A method of determining the position of a mobile unit (MS1) having a GPS receiver (24) is disclosed. The method comprising the steps of measuring mobile unit pseudoranges at the mobile unit (MS1) using the GPS receiver (24); providing reference pseudoranges corresponding to a reference location situated remote from the mobile unit (BS1); and determining the position of the mobile unit (MS1) relative to the reference location (BS1) as a function of both the reference pseudoranges and the mobile unit pseudoranges.