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According to the National Risk Estimate: Risks to U.S. Critical Infrastructure from Global Positioning System Disruptions
report prepared back in 2011, the widespread and growing use of GPS, coupled with threat actors possessing technologies that can disrupt GPS now and in the future, pose a long term threat that cannot be ignored. Since that report was issued, the threat to GPS civilian and military systems has grown considerably.

“We’re doing things with GPS that were never imagined when it was put in place over 20 years ago,” says Peter Soar, business development manager of Military & Defense at NovAtel. “GPS is an integral part of high speed financial trading services, bank transaction time stamps, transportation system schedules, and of course, nearly every military system.”

The pervasiveness of GPS has led to some unforeseen vulnerabilities. “We have discovered several instances where customers are unaware that they have GPS embedded in the tools they use to perform certain functions on the job. Civil and military organizations, in particular, must account for vulnerabilities due to the pervasiveness of GPS aided
applications” Soar said.

For civil and military organizations in the U.S. and other countries, the questions that must be answered are: Can my GNSS receiver be jammed or spoofed, and is there an affordable solution to prevent such attacks? Until recently, size and cost considerations meant GPS anti-jam systems were only feasible for expensive assets, such as strategic aircraft and large ships. That’s begun to change. As the potential for jamming signals has risen along with the number of at risk systems, anti-jamming technology has also begun to evolve in both affordability and capability.

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.

An Anti-Jam Approach

When evaluating anti-jam systems that can defeat simultaneous jammers, a simple rule of thumb to remember is that a CRPA can null in n-1 directions where n is the number of elements. So a 7-element system can null in 6 directions and so on.

Soar says, “The real proof of anti-jamming effectiveness is in the application. If an anti-jamming system shows promise, test it. Evaluate how it works in critical situations and verify ease of use and accuracy.”

Don't Neglect Training!

Training often gets overlooked when it comes to the use of GPS systems. For instance, users need to learn what jamming can do to their systems and practice the mitigation methods so that their responses to alerts are spontaneous, just like any other drill.

Consider the following point. Intuition would generally suggest that the user elevate the antenna when a communication signal is lost—but with GPS, the opposite is typically the case. Since jammers are usually on the horizon, a more effective response might be to place the GPS receiver on the ground or even in a shallow hole, or hold it so that your body or a vehicle is between you and the jammer.

Some receivers flash the same message—“jamming detected”—when the antenna loses signal, even though the problem might be that the user has moved indoors. Exposure to jamming as well as moving the GPS receiver in complex environments will give users the experience to judge between the scenarios.

Setting up jamming for training used to be complex and time consuming. But now authorized users, with appropriate permissions in place, can take advantage of the NAVWAR Electronic Attack Trainer (NEAT) designed by Defence Research and Development Canada and built by NovAtel. NEAT is a simple handheld unit that emits a variety of low power signals.