Integration of GNSS Bistatic Radar Ranging into an Aircraft Terrain Awareness and Warning System

Dallas Masters, Dennis Akos, Stephan Esterhuizen and Eric Vinande

Abstract:	GNSS signals reflecting from the land surface provide a source of ranging for aircraft terrain awareness and warning systems (TAWS). Past experiments have shown reasonable levels of ranging accuracy, but these previous measurements were limited to single satellite collections due to hardware limitations. The present work used the NordNav GPS sampling architecture and a software receiver to capture direct and surface-reflected signals from multiple satellites during aircraft flights over rugged terrain. The reflected signals represented a possible worst-case for GPS bistatic ranging over terrain due to weak reflections and variable terrain slopes. The reflected signal waveforms were distorted from typical C/A code correlation waveforms due to simultaneous scattering of signals from varying terrain heights and possible volume scattering from vegetation. Bistatic delay estimates from the measured reflected signal waveforms were made using a simple threshold technique borrowed from satellite radar altimetry tracking of complex ice surfaces. A threshold of 0.2 on the leading edge of the GPS reflected waveforms produced the minimum error between the measured bistatic ranges and those computed as ìtruthî using the aircraft's GPS position and a high resolution digital elevation models (DEM). Bistatic ranges estimated from four satellite reflection signals yielded similar root mean square errors of approximately 30 m. Uncertainty in georeferencing the reflections on the DEM due to multiple possible reflection locations from sloped terrain facets opened questions as to the validity of the true bistatic range computation used in this analysis. Future work will address this issue in more detail using the present data sets and also investigate combinations of multiple satellite bistatic range estimates to help monitor the integrity of the TAWS application in real-time.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	1739 - 1748
Cite this article:	Masters, Dallas, Akos, Dennis, Esterhuizen, Stephan, Vinande, Eric, "Integration of GNSS Bistatic Radar Ranging into an Aircraft Terrain Awareness and Warning System," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 1739-1748.
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