There are some exceptional anti-jamming systems in the market today that are used in military vehicles, ships and aircraft to prevent signal loss in critical systems where the consequences of inaccurate data could cost lives. In years past, the cost of these systems was prohibitive for all but the most extreme conditions such as on military fighter jets.
However, the military, for instance, can’t afford those systems on the average armored vehicle, small ship or UAV. Yet, jamming is still significant to troops if the GNSS signal is disrupted or spoofed. Hence, the development of requirements for anti-jamming solutions that are still effective but at a lower cost compared to premium systems used thus far by the military.
Most anti-jam antennas are “nulling” systems that work with multiple antenna arrays called controlled reception pattern antennas (CRPA). If the system detects interference from a jammer, the antenna pattern can be shaped to null the signal in
that particular direction, which preserves the antenna’s view of the GPS satellites.
But, how do you compare one system to another? Soar says, “It’s very difficult to measure anti-jam technology effectiveness using one scale because every system and scenario is different.”
Companies often quote a jamming-to-signal ratio (J/S) figure in decibels. At first glance, it seems that the higher the number of decibels, the better. However, the effect of different parts of a position, navigation and timing system (e.g., GNSS, inertial aiding, CRPA, etc.) are additive, so to determine the overall ratio, one needs knowledge of the performance of each part. Plus accurately measuring J/S in a repeatable way is highly scenario-dependent.
Some manufacturers of anti-jam systems quote J/S for combined systems, which include the anti-jam antenna performance plus the performance of the receiver, oscillator, any external aiding, as well as the inherent J/S performance of the spread spectrum GNSS signals themselves. The end user therefore does not know how much of the additional J/S
performance is attributed to the anti-jam antenna alone.
“Where possible, try to decouple the CRPA performance from the rest of the position, navigation and timing chain so that the anti-jamming device can be judged on its individual merits,” explains Soar.