State Vectors versus Keplerian Elements

Satellite orbits may be uniquely described by six (6) independent parameters. These orbital parameters are usually expressed either by Keplerian elements or by State Vectors. State Vectors represent the 3-D Position and Velocity components of the orbital trajectory at a certain snapshot in time or epoch. Keplerian elements are descriptive of the size, shape, and orientation of an orbital ellipse. State Vectors and Keplerian Elements each have their own unique advantages; either one can be computed from the other.


Pre-Launch Orbit Determination

Vectors are an excellent tool for pre-launch orbital predictions. Vectors can use an Earth-Fixed reference frame, with the epoch time expressed as an offset to the Launch date and time. Times offset relative to Launch are labeled as Mission Elapsed Time (MET). Referencing to both the Earth-fixed reference frame and MET makes the Vector completely time-independent and thus a constitutes general orbital description.

Complete sets of Keplerian elements are valid only for a specific Launch time. Therefore, pre-Launch Keplerian elements have limited usefulness. If the Launch time changes, updates need to be made before the elements are useful for orbital predictions.

A time-independent State Vector can be readily combined with the Launch date and time, using software such as VEC2TLE, to produce a valid set of Keplerian elements. The Keplerian elements can then, in turn, be used by tracking software for orbital predictions. Nominal time-independent State Vectors may be downloaded for the mission(s) indicated below:

AMSAT Phase 3D. Phase 3D Estimates STS Launch. STS-90

Further details regarding the capabilities of VEC2TLE are available on the Orbitessera Web site.


Sources of Space Shuttle State Vectors

Space Shuttle State Vectors are obtained from several NASA Sources. While every effort is made to keep the Orbital Data on the AMSAT Web Page current, this is not always possible. The interested user may wish, at some point, to generate Keplerian elements by manually entering a State Vector. If this is attempted, be sure that the Settings in VEC2TLE conform to those applicable to the vector. Sources of Shuttle State Vectors Include:


Questions or comments regarding the information on this page should be addressed to Ken Ernandes, N2WWD at: n2wwd@amsat.org

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