The baseline of a GPS attitude determination system is extended beyond the confines of the platform to improve attitude determination. A structurally detached GPS antenna is located away from the platform defining an extended baseline to a body-fixed GPS antenna attached to the platform with an unknown orientation with respect to the platform. An electro-optical (EO) sensor on the platform acquires an EO observable reference feature on the remote GPS antenna and measures the extended baseline orientation with respect to the orientation of the platform. A processor processes GPS data, positional or interferometric, from both GPS antennas to determine the extended baseline orientation in a reference coordinate system. The first and second baseline orientations determine platform attitude.