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There are various ways to determine the orbit with different accuracies:

 real-time orbit determination: it provides the best estimate of where a satellite is at the present time and may be important for spacecraft and payload operations, such as accurate pointing at some target;

 definitive orbit determination: it is the best estimate of the satellite position and orbital elements at some earlier time, it is done after gathering and processing all relevant observations;

 orbit propagation: it refers to integrating the equations of motion to determine where a satellite will be at some other time. Usually orbit propagation refers to looking ahead in time from when the data was taken and is used either for planning or operations. Occasionally orbits will be propagated backward in time, either to determine where a satellite was in the past or to look at historical astronomical observations in the case of comets or planets.

Traditionally, ground stations from around the world provide tracking data to a mission-operations centre. When all data is available, definitive orbit determination provides the best estimate of the orbit. This is used to process the payload data for science or observation missions. The best estimate of the orbit is then propagated forward for real-time operations (such as star catalogue selection or manoeuvre timing) and further forward for mission planning.

In 1983 NASA launched the first Tracking and Data Relay Satellite, TDRS, to begin replacing the worldwide ground tracking network. TDRS provides the same functions as the traditional ground- station network. As the name implies, it tracks low-Earth orbiting satellites and relays data between the satellite and the TDRS ground station in White Sands, NM.

GPS, GLONASS, and other more autonomous systems are also becoming operational, so orbit determination for future systems will differ significantly from what it has been in the past.

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The observations used for orbit determination can be obtained by tracking from the ground, tracking from space, or from autonomous or semi-autonomous systems on the spacecraft. Each of these approaches will be described.

 Ground tracking is the traditional way to obtain data for orbit determination. We either track

the spacecraft's telemetry signals or use radar tracking from a site not associated with the spacecraft. In both cases, the principal data used for orbit determination are range and range

rate, that is, the distance from the ground station to the satellite and the satellite's line-of-sight

velocity during the overhead pass. Angular measurements are also available at times but are typically far less accurate than range or range-rate measurements. Accurate orbit determination using ground-station data ordinarily requires a number of passes. We may accumulate data from multiple passes over a single ground station, or may receive data at a central location from multiple ground stations around the world. In either case, data from a number of passes goes to one place for processing through a large system such as GTDS (Goddard Trajectory Determination System - NASA). Ground-based systems necessarily operate on historical data and therefore will use propagated orbits for real-time operations and mission planning. Accuracies achievable with ground-based tracking vary with a spacecraft's orbit and the accuracy and amount of data. However, accuracies typically range from several kilometres for low-Earth orbits to approximately 50 km for geosynchronous orbit.

 The Tracking and Data Relay Satellite, TDRS, has now replaced NASA's worldwide ground

tracking network. A major advantage of this system is that the two operational TDRS satellites can provide tracking data coverage for 85% to 100% of most low-Earth orbits. (TDRS does not work for satellites in geosynchronous orbit). The system collects mostly range and range- rate data from the TDRS satellite to the satellite being tracked. Angular information is available, but is much less accurate than the range and range-rate data. If atmospheric drag effects on a satellite are small, TDRS can achieve accuracies of about 50 m. This is considerably better than most ground-tracking systems. Another way to track from space is to use satellite-to-satellite or crosslink tracking.

Manufacturers have developed a number of autonomous navigation systems for spacecraft. Determining the orbit on board is technically easy with the advent of advanced spacecraft computers and higher-order languages. The principal problem is to provide orbit determination that is reliable, robust, and economical in terms of both cost and weight.

Autonomous navigation is inherently real-time. Thus, definitive orbit solutions and payload data are available simultaneously. Moreover, measurements can be less accurate that those for systems that work on old data, because solutions propagated forward in time lose accuracy. With real-time systems highly accurate orbit propagation is less critical, although we will still need some forward propagation for prediction and planning.

 GPS receivers use signals from four different GPS satellites to solve simultaneously for the three components of the observer's position and the time. This can be done several times, providing position and velocity data which determines the orbit elements. The GPS constellation is in a 12 hour orbit at approximately half-geosynchronous altitude. The system provides a moderate accuracy signal (50 m - 100 m) for general navigation and a high- accuracy coded signal (15 m) for military applications. Commercial GPS receivers are now available for spacecraft, and are gaining in popularity in low-Earth orbit.

 A number of proposals have been made for using satellite crosslinks to provide orbit

determination. This is of interest because it can be done with crosslink equipment used for inter-satellite communication, and, therefore, requires minimal additional hardware. Crosslink tracking tends not to be implemented because of several practical problems. One problem is that satellite-to-satellite tracking provides only the relative positions of the satellites in the

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constellation. This means that if the absolute position is needed for any purpose, then an additional system must be provided to establish the orbit relative to the Earth's surface. A second problem is that the satellites become interdependent, so satellite-to-satellite tracking may not work well for the first satellites or may degrade if a satellite stops working. Therefore, an alternative system not based on satellite-to-satellite tracking is required. If additional systems must be provided, there is less benefit from the satellite-to- satellite tracking.

 A number of approaches for orbit and attitude determination have been proposed, based on the

interaction of starlight with the Earth's atmosphere (Stellar Refraction Systems). Specifically, as stars approach the edge of the Earth a

 s seen from the spacecraft, refraction will cause their position relative to other stars to shift,

producing an effect which can be measured with considerable accuracy. Theoretical accuracies for such systems are projected to be in the vicinity of 100 m. However, none of these systems has been fully developed for flight as yet. The combination of Earth and star

sensing works similarly to sensing the Earth, Sun, and Moon. The direction and distance to the

Earth are sensed relative to the inertial frame of the fixed stars. This is then used to directly determine the direction and distance to the spacecraft. The Earth and stars are available nearly continuously in any Earth orbit and star identification is becoming less of a problem with the introduction of substantially better computers for space use.

 Landmark tracking has also been proposed for orbit determination. This has been established

as feasible by using data returned from satellite payloads. However, it has not been used as a normal method for satellite navigation, due in part to the difficulty of establishing automatic, unambiguous identification of landmarks to ensure that tracking accuracy can be maintained in the presence of adverse weather or poor seeing conditions.