arduino based SONAR
I remember coming across an arduino based project that would seem to fit the bill nicely but can't for the life of me remember the search phrasing that it came up through but it went basically like this:
# Mobile unit sends out an ultrasonic ping
# arduino with three (or four) receivers around 'pitch' takes the differential timings of the signal to each receiver
# arduino calculates the location of mobile unit from the differential timings and locations of receivers
# arduin transmits coordinates back to mobile unit via Xbee
This method only requires accurate measurement of the locations of the receivers, not too difficult, I remember it saying they managed accuracy in the 10s of centimetres. Not too shabby!