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Yohkoh Temperature and Pointing Drift

Image name: Attitude_vs_Temperature.png (click image to enlarge)
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Date submitted: 03-Mar-1995


                             L. Acton

Since launch the front end of SXT has undergone temperature variations
related to orbital parameters plus a long term increase.  This variation
is illustrated in the LH panel.

There are also short and long term variations between the commanded
pointing of Yohkoh and the actual pointing as revealed by the position
of suncenter on the SXT images and the HXA limb sensors.  [These two
ways of measuring pointing agree to an accuracy of one or two SXT pixels
and, with corrections, are better than one SXT pixel.  (J.-P.  Wuelser)]
The difference between the actual and commanded pointing the N-S and E-W
directions are plotted in the lower two panels of the middle section. 
Note that this attitude discrepancy in the N-S direction is about four
times larger than it is in the E_W direction. 

It is obvious that the sense of the pointing discrepancies are
correlated in opposite directions for the 40-day orbital thermal peaks
and the annual and long term thermal trends.  This accounts for the very
great scatter in the correlation diagrams of the RH panel -- although
the basic trends can be clearly seen.  The solid lines show the effect
of smoothing both the temperature and pointing data with a 50 point
(roughly 100 days) smoothing width. 

The dominant periods in the temperature data are close to

	 25 days
	 44 days
	179 days
	323 days

although I would not trust the accuracy of these numbers until the power
spectrum analysis is more carefully done with more data points than the
632 of this analysis. 

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