Attitude

The attitude of satellites is used in the determination of the antenna phase center's offset from the center of mass of the satellite. Antenna phase center locations are specified in the body frame of the satellite. The attitude specification provides the means of computing the rotation between the inertial frame in which the equations of motion are employed and the body frame.

Attitude Options
Option Description
Source

Specify the method of computing the satellite attitude:

  • AlignedConstrained - Use this option to specify that the satellite follows a prescribed profile based on orienting particular directions in the body frame with inertial directions related to the trajectory of the satellite.
  • File - Use this option to import a time-ordered list of attitude information in the STK external attitude (.a) format.
  • GPSBlockIIANominal - GPS satellite block IIA nominal attitude profile, used by MCS before introduction of Max Yaw Rate during eclipse season. This Z body axis direction is aligned with the direction to the center of the Earth. The X body axis direction is nominally constrained towards the direction of the Sun with zero yaw bias.
  • GPSBlockIIRNominal - GPS Satellite block IIR nominal attitude profile. The Z body axis direction is aligned with the direction to the center of the Earth. The -X body axis direction is nominally constrained towards the direction of the Sun with zero yaw bias.
  • GPSBlockIIAGYM95 - GPS satellite block IIA nominal attitude profile, used by MCS after introduction of Max Yaw Rate during eclipse season. This Z body axis direction is aligned with the direction to the center of the Earth. The X body axis direction is nominally constrained towards the direction of the Sun, with a yaw bias offset of 0.5 deg, but more complicated motion occurs during eclipsing and when the satellite passes nearly under the Sun direction (the noon turn).
  • During the noon turn, the yaw motion is constrained by a maximum yaw rate of 0.13 deg/sec. When the nominal profile dictates that a higher yaw rate is required to maintain the X axis in the direction of the Sun, the satellite yaws at the maximum rate and maintains the maximum yaw rate until the nominal attitude is recovered.

    At the beginning of an eclipse, the satellite transitions from its nominal yaw rate to the maximum yaw rate in a direction opposite the yaw bias. This transition happens at the maximum yaw acceleration of 0.00165 deg/sec^2. The satellite continues to yaw at the maximum rate until eclipse exit at which time two recovery paths back to the nominal attitude are computed. The first recovery path maintains the current yaw direction, while the second reverses the yaw direction at the maximum yaw acceleration.  The path requiring the least time to recover the nominal yaw profile is selected.

    A full description of the GYM 95 profile is given in:

    Bar-Sever, Y.E. "A New Model for Yaw Attitude of Global Positioning System Satellites",
    TDA Progress Report 42-123, November 15, 1995
  • GalileoIOV – Galileo in-orbit validation nominal attitude profile. The Z body axis direction is aligned with the direction to the center of the Earth. The -X body axis direction is nominally constrained towards the direction of the Sun with zero yaw bias. Note that it is the same as the GPS Block IIR nominal attitude profile.
  • QZSS – Attitude profile for vehicles in the Japanese QZSS for GPS augmentation. For beta angle (angle of the direction to the Sun out of the orbit plane) above 20 degrees, the Z body axis direction is aligned with the direction to the center of the Earth. The -X body axis direction is nominally constrained towards the direction of the Sun with zero yaw bias (same as GPS Block IIR and Galileo). For a beta angle under 20 degrees, the Z body axis direction is still aligned with the direction to the center of the Earth, but the –Y body axis is aligned with the positive orbit angular momentum direction.
  • Spinning - Use this option to specify a spinning attitude where the spin axis is fixed in both the satellite body fixed reference frame and in the central body inertial reference frame. This profile is mainly useful for simulation purposes since the associated parameters cannot currently be estimated.
Filename Name of the attitude file in STK *.a format to read for specification of attitude information when the Source is specifed as File.

StartTime

Time of earliest data in the attitude file, displayed when the Source is specified as File.
StopTime Time of latest data in the attitude file, displayed when the Source is specified as File.
BodyAlignmentVec

Specify a direction in the body frame, via a basis vector, to be aligned with the selected InertialAlignmentVec (below) when the Source is specified as AlignedConstrained. The alignment vectors will be perfectly aligned in the computed attitude.

InertialAlignmentVec

Specify a direction in the inertial frame, via a trajectory based vector, to be aligned with the selected BodyAlignmentVec (above) when the Source is specified as AlignedConstrained.Options for inertial directions are:

  • Radial direction - along the satellite position vector
  • Intrack - in the orbit plane, perpendicular to radial
  • Crosstrack - perpendicular to the orbit plane in the RxV sense
  • Tangential - along the inertial velocity direction
  • Intrack ECF Vel - perpendicular to the radial direction, towards the central body fixed velocity direction
  • Tangential ECF Vel - along the central body fixed velocity direction
BodyConstraintVec Specify a direction in the body frame, via a basis vector, to be constrained in the direction of the selected InertialConstraintVec (below) when the Source is specified as AlignedConstrained. The constraint vectors will be aligned as closely as possible in the computed attitude.
InertialConstraintVec Specify a direction in the inertial frame, via a trajectory based vector, to be constrained towards the selected BodyConstraintVec (above) when the Source is specified as AlignedConstrained. Options for inertial directions are the same as those for InertialAlignmentVec (above).
BodySpinAxisDir

Specify a direction in the body frame, via a basis vector, about which the satellite will spin, in a right handed sense, when the Source is specified as Spinning. This direction will be aligned with the inertial direction of the spin axis as specified via InertialSpinAxisRA and InertialSpinAxisDec.

InertialSpinAxisRA

Specify the right ascension of the inertially fixed spin axis when the Source is specified as Spinning.

InertialSpinAxisDec

Specify the declination of the inertially fixed spin axis when the Source is specified as Spinning.

SpinRate

Specify the rotation rate of the satellite when the Source is specified as Spinning.

InitialSpinOffset

The spin offset is an angular measure of the difference between the satellite orientation at the InitialSpinOffsetEpoch from the orientation achieved by orienting the spin axis. The latter is defined by using a yaw-pitch-roll sequence that achieves the desired spin axis orientation when yaw is set to 0.

InitialSpinOffsetEpoch

Epoch at which the InitialSpinOffset applies.

CenterOfMassInBodyFrame

Cartesian coordinates of the satellite center of mass in the body frame of the satellite.

ODTK 6.5