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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.