Use of Multiple Antennas to Mitigate Carrier Phase Multipath in Reference Stations

GPS carrier phase multipath is a major source of error in high precision static and kinematic differential positioning. It is especially prevalent in static applications whereby the multipath phase change is due only to satellite dynamics and can thus cause slowly varying bias errors. Though some of the currently available correlatorbased techniques reduce high frequency multipath caused by distant objects, lower frequency multipath due to nearby objects still poses a problem. In this paper, an algorithm to estimate carrier phase multipath in a static environment using measurements from multiple closelyspaced antennas is developed and demonstrated. It is shown to significantly remove carrier phase multipath, especially the longer period effects. In the proposed technique, the correlated nature of the multipath error across the antenna array is exploited along with the known geometry among the antennas. A Kalman filter was developed to estimate various parameters of the reflected signals using single difference (between antennas) carrier phase measurements available from the antenna array. The total multipath effect for every measurement at each antenna is then estimated and removed from the raw data. Carrier phase residual analysis performed before and after removing the multipath shows that up to a 60% improvement can be realized. A baseline test of the antenna assembly with respect to a second receiver shows that up to a 70% ION GPS '99, 14-17 September 1999, Nashville, TN 269 improvement gain can be achieved in terms of position accuracy which demonstrates that the system is well suited for a reference station application.