Passive Altimeter Study Using GPS Flight Data (Preprint)
AIR FORCE RESEARCH LAB WRIGHT-PATTERSON AFB OH SENSORS DIRECTORATE
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Software GPS receiver results are presented for an over-land, passive altimeter application utilizing GPS signals. The flight data were collected using a DC-3 aircraft operated by Ohio University. The flight took place over southeast Ohio, where the terrain is hilly and forested. The GPS receiver on board the aircraft contains two channels. One channel has a right-hand-circular-polarized antenna facing upward, and the second has a left-hand-circular-polarized antenna facing downward. The upward antenna primarily receives the GPS direct signals, while the downward antenna primarily receives the ground-reflected GPS signals. Software radio GPS algorithms are used to process the data received from the upward channel. The information obtained after processing includes receiver position data, visible satellites, CA code delay, Doppler frequency and navigation data bits as functions of time. This information is used to generate a reference signal to correlate the data received from the downward channel. Since the Doppler shift can be different between the reference and the reflected signals, the length of the coherent integration time is limited. Furthermore, the Doppler shift of the reflected signal can change rapidly as a function of the terrain. Therefore, a combination of coherent and noncoherent integration is used to enhance the signal-to-noise ratio of the reflected signals. The correlation result shows a CA code delay corresponding to the path length difference between the direct and reflected signals. From the known locations of the receiver and the satellites, the height above ground of the receiver can be calculated. The flight data cover heights ranging from 0 to 3000 meters above the terrain.
- Flight Control and Instrumentation
- Navigation and Guidance