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Periods of anomalous pointing during the Herschel mission due to star-tracker switch-overs and lack of aberration correction.

Rationale

The pointing data of several ODs are affected by offsets due to two causes:

  1. Switch from the main star-tracker (STR1) to the backup unit (STR2) either driven by the autonomous Failure Detection, Isolation and Recovery (FDIR) or commanded from ground for testing and calibrating the backup unit. This produces an offset of some 15 arcsec between the desired (nominal) and actually commanded coordinates.
  2. Incorrect upload of the Spacecraft Velocity Vector (SVV) to the star-tracker (STR) resulting in a lack of light aberration correction in the coordinates of the STR guide stars. This produces a variable error in the range 0 - 20 arcsec between the desired (nominal) and actually commanded coordinates.

NEW In HCSS 13.0, the auxiliary processor that creates the pointing product was updated to automatically check for those events and apply the corrections whenever appropriate. However, this was only implemented for the "simple-corrected" attitude (column simpleCorrFilterQuat) and not for the "gyro-propagated" one (column filterQuat). Hence, the astrometry of the products retrieved from the archive (processed with the gyro-propagated attitude) are still affected by the problems outlined above.

If you have observations in one of the ODs referred below please contact the HSC Helpdesk for more information and support. Please refer to the HSC Helpdesk web link

The problem was been addressed in HCSS 14.0 . The tests performed indicated that the STR misalignment was not properly handled in early ODs (<= 113) and has been fixed in HCSS 14.1. (see SPR HCSS-20990). However, some issues still exist for a small set of SPIRE parallel observations in OD 111 (investigation on-going). In case that you find any problem with the astrometry of your observation, please contact the HSC Helpdesk.

STR switch-overs

OD Duration (h)
44 0.0001
81 11.31
82 19.64
83 24.01
84 1.67
93 24.04
94 24.04
95 24.04
111 10.21
112 22.70
161 2.08
585 14.10
586 2.12
732 23.23
733 0.56
The full list of observations affected by STR switch-overs, including science, calibration and engineering ones can be found here.

SVV resets

OD Last_DPT_update_TC_time (UTC) First_SVV_update_TC_time (UTC) Duration (h)
399 2010-06-16T16:43:53.432 2010-06-16T23:25:47.000 6.698
427 2010-07-14T16:02:01.681 2010-07-14T22:31:16.000 6.487
448 2010-08-04T15:21:47.767 2010-08-04T22:06:17.000 6.741
480 2010-09-05T18:07:30.264 2010-09-06T01:04:48.000 6.955
499 2010-09-24T17:51:15.752 2010-09-25T00:49:03.000 6.963
543 2010-11-07T17:15:11.854 2010-11-08T00:09:32.000 6.906
1308 2012-12-11T14:41:58.644 2012-12-11T19:56:45.000 5.246
The full list of observations affected by SVV resets, including science, calibration and engineering ones can be found here.

-- Miguel Sanchez - 2015-12-15

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Topic revision: r11 - 2016-04-12 - MiguelSanchez
 
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