First, thanks to everybody who replied - much appreciated.
There is, unfortunately, a rather serious reason for my posting this question. I fear someone has planted a tracking device in my vehicle, and a physical search has so far failed to locate it. If you ever have to go digging for one of these yokes in a vehicle, you'll quickly realise how many nooks and crannies it could be hidden in.
Plan A was to eliminate the GPS signal reception near the vehicle. This doesn't seem to do the trick.
MikeMI's APRS method ultimately relies on GPS for the positioning info, except the GPS data is transmitted by 2-Meter digital radio modem, rather than via the cellphone network, if my understanding is correct. And this is why I'm ruling out GPS, for now.
It could be cellphone triangulation method alright, and services exist to do just that. Yes, its crazy. Cellphone network operators will sell positioning data to make a few extra bucks (Location Based Services I believe it is referred to). So anyone can hide a cellphone in your vehicle and get your position to within about 50 meters in a built-up area or within a kilometer or two (depending on cell tower density) out in the country side. This can be sufficiently accurate for many (usually bad) applications. If you happen to leave your vehicle unlocked sometime and they manage to wire the cellphone into the vehicle electrical system, you are permanently bugged. Even conversations within your vehicle can be heard. Be warned, folks, this is downside of the digital mass communications age we now live in.
So I was wondering about other possible tracking methods, like those animal trackers where the animal is fitted with a small transmitter and some radio method is used to track it, based on received signal strength or something. Does anybody know anything about those?
Thanks again for all the replies. I really need to find this thing and remove it (ideally) or else shut it up, somehow.