Draper Laboratory is under contract to the Global Positioning System (GPS) Joint Program Office (JPO) to develop the interface requirements between the GPS X-set being developed by Magnavox Advanced Product Division under subcontract to General Dynamics and the Advanced Inertial Reference System (AIRS). Since the X-set is still under development, a suitable description of it must be created in order to develop this interface. With the exception of the data processor and the X-receiver calibration and automatic-fault indicator operations, this report functionally describes the operations performed by the X-set. The descriptions contained herein are based upon the documentation and specifications currently available and technical discussions with the GPS JPO.