Precise Real-Time Low-Earth-Orbiter Navigation With the Global Positioning System (GPS)

Technology currently is available to support real-time onboard knowledge of the position of a low Earth orbiter at the 5- to 15-m level using the civilian broadcast Global Positioning System (GPS) signal with sophisticated models and flltering techniques onboard the spacecraft. Without these techniques, the standard posi-tioning service yields 50 to 100 m with the current level of selective availability (SA). Proposed augmentations and/or enhancements to the GPS system will make rms accuracies of from 10 centimeters to a few decimeters available to the real-time onboard user. Presently, near-real-time processing of GPS tracking data can routinely pro-vide low-Earth-orbit determination accuracy at the level of 5 cm. Such processing systems can, in fact, be fully automated; recent results from the Jet Propulsion Laboratory (JPL), where ongoing daily processing of low Earth GPS tracking data has been undertaken for several years, are presented in this article, showing orbit determination accuracies at the sub-10-cm level. At the present time, such solutions can be produced with about a 10-h delay after real time, but recent improvement