Mitigation of Static Carrier Phase Multipath Effects Using Multiple Closely-Spaced Antennas

The effect of carrier phase multipath in static mode is investigated and a system is developed to reduce the effect using multiple closely-spaced antennas. The correlated nature of multipath, along with the known geometry among the antennas, are used with the measured relative carrier phase differences to aid in the extraction of the direct carrier phase from the multipath-corrupted carrier phase measurement. The mathematical model of the multipath effects on carrier phase measurements and the Kalman filter implemented to estimate these errors are described. The model is first tested on simulated data in which the antenna geometry and reflector parameters were varied. It was then tested on field data collected on a roof whereby the model adaptively estimates the parameters of the composite multipath signal. Initial results demonstrate substantial mitigation of carrier phase multipath signals from multiple sources using this technique under static conditions. The RMS value of the average multipath error on a single differenced carrier phase measurement data is typically reduced by over 70% percent. This technique estimates the parameters of the composite multipath signal and removes the error due to all multipath signals. It is particularly useful for reference stations which transmit carrier phase data for kinematic positioning applications. INTRODUCTION The influence of multipath signals on carrier phase measurements is one of the limitations to achieving high accuracy positions in a wide variety of applications. The problem is especially a concern for GPS reference stations whereby the static environment may induce slowly changing specular effects which do not easily average out. Significant work has been done to characterize the effects of multipath on a Delay Lock Loop (DLL) in a PRN receiver (e.g. Hagerman, 1973; Breeuwer, 1992; Braasch, 1996) and to reduce these effects using various methods which can be broadly classified as • Antenna-based mitigation • Improved receiver technology • Signal and data processing Antenna based mitigation involves improving the antenna gain pattern to counter the multipath. A choke ring with a ground plane has been very effective in this regard. By designing an antenna with a very low gain for left hand circularly polarized (LHCP) signals and using an antenna array to have a sharp cutoff below a certain elevation angle, significant improvements can be achieved (Bartone and van Graas, 1998). Moelker (1997) describes various methods to mitigate code multipath by using a Multiple Signal Classification (MUSIC) technique with multiple antennas and an extended Multipath Estimation Delay Lock Loop (MEDLL).