Navigation Accuracy and Interference Rejection for an Adaptive GPS Antenna Array

In the Joint Precision Approach and Landing System (JPALS) both the differential GPS reference station and the airborne user will employ Controlled Reception Pattern Array (CRPA) antennas. In a high precision system such as JPALS, CRPA antennas may suffer phase center biases that introduce significant integrity risk. These phase center biases result from both hardware design and from algorithm selection (beamsteering and/or nullforming). This study shows that there is a clear tradeoff between radio frequency interference (RFI) rejection and the introduction of biases in the pseudorange and carrier-phase navigation outputs from a space-time adaptive processor (STAP) GPS receiver. Deterministic corrections based either on single-element or array calibration (and implemented as a line-of-sight-based lookup table) will reduce pseudorange and carrier-phase biases in the tracking output. For the STAP algorithms and patch-element-based antenna array considered here, the carrier-phase bias residuals are on the order of 0-10° and the pseudorange bias residuals are in the 10’s of cm. While the carrier-phase residuals are likely tolerable for high-integrity carrier-phase-differential integer resolution, the code-phase residuals are troubling and will need further work in regards either to algorithm development, to antenna design improvements, or to both.