High Accuracy GPS Performance using a Digital Adaptive Antenna Array

NAVSYS' High-gain Advanced GPS Receiver (HAGR) has been designed to take advantage of digital adaptive antenna array spatial-processing to enhance the accuracy of the GPS observations. The HAGR has the capability to create an optimized digital antenna pattern for up to eight GPS satellites simultaneously. This is used to perform dynamic beam-forming to the GPS satellites, in both static and mobile environments, providing over 10 dB of additional signal gain on each satellite tracked. The additional gain reduces the receiver noise on both the code and carrier tracking loops. This paper includes test results comparing the performance of the HAGR phased array antenna versus two conventional surveying GPS antennas. NAVSYS have also developed an adaptive spatial processing algorithm that minimizes the antenna pattern in the direction of any detected multipath signals. This adaptive spatial processing minimizes the effect of multipath errors on the code and carrier tracking loops. Preliminary test results showing adaptive multipath beamforming/nulling using a 7-element antenna array are also included. INTRODUCTION The precision provided by current generation GPS receivers, and the use of high accuracy processing methods such as differential and kinematic GPS to remove GPS system errors, leaves multipath as the dominant remaining error source affecting GPS performance. Multipath errors are caused by the receiver tracking a composite of the direct GPS signals and reflected GPS signals from nearby objects, such as the ground or nearby buildings, as illustrated in Figure 1. GPS RECEIVER DIRECT SV SIGNALS MULTIPATH SV SIGNALS ANTENNA MULTIPATH SV SIGNALS Figure 1 GPS Multipath Errors Previous techniques for minimizing the multipath effect have used either antenna designs, to cut-off low elevation GPS signals, and signal processing to minimize the multipath tracking loop errors. Antennas that have large ground planes or choke rings provide low elevation signal rejection which will reduce the power of low elevation multipath signals. However, this technique has no effect on errors introduced by high elevation multipath from sources such as buildings. Signal processing techniques include the use of multiple correlators for correlation shape correction or temporal filtering using code/carrier smoothing. These effects can reduce the effect of multipath on the pseudo-range errors but they have no effect on carrier multipath errors. In this paper, test data is presented that compares the performance of a digital adaptive antenna array for high accuracy GPS applications against these previous approaches. DIGITAL BEAMFORMING GPS RECEIVER (HAGR) The test results presented in this paper were collected using NAVSYS’ High-gain Advanced GPS Receiver (HAGR). The reprogrammable digital spatial processing capability inherent in the HAGR allows GPS signals to be combined from as many as 16 antennas and create a multi-beam antenna pattern to apply gain to up to eight GPS satellites simultaneously. The test results presented were collected with the HAGR operating with the C/A code. A Precise Position System (PPS) version of the HAGR (the HAGR-200) is also in development with 12 channel L1 and L2 capability. DFE Module DFE Module