GPS Troubles, Revisited

August 15, 2011

Back in July, I posted a note about a controversy involving the Global Positioning System [GPS] and a proposed new service to provide wireless broadband Internet access, offered by a firm called LightSquared.  The company holds licenses for a portion  of the spectrum reserved for Mobile Satellite Services, using frequencies just below those used by the GPS.  When the licenses were originally acquired by SkyTerra Communications, a predecessor company to LightSquared, the plan was to make connections primarily with satellite links, with some small ground stations to fill in holes in the coverage. The controversy has come about because LightSquared persuaded the Federal Communications Commission[FCC] to amend its license to allow a service based almost entirely on a network of 40,000 ground transmitters.

A recent article in the “Babbage” science and technology blog at The Economist provides some updated information on the situation.  The evidence that implementing the LightSqaured plan would create an enormous interference problem with the GPS continues to mount.  The fundamental problem is that, as I’ve discussed before, GPS signals as received at the Earth’s surface are quite weak, coming as they do from satellites at altitudes of ~20,000 km.  The original SkyTerra plan, which also used satellite transmitters, would not have presented a major interference problem, but the current LightSquared plan is a different animal altogether, using transmitters much more powerful than GPS.

The company intends to build a broadband wireless network comprising 40,000 base-stations across the United States. These stations will put out 15,000 watts apiece. Typical mobile-phone transmitters in urban areas radiate between five and ten watts. Even the 100-foot towers used in open countryside transmit no more than 60 watts.

As the article points out, LightSquared’s complaint that existing GPS receivers are not very well protected against interference has some validity; however, to make a receiver that can reject a signal from an adjacent frequency that is a billion times as strong, and still do a decent job at receiving the desired weak signal from the GPS satellites is not feasible.

The FCC ordered that a technical review be conducted to evaluate the potential for interference.  A report of the review, prepared by RTCA, Inc., a consultant to the government, was submitted in late June (the public version is available here [PDF]).  The conclusion of the review was that implementation of the LightSquared system would cause major interference with GPS-based aviation systems.  From the Executive Summary of the report:

The study concludes that the current LightSquared terrestrial authorization would be incompatible with the current aviation use of GPS, however modifications could be made to allow the LightSquared system to coexist with aviation use of GPS.


The impact of a LightSquared upper channel spectrum deployment is expected to be complete loss of GPS receiver function. Because of the size of the single-city station deployment, GPS-based operations below about 2000 feet will be unavailable over a large radius from the metro deployment center (assuming no other metro deployments are nearby). Given the situation in the high altitude U.S. East Coast scenario, GPS-based operations will likely be unavailable over a whole region at any normal aircraft altitude.

In essence, the planned deployment could make GPS unavailable over a large portion of the eastern United States.  A separate study [PDF], conducted by the National Executive Committee on Space-Based Positioning, Navigation, and Timing’s Systems Engineering Forum, recommended that deployment of the LightSquared system not be allowed to proceed, due to major adverse effects on GPS.  The group also found that proposed mitigations based on modification or replacement of existing GPS equipment were impractical, and probably insufficient for applications requiring high precision.

It seems clear, both from the reports of various study groups and from first principles of physics, that LightSquared’s planned system would cause serious disruption of the GPS, leading to many problems.  As the “Babbage” article noted, the costs just in the aviation sector would be huge.

The Federal Aviation Administration (FAA) reckons it would cost airlines, in particular, more than $70 billion over the next ten years if they had to find fixes to cope with a GPS blackout.

Many others would be affected, too, including mobile phone users, drivers, the armed forces, and emergency services.  “Babbage” concludes, and I have to agree, that, although the US badly needs an alternative broadband supplier, especially in less-populated areas, degrading the GPS is a price too high.

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