Precise phase calibration of a controlled reception pattern GPS antenna for JPALS

The Joint Precision Approach and Landing System (JPALS) is being developed to provide navigation to support aircraft landings for the U.S. military. One variant of JPALS is the Shipboard Relative GPS (SRGPS), which will be implemented on an aircraft carrier. In order to meet strict accuracy, integrity, continuity, and availability goals in the presence of hostile jamming and in a harsh multipath environment, advanced technologies are required. One of those being studied is a controlled reception pattern antenna (CRPA) array with beam steering/adaptive null forming capabilities. The Stanford University GPS Laboratory has developed a software tool to study CRPA algorithms and their effects on GPS signal and tracking characteristics. A testbed has been constructed to investigate hardware issues including the phase center offset of the antenna elements and mutual coupling effects. This testbed consists of a 3 element antenna array with a baseline of 1m, using high-quality survey-grade or lower-quality patch antennas. Data has been taken using this array in conjunction with sufficient satellite constellation and antenna array motion to ensure complete azimuth and elevation signal coverage. A carrier phase-based attitude determination algorithm was used to generate inter-antenna bias residuals, allowing characterization of the virtual phase center of the array. Repeating the testing procedure both with survey-grade antennas, for which the phase center characteristics are well known, and with a patch antenna possessing unknown phase center behavior, allows characterization of the azimuth- and elevation-dependent properties of the patch antenna phase center. In addition, mutual coupling effects have been investigated by adding inactive patch elements around the active patch antenna. All results are compared to predictions from detailed simulation of the patch antenna used using an EM modeling software package.