The present invention relates to portable wireless communication systems, in particular for sending and receiving data including position information using radio frequencies. The invention comprises: (i) a hand held digital computer; (ii) a position-locating device providing position information to the digital computer; (iii) a VHF and/or UHF radio frequency transmitter; and (iv) a radio modem having an input for digital data from the hand held digital computer, a modulator for converting digital data from the computer into a modulated analogue RF signal, and an output connected to the transmitter for transmission of said modulated signal.