Patent: Underground GPS

13:48 12 February 2007
NewScientist.com news service
Barry Fox

Underground GPS

Satellite navigation is becoming a vital tool for the modern motorist. But GPS (Global Positioning System) receivers need to compare signals from at least three orbiting satellites to determine their position. This means satellite navigation does not normally work inside a tunnel, underground or in a heavily built up area.
Two inventors from Cambridge in the UK are now patenting a system that could let satellite equipment acquire positioning information even when satellite signals are blocked. The roof of the building, or the ground above the tunnel, is fitted with at least four directional antennae focused on different patches of the sky. These antennae receive GPS signals, then amplify and re-broadcast them using transmitters positioned at specific points below ground. A GPS device is then fooled into behaving as if it were out in the open, providing accurate positional data from inside a tunnel, in an underground car park or in a heavily built up city. The same trick could also let GPS devices work inside buildings.

See the patent application.