Use of Two-Way Time Transfer Measurements to Improve Geostationary Satellite Navigation

An emerging use of GPS is to provide accurate navigation information for satellites in orbit. The GPS satellites are designed to provide service to terrestrial users, so the antenna array points directly towards the Earth and uses a narrow primary beamwidth. Because GEO altitudes are well above the GPS constellation, the Earth occludes most of the GPS signals to the satellite. Decreased satellite visibility is debilitating, as GPS navigation requires at least four visible satellites to determine position. To assist with the visibility problem, the receiver can look at the GPS satellite transmit antenna side lobes, but this does not entirely solve the navigation problem. GPS measurements are inherently bound by receiver clock errors. The clock error must be known or estimated in order to obtain meaningful ranging information. To obtain threedimensional positioning, at least four satellites must be tracked to solve for three dimensions of position plus the receiver clock error. A new method for improving geostationary navigation accuracy using GPS is to correct the time error by including Two-Way Time Transfer (TWTT) measurements. TWTT is a technique in which signals are simultaneously exchanged between two clocks, and is one of the most accurate methods of comparing clocks. By effectively removing the clock error between the GPS satellite and the GPS receiver, TWTT allows meaningful information to be gathered when less than four GPS satellites are available. The results show a 21-38% improvement in the 3-D RMS position accuracy while using TWTT between the GEO satellite and an atomic clock on the ground. There was a 60-70% improvement when the clock on the ground was synchronized to GPS time.