There are two concepts of free space radio communications that must be understood to appreciate the gravity of the problem with LightSquared's currently proposed system.
Radio signals lose strength as the square of the distance. GPS satellites are some 10,000 miles up. That puts GPS signals at a disadvantage of 10 billion to one over a cell tower a mile away, and LightSquared are proposing to build 40,000 of them. That
represents a 100 decibel difference in signal strength. Can you hear the rustle of leaves over the roar of a jet?
The other factor is the closeness of the frequencies as a percentage of the frequencies, as well as the bandwidths involved.
Tune in the strongest station on your AM dial, then try to get a station 100 miles away up or down 20 kHz. Broadcast band
DXers (long distance reception hobbyists) use precisely aimed loop antennas to null out nearby stations. Unfortunately that trick won't work for practical GPS navigation receivers.