This model relates 2D Keplerian orbits to their orientation in 3D space. 2D orbits are expressed in the perifocal (PQW) coordinate system whose origin is the orbit’s focus and whose unit vectors lie in the plane of the orbit and correspond to the θ=0° direction (periapsis) and θ=90°. Quaternion algebra is used to rotate between PQW coordinates and other coordinates relative to either an equatorial plane (if the orbited body is a planet) or the ecliptic plane (if the orbited body is the Sun). These rotations correspond to the definitions of orbital inclination, longitude of ascending node, and so on.
When it is Earth that is orbited (the default), nodal and apsidal precession models enable Sun-synchronous and Molniya orbits to be calculated.
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