Abstract : The purpose of this research is to compare the effects of two filtering routines which may be used to integrate Inertial Navigation System (INS) and Global Positioning System (GPS) data to determine certain state vector elements. The two filtering routines are: 1) the ordinary Kalman Filter, and 2) a Two-Stage Least- Squares Procedure, which will be referred to as the 2-Stage Filter. Using the Kalman Filter to determine state vector elements, the vector quantities can be affected when system error is introduced into the model. Theoretically, the 2-Stage Filter is more robust, that is, it should be able to determine accurately the state vector elements despite the presence of errors (Schaffrin 1991) This research will attempt to verify the 2-Stage Filter is, in fact, a more robust filter than the Kalman Filter.