This is info you may already know but I thought I would post it.
When your within your cellular reception area you are able to receive AGPS from the towers which will already have "almanac" type data embedded in it speeding up the GPS acquisition times. Outside of that you are relying completely on the GPS receiver.
One of my thoughts is that the onboard GPS receiver is extremely weak (at best) there for its relying on the AGPS to do the majority of the work. If AGPS is not present you may have a hell of a time getting a location fix.
Quote:
. AGPS uses a mobile network to reduce the initialization time that a normal GPS receiver needs to receive navigation data. In AGPS, information about satellite orbits and errors previously measured by a base station is provided to the terminal through a communication means. The GPS chipset or module mounted in the terminal determines the location of the terminal in a short time using the data received from the base station and the calculated pseudo distance from the GPS satellites.
Since satellite orbit information and correction information are transmitted from a position determination entity ("PDE"), installed in the base station, to a mobile station, AGPS ensures faster response times than stand-alone GPS and can track GPS signals even in weak signal conditions such as downtown areas or indoors.
When location or time information is not provided, a mobile terminal has to track search spaces with frequencies and codes of all GPS satellites to acquire GPS signals and calculate its location. AGPS combines rough location and time information from the mobile network with satellite orbit information to predict the GPS satellites currently in view with the Doppler shifts for the visible satellites to reduce the number of searchable satellites and speed up the response. Knowledge of currently operating satellites reduces the number of satellites to search for and reduces response times.
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