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.