Pseudolites have been studied by the US military, which curiously has declined to adopt them beyond a few prototypes that might be used in regional conflicts. A pseudolite is a "pseudo-satellite," an aircraft able to replace a GPS satellite in navigational systems. Basically an unmanned aircraft orbiting for long periods, a pseudolite could carry a transmitter for GPS data which is 10,000-times more powerful than the satellites. This could allow GPS to continue functioning in the face of heavy jamming. --Stephen V Cole