Police Tech & Gear
with Tim Dees
GPS may be an endangered species
A new cell phone network could jam GPS signals
In a column a few weeks back, I described some of the services and industries that have little to do with navigation but are served by GPS signals. According to GPS industry vendors, those services, as well as the primary application of mapping and tracking people, vehicles, aircraft and marine vessels, could be jeopardized by a proposed communications network.
The controversy is between the GPS industry and a communications company called LightSquared. LightSquared has received FCC approval to build a nationwide network of up to 40,000 cell sites supplemented by their Sky Terra 1 satellite that was placed in geostationary orbit last November. The satellite has a 22-meter L-band antenna that makes it the largest commercial communications reflector in use. When fully operational, LightSquared voice and data service subscribers will stay connected via the cell sites when they’re close enough to a site to receive a signal. When they’re not, they’ll still stay connected via the satellite. LightSquared says the Sky Terra 1 will provide coverage over the continental United States.
Unlike most premium satellite phone services that have rates in excess of a dollar per minute or air time, LightSquared is targeting the consumer market, selling their service through Best Buy and similar outlets. Air time rates are expected to be priced competitively with other wireless carriers. Most traffic will flow through the ground-based network, with signals passing through the satellite only when there is no cell site in range.
LightSquared has asked the FCC to grant them a license to use the L Band 1 spectrum for this service. This takes up the bandwidth from 1525 to 1559 MHz. Right next door, at 1575.42 MHz, is the GPS frequency that carries signals from the 30 or so GPS satellites orbiting the earth.
GPS signals travel around 13,000 miles before they reach one of the millions of receivers in use around the world, and they’re not very strong. The ground transmitters that LightSquared proposes to use are licensed at over 15,000 watts, and the GPS industry says this will effectively jam any GPS signals in the vicinity of a transmitter. Signals from one of the proposed LightSquared transmitters would be one billion times more powerful than those from GPS satellites, according to information published by the GPS industry.
Tests conducted by Garmin International indicate that a typical consumer navigation device will be jammed at 3.57 miles from a LightSquared transmitter site, and will lose its fix completely at 0.66 miles. More sensitive aviation receivers are similarly affected at 13.76 and 5.60 miles, respectively.
It’s not clear why the FCC is giving preliminary approval to LightSquared’s application, given the widespread use of GPS services and the GPS industry’s objections. The L Band 1 spectrum has long been designated for satellite communications use, but pairing a satcomm network with terrestrial transmitters is a game-changer. When there is a potential conflict between users of the radio spectrum, the usual procedure is to test for the level of interference that will result from the operation, then issue approval as appropriate. In this case, approval was granted conditionally, based on proposed testing.
There is obviously a huge market already established for GPS, but LightSquared’s network represents an investment running into billions. The company has signed agreements with Leap Wireless International, which markets Cricket cell phones, and there is industry buzz that this could mean the ability to buy cell service without having to commit to a contract with a single carrier such as AT&T or Sprint.
Multiple requests to LightSquared for comment on this issue got no response from the company.
The GPS industry has established a web site called Coalition to Save Our GPS. Available from the site are white papers and other information on the technical and political aspects of the issue, and links to contact the FCC and express your concerns.