GPS

GPS allows you to add the following additional GPS attitude profiles to an existing vehicle.

• GYM 95. This attitude profile describes that complex motion of the Block IIA satellites.  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

  

• Block IIR Nominal. 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.

  

• Block IIA Nominal. 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.