A portable GPS/radio unit (10) communicates over a wireless radio network (12) with at least one other such unit. The unit (10) may transmit radio signals over the network (12) indicative of the unit's location. The unit (10) includes a single, push-to-talk button (38) that may be pressed to both transmit a voice communication and to initiate transmission of location data. The unit (10) also includes a "polling" function that requests location data from other units, an auxiliary coding system that codes or encrypts all transmitted data, and an emergency message feature that facilities the transmission and receipt of emergency messages.