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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel Pointing : Status, Impact on Map-Making,
and Coming Improvements Hervé Aussel
AIM Paris-Saclay
reporting on PACS-ICC investigations
H. Feuchtgruber, M. Tuttlebee
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel pointing accuracy
• There are two types of “pointing accuracy”:
• How accurate is Herschel ability to point toward a commanded attitude ?
• How accurate is our determination of where Herschel is pointing at any given time ?
• Only the second one matters for map making.
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Pointing Accuracy Impact in Map Making
• Astrometric issues
• Signal to noise loss
• Artifacts on bright sources
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Cross-like artifacts in inversion methods
• Scan and cross-scan over a bright gaussian source
• On board compression: 4 frames
• Perfect pointing accuracy
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Cross-like artifacts in inversion methods
• Scan and cross-scan over a bright gaussian source
• On board compression: 4 frames
• 2.45” 1 σpointing
offsets between scans
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Components of a detector pointing
• The accuracy of the position of a detector on the sky depend on four elements:
• Herschel attitude and velocity accuracy
• Instrument position in Herschel focal plane
• Time synchronization between satellite and instrument clock
• Instrument focal plane geometry
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel Attitude Accuracy
• ESA uses three different types of errors to describe how accurate the pointing (in the first sense) is:
• APE : Absolute Pointing Error
• RPE : Relative Pointing Error
• SRPE : Spatial Relative Pointing Error
• The physical origin of SRPE and APE is the same, so APE is the error that we are getting when making maps larger than a few arcmin.
• Remember that all errors are 1 σ, so the odds are good that your actual error is larger.
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel pointing products
• The attitude we get in the pointing product has been for a long time just the attitude derived on board by the ACMS
Thursday, January 31, 2013
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Place & DateH. Aussel Meeting Name
Gyroscopes
Herschel ACMS
Star Tracker
Real Time Kalman Filter
Synchronization Synchronization
Herschel Attitude
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel Star Tracker (STR)
• Herschel uses 1 STR from Galileo Avionica
• STR is a small CCD camera with its software.
• Can figure out the direction it is pointing to.
• Can track 9 stars at 4Hz or 18 stars at 2 x 2Hz
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel STR
• Wide field: 16º• Individual star positions
are measured with an accuracy of 2 to 4”
• Best theoretical accuracy is 0.9” (1σ) for direction and 6.5” (1σ) for roll
• Requirement are 1” (1σ) for direction and 10” (1σ)
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel Gyroscopes (GYR)
• Herschel uses 4 Gruman-Northtop Space Inertial Reference Units
• Can measure a rotation of the spacecraft very accurately: 0.0005”/s
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel GYR
• GYR are subject to drift: even when the spacecraft is not moving, they register a rotation.
• This bias is not constant, but varies slowly.
0 500 1000 1500t (s)
3.60•10-3
3.65•10-3
3.70•10-3
3.75•10-3
3.80•10-3
3.85•10-3
3.90•10-3
Angl
e (ra
d)
Fitted Bias A = 1.6227108e-07rad / s
ACMS Bias A = 1.1675929e-07rad / s
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel ACMS
• The ACMS job is three-fold
• synchronize the output of the STR and GYR subsystem
• determine the attitude of Herschel and the GYR biases
• command trajectory adjustements to stick to the planned one.
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel ACMS
• The state vector is the attitude quaternion plus the GYR bias
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel ACMS
• The Kalman filter is used to determine the GYR biases from the STR, and increase the accuracy of the STR from the GYR
• The gains of the Kalman filter give a heavy weight on the STR, so that the satellite is immune to GYR unknown drift, but sensitive to STR glitches.
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Herschel STR output
• Star positions measured by the STR during a long staring observation
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Speedbumps.
• In the beginning of the mission, there were a lot of hot pixels in the CCD STR, causing some erroneous position measuements
Planned trajectory
STR measure
Real trajectory
Measured Trajectory
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Speedbumps
• Because of the high weight on STR vs. GYR, the Kalman filter incorrectly change the GYR bias instead of filtering the STR data
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
A provocative statement...
• The ACMS is continuously calibrating a very accurate sensor (the GYR) with a poorly accurate one (the STR)
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Solving the speedbumps
• In order to lower the number of STR CCD hot pixels, it was decided to lower its operating temperature from 20ºC to -10ºC from OD 320 onward
• At the time, nothing was changed in the ACMS parameters
• The PACS-ICC, given the difficulties of having a source in the slit, launched a dedicated analysis of the pointing
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Calibrating the STR
• A complete analysis of all the ACMS telemetry was done
• Resulted in corrected distortion maps of the STR CCD
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Analysis of the STR star catalog
• The stars used for guiding were checked.
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
The timeline of pointing accuracy
Period (OD) Info APE
1-319 CCD 20ºC 2”
320-761 CCD -10ºC 2.4”
762-865 1D corr. 1.5”
866-1010 2D corr. 1.0”
1011-1031 Full dist. 0.9”
1032- clean cat 0.8”
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Pointing Reconstruction
• The improved calibration that are now used on board are also used to derive a correction to the pointing during OD 320-1011.
• This correction is a bias, applied to the on board derived attitude
• The pointing we get is still using on baord GYR info with bias derived from the wrong STR info
• There is a new ACMS product containing all the STR and GYR telemetry
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
The ACMS product
• Contains the STR star measurements at 1 Hz• Contains the STR star J2000 positions
• The full raw GYR data at 4 Hz
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Next steps
• The PACS-ICC is developing a new approach using a least-square estimation of the pointing.
• We use a full correction of the STR output, taking into account sub pixel CCD distortions (not implemented on board)
• Uses the 1 Hz STR and 4 Hz GYR
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
An almost linear problem
• The measure:
• The attitude:
• The system:
!yi = ("STR, #STR,$STR, %1, %2, %3, %4)T
!y = H(", #,$, b1, b2, b3, b4)!a
!ai = (", #,$)T
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Rewrite the system
• The measure vector
• The acquisition equation
!yi =
!
""""""""#
"STR
#STR
$STR
%1,i ! %1,i!1 ! b1!t%2,i ! %2,i!1 ! b2!t%3,i ! %3,i!1 ! b3!t%4,i ! %4,i!1 ! b4!t
$
%%%%%%%%&
!y = (HSTR, HGY R(", #,$))!a
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Do a classical LS and iterate
• Assume b, use the STR data (interpolated) to build H.
• Solve with a pre-conjugate gradient method
• Iterate with the new biases and attitudes
!x = (HTN!1H)!1HTN!1!y
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
R Dor observations
1.728571•1015 1.728571•1015 1.728571•1015 1.728571•1015 1.728571•1015 1.728572•1FineTime
-1.0
-0.5
0.0
0.5
1.0D
elta
Dec
(arc
sec)
pointing product
Source measure on PACS array
New reconstruction
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
R Dor Observations
1.728571•1015 1.728571•1015 1.728571•1015 1.728571•1015 1.728571•1015 1.728572•1FineTime
-1.0
-0.5
0.0
0.5
1.0
Del
ta a
lpha
cos
del
ta (a
rcse
c)
Note : the full inversion iterated 20 times takes ~20s on my laptop
Thursday, January 31, 2013
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ESAC, Jan 28, 2013H. Aussel Herschel PACS and SPIRE Map-Making Workshop
Conclusions
• The final pointing reconstruction accuracy should be of the order of 0.3”
• An approximate solver is being implemented in hipe - may not work on fast scans as it does not synchronizes the sensors
• The PACS ICC will soon revise the focal plane geometry of the instrument
• The ACMS of future missions should be treated as an instrument
Thursday, January 31, 2013