An accessory module for a user terminal is disclosed that utilizes information broadcast from the GPS system to determine the user terminal's position on the earth. An improved electrical interface between the GPS accessory module and the user terminal is utilized that takes advantage of pre-existing electrical interfaces within the user terminal. The accessory module also includes a method and apparatus for managing its power consumption. A set of mechanical interfaces among the components of the GPS accessory module are disclosed for minimizing electrical path lengths, noise and crosstalk in the accessory module, and for providing for mechanical mounting of the components. A mechanical interface between the user terminal and the accessory module is also disclosed for coupling to an existing user terminal in place of that user terminal's removable battery pack. The invention further includes a method and apparatus for improving the accessory module's accuracy by providing position assistance data from a position reference server. The assistance data is provided to the accessory module by way of a transmission channel communication between the user terminal and the position reference server.