The so-called pseudolites are GPS navigation signal transmitters installed on aircraft or on the ground to replace GPS satellites for navigation. DARPA uses drones to study pseudolites, called GPS pseudolites, to enable its own troops to have accurate navigation capabilities in a jammed battlefield environment. The method is that four pseudolites on the flying drones broadcast high-power signals, thus creating an artificial GPS constellation over the battlefield area. Four "Hunter" drones can cover a 300-kilometer square war zone. cell phone jammer

The signals emitted by pseudolites can be used by making some changes to the software of existing GPS receivers. Usually when using the actual GPS constellation for navigation, the receiver needs to know the satellite position, that is, the ephemeris situation, so the challenge faced by pseudo-satellites is to use the available low data rate information to tell the receiver the position of the four moving pseudo-satellites. Therefore, the key task of DARPA and Collins designers is to send pseudo-satellite ephemeris in the available 50 bits/second information. The stability of the drone is quite good and it will not maneuver like a fighter; but any movement will make the position a little uncertain. Therefore, compared with navigation using a satellite constellation, its total positioning error will increase by about 20%. DAPRA has tested a single pseudo-satellite on a business jet at an altitude of 7,500 meters and a "Hunter" drone at an altitude of about 3,000 meters. The navigation accuracy dropped from 2.7 meters when using real satellites to 4.3 meters.signal jammer

Of course, pseudo-satellites do not have to be all airborne, and a mixed solution of ground and airborne transmitters can also be used. The disadvantage of setting some pseudo-satellites on the ground is that the coverage is reduced, but the navigation accuracy is improved. To overcome interference, pseudo-satellites can transmit 100-watt signals, which increase the signal strength at the ground receiver by 45 decibels compared to the signal strength from the satellite.

Northrop Grumman is developing a GPS receiver that can provide 30-40 dB of anti-jam improvement. This anti-jam method, called "anti-jam autonomous integrity monitoring extrapolation," will be achieved by fully coupling the inertial navigation and GPS receiver at the carrier phase level. The fully coupled filter will reduce the bandwidth of the GPS tracking loop, thereby reducing the chance of jamming signals entering the GPS receiver.