High Accuracy GPS and Antijam Protection using a P(Y) Code Digital Beamsteering Receiver

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 multipath signals and GPS jamming and interference. This paper describes the operation of the HAGR digital beam steering array and includes test data demonstrating the high accuracy observation that it produces, Test results showing the benefits of the digital spatial processing in reducing multipath errors and GPS jamming are also included. INTRODUCTION A key requirement for aircraft precision approach and landing systems is to provide high quality GPS pseudorange and carrier phase observations in both the ground reference station and the aircraft making the approach. For military applications, such as the Joint Precision Approach and Landing System (JPALS) and the Navy’s Shipboard Relative GPS (SRGPS) carrier landing system, the measurement precision must be maintained in a hostile environment, where GPS jamming may occur, and also using GPS reference stations installed in less than ideal locations, for example on the mast of a ship where significant signal multipath can corrupt the measurement performance. NAVSYS has developed a digital beam-steering GPS receiver which processes the GPS data from a multielement phased array antenna. This has significant performance advantages over previous GPS reference station architectures which used a single reference antenna. In particular, the digital beam-steering approach has the following benefits in meeting the military JPALS or SRGPS key requirements. 1. Must provide high accuracy pseudo-range and carrier –phase observations The beam-steering provides gain in the direction of the GPS satellites increasing their effective C/N0Gain from beamforming The increase in C/N0 on the GPS satellites reduces the pseudo-range and carrier-phase measurement noise improving the navigation solution accuracy. 2. Must be able to maintain precision in the presence of close-in multipath The digital beam-steering optimizes the adaptive antenna pattern for each satellite tracked. This provides gain in the direction of the desired satellite signal and will attenuate signals arriving from other directions, such as closein multipath. This allows the GPS signal integrity to be maintained even under non-ideal antenna installation scenarios. 3. Must be able to maintain performance in a jamming environment. With conventional analog null-steering electronics , significant segments of the sky are “blanked” out when a jammer (or jammers) are detected and nulled. This will cause the GPS UE