GPS has become a vital component for U.S. combat forces, and several nations have developed and sell GPS jammers. The U.S. Air Force has equipped its GPS weapons with electronic and software features that help overcome this jamming. But DARPA (Defense Advanced Research Projects Agency) is developing an even more powerful anti-jamming system that also solves a more common problem; weak GPS signals. When a GPS guided weapon goes after a target in a canyon, the GPS signals are often so weak that the guidance system must revert to the backup, and several times less accurate, inertial guidance system.
RSN (The Robust Surface Navigation) system uses math, statistics and modified antennae to take signals (GPS and non-GPS) and create accurate estimates to make up for the lost GPS data. If this substitute is judged, by the software, as more accurate than the backup system, it uses the RSN estimate, to maximize accuracy. Other signals can come from nearby aircraft, or navigation beacons set up on the ground. The RSN system would enable the U.S. to jam GPS signals in a combat zone where the enemy was using GPS guided weapons, but does not have RSN.