A method for radio positioning of a mobile receiver, the mobile receiver receiving transmissions from a plurality of geographically spaced terrestrial radio signal transmitters (7, 8, 9), each transmitter of said plurality of transmitters having a known position and transmitting predictably repeated code words that are distinguishable from one another and have different code word repeat rates, the method comprising: when the mobile receiver is at a first known position, the first known position being derived via a primary positioning resource local to the mobile receiver establishing, in respect of each of said plurality of transmitters, a virtual positioning reference by logging the first known position of the mobile receiver together with a local time at which a first instance of a respective code word is received by the mobile receiver from the respective transmitter, the local time being measured relative to a clock local to the mobile receiver, and determining for each of said transmitters a repeat rate of the respective code word; running the local clock during movement of the mobile receiver from the first known position to a second unknown position, the second unknown position being a position at which the primary positioning resource of the mobile receiver is rendered ineffective; and when the mobile receiver is at the second unknown position, opening a radio signal capture window for a period limited to that of the most slowly repeating code word and using the local clock to determine the time shift of a code word in the radio signal capture window by measuring the difference between when a second instance of the code word is predicted to be received at the virtual positioning reference and when the mobile receiver actually receives the second instance of the code word, using the time shift to determine a distance travelled by the mobile receiver relative to the respective transmitter, and thereby determining the second unknown position of the mobile receiver. when the mobile receiver is at a first known position, the first known position being derived via a primary positioning resource local to the mobile receiver establishing, in respect of each of said plurality of transmitters, a virtual positioning reference by logging the first known position of the mobile receiver together with a local time at which a first instance of a respective code word is received by the mobile receiver from the respective transmitter, the local time being measured relative to a clock local to the mobile receiver, and determining for each of said transmitters a repeat rate of the respective code word; running the local clock during movement of the mobile receiver from the first known position to a second unknown position, the second unknown position being a position at which the primary positioning resource of the mobile receiver is rendered ineffective; and when the mobile receiver is at the second unknown position, opening a radio signal capture window for a period limited to that of the most slowly repeating code word and using the local clock to determine the time shift of a code word in the radio signal capture window by measuring the difference between when a second instance of the code word is predicted to be received at the virtual positioning reference and when the mobile receiver actually receives the second instance of the code word, using the time shift to determine a distance travelled by the mobile receiver relative to the respective transmitter, and thereby determining the second unknown position of the mobile receiver.