GPS C/A Code Multipath Error Estimation for Surveying Applications in Urban Canyon

Global Positioning System (GPS) is satellite based navigation system implemented on the principle of trilateration, provides instantaneous 3D PVT (position, velocity and time) in the common reference system anywhere on or above the earth surface. But the positional accuracy of the GPS receiver is impaired by various errors which may be originating at the satellite, receiver or in the propagation path. These errors have assumed importance due to the high accuracy and precision requirements in number of applications like the static and kinematic surveying, altitude determination, CAT I aircrafts landing and missile guidance. In this paper, the error originating at the receiver due to multiple paths of the satellite transmitted radio frequency (RF) signal is estimated. Multipath phenomenon is prevalent particularly in urban canyons, which is the major error among other GPS error sources originating at the receiver. The algorithm proposed in this paper estimates the error using coarse/acquisition (C/A) code range, carrier phase range and Link1 (L1) and Link2 (L2) carrier frequencies. This algorithm avoids the complexity of the error estimation using conventional methods where sensitive parameters such as the geometry or the reflection coefficient of the nearby reflectors are considered. The error impact analysis presented in this paper will be useful in selecting the site for GPS receiving antenna where the reflection coefficients are hard to measure up to the required accuracy. Analysis of the change in intensity of this error with respect to elevation angle of the satellite will facilitate in selecting pseudoranges with least error. Error estimation and range modeling proposed in this paper will be a valuable aid in precise navigation, surveying and ground based geodetic studies.