A true GPS (Global Positioning System) uses satellites. Assisted GPS uses cell tower triangulation. Portable GPS devices use both. However if at least two cell tower connections are available this provides a faster connection and location service. To use satellite based GPS the device must first locate three satellites out of at least than 24 in orbit. Then the satellite projected locations must be downloaded into the device. Once the locations are downloaded, the GPS receivers use this information and trilateration to calculate the device's position.
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