Submarines have been using gravity gradiometers for navigation for decades. Tom Clancy revealed what was then classified information about these in The Hunt fo

Submarines have been using gravity gradiometers for navigation for decades. Tom Clancy revealed what was then classified information about these in The Hunt for Red October. They can serve as passive collision avoidance systems or let the navigators use geological features to correct drift in inertial navigation. The later application requires gravity gradient maps. They are useful for submarines because they are completely passive, unlike active sonar that produces a signal others can detect.