The US Department of Defense constellation of Global Positioning System (GPS) Satellites has become a global utility currently used to provide position, velocity and time information to a wide range of commercial and military applications. The GPS system’s signal is extremely weak and is vulnerable to both intentional and unintentional interference. As dependence on GPS increases, the need to ensure the availability of GPS also grows and therefore, GPS jamming is becoming a major concern for many armies. The US military maintains exclusive access to the accurate “P-code” pseudo random code, which has ten times the frequency of the civilian Coarse/Acquisition (C/A) code and therefore is potentially more accurate and much more difficult to jam. An encrypted military “Y-code” is also available to receivers equipped with suitable encryption key.
The DOD GPS Joint Program Office established the Navigation Warfare (NAVWAR) program in 1996 to address the electronic warfare threat to the GPS system. The NAVWAR program was tasked with protecting DoD and allied use of GPS during times of conflict, preventing its use by adversaries, and maintaining normal availability to the civil user outside the area of conflict. The US Air Force is planning the deployment of a new, more powerful, x100 jam-resistant GPS Block III satellite constellation, but has been subjected to funding delays and will not be deployed before 2015. Another countermeasure aspect is the deployment of anti-jam equipment with existing and new receiver systems.
Anti-Jam GPS address both narrow band interference signals which occupy a small portion of the overall GPS frequency spectrum, (such as harmonious frequencies from TV stations, FM radios etc.) and broadband jammers, occupying the entire GPS spectrum. Current anti-jam analog technology use several antennas, and receiving channels on each receiver device, employing nulling techniques, to eliminate the interfering signal. The number of antenna elements usually determines the maximum number of signals which can be eliminated with such systems. New digital anti-jam receivers and such as the DAR system known as G-STAR is currently fielding, offering significant improvement over existing anti-jamming capabilities. Other techniques are approaching the problem by providing stronger GPS signals over the area of interest. Such signals can be transmitted by Pseudolites deployed on UAVs or ground systems and augment the weak satellite signals by providing local reference points.
