An Investigation into the Feasibility of using a Modern Gravity Gradiometer Instrument for Passive Aircraft Navigation and Terrain Avoidance

Marshall M. Rogers, Richard E. Huffman, Jr., Christopher M. Shearer

Abstract:	Over the past decade, Gravity Gradient Instruments (GGIs) have improved remarkably in accuracy due to the development and refinement of a variety of accelerometer technologies. Some specialized GGIs are currently flown on aircraft for geological purposes in the mining industries and, as such, gravity gradient data is recorded in flight and detailed gradient maps are created after post mission processing. These maps, if stored in a database onboard an aircraft and combined with a GGI, form the basis for a covert navigation system using a process known as the “map matching” method. This system, if it could be successfully implemented, would be completely passive – impervious to conventional jamming methods and relying only on local gravity gradient measurements from an onboard sensor. This paper entails an investigation into the feasibility of using a modern GGI on an airborne platform for covert navigation and terrain avoidance by quantifying navigation performance in different flight scenarios (low, medium, & high altitudes and velocities). Previous studies using gravity gradiometers have been accomplished with promising results (some theoretical gradiometers have been predicted to produce GPS-like navigation accuracy). However, “rotating disk” style gradiometers are the only type currently used in airborne surveys. While major improvements have been made to these instruments, they still produce noise at least an order of magnitude too high for useful aircraft navigation purposes. This research focuses on the implementation of an airborne superconducting GGI, currently in flight test, which has demonstrated approximately 10x better sensitivity than the rotating disk GGI in a laboratory setting. To demonstrate whether or not this technology is currently feasible, a model of the GGI sensor was developed to investigate signal levels at representative flight conditions. Using the sensor model, representative aircraft trajectories were “flown” over modeled gravity gradient maps to determine the utility of flying current GGIs in the roles of terrain avoidance and navigation. The results of the GGI simulations at different altitudes, velocities, gravity gradient map resolutions and gradiometer sensitivities will be presented and discussed. It is shown that the map matching superconducting GGI navigation system has the potential to provide a marked improvement over a non-aided INS in some cases but is limited by the drop in gravity gradient strength at higher altitudes, particularly in areas of smooth terrain, and the availability and accuracy of gradient maps. It was originally hypothesized that the GGI could also be used for terrain avoidance due to the rapid signal change as rising terrain is approached. However, GGI gradient production rate and bandwidth limitations, along with the inverse nature of the terrain avoidance problem, rendered GGI aided terrain avoidance unfeasible for the time being.
Published in:	Proceedings of the 2009 International Technical Meeting of The Institute of Navigation January 26 - 28, 2009 Disney's Paradise Pier Hotel Anaheim, CA
Pages:	49 - 60
Cite this article:	Rogers, Marshall M., Huffman, Richard E., Jr.,, Shearer, Christopher M., "An Investigation into the Feasibility of using a Modern Gravity Gradiometer Instrument for Passive Aircraft Navigation and Terrain Avoidance," Proceedings of the 2009 International Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2009, pp. 49-60.
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