Passive Airborne Navigation and Terrain Avoidance Using Gravity Gradiometry

The Air Force is studying the integration of an inertial navigation system (INS) and a gravity gradiometer for passive, purely autonomous navigation and terrain avoidance. In such an inertial/gradiometric system, navigation errors are bounded by passive updates consisting of real-time matches between the single flight-path profile of measured gravity gradients and stored, constant-altitude grids of estimated gradients based on all available ground gravity, terrain elevation, and mass density data. An envisioned mechanization is block diagrammed and reviewed. The relationship between the system's three-dimensional position errors, the self-generated errors of the INS and gradiometer, and the errors in the stored gradient grids is discussed. If high-resoluti on grids of ground gravity and terrain elevation data exist for the fly-over area, then airborne grids of gravity gradients can be rigorously estimated without any assumption involving terrain mass densities. If ground gravity data is lacking, a nominal terrain density value is assumed and terrain-implied grids of gradients can be nonoptimally estimated. The two estimation techniques possess no relative bias and produce airborne grids having similar gradient trends and statistical properties. The power spectrum densities of local terrain-implied gravity fields are shown to dominate those of local geology-implied (subterranean) fields along the shorter wavelengths where most of the spectral sensitivities of the gradiometer's signal reside. The effects of aboveground altitudes, aircraft velocities, and low-noise gradient production rates on terrain feature resolvability are discussed. The performance specifications of gradiometers currently being developed are reviewed. Although the requirements of this application are demanding, it is felt they can be engineered into an optimal 21st century gravity gradiometer design.