Card 6t - ORBDEF1 Major Orbit Parameters

This optional card defines the major orbit parameters for a spacecraft.

Parameter Value Description
ORBDEF1 ORBDEF1 (or 41) mnemonic
ID integer ID number
SHADOPT N1 No shadowing for some elements
SALL Shadowing for all elements
NALL No shadowing for all elements
PNAME MERCURY Planet Mercury
VENUS Planet Venus
EARTH Planet Earth
MARS Planet Mars
JUPITER Planet Jupiter
SATURN Planet Saturn
URANUS Planet Uranus
NEPTUNE Planet Neptune
PLUTO Planet Pluto
GENERIC Generic planet
MOON Moon
V1OPT NADIR vector1 points to the nadir
SUN vector1 points to the Sun
STAR vector1 points in the star direction
PVEL vector1 points in the velocity vector direction
PNORMAL vector1 points along the orbit normal
NORTH vector1 points North
SOUTH vector1 points South
V2OPT PVEL vector2 points close to the positive velocity vector direction
PNORMAL vector2 points close to the positive orbit normal direction
NORTH vector2 points close to the North direction
SOUTH vector2 points close to the South direction
NADIR vector2 points close to the nadir direction
SUN vector2 points close to the Sun direction
STAR vector2 points close to the star direction
FRONTGRP mnemonic or integer Element ID or group ID of front facing elements
REVGRP mnemonic or integer Element ID or group ID of reverse facing elements

SHADOPT

SHADOPT is the shadowing option to be used for the selected elements.

PNAME

PNAME is the planet name around which the spacecraft orbits. This field has the following effects:

  • For planets other than the Moon or Mercury, the dark side IR radiation value is assumed to be the same as the sunlight side radiation value. For the Moon and Mercury, an interpolation is performed between the sunlight and dark sides, based on the algorithm described on Card 9 PARAM PIRSUBSOLAR Card.
  • For the planet Earth, options exist to specify the time of the year when the orbit occurs. In addition, it is necessary to specify only either the sun declination angle or the sun right ascension angle, as opposed to other planets, where both need to be specified. The reason is that the maximum and minimum sun declination angles are known to be 23.44 degrees.

V1OPT

V1OPT is the flag for target1, the target with which vector1 is aligned. vector1 is the first of two vectors fixed with respect to the spacecraft (global) coordinate system needed to specify the alignment of the spacecraft with respect to the orbit.

The components of vector1 are specified on the ORBDEF5 Card.

If no rotation is specified on ORBDEF7, the spacecraft is so oriented that vector1 is always locked onto target1.

If a rotation is specified on ORBDEF7, vector1 is aimed at its target only at time = 0. At subsequent times, the spacecraft is rotated about the rotation axis by the rate specified on ORBDEF7.

V2OPT

V2OPT is the flag for target2, the target towards which vector2 is aimed. vector2 is the second of two vectors fixed with respect to the spacecraft (global) coordinate system needed to specify the alignment of the spacecraft with respect to the orbit.

The components of vector2 are specified on the ORBDEF5 Card.

The spacecraft will be aligned to minimize the angular deviation between vector2 and the spacecraft-to-target2 vector. However, it may not be achievable to have perfect alignment between the spacecraft-to-target2 vector and vector2. Then, the spacecraft will be oriented with respect to the orbit that:

  • The spacecraft-to-target2 vector always lies in the plane of vector1 and a vector2.
  • The dot product of the spacecraft-to-target2 vector and vector2 is always positive. If this is not possible, an error message will be issued.

FRONTGRP

FRONTGRP is the single element ID or the group name of the front facing elements for which the orbital fluxes are to be calculated.

This field should be blank if the SHADOPT is NALL or SALL.

This field should be 0 if only a reverse group is selected.

REVGRP

REVGRP is a single element ID or the group name of the elements for whose reverse sides orbital fluxes are to be calculated.

The field should be blank or 0 if no reverse sides are to be included in the orbital flux calculations.

If both FRONTGRP and REVGRP are blank or 0, and SHADOPT is S1 or N1, then orbital positions are computed, in case the orbit is referenced in an articulation object, but orbital heating is not taken into account.

Notes

All fields are required on all the Cards, except for the FRONTGRP and REVGRP on ORBDEF1, which may be blank. If the value in a field is to be ignored, specify 0.

A simpler alternate method is available in Cards 6k and 6l.

It is helpful to reference the diagram in Card 6l - Additional Orbital Parameters ORBADD when perusing these cards.

ID is the orbit and attitude modeling request number. ID is an integer value common to all associated ORBDEF Cards.