antenna on big-rig trucks

I have occasionally seen on the roof of the tractor part of a big-rig a device that looks like a large white hamburger bun, probably plastic or fiberglass mounted on a short tripod. I must assume it is an antenna of sorts. An antenna for what I do not know. Any operators out there can answer? Thanks.
Mike30

That would be the antenna for the satellite communication/tracking system that is popular with many large fleets now. The main office can track where the truck is and send & receive text messages to the truck.

Qualcomm is one major manufacturer of these systems –
http://www.qualcomm.com/qwbs/

And a link with a pic of the antenna and in truck display –
http://www.qualcomm.com/qwbs/products/omnitracs-vehiclehdwr.html