The Use of Pseudo-Satellites for Improving GPS Performance

Abstract

The NAVSTAR Global Positioning System has proven itself to be extremely accurate under tests of normal operations conducted at the Yuma proving ground. During the same tests of several years ago, a special technique called Differential GPS was also evaluated and shown to have even more startling accuracies. The basic concept of Differential GPS is to have a local receiving station continuously calibrate the remaining bias in the GPS signal and transmit it to the local users. This allows aircraft, ships or land vehicles to refine their estimate of position to the last meter or two. One technique for transmitting such a correction is to use a GPS-like transmitter itself. The exact form of the signal used may be quite significant to operations and accuracy enhancement. The “pseudo-lite” or GPS ground transmitter also provides powerful geometric leverage for the total navigation solution. A pseudo-lite will be called a PL in this paper. This paper will address the geometric problems encountered by the current GPS satellite constellation. Simulation results will show the value of PLs to deal with both poor satellite geometry and potential satellite outages (in the event of failure). Quantitative examples of the performance improvements are included. This paper will also outline potential design alternatives for the PLs.