A system determines three-dimensional attitude of a moving platform using signals from two closely spaced Global Positioning System (GPS) antennas. The system includes three rate gyroscopes and three accelerometers rigidly mounted in a fixed relationship to the platform to aid in determining the attitude. The system applies signals from one of the two GPS antennas to sufficient channels of a GPS receiver to support navigation. The system applies signals from a second of the two GPS antennas to the additional receive channels to support interferometry. The system resolves the ambiguity normally associated with the interferometric heading solution by having closely spaced GPS antennas, and uses interferometry to refine a coarse heading estimate from a GPS plus Inertial Measurement Unit (IMU) transfer alignment solution. The system achieves sub-meter spacing of the two GPS antennas by merging many temporal interferometric measurements and the attitude memory provided by the IMU time-history solution.