Low-powered incoming GPS signals are allocated 1559-1610 MHz in the L-Band radio spectrum. The neighboring spectrum–from 1525-1559 Mhz–is allocated to other, equally low-powered, incoming satellite signals, such as Sky Terra’s. GPS receivers are open to signals above the blue line shown, but filter out signals below it, and require modification to accept Glonass. GPS industry officials state that current filter technology could not suppress LightSquared’s interference.
The right-hand red column (stylized here but actually bell-shaped) represents the power of just one of LightSquared’s proposed 40,000 nationwide transmitters. In tests, one transmitter jammed virtually all GPS receivers. LightSquared proposes moving to the left-hand column, farther away from GPS, but this will still disable high-precision surveying, agriculture and other specialized GPS applications, according to engineers in those areas. While rarely appreciated by the larger GPS user community, satellite guidance has become an essential element in U.S. agriculture, injecting tens of millions of dollars in new money into rural and state economies across the nation.
Perhaps recognizing this, and anxious to reassure farming communities after recent field tests revealed serious GPS interference from one of its transmitters, LightSquared in early July announced the creation of the Empower Rural America Initiative, to be led by former Congressmen from North Dakota, Washington State and Texas. The group has several aims, including “Working with LightSquared and other parties to make sure device filters and other approaches are developed that will resolve any GPS issues related to precision agriculture and other areas.” This is certainly an encouraging step, but it imposes an onerous condition on the FCC’s pledge not to license LightSquared until all questions of GPS interference have been resolved, since experts at leading agricultural equipment manufacturer John Deere have stated that there is simply no technical solution in sight to prevent LightSquared interference to GPS.
Defense, Transportation and other government departments want LightSquared to move completely away from the satellite spectra. Perhaps the first task of the former Congressman should be to talk to the people at John Deere.