An electronic system that comprises airborne and ground-based subsystems generates data for a global real-time interactive map (GRIM) that displays the current position and velocity of the NASA F-18 Systems Research Aircraft (SRA). This system utilizes the Global Positioning System (GPS) to determine the position and velocity of the aircraft; it serves as a prototype for the development of other, similar GPS-based systems that could be used for tracking and guidance of aircraft during flight research. Such GPS-based systems could be used to augment, back up, or substitute for radar-based tracking systems; indeed, as described below, the performance of the prototype GPS-based system has been compared with that of a radar-based tracking system in initial tests.
Located aboard the SRA, the prototype airborne subsystem includes a GPS receiver (see Figure 1). The output data from the GPS receiver are melded into a stream of telemetry data, which are transmitted from the SRA to a ground station on a pulse-code-modulated radio signal. The ground-based subsystem includes the GRIM display equipment, which resides in a control room at Dryden Flight Research Center. Previously, the GRIM was driven by data from a ground-based FPS-16 radar subsystem. The development of the present system included modification of the GRIM to make it possible to track the position and velocity of the SRA simultaneously by use of both radar and GPS information. The GRIM software was modified to display a triangle to represent the position based on radar data and a circle to represent the position based on GPS data (see Figure 2).
At the time of reporting the information for this article, the performances of the GRIM as driven by GPS and radar-based data had been evaluated and compared in more than ten research flights. Post-flight analysis revealed that GPS data differed from radar data by the following:
Time lag.........................................2 seconds
Horizontal position..........................GPS horizontal velocity X time lag
Vertical position (see figure)............±300 ft (±91 m)(see figure)
Velocity..........................................±20 ft/s (±6 m/s)
The system is not limited to a single aircraft. Multiple aircraft can be tracked on the GRIM, using GPS data only, radar data only, or both GPS and radar data. The capability to utilize GPS data is particularly advantageous in situations in which radar resources are insufficient or are unavailable because of competing priorities or schedules.
This work was done by John McGrath; Ed Haering, Jr.; Harry Miller; Jack Trapp; Dave Webber; Glenn Bever; and Joe Collura of Dryden Flight Research Center; Jules Ficke of SPARTA, Inc.; and George Aragon of OAO. DRC-98-23