GPS/INS integration with fault detection and exclusion in shadowed environments

This paper presents a method for GPS/INS operation in shadowed environments such as urban canyons and rural foliage cover. Shadowing causes a combination of multipath and signal attenuation which results in increased uncertainty in the GPS observables and sometimes complete loss of satellite tracking. Environment layout and the line-of-sight vector to the affected satellite determines the degree of shadowing in the range domain. Details are provided for the failure modes and effects in such environments. These results are used in the analysis of a fault detection and exclusion (FDE) algorithm to provide integrity to the GPS observables. Also, a closely coupled GPS/INS integration is described in which inertial measurements are combined with available GPS ranges even when less than four satellites are in view; a common difficulty in shadowed environments. By using the raw GPS observables in the integration, GPS information is still available for aiding even during severe loss of satellite visibility. The closely coupled integration method is chosen since raw range information is used in both the navigation solution and the FDE algorithm. Using the closely coupled system with an FDE algorithm provides persistent position estimation. Solutions are available even when a stand-alone GPS solution could not be provided; a necessity in position critical applications. It is shown that the effects of shadowed environments can be detected and mitigated and still navigated through the use of inertial coupling with FDE.