GPS Anti-jamming Technology

As the use of precision guided munitions grows, so to do the attempts to jam the systems that guide them. Military Aerospace Technology looks at what’s being done to protect those navigation systems.

By Anthony Abbott

Although the GPS spread-spectrum signal offers some inherent anti-jam protection, an adversary who is determined to negate a GPS system need only generate a jamming signal with enough power and suitable temporal/spectral signature to deny the use of GPS throughout a given threat area. The reason for this problem is clear: GPS satellites produce low-power signals that must travel great distances to reach the receiver. A jammer, on the other hand, can produce a stronger signal much closer to the receiver, and since signal power diminishes as the square of the distance traveled, the jammer has a distinct advantage.

This vulnerability has been identified as a high priority within the DoD, and numerous programs have been established to develop near-term solutions for today's potential threats and more extensive long-term solutions for projected future threats. The Aerospace Corporation has been spearheading many of these development efforts.

Traditional Approaches

The first system developed to increase GPS anti-jam capability for users on the ground or in the air was the controlled reception pattern antenna. This device consists of an array of six antenna elements arranged in a hexagon around a central reference element.

The underlying principle is fairly straightforward: Received GPS signals are rather weak and cannot be detected or measured without a signal-correlation process; therefore, the processing algorithm assumes that any measurable energy above the ambient noise must be a jamming signal, and so it computes the necessary weights to null the source.

Certain factors limit the usefulness of these antennas for some vehicles. Controlled reception pattern antenna arrays are physically quite large (on the order of 35 centimeters in diameter) and generally cannot be used, for example, on small missiles that lack the necessary mounting space. In addition, a controlled reception pattern antenna can only counter a limited number of jammers, as it eventually runs out of "degrees of freedom" or anti-jamming options when the number of spatially distributed jammers grows too great. This is because the array must use at least two elements to null one jammer. Moreover, the antenna must devote a degree of freedom to a jammer regardless of the jammer type (broadband or narrowband). This approach is less effective than other, more advanced processing techniques that can attack a broadband jammer with spatial resources and a narrowband jammer with time/frequency resources.

Various alternatives are being researched as part of the GPS Modernization and Navwar programs. No one method is right for all circumstances because each application presents its own unique requirements and constraints. Moreover, a given technique may be effective against a particular class of threats, but may not necessarily address all threats. For example, an adaptive narrowband filter is effective against a jammer that has some repetitive or predictable signal structure, but is ineffective against a broadband noise jammer, whose signal cannot be predicted from previous samples. Likewise, spatial adaptive antenna arrays are effective against a limited number of broadband noise and structured signal jammers, but eventually run out of degrees of freedom as the number of jammers increases.

Jammer Signal Power Reduction

Among the advanced techniques for reducing jammer power, the most promising employs a technology that was originally developed for radar, called space-time adaptive processing. With this technique, the output of each antenna array element is delayed using a series of tapped delay lines, each stage of which outputs a version of the input signal slightly later than the previous stage. The output of each tap is available as a separate signal, and each can be processed with a unique complex weight and combined into a composite signal. A close variant of this technique, called space-frequency adaptive processing, performs equivalent processing in the frequency domain.

A very similar anti-jamming technique-actually a subset of space-time and space-frequency adaptive processing-is known as adaptive narrowband filtering. Adaptive narrowband filters work with a single antenna element, so they are typically used in applications that lack sufficient space for a spatial antenna array. They are effective against structured interference signals, such as continuous (e.g., sine) waves or pulsed signals, but they are ineffective against broadband interference, which does not have an identifying signature that can be tracked and eliminated.

Beam steering uses the direction to the desired satellite as an additional constraint on the complex weight applied to each tap output. To perform these calculations, the processor needs to know the direction to the desired GPS satellite and the position and attitude of the host vehicle.

Beam steering is a "precorrelation" technique, meaning it does not require GPS signal detection to compute the phase and gain for each tap on each array element. Beamforming, on the other hand, is a "postcorrelation" technique, meaning it attempts to maximize the signal-to-noise ratio after signal capture. Both techniques maximize the GPS signal while simultaneously minimizing the jammer power for multiple jammers of various types.

Processing Gain

The second major anti-jamming strategy involves processing gain improvement. The GPS spread-spectrum signal derives some inherent jam protection from the "despreading" process, which converts it from a 20-megahertz bandwidth to a narrower bandwidth. Signal power grows stronger as bandwidth is reduced, so for maximum anti-jam performance, the narrowest possible bandwidth should be used in the despreading process.

In general, greater anti-jam performance can be achieved by narrowing the bandwidth of the code and carrier tracking loops. Unfortunately, narrow tracking-loop bandwidths imply sluggish response time, and if a vehicle is undergoing high acceleration, the narrow-bandwidth tracking loop cannot keep pace. If the tracking-loop bandwidth were widened, it would be more responsive to high acceleration, but it would not filter the noise as effectively.

New Approaches

To meet the future challenge of GPS applications that must operate in projected jamming environments, the GPS Joint Program Office is pursuing several promising technologies and a future GPS set architecture that will yield further improvements in anti-jam performance. Aerospace is actively involved in defining advanced architectures and technologies that will economically provide better antijam performance. Two approaches in particular are generating considerable interest in the field.

Microelectromechanics

With the recent advances in microelectromechanical systems, new architecture concepts that were unimaginable five years ago have now come within reach. One such technology, the microelectromechanical interial measurement unit (IMU-a set of gyros and accelerometers that feed the inertial navigation system in an aircraft or missile), will have a significant impact on the future design of user navigation sets.

The best way to reduce the bandwidth of the tracking loops (and thus improve anti-jam performance) is to keep the GPS antenna and the IMU together, thereby forcing the lever arm (a motion compensation vector between the inertial sensors and the GPS antenna) to zero. This placement eliminates the need for the lever-arm correction and its associated errors. Of course, when IMUs were first invented, they were very large, and although they've become smaller over the years, they remain large enough to require special attention concerning their placement in a host vehicle or missile. The ability to place an IMU in the same box with the GPS receiver was viewed as a significant step forward. But until recently, no one considered the possibility of embedding the IMU in the antenna itself.

The cost, size and performance of microelectromechanical IMUs are improving to the point where they'll soon be good enough to embed in a GPS antenna. This new architecture overcomes many of the factors that prevented the narrowing of tracking-loop bandwidths in older systems.

Ultratight GPS/Inertial Coupling

The other technology that has recently emerged to address the need for anti-jam performance. This new technique, called ultratight GPS/inertial coupling, is a different method to jointly process GPS and IMU data. Several organizations throughout the United States have been performing research in this area, either through independent research and development funds or DoD research contracts. Although each approach is unique in its implementation, they all share certain common traits. For example, they all eliminate the code and carrier tracking operations, which are susceptible to jamming even when aided. All use estimated navigation parameters to generate the local replica signal needed to track the satellite signal. All directly use the correlator outputs (i.e., comparisons of the local and satellite signals) to compute the range and range-rate errors for the navigation processing algorithm.

What of the Future

Future GPS systems-particularly for weapon delivery-will benefit from the optimal integration of GPS receivers with inertial measurement units and the use of adaptive processing algorithms and antennas that reject unwanted signal interference while maximizing the power of the desired satellite signal. The combination of all these technologies and the associated system architecture will be the blueprint for DoD GPS sets for the next several decades.

This article is based on two articles by Anthony Abbott that appeared in the Summer 2002 issue of Crosslink, a quarterly publication of The Aerospace Corporation. Anthony Abbott is the principal engineer in the GPS III and Military Applications Directorate, supporting the GPS Joint Program Office in advanced anti-jam technology and user equipment architecture with The Aerospace Corporation, is an independent, nonprofit, federally funded research and development center (FFRDC) sponsored by the Department of Defense.