I’ve written here before about the many ways in which the Global Positioning System [GPS] has come to be used, and some of the interference problems, both accidental and otherwise, that have grown along with the system’s usage. A new controversy, concerning potential interference from a new broadband wireless networking service, has arisen in the last year.
The proposed new service would be provided by a company called LightSquared, The company has two satellites in geostationary orbit that can relay data on the Mobile Satellite Services [MSS] band, using uplink frequencies 1631.5 – 1660.6 MHz, and downlink frequencies 1530 – 1559 MHz.. (Rights to the spectrum were acquired by SkyTerra Communications, a predecessor company to LightSquared.) LightSquared’s plan is to distribute streaming 4G Internet service from these two satellites via a network of ~40,000 ground stations (called “Ancillary Terrestrial Components”, or ATCs), which would in turn relay the signal to user devices. The company’s business plan calls for it to act as a wholesaler, with the service resold via third parties (Best Buy was a suggested example). The attraction of the plan is its potential for providing broadband service in more rural areas where the current provider oligopoly doesn’t provide coverage.
Since the plan was introduced, there have been concerns voiced by the Department of Defense and the GPS devices industry about potential interference with GPS operations. The downlink frequency band, to be used by the LightSquared satellites and ATCs, is just below the band used by GPS. The satellite signals are not a concern; like the signals from the GPS satellites, they are fairly weak. But the ATC ground stations would re-transmit the signals with orders of magnitude more power than the satellites.
According to an article on the “Law & Disorder” blog at Ars Technica, the National Telecommunications and Information Administration [NTIA] of the US Department of Commerce has released a report [PDF], addressed to the Federal Communications Commission [FCC], stating that the proposed system would produce serious problems for GPS operations.
The NTIA assessment contains testing results overseen by the National Executive Committee for Space-Based Positioning, Navigation, and Timing (EXCOM). “The LightSquared Network initial deployment would cause severe operational impact over significant regions of the United States,” EXCOM’s testing team advises.
The report includes a map, for example, showing a region around Washington DC in which the early-stage deployment of the LightSquared system would prevent usage of GPS services by aircraft flying at 500 feet; the affected area extends well beyond Baltimore. GPS services would be degraded at even longer distances.
The report’s main conclusion: “LightSquared should not commence commercial services per its planned deployment for terrestrial operations in the 1525 – 1559 MHz Mobile-Satellite Service (MSS) Band due to harmful interference to GPS operations.”
LightSquared has submitted a revised plan (which the NTIA report acknowledges), which the company claims “poses no risk to the users of over 99 percent of GPS devices”, a claim which the NTIA proposes should be evaluated before deployment is allowed to commence. I have had a quick look at the revised plan, though I have not read it all; I am somewhat skeptical of its claims, solely because most of the first half of the plan is devoted to blaming everyone else for the problem. It seems that the controversy stems in part from the fact that, when the spectrum in question was originally auctioned, the intent was that it be used for purely satellite-based services. Subsequent FCC decisions have made the addition of some sort of ATC possible, without being very specific about what form those additions might take.
I hope that a reasonable solution can be worked out here. The GPS had turned out to be a very useful service in many important applications, and certainly is worth preserving. On the other hand, providing broadband Internet access to those areas which have none is also a worthy goal.