Optimal Locations of Pseudolites for Differential GPS

Because of the announced DOD policy to degrade GPS accuracy, Differential GPS has assumed increasing importance. Furthermore, the delay in operational deployment of the NAVSTAR satellites has focused increased attention on the use of pseudo-satellites (pseudolites or PLs) to enhance accuracy and provide earlier operational capability. Pseudolites perform a double function: they both calibrate the local ranging errors and significantly improve the user's geometry as measured by GDOP (Geometric Dilution of Precision). This paper examines the following problem: What is the best location for a PL when one considers the worst possible satellite outage in twenty-four hours? What is the resulting worst case GDOP? Both single and dual PL systems are considered, and the results are extrapolated to expected user receiver performance (horizontal and vertical errors). The results indicate that a dual PL system will outperform the satellite-only GPS system, even with a worst case satellite outage. In addition, there should be negligible degradation during periods when the military intentionally reduces ephemeris or clock accuracy. Furthermore, the accuracy of such a system would provide aircraft positioning very close to Category I FAA landing requirements.