Orbit Element Types 
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The orbital elements are six independent parameters that describe the shape and orientation of a Spacecraft's orbit. The orbital element systems available in FreeFlyer are described below. The ElementConvert function can be used to convert orbital elements between the different systems.
FreeFlyer supports the following orbit element types, described on this page:
The diagram below illustrates the orbital element systems that are available for each central body in FreeFlyer. Note that the Spherical (Latitude, Longitude), BrouwerLyddane Mean, J2 BrouwerLyddane, and SGP4 elements are intended for use only when the Spacecraft's central body is Earth.
The Cartesian element set defines the position and velocity of a Spacecraft with respect to the origin of an inertial rectangular coordinate system. Positions and velocities are expressed by their components along the three principal axes of the MJ2000 Earth Equator reference frame.
You can also set and get a Spacecraft's or CelestialObject's Cartesian state with respect to other reference frames using the methods:
The Keplerian element set defines six parameters that describe the shape, size, and orientation of a Spacecraft's orbit, as well as the Spacecraft's current location in the orbit (represented by the True Anomaly). The Keplerian element set assumes perfect Keplerian orbital motion, so all gravitational forces are treated as point masses and no perturbing forces are considered when calculating these parameters.
You can also set and get a Spacecraft's or CelestialObject's Keplerian state with respect to other reference frames using the methods:
The spherical element set defines the position and velocity of a Spacecraft with respect to the MJ2000 Earth Equator reference frame in spherical coordinates.
Spherical (Latitude, Longitude) The spherical element set defines the position and velocity of a Spacecraft with respect to the Earth Fixed in spherical coordinates, using the Earth's latitude and longitude in place of right ascension and declination. This element set is intended for use only when the Spacecraft's central body is Earth.
A representation of the Keplerian element set which removes the mathematical singularities at E = 0 and I = 0 degrees.
A representation of the Keplerian element set which removes the mathematical singularities at E = 0 and I = 0 and 90 degrees. This element set has a singularity at an inclination of 180 degrees.
Direct Equinoctial Reference Frame
Note: The equinoctial reference frame in FreeFlyer is the direct (prograde) equinoctial reference frame.
A representation of the Keplerian element set which removes the mathematical singularities at E = 0 and I = 0 and 90 degrees. This element set has a singularity at an inclination of 180 degrees.
BrouwerLyddane MeanThe BrouwerLyddane Mean element contains similar parameters to the traditional Keplerian elements, but represent values that are averaged over time rather than instantaneous values. These elements account for gravitational perturbations due to the J2J5 oblateness terms. This element set is intended for use only when the Spacecraft's central body is Earth. Mean elements are particularly useful when designing orbits that are sensitive to gravitational perturbations.
This element set is a mean element set similar to the BrouwerLyddane Mean element set, but accounts for only the J2 gravitational perturbation term. This element set is intended for use only when the Spacecraft's central body is Earth. Mean elements are particularly useful when designing orbits that are sensitive to gravitational perturbations.
The SGP4 derived elements are associated with the TwoLine Element (TLE) definitions. This element set is intended for use only when the Spacecraft's central body is Earth, and should be used with the SGP4 propagator.
See Also
