so with GPS tracking devices it's not the GPS signal thats transmitting but some other signal eg gsm or some other signal. so it may be possible to detect these with the n900 and confirm dB levels from said signal. I think it would interesting to display the signals that the n900 can pick up, their frequency, and dB etc, and what card it's picking it up through, gps,wifi,bt,cell etc, making the n900 into an all round signal checker with raw data of those signals.