[Ibex-maneuver-engineering] Biased repointing vector for post-spindown

[Mark Tapley]

All,
At 6:09 +0000 6/16/11, fair-play at comcast.net wrote:
>I commented out all the lines in the orbit 129 STF e vector is
>
>0.042788,0.92168,0.385584

	I have generated another pointing vector to use after the spin-down.

	The objective is to point the spin axis at where the sun was 
2.5 days before the perigee happening 21-Jun at 14:43:31 Z.

	Using ibex_rotate on ena, I think that vector is

RA:	87.219
Dec:	23.378

which is the same as

+0.044536,+0.916826,+0.396795



[tapley at ena ~]$ ibex_rotate -o -r 87.219 -d 23.378 -w ibex-sun -t 
2011-06-19T02:43:31.000Z
Quaternion    +0.000000,+0.000000,+0.000000,+0.000000
  points axis  +1.000000,+0.000000,+0.000000
  towards ECI  +0.044536,+0.916826,+0.396795
  which is     R.A.  +87.219 Decl. +23.378
  missing      R.A.  +87.219 Decl. +23.378 (ibex-sun)
  error is     R.A.   +0.000 Decl.  +0.000
  for a total  0.000 deg


	To get this as the result, I think we need to tell ADC to point at

RA:	86.6211699309974
Dec:	23.076131027330046

which is the same as

+0.0542216, +0.918386, +0.391954

	A mathematica model showing what I think will happen is 
attached, both as a mathematica notebook and as a .pdf printout of 
the notebook.

	The first part of the notebook shows a model of our repoint 
to get to the Delta-V pointing. This should compare reasonably to the 
standalone simulation Jim Bobbett described in his "Notes ... " word 
document, Figure 3.
	Note that to get this to match, I used a Control-loop delay 
of 1.0 seconds (vs. the nominal single loop of 0.5 seconds). After 
talking with Jim, this seems somewhat reasonable, given filter 
(analog and digital) and processing delays.

	The second part of the notebook shows what should happen 
given the same ADC parameters but the target vector I suggest. The 
orange dot shows the desired final spin vector, the blue dots show 
the history of the maneuver, and the final line shows the error in 
the pointing at the conclusion of the maneuver.

	Comments are most welcome!
-- 
						- Mark     210-379-4635
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