Abstract
In this review on Global Positioning System (GPS) satellites' orbits, basic concepts of orbit determination are introduced first. Next, a history of orbit determination activities from 1985 to 1992 is reviewed with the emphasis on strategies which enable accuracy improvements. The initial accuracy of GPS ephemeris was around 10 m in orbits or 0.1 ppm in baseline components. The current orbit accuracy reaches some tens of cm, producing baseline accuracy of 0.01 ppm. Twenty times accuracy improvement within seven years is astonishing. The evolution is still going on and leads to an international effort to establish a GPS orbit information service. Given the global tracking network and precise orbit service through international cooperation, GPS will be an indispensable utility of geodesy and geodynamics with mm accuracy in the near future.
