Instead of trying to track using radio signal strength, etc., design a device that can determine it's own position (like a sort of GPS). Then, it could transmit it's location data to the other unit(s) digitally. GPS itself may have the accuracy/resolution you need. You might have problems with the GPS receiver being shielded from the satellites' signals by the metal structure of the building, though. I don't really know much about the current generation of GPS, however.
Jeff