GPS Antenna Design Characteristics for High‐Precision Applications

Antenna characteristics required for geodetic and subcentimeter accuracy GPS (Global Positioning System) applications are discussed, including the types of measurements needed for precise antenna calibration. The antenna phase characteristics over the upper hemispherical coverage region, which are of greatest importance in precise applications, are described in terms of a best‐fit (least mean square sense) sphere with associated residuals that may then be used for very precise correction of direct GPS carrier phase measurements. A criterion of $\pm 5°$ phase ripple (maximum) and overall 5° rms deviation from the best‐fit sphere is presented for acceptable high‐accuracy performance. Measured phase responses are presented for two representative commercial GPS antennas; a quadrifilar helix (volute) at L1 frequency and a microstrip patch at L1 and L2 frequencies. Theoretical results obtained from analytic models are presented to identify the major factors that affect precise GPS antenna performance, including the electrical feed network balance, the use of ground planes, and signal multipath.