Characterising the Signal Structure of Locata's Pseudolite-based Positioning System

Locata is a Pseudolite-based Positioning System (PPS) operating in the licence-free 2.4GHz Industrial Scientific and Medical (ISM) frequency band. Unlike its predecessors, Locata utilises time-diversity methods to overcome the near-far problem. Therefore Locata is able to operate over larger regions than previous PPSs. Since the School of Surveying & Spatial Information Systems (SSIS) at the University of New South Wales (UNSW) has considerable experience in designing GNSS receivers, the integration of PPS signal processing within its own GNSS receiver (known as “Namuru”) is currently being investigated. Research into Locata’s signal structure is aimed at gaining insight and detailed technical knowledge to support alternate receiver designs for processing terrestrial PPS-type signals. In this investigation, several important signal variables were identified via coaxially connecting the Locata transmitter to a Radio Frequency (RF) down-converter, high speed digitiser and signal recorder. Using spectral analysis methods, and having apriori knowledge of the signal’s operating frequency range (i.e. the ISM band), the centre frequency of the signal was found. It was assumed that the signal has CDMA-type characteristics. This was further verified by fine tuning the centre frequency, and sampling the down-converted signal at very high frequency. Exploiting its autocorrelation function, the code sequence and code period was also deciphered. Other signal parameters (e.g. code chipping rate, code sequence generator, and Time Hopping sequence) were then progressively discovered using various signal processing/diagnostic methods.