Test Results of a Digital Beamforming GPS Receiver in a Jamming Environment

NAVSYS High Gain Advanced GPS Receiver (HAGR) uses a digital beam-steering antenna array to provide additional gain in the direction of the GPS satellite signals. This increases the received signal/noise ratio on the satellites tracked and also improves the accuracy of the pseudo-range and carrier-phase observations. The directivity of the digital beams created from the antenna array also reduces the effect of jamming and interference. This paper describes the operation of the HAGR digital beam steering array in a jamming environment and includes test data collected from the HAGR at the Electronic Proving Grounds, Fort Huachuca, that demonstrate the performance of a digital, beam-steering receiver against interference sources. INTRODUCTION The susceptibility of the GPS signals to interference is of concern to the GPS user community. Because of the low received power of the GPS signals, outages can easily occur due to unintentional interference, and even low power jammers can deny GPS operation over significant areas of operation. Current generation GPS military User Equipment (UE) uses nulling electronics to increase the Jammer/Signal (J/S) margin under which they can operate. These analog electronics use a Controlled Reception Pattern Antenna (CRPA) to create an adaptive pattern which provides nulls in the direction of a detected GPS jammer. With a 7-element antenna array, such as the GAS-1 antenna in use by the DoD, nulls can be placed on up to 6 different jammers. In this paper, an alternative GPS jammer protection solution is described where digital beam-steering is used to apply gain in the direction of the GPS satellites. This improves the J/S margin for each of the GPS satellites tracked by applying optimized antenna weights on a channel-by-channel basis. Digital beam-steering has the following advantages for GPS anti-jam applications. 1. Increases GPS satellite signal power. The beamsteering provides gain in the direction of the GPS satellites increasing their effective C/N0. 2. Improved GPS measurement accuracy. The increase in C/N0 on the GPS satellites reduces the pseudo-range and carrier-phase measurement noise improving the navigation solution accuracy. 3. Improves satellite coverage factor. With nullsteering electronics, significant segments of the sky are “blanked” out when a jammer (or jammers) are detected. This will cause the GPS UE to lose lock on multiple satellites whenever jammers are detected, reducing the satellite coverage factor. With the beam-steering approach, the antenna pattern is optimized to increase the satellite gain. This improves the amount of time that four or more