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 3D
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.
PreLaunch Orbit
Determination
Vectors are an excellent tool for prelaunch orbital predictions. Vectors
can use an EarthFixed 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 Earthfixed
reference frame and MET makes the Vector completely timeindependent
and thus a constitutes general orbital description.
Complete sets of Keplerian elements are valid only for a specific Launch
time. Therefore, preLaunch Keplerian elements have limited
usefulness. If the Launch time changes, updates need to be made before the
elements are useful for orbital predictions.
A timeindependent 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 timeindependent State Vectors may be downloaded for the mission(s) indicated below:
.
Phase 3D Estimates
.
STS90
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:
 NORAD's Space Track can be accessed by their Web Site [NOTE: vector data is not currently available from the Space Track Web Page], In order to log on to the web site, you must be assigned an account. An account may be obtained by filling out the online application. The Vectors are in the EarthCentered Inertial (ECI), True Equator and Equinox of Date Coordinate frame in units of km and km/sec. Vector data may not be redistributed except by agreement. See the site for additional information.
 NASA Human Spaceflight
Web Page has information about current flights. The Realtime Data section also provides periodic updates to State Vectors and Keplerian Elements. The Vectors are in the EarthCentered Inertial (ECI), Mean of 1950 (M50) Coordinate frame in units of meters and meters/sec.
 The Rockwell Mission Support Room provides vector data
on a cooperative basis to a few individuals who, inturn, reciprocate with the
corresponding Keps and also release these data publicly. The Vectors are in
the EarthCentered Inertial (ECI), Mean of 1950 (M50) Coordinate frame in
units of ft and ft/sec.
 DOD CBand Radar Network
provides vector data on a cooperative basis to a few individuals who,
inturn, reciprocate with the corresponding Keps and also release these data
publicly. The Vectors are in the EarthFixed Greenwich (EFG) Coordinate
frame in units of ft and ft/sec.
Originally contributed by Ken Ernandes, N2WWD. Additional updates by Emily Clarke, W0EEC.
