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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.
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