AES/newspost wrote:
How did the GPS units installed in Scott Peterson's car transmit the
location information back to the police?
Some kind of continuous real-time transmitting capability? (Is this
built into the GPS system itself?)
Or did they have to surreptitiously recover the units from time to time
and downloaded data stored in memory? (And if so, are such units
available to the general public?)

There are several commercial tracking operations in business providing
the service. There are different methods of obtaining the tracking data.
Some are realtime transmitters, others are via interrogation.
Some services on high value rental cars will remotely shut the engine
off when the vehicle gets within so many miles of the Mexican border to
prevent auto theft.
Others, like OnStar, can detect airbag inflation, unlock car doors, etc.