Coordinate Systems
One of the first requirement to describe a spacecraft in orbit is to define the different coordinate systems to work with:
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The equatorial coordinate system (ECS) has its origin at the center of the planet and its first axis pointing towards the Vernal point on the celestial sphere. The third axis is parallel to the rotation of the planet, pointing towards North. The second axis completes a right-handed coordinate system.
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The planetary coordinate system (PCS) is a spherical coordinate system, in terms of latitude and longitude with the same center as the ECS. It is assumed that planets are perfect spheres. The cartesian PCS can also be used, with the first axis pointing to longitude 0, latitude 0, the second axis pointing to longitude +90, latitude 0, and the third axis to the North pole (parallel to the third axis of the ECS). It is rotating in time about the third axis related to the ECS.
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The spacecraft coordinate system (SCS) is a cartesian coordinate system with its origin at the location of the spacecraft in orbit. The primary and secondary axis are defined by the user in the Spacecraft Orientation feature. The secondary axis is defined as the projection of the user-specified direction onto the plane perpendicular to the primary axis. The third axis completes the right-handed coordinate system. The selection of these axis can change during orbit.
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The geometry coordinate system is the material frame, where the model exists. All vectors describing the thermal loads are to be transformed into this system.
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The orbital plane coordinate system is parallel to the orbital plane. The first axis points from the planet center to the periapsis and the spacecraft rotates about the third axis, which is normal to the plane of orbit. The elliptical orbit is computed in this plane, and then rotated into the ECS.