Frames and conventions

Body frame of the models

All models share one body frame:

Axis	Direction
+Z	Toward the Earth (nadir deck)
+Y	Along the solar-array axis; the arrays turn about +Y
+X	Completes the right-handed frame; solar cells face +X at zero array angle

Exceptions follow the shape of the object:

• rocket stages and bodies of revolution (including DISCOS cylinders and cones): long axis along +X, nozzle toward −X;
• ISS and Tiangong: local vertical, local horizontal frame — +X along the velocity, +Y along the truss or the laboratories, +Z nadir;
• Hubble: tube along +X, aperture forward.

The Euler angles of the attitude apply in the order yaw (Z), pitch (Y), roll (X).

Inertial frame

The simulator works in the geocentric equatorial frame of date (mean equator and equinox of date, nutation neglected). Earth rotation uses the Greenwich mean sidereal time (IAU 1982). Data given in other frames is converted:

Frame	Conversion
EME2000, ICRF, GCRF	Precession to the date (IAU 1976 angles); frame bias neglected
TEME, TOD, MOD	Used as they are (a few arcseconds of difference)
ITRF, EFG	Rotated by the sidereal time; velocity + ω × r

Units and time

• Distances in km (positions) and m (models); angles in degrees in the interface.
• Times in UTC. UT1 − UTC and polar motion are neglected.
• Azimuth from north toward east.