The role of pseudolites in the implementation of differential GPS

When the Radio Technical Commission for Maritime Users (RTCM) took on the task of standardizing the broadcast of differential Global Positioning System (DGPS) corrections, the use of pseudolites (PLs) for broadcasting corrections was explored extensively, and a preliminary specification for PLs was established. In the present work, it is reported that these specified requirements were analyzed to verify proper GPS receiver operation in the DGPS environment and found to be inadequate. This is because of near-far signal interference problems, and because of the requirement for a minimum distance between PLs. Alternate signal structures more compatible with all GPS receivers, and more compatible with closely located PLs, are derived that minimize the impact of these problems. The improved signal structure features a lower duty cycle, a modest transmission frequency offset, a revised pulse positioning algorithm, and a new data rate. The revised pulse positioning algorithm eliminates the minimum distance between PLs requirement imposed by the RTCM signal structure. Results of the analysis, proposed alternatives, and PL Architectures are presented.<>