Navigating by Earth’s gravity when GPS is denied

I like the idea of navigating by Earth’s gravity when GPS is unavailable. It has the obvious benefit of being passive and difficult to jam, and it is also a wonderfully nerdy way to find your position. A gravimeter measures tiny variations in gravitational acceleration and a navigation system matches them against a map, much as a hiker matches a landscape against a chart. Q-CTRL describes a recent GPS-free gravimetric navigation trial, while earlier gravity-mapping work showed how much the usefulness depends on the resolution of the map and the sensor.

My instinct is that the engineering becomes difficult very quickly. A skyscraper or a large piece of infrastructure can create a local gravity signal measured in hundreds of microgals, which is large enough to complicate any attempt at metre-level positioning in a built-up area. The global reference map is another problem. ESA’s GOCE mission achieved roughly 1 mGal accuracy at a 100 km spatial resolution, not a street-level map, and the field changes as mass moves. USGS records about 20,000 earthquakes around the world each year, though only a fraction would matter to a particular navigation map. I may be missing clever filtering and sensor fusion that makes this practical. Positioning at roughly kilometre scale feels plausible; metre-level positioning everywhere feels like a much harder promise.