Card 6l - Additional Orbital Parameters ORBADD

L

L is the code ORBADD (or 23).

STARTANGLE

STARTANGLE is the initial orbit position in degrees.

STARTANGLE is measured in the orbit plane, starting from the periapsis, and is positive in the direction of the motion of the spacecraft.

For ORIENT=SURFACE, STARTANGLE is the number of degrees the planet has rotated from local midnight for the start of the orbit. For a 6:00 AM start STARTANGLE should be 90 degrees.

ENDANGLE

ENDANGLE is the final orbit position in degrees.

ENDANGLE is measured in the orbit plane, starting from the periapsis, and is positive in the direction of the motion of the spacecraft.

For ORIENT=SURFACE, ENDANGLE is the number of degrees the planet has rotated from local midnight for the end of the orbit. For a 5:00 PM end ENDANGLE should be 255 degrees.

TIMEI

TIMEI is the TIME parameter value at STARTANGLE.

If ENDANGLE–STARTANGLE = 360, TIMEI must be 0, i.e. a full orbit must start at TIMEI = 0.

ANGX, ANGY, ANGZ

ANGX, ANGY, ANGZ are the angles in degrees which define the orientation of the vehicle coordinate system (as defined by the ORIENT on Card 6k) at TIME = TIMEI with respect to the global (model’s) coordinate system.

If the two coordinate systems coincide and are stationary relative to each other, i.e. DANGX, DANGY, DANGZ are blank, then ANGX, ANGY, ANGZ, and ORDER must also be blank.

ORDER

ORDER determines the order of the rotations by which the vehicle coordinate system is rotated into the global (model's) coordinate system. ORDER may be the codes: XYZ, XZY, YXZ, YZX, ZXY, or ZYX (or 1 to 6), defining the axes of rotation in the order the rotations are performed.

For example, if ORDER is XZY, to arrive at the global coordinate system you must first rotate counterclockwise about the vehicle coordinate system's X axis by ANGX, then next about the new Z axis by ANGZ, then about the new Y axis by ANGY.

DANGX, DANGY, DANGZ

DANGX, DANGY, DANGZ are the rotational angle increments in degrees for ANGX, ANGY, ANGZ after one complete orbit.

If these are blank or zero, the vehicle coordinate system is fixed with respect to the global coordinate system. For example, the rotation AX(t) about the X axis at TIME = t is computed with:

OMEGASTAR

OMEGASTAR is the angle of the vernal equinox in degrees from the Reference Line. The vernal equinox is a vector along the line of intersection of the ecliptic and equatorial planes, pointing in the direction where the sun crosses the equator going Northwards.

PHISTAR

PHISTAR is the angle between the projection of the star vector on the equatorial plane and the angle of the vernal equinox measured counter clockwise when looking southwards.

THETASTAR

THETASTAR is the angle between the vector pointing towards the star and the North vector.

Code example

In this example, the spacecraft is in geosynchronous (SMAR = 6.625, PERIOD = 24) equatorial (ORBINCL = 0) orbit, during equinox (SUNDECL = 0). Solar eclipse calculations with shadowing (L = ORBSALL), are performed every 2 hours (NINT = 12), plus just before and just after entering and exiting eclipse. No Earth view factor or albedo factor calculations are performed (ALB = 0). The vehicle coordinate system's X axis points towards the Earth, its Z axis North, and its Y axis West (ORIENT = PLANET). The spacecraft's Z axis points away from the Earth, its Y axis is positive in the direction of the motion (towards East), and its X axis points southward. To get to the model's coordinate system, the vehicle coordinate system is rotated 180 degrees counterclockwise about its Z axis, then 90 degrees counterclockwise about its new Y axis.

 ORBSALL PLANET 0 12 0 0 0 6.625 180 0 24.0      $ CARD 6K EXAMPLE
 ORBADD 0 360 0 0 90 180 ZYX            $ S/C ORIENTATION IN ORBIT