Orbital
Elements
Information
regarding the orbit trajectories of the International Space Station
and the space shuttle is provided here courtesy of the Johnson
Space Center's Flight Design and Dynamics Division -- the same
people who establish and track U.S. spacecraft trajectories from
Mission Control. This data can be used in a variety of programs
that require accurate knowledge of the orbit of the satellite.
Such applications might include ground track plotting programs,
visual sighting programs and programs that predict past or future
spacecraft trajectory information.
 | Go here
to get the orbital elements to track the space shuttle during
a mission. |
|  | Get
the latest International Space Station orbital elements as
it grows over the next several years. |
The trajectory
data, which describes the motion of a satellite, can take on a
variety of formats. We provide this data as either Keplerian osculating
or mean element sets, which include the TWO-LINE and AMSAT formats.
In general, it takes at least six parameters to uniquely define
an orbit and a satellite's position within the orbit. The mean
element set format also contains the mean orbital elements, plus
additional information such as the element set number, orbit number
and drag characteristics. Both of these formats are common and
most applications requiring trajectory data will be able to accept
data of this kind.
The six orbital
elements used to completely describe the motion of a satellite
within an orbit are summarized below:
| Orbital
Elements : | | Semi-major
axis | a | Defines
the size of the orbit. | | Eccentricity | e | Defines
the shape of the orbit. | | Inclination | i | Defines
the orientation of the orbit with respect to the Earth's equator. | | Argument
of Perigee |  | Defines
where the low point, perigee, of the orbit is with respect
to the Earth's surface. | | Right
Ascension of the Ascending Node |  | Defines
the location of the ascending and descending orbit locations
with respect to the Earth's equatorial plane. | | True/Mean
Anomaly |  | Defines
where the satellite is within the orbit with respect to perigee. |
If maneuvers
that change the size of the orbit are planned, a detailed table
of maneuver parameters is included plus new orbital elements and
mean element sets after each maneuver. These maneuver parameters
include the LVLH maneuver delta V components at the impulsive
ignition time, the M50 delta V components, the maneuver magnitude
and the resulting mean apogee and perigee altitudes. In general,
new data is provided after each maneuver or 48 hours, whichever
occurs first.
Additionally,
a satellite's motion can be described using a 3-D inertial coordinate
system or frame of reference. This data set is called a Cartesian
state vector and provides the position and velocity of the satellite
with respect to a three-axis Cartesian coordinate system. For
informational purposes, the following technical definitions are
provided to define the frames of reference of the displayed Cartesian
state vectors.
| Definitions
: | M50
|
= | Inertial
mean of year 1950 frame of reference. | | Epoch
|
= | beginning
of Besselian year 1950. | | X
axis |
= | mean
vernal equinox of epoch. | | Z
axis |
= | earth
mean rotation axis of epoch. | | Y
axis |
= | completes
right handed Cartesian Earth-centered system. |
| | J2K |
= | Inertial
mean of year 2000 frame of reference. | | Epoch |
= | Julian
date 2451545.0 TDT. | | X
axis |
= | mean
vernal equinox of epoch. | | Z
axis |
= | earth
mean rotation axis of epoch. | | Y
axis |
= | completes
right handed Cartesian Earth-centered system. |
| | LVLH |
= | Local
Vertical Local Horizontal rotating, instantaneously inertial,
frame of reference. | | Epoch |
= | Vector
epoch time. | | X
axis |
= | completes
the right handed orthogonal system. | | Z
axis |
= | lies
along the radius vector, positive toward the center of the
earth. | | Y
axis |
= | lies
along the instantaneous angular momentum vector, negative
in the direction of the angular momentum. |
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Please
direct any comments to:
DM33/ Orbit Analysis and Navigation at email: jonathan.k.weaver@nasa.gov
|
