A radio beacon (105(5)) is for use in location determination inside a building (103) or other enclosure. The beacon transmits a RF signal equivalent to a partially coded GPS (Global Positioning System) signal which includes an identity code identifying the beacon but null data in at least one GPS data field. A mobile station (101) includes a GPS receiver (208) for receiving GPS signals and a processor (202) for processing data extracted from signals received by the GPS receiver. The GPS receiver is operable to receive a signal from a transmitting beacon (105(5)). The processor is operable to recognise a beacon identity code from the data extracted from the received signal and thereby detect that the signal is from a radio beacon rather than a GPS satellite and thereby to apply a further procedure to calculate the location of the mobile station based upon a known location of the identified beacon.