Predefined Attitude Profile Definitions

This topic provides technical notes that define and describe all predefined vehicle attitude types that are installed with STK. See Predefined Types for a table that lists which vehicle objects can use each of the predefined attitude profile types.

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GPS

Attitude profiles for GNSS satellites (GPS, Galileo, QZSS)

Block IIA. This attitude profile accounts for the complex motion of Block IIA satellites. The Z body axis is aligned with the direction to the geodetic center of the Earth. The X body axis is nominally constrained toward the direction of the Sun, with a yaw bias offset of 0.5 deg. However, more complicated motion occurs during eclipsing and when the satellite passes nearly under the Sun direction, know as "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/sec². 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.
For a full description of the GYM 95 profile, see:
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 profile has Z body axis aligned with the direction to the geocentric center of the Earth. It constrains the -X body axis nominally toward the direction of the Sun with zero yaw bias.

Block IIA Nominal. This profile has the Z body axis aligned with the direction to the geocentric center of the Earth. It constrains the X body axis nominally toward the direction of the Sun with zero yaw bias.

Block IIR: This attitude profile follows the Block IIR nominal attitude profile with the exception of noon and midnight turns. During both the noon and midnight turn, the yaw motion is constrained by a maximum yaw rate of 0.2 deg/sec. The attitude follows the nominal profile until the maximum turn rate is reached. Slewing continues at the maximum turn rate until the yaw angle catches up with the nominal yaw angle, at which point the nominal profile resumes. For a full description of the Block IIR attitude profile, see:
Kouba, J., "A Simplified yaw-attitude model for eclipsing GPS satellites", GPS Solutions, 13:1-12, 2009.
Galileo IOV:. These are in-orbit validation vehicles in the Galileo GNSS, built by the European Union and ESA. The attitude profile is the same as GPS Block IIR.
Block IIF Nominal This profile has the Z body axis aligned with the direction to the geocentric center of the Earth. It constrains the X body axis nominally in the direction of the Sun with zero yaw bias.
Block IIF This attitude profile follows the Block IIF nominal attitude profile with the exception of noon turns and during special low beta-angle eclipse behavior. During the noon turn, the yaw motion is constrained by a maximum yaw rate of 0.11 deg/sec. During low beta-angle eclipse periods (|b| < 8 degrees), the yaw rate is chosen such that the yaw angle catches up with nominal yaw angle at the end of the eclipse period, at which point the nominal profile is resumed. Eclipse operations apply a yaw bias of magnitude 0.5 degrees and beta angle sign determination based on a U.S. Air Force memorandum (see reference below). The Block IIF attitude profile mainly follows one described by Dilssner. However, the computed yaw rates for each specific eclipse period are as described in Weiss et al.
Beta angle is defined as the angle of the direction to the Sun out of the orbit plane.
U.S. Air Force Memorandum in response to 14 AF/A30 Tasker 14AF_2011-01-20.
Dilssner, F., "GPS IIF-1 Satellite Antenna Phase Center and Attitude Modeling", Inside GNSS, September 2010.
Weiss, J.P., Bar-Sever, Y., Bertiger, W., Fesai, S.D., Harvey, N., Sibthorpe, A., "Modeling and Characterization of the GPS Block II/IIA/IIF Attitude", IGS Workshop, 2012.
QZSS. These are vehicles in the Japanese QZSS system used for GPS augmentation. For beta angle above 20 degrees, the Z body axis is aligned with the direction to the geocentric center of the Earth. The profile constrains the -X body axis nominally toward the direction of the Sun with zero yaw bias (same as GPS Block IIR and Galileo). For beta angle under 20 degrees, the Z body axis is still aligned with the direction to the geocentric center of the Earth, but the -Y body axis is aligned with the positive orbit angular momentum direction.

Precessing Spin

This type of attitude profile describes attitude motion as a composition of two rotations: spin and precession.

Spin occurs about a body-fixed direction specified by the Body Spin Axis of the vehicle's body-fixed axes. The spin axis itself precesses, executing a coning motion with constant cone angle specified by the Nutation Angle, about a direction fixed in the Reference Axes (specified by the Precession Axis direction of the Reference Axes). Both spin and precession occur at constant rates. For example, when the Body Spin Axis is the Z-axis and the Precession Axis is Z-axis of ICRF, and the nutation angle is 20 deg, then the Body Z-axis precesses about the ICRF Z-axis with a constant cone angle of 20 deg and the body spins about its Body Z-axis. Both spin and precession include specifications for their rotation Rates and initial Offsets at the specified Epoch. Rate and offset definitions are similar to those described for the spinning attitude profile type.

The orientation of the Body Axes is found from the Reference Axes by composing two rotations: precession then spin. Precession occurs by rotating the BodyPrecessionRef axes (defined below) about the precession rotation axis (both are fixed with respect to the Reference Axes); spin occurs by rotating about the Body Spin Axis.

The BodyPrecessionRef Axes are found as follows. The Reference Axes are rotated into Precession Axes using a YPR sequence where the YPR angles represent the direction of the Precession Axis. The Z-axis of the Precession Axes aligns with the Precession Axis. Second, the Precession Axes are then rotated about its Y-axis by the Nutation angle (i.e., cone angle). The resulting Cone Axes has its Z-axis lying on the cone. Third, the Cone Axes are rotated into BodyPrecessionRef Axes using a RPY sequence where the YPR angles are the negatives of the YPR angles representing the direction of the Body Spin Axis. The Body axes would coincide with the BodyPrecessionRef axes at the Epoch specified for the Precessing Spin attitude if the precession offset and spin offset were both 0.0 deg. The BodyPrecessionRef axes do not rotate with respect to the Reference Axes.

The precession rotation axis is the Precession Axis direction of the Reference Axes, expressed in BodyPrecessionRef Axes. Define the precession angle as the precession offset plus the precession rate times the time since Epoch; similarly, the spin angle is defined as the spin offset plus the spin rate times the time since Epoch. The Body axes are found by first rotating the BodyPrecessionRef axes by the precession angle about the precession rotation axis and then subsequently rotating by the spin angle about the Body Spin axis.

This type of attitude can represent torque-free motion of the axisymmetric rigid body. To do that, the precession rate, spin rate, and nutation angle must be related to the ratio of the axial and transverse moment of inertia via the following formula:

(Axial Moment of Inertia) / (Transverse Moment of Inertia) = (Precession Rate) * cos(Nutation Angle) / (Spin Rate)

If this relationship is satisfied, then the angular momentum vector is aligned with the inertially fixed precession axis. You can enter the elements of the inertia matrix on the Mass property page.

The illustration below shows an example of this attitude type as displayed in the 3D Attitude Graphics window. It uses an attitude sphere and components created with the Vector Geometry Tool.

Variable Time Slew

The Variable Time Slew profile performs an attitude motion that slews between the end attitude of the preceding segment and the initial attitude of the next segment. If the slew is the first segment of a multisegment attitude profile, then it assumes that the slew is from an inertially fixed attitude. If the slew is the last segment, then it assumes that the slew is to an inertially fixed attitude. Select a Slew mode from the menu. The modes are described in the table below.

Mode	Description
Fixed Rate	The time required to change attitude will be calculated by applying a fixed slew rate, limited to the Maximum slew time. From the Slew segment timing menu, select an event within the window of opportunity to trigger each slew, or select Optimal to change attitude whenever the slew can be performed most efficiently. Define the maximum slew rate by entering a maximum overall Magnitude. You can constrain the slew rate in specific directions by selecting one or more Per Axis rates and defining separate maximum rates for those axes.
Constrained	STK calculates the time required to change attitude by applying a constrained slew rate and acceleration, limited to the Maximum slew time. From the Slew segment timing menu, select an event within the window of opportunity to trigger each slew, or select Optimal to change attitude whenever the slew can be performed most efficiently. From the Slew type menu, select to use a second- or third-order spline as the interpolation method of the slew's data points. Define the maximum slew rate and slew acceleration by entering for each the maximum overall Magnitude. You can constrain the slew rate or acceleration in specific directions by selecting one or more Per Axis rates and defining separate maximum rates and accelerations for those axes.

Mode

Description

Fixed Rate

The time required to change attitude will be calculated by applying a fixed slew rate, limited to the Maximum slew time. From the Slew segment timing menu, select an event within the window of opportunity to trigger each slew, or select Optimal to change attitude whenever the slew can be performed most efficiently. Define the maximum slew rate by entering a maximum overall Magnitude. You can constrain the slew rate in specific directions by selecting one or more Per Axis rates and defining separate maximum rates for those axes.

Constrained

STK calculates the time required to change attitude by applying a constrained slew rate and acceleration, limited to the Maximum slew time. From the Slew segment timing menu, select an event within the window of opportunity to trigger each slew, or select Optimal to change attitude whenever the slew can be performed most efficiently. From the Slew type menu, select to use a second- or third-order spline as the interpolation method of the slew's data points. Define the maximum slew rate and slew acceleration by entering for each the maximum overall Magnitude. You can constrain the slew rate or acceleration in specific directions by selecting one or more Per Axis rates and defining separate maximum rates and accelerations for those axes.