Herschel Pointing : Status, Impact on Map-Making, and...

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

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.

Pointing Accuracy Impact in Map Making

• Astrometric issues

• Signal to noise loss

• Artifacts on bright sources

Cross-like artifacts in inversion methods

• Scan and cross-scan over a bright gaussian source

• On board compression: 4 frames

• Perfect pointing accuracy

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

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

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.

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

Place & DateH. Aussel Meeting Name

Gyroscopes

Herschel ACMS

Star Tracker

Real Time Kalman Filter

Synchronization Synchronization

Herschel Attitude

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

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)

Herschel Gyroscopes (GYR)

• Herschel uses 4 Gruman-Northtop Space Inertial Reference Units

• Can measure a rotation of the spacecraft very accurately: 0.0005”/s

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

Fitted Bias A = 1.6227108e-07rad / s

ACMS Bias A = 1.1675929e-07rad / s

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.

Herschel ACMS

• The state vector is the attitude quaternion plus the GYR bias

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.

Herschel STR output

• Star positions measured by the STR during a long staring observation

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

Speedbumps

• Because of the high weight on STR vs. GYR, the Kalman filter incorrectly change the GYR bias instead of filtering the STR data

A provocative statement...

• The ACMS is continuously calibrating a very accurate sensor (the GYR) with a poorly accurate one (the STR)

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

Calibrating the STR

• A complete analysis of all the ACMS telemetry was done

• Resulted in corrected distortion maps of the STR CCD

Analysis of the STR star catalog

• The stars used for guiding were checked.

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”

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

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

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

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

Rewrite the system

• The measure vector

• The acquisition equation

""""""""#

%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

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

R Dor observations

1.728571•1015 1.728571•1015 1.728571•1015 1.728571•1015 1.728571•1015 1.728572•1FineTime

pointing product

Source measure on PACS array

New reconstruction

R Dor Observations

1.728571•1015 1.728571•1015 1.728571•1015 1.728571•1015 1.728571•1015 1.728572•1FineTime

Note : the full inversion iterated 20 times takes ~20s on my laptop

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

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