A New Pseudolite-Based Positioning Technology For High Precision Indoor and Outdoor Positioning

A real-time positioning technology that can operate indoors and outside anywhere in the world, with sub-cm accuracy, and at low cost, is the ultimate goal for many researchers. GPS can achieve cmlevel kinematic positioning accuracy, but with some major constraints. First and foremost the use of GPS signals for indoor positioning poses very difficult challenges, due to the very weak signal levels. Indoor positioning using high sensitivity GPS receivers cannot be guaranteed in all situations, and accuracies are typically of the order of tens to hundreds of metres at best. Of course GPS is widely used outdoors for real-time cm-level positioning in numerous applications. In these situations the realtime kinematic (RTK) GPS technique is used, where a base station transmits data to a rover unit via a radio modem. In situations where GPS satellite geometry is poor or the signal availability is limited, ground-based transmitters of GPS-like signals (called “pseudolites”) can be used to augment GPS. They therefore have the potential to be used for both outdoor and indoor positioning. With enough pseudolites it is theoretically possible to replace GPS entirely, though in practice this has been difficult to achieve. Typically pseudolites use inexpensive crystal oscillators and operate independently (in the so-called “unsynchronised mode”). In this case, double-differencing must be used to eliminate the pseudolite and receiver clock biases. Therefore real-time cm-level positioning with unsynchronised pseudolites can only be achieved with a base station that provides data to a mobile user unit via a radio modem (as with standard RTK-GPS). If pseudolites can be synchronised, stand-alone positioning can be achieved without base station data (and without the radio modem datalink). This paper describes a time-synchronised pseudolite transceiver developed by the authors, called a LocataLite. A network of LocataLites forms a LocataNet which transmits GPS-like signals that are time synchronised to within several picoseconds. This allows single point positioning with sub-cm precision (one sigma) for a mobile unit. The LocataLite hardware is designed to be inexpensive and currently uses temperature compensated crystal oscillators. The Satellite Navigation and Positioning group (SNAP) at UNSW has assisted in the testing of LocataNet and the development of a mobile unit, initially based on a standard GPS receiver with modified firmware. In this paper the LocataNet technology is described and proof-of-concept for the time synchronised approach is demonstrated. Results are presented of real-time stand-alone positioning with sub-cm precision both outdoors and indoors.