GPS/INS Operation in Shadowed Environments

Abstract

This thesis presents the analysis techniques developed for monitoring GPS signals in harsh shadowed environments such as heavy foliage. It also details a method selected for improved performance by combining raw GPS information with an Inertial Navigation System (INS). Normal GPS operation in shadowed areas suffer from position jumps of tens to hundreds of meters. The developed analysis reveals that these errors are due to the quickly changing local errors that cause a GPS receiver to report erroneous position spikes. Monitoring variables for the signal strength and change in multipath are employed to keep track of the environmental effects on the GPS measurements. A new visualization technique is also developed to qualitatively monitor the environmental effects.

From the visualization technique, the effects of the shadowing environment are shown to simultaneously affect the signal strength and multipath. It is shown that foliage cover causes these effects to occur spontaneously as a signal travels through and around obstacles. To mitigate these errors, a GPS/INS closely coupled system is implemented which uses inertial sensors to smooth the erroneous GPS jumps. Introduction of alternative sensors allows for integrity monitoring in the form of GPS outlier measurement rejection so that local environmental effects can be detected and removed from the navigation solution. The resulting implementation reveals that it is possible to operate in these GPS harsh environments without suffering from the position jumps of tens or hundreds of meters. This implementation allows for navigation in foliage cover comparable to the under ten meter accuracy of standard GPS in clear environments.