If your talking about the white dome on top of semi trucks, that is used by the company to track there trucks. There is also an emergency button they have in the cab encase someone tries to hijack the truck. The white dome recieves and transmits location, speed and alot more.
I do think onstar does send and recieve. I'm not exactly sure how it works.
The GPS that people here use to find there way only recieves, it doesn't transmit any signal. Therefore, there is no way to be tracked.