A relative technique for characterization of PCV error of large aperture antennas using GPS data

The Federal Aviation Administration's (FAA) local area augmentation system (LAAS) is a code-based differential global positioning system (DGPS) to be used for guidance of aircraft during the approach and landing phase of flight. Code-based multipath error was a limiting factor in the LAAS meeting Category III accuracy requirements for this phase of flight. Consequently, a large aperture antenna was proposed for use at each ground-based DGPS reference site to reduce the impact of code-based multipath error to less than 0.25 m. The large aperture antenna accomplishes this task at the expense of providing limited coverage in the vertical plane (0-35 degrees). An ancillary high-zenith antenna is then necessary to track satellites from 35-90 degrees in elevation. Phase center variation (PCV) has been observed to be a significant source of GPS error when translating the accumulated carrier phase (ACP) and pseudorange (PR) data between the two aforementioned antennas to a common phase center. This paper presents a relatively simple method for reducing the error caused by the PCV of a large aperture antenna. This is done by comparing the GPS ACP data from a large aperture vertically polarized dipole array antenna with undesirably large PCV (more than 20 cm), with similar data from a right-hand circularly polarized high-zenith antenna with small PCV (relative to physical center of array). A triple-differencing technique of the ACP data between the two antennas across successive epochs is used to characterize the PCV of the large aperture antenna as a function of elevation angle. The PCV correction factor is applied to the ACP data from the large aperture antenna array. The ACP data from the two antennas are then combined in software to appear as one antenna with minimal offset from PCV. A field test that employed this technique is described and test results are provided. This technique confers an advantage over other methods for PCV determination, which require precision mounting and/or robotic motion capabilities, or the use of an antenna range.