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.
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.
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
Further details regarding the capabilities of VEC2TLE are available
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.
Spacelink may be accessed by their
Web Page. Both pre-launch
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: