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:

. Phase 3D Estimates . 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:

• NASA OIG RAID RBBS can be accessed by their Web Page [NOTE: vector data is not currently available from the OIG Web Page], OIG telnet access: oig1.gsfc.nasa.gov [User ID = oig / Password = goddard1], or by Telephone: (301) 262-6782. In order to log on to the BBS, you must be assigned an account. An account may be obtained by written request to: NASA / Goddard Space Flight Center, Operations Management Section Code 513.1, Attn: Orbital Information Group (OIG), Greenbelt, MD 20771, USA. The Vectors are in the Earth-Centered Inertial (ECI), True Equator and Equinox of Date Coordinate frame in units of km and km/sec.
• NASA Shuttle Web Page has periodic updates to State Vectors and Keplerian Elements. The Vectors are in the Earth-Centered Inertial (ECI), Mean of 1950 (M50) Coordinate frame in units of meters and meters/sec.
• NASA Spacelink may be accessed by their Web Page. Both pre-launch and real-time vector data are available from a variety of sources. NASA Spacelink also provides the STSTLE mailing list as a means of getting automatic updates of Space Shuttle orbital data. You may subscribe to this service by sending an E-Mail message with the following text: subscribe STSTLE Your Name to the STSTLE Mailing List manager. Potential subscribers should be cautioned that this typically provides three (3) E-Mail updates per day for the Shuttle Orbiter, plus updates for any satellites associated with the Shuttle mission. These updates are provided regardless of whether or not the mission has SAREX equipment. If this frequency of updates is undesirable, you should probably use the AMSAT Shuttle Orbital Data Web page for updates.
• The Rockwell Mission Support Room provides vector data on a cooperative basis to a few individuals who, in-turn, reciprocate with the corresponding Keps and also release these data publicly. The Vectors are in the Earth-Centered Inertial (ECI), Mean of 1950 (M50) Coordinate frame in units of ft and ft/sec.
• DOD C-Band Radar Network provides vector data on a cooperative basis to a few individuals who, in-turn, reciprocate with the corresponding Keps and also release these data publicly. The Vectors are in the Earth-Fixed Greenwich (EFG) Coordinate frame in units of ft and ft/sec.

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