JPL's Tracking Systems and Applications Section has been an innovator in GPS (Global Positioning System) technologies for nearly two decades. With the advent of the International GPS Service in the early 1990s, regular JPL analyses of data from the emerging terrestrial network of permanent GPS receivers resulted immediately in a several-fold improvement in the accuracy with which the positions of the GPS transmitters are known, from several meters in the broadcast GPS message to about 40 cm, as indicated in Figure 1 for the period 1992-1994. The "orbit overlap" metric is the degree to which nearly independent estimates of the GPS satellite positions agree from day to day.
During the last half of the 1990s, and continuing on into the current decade, continual improvement in this metric resulted from an increase in the density and uniformity of the terrestrial network as well as improved modeling of the GPS data. Currently our analyses result in knowledge of GPS transmitter positions to about 5 cm! These estimates are used in interpretation of data from missions like TOPEX/Poseidon, JASON-1, and GRACE, and make valuable contributions to the science products of these missions.
Jim Zumberge and Michael Heflin