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This paper summarizes the development of a Kalman combining algorithm, the derivation of a GPS selection technique and the results of analyzing 12 cases in demonstrating the accuracy of using GPS, totally independent of ground-based equipment, in determining the relative state of two vehicles during rendezvous The results show expected-error values ranging from 1 meter and .001 meter/sec to 24 meters and 3.1 meters/sec. The technique is sensitive to antennae offsets (unknown), GPS selection/availability and gravity gradient modeling. It is less sensitive to filter initial values than expected. Most interestingly, the relative state errors are significantly less than the individual errors in the two spacecraft absolute states and that the technique is virtually immune to refraction and relativistic effects.