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Jam-Proof Signals To Guide Navigation

November 2001
By Robert K. Ackerman
E-mail About the Author

Both inadvertent and deliberate interference threaten steering by satellite.

New algorithms and signal processing technologies may reduce the vulnerability of global positioning system devices to electronic countermeasures. Antijam systems already are in production and are being incorporated into the latest U.S. Defense Department weaponry. Future iterations may bring signal assurance to even the smallest handheld consumer devices.

The growing reliance on the global positioning system (GPS) for a wide range of applications increases the need for interference-resistant signals. Military operations face threats both from adversaries intent on jamming the signals and from other forms of interference that can corrupt or drown out the satellite signal. Nonmilitary and commercial uses of GPS face a similar interference challenge, especially in urban areas, which are characteristically active wireless communications environments.

On Earth, the GPS satellite signal is received about 30 decibels below the background noise level. This translates to a signal strength of about 1,000 times weaker than that of thermal noise normally inherent in electronic equipment. Conventional GPS digital signal processing allows receivers to pluck these signals out of the background noise. When interference raises the level of background noise, however, a receiver may be unable to track the GPS signal.

This problem is especially acute with low-end receivers using omnidirectional antennas. These antennas lack the ability to provide directional discrimination away from sources of interference and toward satellites. Interference sources can be narrowband—affecting only a small part of the overall GPS frequency spectrum—or they can be broadband, affecting the full GPS spectrum. The civilian GPS arena is particularly susceptible to narrowband interference, as it occupies only 2 megahertz of spectrum. Military GPS is spread across 20 megahertz.

Direct countermeasures against GPS signals can be highly effective. A 1-kilowatt jammer can block a military GPS receiver from as far away as 80 kilometers (50 miles). A Russian company recently marketed a 4-watt jammer that can deny a standard GPS signal within up to 200 kilometers (125 miles).

Because GPS signals are beamed earthward from satellites, the key to defeating jamming and inadvertent interference lies with earth-based signal processing. Engineers at Lockheed Martin Systems Integration, Owego, New York, have developed the GPS spatial temporal antijam receiver, known as G-STAR. James Naylor, manager of navigation warfare systems at Lockheed Martin Systems Integration, explains that the technology maximizes the signal level in the direction of the satellites and ignores interference. The company teamed with Rockwell Collins to develop G-STAR based on research that Lockheed Martin performed with General Electric.

A version is being incorporated into the joint air-to-surface standoff missile, or JASSM, which is being built by Lockheed Martin Missiles and Fire Control. An autonomous precision strike weapon designed to attack both fixed and relocatable targets, JASSM must be able to carry out its mission through intense air defense environments. Equipped with an infrared seeker, the 14-foot missile is designed to be launched from B-2, B-52H, B-1B, F-16C/D and F/A-18E/F aircraft. Its objectives could range from nonhardened above-ground targets to hardened shallow underground assets.

A key aspect of G-STAR is its spatial temporal adaptive processing capability. Naylor notes that most analog systems involve spatial processing, which requires an antenna array to ignore interference. Adding the temporal element permits neutralizing many more jammers by removing narrowband interference.

The company examined several different approaches to GPS antijamming, Naylor relates, before settling on G-STAR. The Johns Hopkins University Applied Physics Laboratory also conducted a risk assessment that was factored into Lockheed Martin’s work.

One technique, adaptive transversal filter technology, works well against a large number of narrowband emitters, but it is ineffective against wideband emitters. Adaptive controlled radiation pattern antenna nullers work well against both narrowband and wideband interference, but they are limited by the number of array elements. Interference canceller technology, low-elevation antenna nuller, and signal polarization cancellation antenna technology all work against a large number of jammers on the horizon, but they are effective against only a limited set of jammer scenarios.

Naylor relates that engineers opted to combine the spatial processing of adaptive controlled radiation pattern antenna nullers with aspects of temporal processing found in adaptive transversal filter systems. The company employed General Electric Corporate Research and Development as a subcontractor to evaluate various interference cancellation architectures. Lockheed Martin teamed with Rockwell Collins to develop the G-STAR system. They designed and built a brassboard of the system in 1998.

G-STAR technology employs two approaches: beam steering and nulling. The system consists of a high-dynamic-range radio frequency (RF) front end, a P(Y) code GPS receiver and a selective availability antispoofing module (SAASM). Rockwell Collins provides the RF front end, the GPS back end and the SAASM, while Lockheed Martin provides the antijam adaptive processing and system integration.

The RF front end converts input signals to an intermediate frequency, after which these signals are passed to the system’s beamformer for analog to digital conversion. The beamformer generates multiple independent outputs to the GPS receiver, and it is able to provide a separate steering solution for each output beam. The output beams can be steered to a particular satellite. The beamformer reduces interference before it passes data to the GPS receiver. This digital beamformer is integrated into an application-specific integrated circuit, which provides considerable size and weight reduction.

The GPS back end is completely digital, as the RF section is removed and digital data is provided directly at baseband. This receiver could be located remotely from the RF front end and beamformer, if necessary.

The digital antijam system provides more capability and flexibility than an analog system, Naylor relates. Its digital character permits more antijam processing to be performed in software, and this also permits modifications and upgrades as well as scaling to suit applications. The system can be scaled from two to 12 RF input channels, and it also offers deeper null depth.

Naylor states that G-STAR currently is effective against a wide range of GPS jamming environments. Its software-driven nature also permits easier upgrades as new threats emerge. The version that is equipping JASSM consumes only 52 watts of power. It weighs 25 pounds and measures 10 inches by 15 inches by 2 inches.

Potential G-STAR applications can take several forms. Military or commercial handheld GPS units could be equipped with G-STAR in the form of a hardware appliqué. Depending on application requirements, a scaled-down G-STAR could operate on less than 10 watts of power.

Other applications could include various types of missiles and even munitions. Larger-scale applications might include aircraft and other mobile platforms. The Defense Department’s joint precision approach and landing system, or JPALS, will require pilots to rely heavily on GPS signals. Even unintentional interference could cripple the system, and G-STAR could reduce that likelihood while defending against hostile jamming. Naylor reports that his company is pursuing several new applications for G-STAR. Civilian C/A code GPS systems also can benefit from protection against unintentional—or even intentional—interference. Commercial aircraft already have been affected by unintentional interference from various ground transmitters inadvertently being left on during aircraft takeoffs and landings. Other reports cite commercial radio and television transmitters jamming GPS signals.

As GPS continues to penetrate the commercial marketplace, the need for anti-interference GPS systems will grow, Naylor predicts. Potential commercial applications include automobile navigation, emergency 911 services, automatic vehicle location, general and commercial aviation navigation and landing systems, surveying and precision timing systems.

Additional information on Lockheed Martin Systems Integration’s work on G-STAR is available on the World Wide Web at http://www.lmowego.com.