A developmental electronic system that includes Global Positioning System (GPS) receivers is designed to track the relative position and velocity to two airplanes in real time. In the original application for which the system is being developed, one F-18 airplane (denoted the "chase vehicle") is to be used to take schlieren photographs of shock waves generated by the another F-18 airplane (denoted the "target vehicle"). When fully developed, the system would afford the precise position and time measurements needed to accomplish the schlieren shock-wave photography.
The system includes two GPS receivers (of types called "AV-12" and "Z-12") connected to the same antenna in the chase vehicle, and a single Z-12 GPS receiver in the target vehicle. Special-purpose software in the AV-12 receiver in the chase vehicle determines the absolute position of the chase vehicle by use of data received via the L1 [coarse-acquisition (C/A) GPS Code]. Before the relative position and velocity can be determined, the AV-12 receiver in the chase vehicle must receive GPS data from the Z-12 GPS receiver in the target vehicle. These data are transmitted to the chase vehicle as part of a message in a proprietary format. The relative-position and relative-velocity data can be used three ways: (1) viewed in real time on a display on the AV-12 receiver, (2) passed on to other computers, and (3) stored for later evaluation.
Previous tests of the system were performed on the ground. Flight tests now under way involve the following operations:
- Simultaneously with the operation of the GPS receivers in the chase vehicle, the Z-12 receiver in the target vehicle logs its own position data. These data are downloaded to a laptop computer after a flight test for post-processing correction.
- The output data from the AV-12 receiver in the chase vehicle are saved in the aircraft data recording system. The data from the Z-12 receiver in the chase vehicle are taken to indicate the position of the chase vehicle and are logged internally. A laptop computer is used to download the data from this Z-12 receiver after a flight test, also for post-processing correction.
Preliminary results of the flight tests include observations of unexpected dropouts in the relative-position calculations. This observation indicates a need for tests to verify the quality of the communication lines between the transmitting and receiving antennas and the radio modems.
When dropouts do not occur, the accuracy achieved in flight tests is characterized by lateral position errors within ±3 m and vertical position errors within±4 m with 2-m bias. This level of accuracy is more than adequate for schlieren photography between aircraft using the Sun as the light source. In planned flight tests, the data generated by the developmental system will be used, during subsonic flight of the chase vehicle and supersonic flight of the target vehicle, to momentarily position the chase vehicle to eclipse the Sun as viewed from the target vehicle. The data will also be used to trigger the photograph at the precise time of the artificial eclipse thus produced. The goal is to make detailed photographs of shock waves about the target vehicle.
This work was done by Edward A. Haering, Jr., and Glenn Bever of Dryden Flight Research Center and Joe Collura of TYBRIN Corp. DRC-98-80