Impact of Near-Field Effects on the GNSS Position Solution

Abstract

near-field effect with respect to the coordinate domain. With the aid of Hannover´s Automated Absolute Antenna Field Calibration Technique developed by the Institut fur Erdmessung (Universitat Hannover) and Geo++® we demonstrate that mechanical structures mounted underneath the antennas (e.g. tribrachs, tripod heads, etc.) can cause significant changes in the phase center offset and variations (PCV) described by the common spherical harmonic model. For the GPS carrier signals L1 and L2 these changes (DPCV) are in the order of several millimeters. Long-term static GPS simulations on a local and global scale prove that during the adjustment process the often quoted averaging effect due to extended observation periods does not actually apply to this DPCV signal. It is instead a bias that falsifies the coordinate estimation. Analysis based on the ionosphericfree linear combination L0 and the estimation of tropospheric zenith delay parameters indicate systematic height errors with a magnitude of 1-2 cm for mid-latitude locations. Independent comparisons between the height difference provided by GPS and the height difference measured with an optical precise leveling instrument clearly verify the impact of the near-field effect. Since GPS satellite geometry degrades with increasing station latitude, the bias can reach up to 4-5 cm in polar regions. Thus, we draw the conclusion that for the purpose of precise height determination an antenna calibration accounting for the near-field is imperative. This is true even if multipath-resistant geodetic antennas are used that are provided with extended ground planes and additional choke-ring-elements.