Adaptive Array
for Wideband Nulling in
GPS Systems



Gary F. Hatke
MIT Lincoln Laboratory
244 Wood Street, Room S3-413
Lexington, MA 02173-9108
tel: (781) 981-3364

Abstract Military GPS systems must operate in the presence of hostile jamming. The spread spectrum nature of the GPS signal allows some anti-jamming (A/J) capability, but the vast power differences between a nearby jammer and distant low-power satellite signals can quickly overwhelm the receiver. Adaptive array processing has been applied to this problem in an attempt to give an adaptive spatial nulling A/J capability. Arrays of seven elements (a controlled radiation pattern antenna, or CRPA) has been used for spatial nulling along with a sidelobe canceller array processor. Past analysis of this system has ignored local multipath generated by the platform on which the array is mounted (typically an aircraft). This multipath can greatly increase the effective aperture of the array system. For wideband systems like GPS, this large effective aperture can lead to decorrelation of the jamming wavefront at the array, thus limiting the null depths achievable by a standard spatial only adaptive processor. We propose using a space-time adaptive processor to increase the nulling capability of the adaptive array. Special care must be taken when using a space-time processor for the GPS problem, however, since the nulling operation may distort the desired GPS signal and degrade the time delay estimates which are necessary for accurate position estimation. The optimal method for time delay estimation in the presence of wideband interference is derived. In addition, a novel technique of using soft constraints to limit signal distortion while allowing sufficient null depths on the interference is proposed which can dramatically decrease computational cost relative to the optimal processing method. The performance of the two techniques is compared in simulation.



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