Real Time Kinematic (RTK) is a technique that uses high precision positioning with Radios & Modems. It involves radio transmission of L1 and L2 carrier phase observation data (normally known as ‘corrections’) from a base station of known co-ordinates, to a mobile receiver where a position is established. Double-differencing the measurements and resolving the uncertainties provide relative positioning accuracies at the centimetre level. In order to acquire real-time corrections, the Rover must communicate to one or more reference stations. GPS Radios receive correction data over short ranges, whilst GPRS modems allow nation-wide RTK coverage via connection to the SmartNet commercial GNSS (Global Navigation Satellite System) network based on data from the Ordnance Survey infrastructure.