High accuracy

Provides time signal as well as position


Depends on long-term maintenance and structure of GPS

Orbit only (see text for discussion)

Must initialize some units






Fully autonomous Uses attitude-sensing hardware Provides orbit, attitude, ground look-point, and direction to Sun

First flight test In 1993 Initialization and convergence speeds depend on geometry

Space Sextant

Could be fully autonomous

Right-tested prototype only— not a current production product Relatively heavy and high power

Stellar Refraction

Could be fully autonomous Uses attitude-sensing hardware

Still In concept and test stage

Landmark Tracking

Can use data from observation payload sensor

Still in concept stags

Landmark identification may be difficult

May have geometrical singularities

SateWte Crosslinks

Can use crosslink hardware already on the spacecraft for other purposes

Unique to each constellation No absolute position reference Potential problems with system deployment and spacecraft failures

Earth and StarSensing

Earth and stars available nearly continuously In vicinity of Earth

Cost and complexity of star sensors Potential difficulty Identifying stars

Ground-Station Tracking

This 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, described above. 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, 3a accuracies typically range from several kilometers for low-Earth orbits to approximately 50 km for geosynchronous orbit

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