| Multipath is a major source of error in GPS code and carrier phase measurements in the differential mode of operation, which can prevent the achievement of the highest levels of accuracy. This is especially prevalent in a static receiver where multipath introduces slow varying errors in the measurements due to satellite dynamics, and which cannot be averaged out. Multipath is spatially correlated within a small area. This relationship can be exploited to mitigate multipath errors. The research aims at reducing the effect of multipath in code and carrier phase measurements in a stationary receiver using a multiple antenna/receiver approach.; A method is developed, which uses five or more antennas, spaced about 5–10 cm apart. The response of the GPS receiver code and carrier discriminator functions in the presence of a multipath signal is analyzed, and multipath errors within a small area are related to the antenna-satellite and antenna-reflector geometry using multipath and geometric parameters, such as, the reflection coefficient, multipath delay, multipath phase, as well as multipath signal azimuth and elevation. A Kalman filter is developed to use multipath-corrupted measurements from multiple closely-spaced antennas to estimate the multipath and geometric parameters, from which the multipath errors in the code and carrier measurements at each antenna can be computed. Field tests in a moderate multipath environment show a reduction in multipath errors up to 73% (average 22%) in the code and up to 52% (average 15%) in the carrier residuals. Improvements are also observed in the position domain, whereby the differential position accuracy is improved by up to 51% (average 21%) and up to 37% (average 24%) for the non-smoothed code and carrier cases respectively. A desirable characteristic of this technique is that it is more effective in a high multipath environment. This technique has potential to be used in real time in reference stations generating corrections for kinematic applications. Furthermore, this technique can be extended to other direct sequence spread spectrum communication systems employing PRN codes for signal spreading. |
