study on GNSS use in the high Arctic by Richard Langley, however, found that conventional horizontal (marine) navigation works well north of the Arctic Circle. Still, others held that “this is another reason why eLoran is so important: someone at USCG/State/Commerce needs to use this as a wake-up call!”

Created from nearly 200 Envisat scenes, this Arctic mosaic reveals that the most direct route of the Northwest Passage (the orange line) across northern Canada is fully navigable. The blue line traces the Northeast Passage along the Siberian coast, which is only partially obstructed by ice; see story, page 16. Envisat advanced synthetic aperture radar mosaic produced by the Danish National Space Center.
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