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 3a 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 as described below.

Spacecraft Autonomous Navigation

As summarized in Table 11-66, 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. A number of systems which can do this now exist—autonomous orbit determination is clearly feasible but becoming less important with the increasing use of GPS for navigation in low-Earth orbit Wertz [2001] and Chory et a). [1986] describe alternative methods of autonomous navigation on board satellites. Table 11-65, earlier in the section, gives the advantages and disadvantages of the primary alternatives.

Autonomous navigation is inherently real-time. Thus, definitive orbit solutions and payload data are available simultaneously, which means that we can generate ground look-points or target positions and immediately associate them with the payload data. In addition, measurements can be less accurate than those for systems that work on old data, because solutions propagated forward in time lose accuracy. For example, to do accurate orbit maneuvers without autonomous navigation, we need a greater accuracy from a definitive solution based on old data that must be propagated forward to meet o â– o a w

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