PLATFORM DEFINITIONS AND CONSTRAINTS

The purpose of this document is to clearup any potential misunderstandings between the SPOF, FOT, and the Platform project scientists concerning the operation of the despun platform and the development of the platform pointing plans.

Definitions:

The boresight direction lies in the platform plane and is in the direction in which VIS,UVI, PIXIE and SEPS are pointed. SEPS also points in the anti-boresight direction.
The platform normal is the unit vector normal to the plane of the platform, it is anti-parallel to the spin axis.
The pitch angle of a vector is the angle of the vector projected into the platform plane measured from nadir, with positive sense determined by the right hand rule about platform normal(anti-parallel to the spin axis).
The latitude of a vector is the complement of the angle between the platform normal and the vector.
The working definition of the north pole is 3.4 anti-sunward of the north magnetic pole, and working definition of the south pole is that of the south magnetic pole.
The Holzworth-Meng auroral oval at a Qindex of 6 was used to model the auroral oval (GRL, Sept. 75, vol. 2, no. 9, pg. 377).

Operational Constraints for Platform Slew (rotation) Maneuvers:

The slew maneuvers must be specified in units of .1 degree increments.
At least 90 seconds must occur between adjacent slew maneuvers.
When a slew manuever is performed the boresight is rotated through the minimun absolute angular distance between the starting point and the end point. Therefore if a rotation is required of greater than 178 degrees in order to avoid sun danger the rotation has to be broken up into smaller rotations separated by at least 90 seconds.
Approximately 2 minutes are required for the platform to settle after a slew maneuver of less than 20 absolute degrees. The maximun setting time is no more than 6 minutes for slew maneuvers greater than 20 absolute degrees. The FOT has generated a graph of setting time versus change in slew angle for slew maneuvers.
The actual maneuver requires about 2 seconds.
The VIS team has requested that no slew maneuvers be made within +/- 10 minutes of perigee.

Instrumental constraints

The current criteria that the SPOF uses for sun danger is that the sun direction can never lie in the following angular region with respect to the boresight direction.

+/- 20 degrees out of platform plane (cross track).
+/- 24 degrees in the platform plane (long track).

The SEPS field of view is in both the bore and anti-bore sight directions. SEPS can never accumulate more than 100 minutes of sun exposure over the previous 100 minutes. Sun exposure occurs when the sun lies within the following angular region with respect to bore or anti-bore sight direction.

+/- 12 degrees out of platform plane (cross track).
+/- 24 degrees in the platform plane (long track).

If SEPS accumulates 100 minutes of sun exposure within the previous 100 minutes , SEPS must not be exposed to the sun for the next 100 minutes (but can be slewed through the sun if the maneuver is performed rapidly (few seconds)).

The VIS sun danger zone lies within the following angular region.

+/- 20 degrees out of platform plane (cross track).
+/- 20 degrees in the platform plane (long track).

The VIS fov can be slewed through the Moon without taking any protective measure, however if the moon lies in the VIS fov outside of slew maneuvers special filters are required. The VIS fov is pyramidal. The VIS Earth camera's center of fov is shifted ~2 degrees out of the platform (spin) plane away from the spin axis. The VIS visible camera's center of fov lies in the platform plane in the boresite direction.

+/- 10 degrees out of platform plane (cross track).
+/- 10 degrees in the platform plane (long track).

The UVI sun danger zone lies within the following angular region.

+/- 20 degrees out of platform plane (cross track).
+17,-23 degrees in the platform plane (long track).

The UVI fov is conical with a halfwidth of 4 degrees. After switching to side B, on day 293 of 1996, the UVI's center of fov is now shifted 1.46 degrees out of the platform (spin) plane towards the spin axis.

PIXIE no sun danger constraints.

Scott Boardsen, boardsen@mail630.gsfc.nasa.gov