I believe you can order it without XM, not sure. Yes, the antenna on top is GPS capable. For XM it is a satillite radio antenna, for onstar it is a GPS antenna with the communications antenna screwed into it.
Onstar operates by the onstar module (VCIM) receiving GPS information and when you connect to onstar, it sends this information to onstar for them to locate you. Air bags go off, a signal is sent along with location. Car is stolen, onstar activates the system to locate the vehicle.
Onstar is quite amazing, and very integrated with the vehicle. If you signed up for the montly reports, the same day every month onstar sends the vehicle information to onstar, and they in turn relay it to you in an email. If your car is not running that day, it will send the next time the vehicle is started.
If your oil percentage drops to 22 percent, the PCM signals the onstar module to notify onstar and a notice is sent via email.
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Black Sheep Hummer Squadron
(ME TOO)
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