…from a domestic source:
An analytical assessment performed by the Defense Department suggested that GPS receivers will be negatively affected if the [LightSquared 4G cellular network] is deployed. That analysis has now been confirmed by recent field testing, Air Force Gen. William Shelton, commander of Air Force Space Command, said during a May 11 hearing of the Senate Armed Services strategic forces subcommittee. Using actual LightSquared hardware at Kirtland Air Force Base, N.M., engineers detected interference to military, civilian and commercial GPS receivers, Shelton said.
“Although the data is still being analyzed, I would tell you that the empirical data appears to be consistent with the analytical data, so we have concerns for commercial applications, civil applications and military applications,”?he said.
— Brinton, Turner. “House Panel Slashes Funding for ASSIST.” SpaceNews.com, 13 May 2011.
The problem is, in essence, the proximity of the satnav and cellular frequency bands. GPS satellites transmit their navigation signals in the 1559 to 1610 MHz band, and LightSquared’s proposed 4G network would operate in the adjacent L-band, from 1525-1559 MHz. The 4G network would involve the erection of 40,000 new base stations, and manufacturers of satnav receivers are concerned that the large increase in base stations would create interference with GPS receivers, resulting in satnav dead zones all across the United States. (The GPS industry’s January 2011 report to the FCC on interference and jamming effects has additional detail.)
According to a February 2011 article in New Scientist magazine, LightSquared characterises the problem as a fault in the GPS receivers themselves, since some have the ability to “see” into the L-band where the cellular network will operate.
The GPS industry’s testing appears to have taken place using L-band simulated transmissions, while USAF’s testing occured using actual LightSquared hardware. According to Gen. Shelton’s May 11th testimony to the Senate Armed Services Committee’s strategic forces panel, the Air Force’s testing seems to have validated the earlier results; it will be interesting to see what this heralds for next-generation telecom networks and GPS receivers.