In a local positioning system, the land-based transmitters include free running oscillators or oscillators free of clock synchronization with any remote oscillator. A reference receiver receives the ranging signals from different transmitters and generates timing offset information, such as code phase...http://www.google.de/patents/US20060022873?utm_source=gb-gplus-sharePatent US20060022873 - Synchronizing ranging signals in an asynchronous ranging or position system
