A GPS unit and a cellular telephone unit each have add-on modules for providing additional functionality. A battery powered hand-held GPS unit has a battery power module with a built-in cellular telephone. The controls on the GPS unit are used to control cellular telephone operations. The GPS controlled cellular telephone transmits position location information obtained by the GPS unit. Alternatively, a battery powered hand-held cellular telephone unit has a battery power module with a built-in GPS device. The controls on the cellular telephone portion are used to control GPS device operations. The cellular telephone portion is able to transmit position location information obtained by the GPS device. The GPS and a cellular telephone device can function with either a standard battery module, or with a battery module having a cellular telephone or a GPS device built into the battery module.