An Adaptive Filter Approach for GPS Multipath Error Estimation and Mitigation

The positional accuracy of Global Positioning System (GPS) is affected by several errors such as delay due to atmosphere, satellite-receiver geometry, receiver clock error and multipath error. Along with the atmospheric errors (≈10–40 m), multipath can cause error (≈2–4 m) in the ranging measurements of the GPS receiver which degrades the positional accuracy. Since, the reception of multipath can create a significant distortion to the shape of the correlation function leading to an error in the receiver position estimate, Multipath is undesirable. The multipath disturbance is largely dependent on the receiver environment since satellite signals can arrive at the receiver via multiple paths, due to reflections from nearby objects such as trees, buildings, vehicles, etc. Although the multipath effect can be reduced by choosing sites without multipath reflectors or by using choke-ring antennas to mitigate the reflected signal, but it is difficult to eliminate all multipath effects from GPS observations. By using data processing schemes such as different adaptive filters, the effect of multipath error can be minimized to centimeter level. Estimation of the effect of multipath interference at the receiver antenna is the objective of this work for which both code range and carrier phase measurements are considered. Along with multipath error, ionospheric delay is also estimated. By using a dual frequency GPS receiver code ranges and carrier phases are extracted and corresponding differences called Code Carrier Difference (CCD) is performed, which results in cancellation of all effects except multipath and measurement noise. Least Mean Square (LMS), Normalized Least Mean Square (NLMS) and Recursive Least Squares (RLS) adaptive filters are considered to mitigate the multipath error. In order to carry out this work, dual frequency data of an IGS station (NGRI, Hyderabad, Lat:/Long:78°33′4″E/17°24′39″N) is collected from the website of Scripps Orbit and Permanent Array Centre (SOPAC) for the entire day of 11th September 2014.