Not that I’m paranoid or anything 
A Wisconsin court has recently ruled that the police do not need a warrant to install a GPS on a suspect’s car. link1 link2
Google suggests that they’re pretty hard to detect, and that you’re better off trying to jam one. I thought I’d check if any of you all had some super secret Doper knowledge on the topic.
There seems to be no shortage of “anti-spy” sites selling solutions but I guess it will depend on the method of tracking. If its passive, ie sending out no active signal, I’m not sure how you’d defeat that aside from a very thorough search of your vehicle. If its active and sends out a signal then a proper detector should revel its general location.
Frankly I’m a little disturbed that the police can do this without a warrant. In my mind your vehicle is your own personal property and for the law to sneak in and plant a tracking device without a warrant seems like they are not allowing for proper oversight.
Wrap your car in tin foil.
I used it all on my head 
Any idea what frequency the GPS signal is broadcast on? I’m sure one could design a faraday cage that’s see-though.
Well, if you are a teenage daughter in the US and your father, the amateur radio operator, has installed a tracker in your car then it is probably transmitting on 144.390 mhz ( the US APRS freq). GPS trackers transmitting valid APRS packets on this freq are usually gated to the internet and available to any web browser. I would think though that the police would not use a transmitter. It would need an external power supply and would not always be in range. I would think that it simply records the gps positions every few minutes and then is recovered from the vehicle at some later date.
GPS antennas need a clear view of the sky. A GPS signal won’t pass though anything metal. Even tree cover is enough to block a signal. If the police were to place a tracker on your car, even the most cursory search would reveal it. Now, if they had enough time to install it so that it used your OnStar antenna, then you might have a tougher time.
I have heard of devices that attach underneath a bumper by means of a quick magnetic mount. The antenna is normally out of sight, but emerges when the vehicle is in motion - thus making the device unlikely to be detected by a casual inspection.
Sorry, but I wasn’t able to Google up any useful links for this.
I think you are correct that many GPS-based tracking devices simply record position for later retrieval.
But it would not be particularly difficult to include a cellphone-based transmitter and a battery sufficient for many days of (say, hourly) position-history reports.
GPS itself is simply a receiver, not a transmitter. If it’s hooked up to a transmitter then it can transmit on any available frequency.
Ed
Now that would be cool. A little on the complicated-and-prone-to-failure side but cool nonetheless.