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Image name: Orbit_Pointing_Variation.png (click image to enlarge)
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Date submitted: 11-Nov-1995
L. Acton 8-Nov-95
(Rev. 11-Nov-95)
ORBITAL VARIATION OF YOHKOH POINTING & COMPUTATION OF TERMINATOR OFFSET
This figure displays the variation of Yohkoh pointing over a 3 day period
during which no offpoints took place. The data were derived from running
SXT_CEN on the pfi roadmap structure for the period:
5-NOV-95 18:21:53 to 7-NOV-95 22:24:45
This effect has been known for some time. Note that the peak to peak E-W
(X) variation of about 3-4 FR pixels is substantially smaller than the N-S
(Y) variation of 8-10 FR pixels.
The purpose of this analysis was to compute a bias to include in the
program PLOT2OFFSET which is used to determine offset pointings for
terminator images. Because terminator images are always taken at the end
of a orbital day they have a systematic offset from the average pointing --
yet they are referenced to the average pointing through LASTSFD which may
be acquired at any time-to-FMS position. Another approach would be to
choose reference SFDs just before sunset. However, because of other
sources of random pointing error, and the need to change tested software, I
think it is better to include this systematic offset in the pointing
determination.
Here are the average values of these pointing data (FR pixels).
X Y
Tim2fms<60 (1251 cases) 537.430 593.183
Tim2fms>60 (27 cases) 537.153 588.262
Terminator offset 0.277 4.921
The difference in X is small enough to ignore. The Y offset is nearly 5
pixels and should be accommodated in the software.
