Spacecraft Attitude, Ephemeris and Coordinate Systems
    6.1  Spacecraft Attitude Control System (ACS)
        6.1.1  Inertial Reference Unit (IRU)
        6.1.2  Two-dimensional Fine Sun Sensor (TFSS)
        6.1.3  Star Tracker (STT)
        6.1.4  Geomagnetic Sensors (GAS)
        6.1.5  HXT Aspect Sensor (HXA)
        6.1.6  Attitude Determination Software (ADS)
    6.2  ACS Performance
        6.2.1  Pointing Accuracy
        6.2.2  Spacecraft Jitter
        6.2.3  Sensor Problems
    6.3  Instrument and Spacecraft Co-alignment
    6.4  Spacecraft Ephemeris
    6.5  Contact Persons
    6.6  References


6  Spacecraft Attitude, Ephemeris and Coordinate Systems

6.1  Spacecraft Attitude Control System (ACS)

The attitude control system uses momentum wheels, magnetic torquers, and control-moment gyros as the actuators. As the attitude sensors, two sun sensors and a star tracker, as well as geomagnetic sensors, are available for determining the spacecraft pointing relative to the direction of the sun and to the ecliptic plane, respectfully. An inertial reference unit comprising four gyros detects changes of attitude with time. HXA??

6.1.1  Inertial Reference Unit (IRU)

Four gyros are present on the Yohkoh spacecraft, one each in X, Y and Z and the fourth one in a 45 degree skew position. The IRU values are used by the IDL software to create the attitude (ATT) database.

6.1.2  Two-dimensional Fine Sun Sensor (TFSS)

More...

6.1.3  Star Tracker (STT)

Used to define spacecraft roll. Tracks star Canopus. Eclipses...

6.1.4  Geomagnetic Sensors (GAS)

What are these??

6.1.5  HXT Aspect Sensor (HXA)

The HXT aspect sensor (HXA) consists of two 1-dimensional CCD arrays located in a cross, 90 degrees to each other. The cross is rotated approximately 45 degrees relative to the SXT and HXT coordinate system. The HXA values are used by the IDL software which creates the attitude (ATT) database. There are fiducial marks which can reduce the resolution of the HXA results if the limb coincides with the fiducial mark.

6.1.6  Attitude Determination Software (ADS)

The attitude determination software (ADS) runs on the FACOM mainframe at ISAS and has been found to have noise problems (probably due to the TFSS). The ATT IDL database was created to replace the ADS results, which uses the HXA and IRU raw data. The ATT database does copy the ADS roll results into the database.

6.2  ACS Performance

6.2.1  Pointing Accuracy

Table of values

6.2.2  Spacecraft Jitter

Due for movement of filter wheel.

6.2.3  Sensor Problems

In Sep-93, it was decided to switch to the X, Y and skew gyros (instead of X, Y, and Z) because of the high drift rate of the Y gyro. Stil believed to be functional??

The two dimensional fine sun sensor had a noise problem shortly after launch and it's usefulness was greatly reduced.

Loss of spacecraft Roll... See appendix ?? for list of dates...

6.3  Instrument and Spacecraft Co-alignment

Summery of paper by Wuelser et al. (1998)

6.4  Spacecraft Ephemeris

Ranging. ORB and FEM files.

6.5  Contact Persons

If you have any question about spacecraft attitude, ephemeris and coordinate systems, contact:

6.6  References


Converted at the YDAC on Oct 4, 2004
(from LaTEX using TTH, version 1.92, with postprocessing)