Gravity Aided Inertial Navigation System (GAINS)

A new autonomous covert Inertial Navigation System (1X3) uniquely suited to underwater applications is described. Unlike the conventional INS, schuler and siderial errors ,are bounded without external navigation aids or active instrumentation of ground speed. As a result the system exhibits excellent long term navigation (both velocity and position) performance while maintaining the inherent covertness of an INS system. This new innovation in INS technology results by integrating a conventional INS with a gravity gradiometer capable of measuring gravity field components independently of platform accelerations. A number of integration schemes use gradiometer measurements to estimate gravity distrubance vector components which in turn are used to compensate INS accelerometer measurements. The resulting INS performance, although much improved, continue to exhibit random walk navigation errors. This new integration scheme goes further by taking advantage of navigation system velocity error observability. Velocity error is manifested in two ways. First. east velocity error results in vertical channel acceleration error through the coriolis term leading to detectable depth error. Second, gravity state estimates based mainly on gradiometer measurements are transitioned forward using estimated velocity. So errors in estimated velocity, both north and east, lead to disturbance vector solution errors and to gradient prediction errors. The vertical disturbance in turn leads to detectable depth error and gravity gradient prediction errors are observable with measured gradients. Parametric performance results are presented for GAINS, varying gyro, gravimeter, gradiometer, depth sensor quality and gravity field activity. If gravity maps are available (e.g. GEOSAT maps) GAINS can be used to implement gravity field based map matching navigation in order to further improve long term navigation performance. A significant added capability of a gradiometer based system is that these covert measurements along the vehicle track can be used to develop surrounding terrain estimates. So stealth enhancing terrain following terrain avoidance modes can be implemented.