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Satellite is used only for GPS location. The signal from the GPS unit is fed into the vehicle control interface module (commonly called the OnStar module), and this is sent to OnStar via the cellular phone link.
OnStar does not beam your location, they can ask the VCIM to update the GPS information, for an update on your location.
As for the phone service, OnStar uses two different circuits; the default is Verizon (connected through carrier circuit A), and if no connection is possible, it will default to carrier circuit B. In some areas such as Alaska, Verizon has almost gone totally digital, and there are limited analog capabilities. Therefore, if OnStar cannot connect to carrier A (Verizon), they will attempt to connect to carrier B. Problem is, carrier B does not have a contract with OnStar, for the personal calling program. So Carrier B can only be used for the primary OnStar functions.
Most of the OnStar units on the road are still analog, due to the numerous numbers of analog towers still operating in this country. Many of the newer modules, known as Generation 6 Dual Mode are dual mode and will operate in both analog and digital. The H3 is a Gen 6 dual mode module, and will operate in either analog or digital.

The cellular operation of the OnStar® system may be inhibited by factors such as the user range from an analog cellular tower, the state of the cellular carrier equipment, and the location where the call is placed. Making an OnStar® keypress in areas that lack sufficient cellular coverage or have a temporary equipment failure, will result in either the inability of a call to complete a data transfer or the complete inability to connect to the OnStar® Call Center. The OnStar® system may also experience connection issues if the identification numbers for the module, station identification (STID) and electronic serial number (ESN) numbers, are not recognized by the cellular carrier local signal receiving towers. OnStar® cellular connection issues such as these require the assistance of the General Motors Technical Assistance Center OnStar® Group, which coordinates with cellular carriers to resolve connection issues.

The satellites that orbit earth providing the OnStar system with GPS data have almost no failures associated with them. In the event of a no GPS concern, the failure will likely lie with the inability of the system to gain GPS signals because of its location, i.e. in a parking structure, hardware failure, or being mistaken with an OnStar® call which has reached the Call Center without vehicle data.

One other thing, if your account is not activated, you cannot make an emergency call. Per Onstar, if you do not have an active account, you will receive a message.
"During an Emergency button press, a demo message will be played indicating the service has been deactivated."
This means if you do not have an active account, if the air bags go off, you will not be called to see if assistance is required.

OnStar uses CDMA, that is why Verizon is the main carrier, but only for the digital network. However, and I'm not sure, they might be able to use either CDMA or TDMA for circuit B. Again, not sure about this.

But you can obtain CDMA technology without Verizon or Spring. Great example is in KY. From south of Louisville to the TN border, if you have Verizon you will roam on CDMA with a local KY company. Costs nothing extra, but your cell phone will say Extended Roaming. When I lived in TN, I roamed into this network quite often with my trips to the Corvette Museum and their special functions, and I was never charged extra.
So, an Onstar Gen 6 Dual Mode should be able to connect with this same carrier if driving through KY.