A performance comparison between a SCAT-1 DGPS system and the Omnistar DGPS system

The Global Hawk UAV (Unmanned Aerial Vehicle) was developed by the Northrop Grumman´s Ryan Aeronautical Center to perform high altitude reconnaissance. The initial design of the Global Hawk navigation system utilized a RTCA DO-217 compliant special category 1 (SCAT-1) differential GPS (DGPS) system to provide sufficient navigation accuracy for taxi, takeoff and landing operations. The SCAT-1 DGPS system consists of a ground station that produces the differential corrections, a data link transmitter to uplink the corrections to the aircraft, a data link receiver in the aircraft and a GPS receiver able to utilize differential GPS corrections. The SCAT-1 DGPS system has been shown, both during flight tests of the navigation system in a company owned King Air, and during actual flights of the UAV to provide more than sufficient accuracy for takeoff and landing operations. At the point in time that the Global Hawk was to fly outside of the Edwards Air Force Base restricted airspace, discussions were held relative to off field landings. The problem raised relative to getting in and out of an emergency landing field was how to provide differential corrections to the aircraft in such a situation. It was undesirable to have to install a SCAT-1 DGPS ground station at each of the anticipated emergency landing fields. It was at this juncture that the use of commercially available world-wide DGPS services was investigated. The system chosen for providing DGPS corrections to the aircraft in the event of an emergency landing is Omnistar. This paper will describe the system implementation of the Omnistar on board Global Hawk, and will provide data comparing the accuracy of this system against the accuracy of the SCAT-1 DGPS system initially designed into the UAV.