GPS Signal Analysis using LHCP / RHCP Antenna and Software GPS Receiver

Abstract

In spite of, continuing improvements in GPS receivers and antenna technology, multipath signal has remained a major source of error in GPS positioning. In order to minimize the error due to multipath, we need to understand the multipath behavior and corresponding signal characteristics. Multipath is the signal that is coming to the receiver antenna not directly from the satellite but reflected by the objects. The reflected GPS signal takes longer time to reach the antenna than the direct line-of-sight signal. Since, GPS computes the pseudorange based on the signal travel time from the satellite to the receiver, multipath adds significant range error in position computation. It degrades the accuracy of both code and carrier-phase based measurements. The amplitude and phase of a GPS signal changes when it is reflected depending upon the reflecting material property and incidence angle. GPS signal is right hand circular polarization and the polarization may change from right hand to left when the signal is reflected. Theoretically, the polarization changes for every reflection from right to left or vice versa if the incidence angle is greater than the Brewster’s angle. The observation of GPS signal using right hand and left hand circular polarization antennas will help us better understand the characteristics of multipath of GPS signal in various environments and conditions. In this paper, we present the analysis of GPS signal acquired by both RHCP and LHCP antenna using NovaTel OEM-3, Ublox, Garmin-V and Software-based GPS receivers. However, we will mainly focus the discussions on OEM-3 and software-based receivers. The results of this analysis will help us to determine whether observation of GPS signal by RHCP and LHCP can lead to GPS multipath mitigation in future or the results may form a base for possibility of polarization diversity scheme for multipath mitigation or minimization in GPS signal.