Adaptive Antennas for GPS

Summary

The utilization of spread spectrum techniques by GPS enables the processing of satellite signals with a power level far below that of the normal background noise. The signal to noise gain provided by GPS correlation receivers is sufficient for operation in a benign electromagnetic environment. Thus, civil users generally need not be overly concerned about the noise performance of their receivers. A quite different situation applies for the military utilization of GPS. Here, users are expected to operate in the vicinity of hostile high-power “Jammers”. The signals radiated by these noise sources may easily drive the SNR at the receiver front-end below an acceptable minimum, in many cases they will even force the receiver into saturation. This will result in a complete loss of the position-fix capability or at least lead to a considerably impaired performance.

In order to mitigate the jamming threat, military high performance GPS receivers will be equipped in the future with adaptive ‘Null Steering’ antennas. These antennas automatically adapt their pattern according to the actual jamming scenario. They generate deep nulls in the direction of all active jammers and thus selectively reduce the noise power passed to the receiver.

Although designed for military use, adaptive Null-Steering antennas might in special cases also be useful for civil applications. They will, for example, allow high precision geodetic GPS measurements in the proximity of strong interferers (e.g. L-Band radars, TACAN/DME transmitters, earth stations for satellite communication as well as harmonics of UHF transmitters etc.).

The paper gives an overview on various null-steering methods available for the GPS application.