I wonder if those OnStar dead zones are due to military installations?
I just read up on it. It is because of CDMA cellular coverage. OnStar uses Verizon primarily and Bell Mobility in Canada. I guess the dead zones are where either there is no CDMA or OnStar has no contracts with the cell providers for those areas.
There is no active satellite action happening when an OnStar alert happens. It is all done by CDMA cellular signals. The onboard computer collects data, including getting a GPS reading, and then sends a cellular message to one of the OnStar call centers.