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GPS APPLICATIONS AND REQUIREMENTS 57 time and contributed to its success.56 An integrated GPS/inertial navigation unit is also being test flown on the Orbital Science Corporation's Pegasus launch vehicle. The company hopes that an operational version of the unit will one day improve the vehicle's en route navigation and orbital injection accuracy. Current and Future Applications and Requirements GPS is currently being tested or used for several spacecraft applications, including orbit determination, attitude determination, launch and reentry vehicle positioning and trajectory determination, and time synchronization. Precise time synchronization, which is required by many spacecraft, such as telecommunications satellites, to an accuracy of 100 nanoseconds was discussed in some detail in the previous section, but the remaining applications are discussed below. Orbit Determination The use of GPS for real-time determination of orbital parameters provides an economical means of determining a spacecraft's orbit very accurately. A properly designed, space-qualified GPS receiver can replace several conventional orbital positioning spacecraft sensors, reducing both weight and cost, and in some cases relieving the requirement for worldwide, ground-based stations to track orbital positions. In addition, the orbital parameters determined with GPS can in some cases be input to an on-board control computer and propulsion system to provide autonomous station keeping. This would alleviate or reduce the need for mission operations personnel to control a spacecraft's orbital position from the ground. In general the requirements for real-time orbit determination are not very stringent, ranging from about 50 meters to several kilometers. Although these requirements are quite lax, the same is not true for post-flight or post-processed solution accuracies. Many spacecraft, in particular those used for scientific missions, require very precise knowledge of where the satellite was when scientific data were being collected. The desire to achieve Â± 1 centimeter orbit determination accuracy for the Topex/Poseidon spacecraft, as discussed in the Earth Science section of this chapter, provides an excellent example. In order to achieve this level of accuracy, GPS measurements from the spacecraft are processed together with GPS data from a worldwide network of ground stations and an extensive set of dynamic models. Future science missions are likely to push this requirement even further towards the millimeter level. 56 "Delta Clipper Contractors Tout Components' Success," Space News, 27 September - 3 October 1993, p. 17. The DC-X is a one-third-scale sub-orbital, single-stage-to-orbit (SSTO) technology demonstrator developed with funding from the Ballistic Missile Defense Organization (BMDO).