Integration of GNSS Bistatic RadarRanging into an Aircraft TerrainAwareness and Warning System

摘要

GNSS signals reflecting from the land surface provide asource of ranging for aircraft terrain awareness andwarning systems (TAWS). Past experiments have shownreasonable levels of ranging accuracy, but these previousmeasurements were limited to single satellite collectionsdue to hardware limitations. The present work used theNordNav GPS sampling architecture and a softwarereceiver to capture direct and surface-reflected signalsfrom multiple satellites during aircraft flights over ruggedterrain. The reflected signals represented a possibleworst-case for GPS bistatic ranging over terrain due toweak reflections and variable terrain slopes. The reflectedsignal waveforms were distorted from typical C/A codecorrelation waveforms due to simultaneous scattering ofsignals from varying terrain heights and possible volumescattering from vegetation. Bistatic delay estimates fromthe measured reflected signal waveforms were made usinga simple threshold technique borrowed from satellite radaraltimetry tracking of complex ice surfaces. A threshold of0.2 on the leading edge of the GPS reflected waveformsproduced the minimum error between the measuredbistatic ranges and those computed as ìtruth? using theaircraft's GPS position and a high resolution digitalelevation models (DEM). Bistatic ranges estimated fromfour satellite reflection signals yielded similar root meansquare errors of approximately 30 m. Uncertainty ingeoreferencing the reflections on the DEM due to multiplepossible reflection locations from sloped terrain facetsopened questions as to the validity of the true bistaticrange computation used in this analysis. Future work willaddress this issue in more detail using the present data setsand also investigate combinations of multiple satellitebistatic range estimates to help monitor the integrity of theTAWS application in real-time.