How to Apply the AATSR VIS-SWIR Calibration Corrections
Dave Smith and Caroline Poulsen
Contact: [email protected]
Last updated: 23/11/2010
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Background
Three modifications to the AATSR VIS-SWIR calibration processing have been implemented
by the operational processing since the launch of ENVISAT in 2002.
14-December-2004 - 1.6um non-linearity correction introduced (PO-TN-RAL-AT-0540). This
was traced to an error in the general calibration file (GC1) that did not properly implement
the known non-linearity effects in the 1.6um channel that was measured during pre-launch
testing. The problem was fixed by a modification to the GC1 file. All L1b data processed
after 14th December 2004 has the non-linearity correction applied.
29-November-2005 – Exponential Drift Correction is applied to VC1 files (PO-TN-RAL-AT0542). This correction was based on the first two years of data aquired over stable desert
and ice targets using the method described in Smith et al. 2002. For the early phase of the
mission the calibration trend followed an exponential decay function as predicted from the
experience of ATSR-2, AVHRR etc. such that D(t) = exp(-kt).
18-December-2006 – Thin Film Drift Correction is introduced to VC1 files (PO-TN-RAL-AT0552). This correction was implemented post 2005 to account for results that showed that
the exponential decay model was incorrect in the case of the AATSR visible channels. The
observed long term trends suggest that the drift is caused by a thin-film interference effect
(Etalon) of the form D(t) = 1+Asin2(2πnxt/λ) (where n is the refractive index and x is the
thickness)
Although the ‘thin film’ model provided a reasonable representation of the observed drift,
analysis showed that a simple parametric model could not adequately account for all
variations in the measured drift.
So, instead of a parametric model to obtain the drift correction, a smoothed average of the
drift measurements are used to provide a drift correction table. Before applying the
‘improved’ drift correction, it is essential to remove any previous non-linearity corrections to
avoid over compensating the drift. This is described in Smith et al 2008.
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Do I need to apply a drift correction?
Yes! if you are using ANY (MERCI RAL etc...) ESA provided AATSR lv1b product then you
should be applying the following drift correction.
The current correction applied to the data is a predicted correction decided at a time early
in the mission. Careful drift monitoring has shown that the drift is not easily predicted and
needs constant monitoring.
The drift correction is constantly being improved and updated so users are advised to use
the latest drift correction and to store in the processed data the calibration table used.
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Drift Correction Table and Tools
The latest version of the AATSR drift correction table and tools can be found at any of the
following locations:
•
The AATSR engineering data system
http://aatsr2.ag.rl.ac.uk/EDSX/OtherInfo/ in the Visible Calibration Information link
•
The NEODC ATSR on-line archive
http://www.neodc.rl.ac.uk/?option=displaypage&Itemid=91&op=page&SubMenu=9
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•
The CEOS Cal-Val portal
http://calvalportal.ceos.org/cvp/web/guest/aatsr-envisat
The AATSR drift correction table is regularly updated and the latest calibration file stored
here. If you would like to be regularly updated with the latest calibration file please email
Dave Smith.
This folder contains plots of the latest derivation of the drift correction as well as
comparisons with MERIS visible channels. The folder also contains the IDL code needed to
apply the corrections. Details on how to apply the drift correction are outlined below.
We ask that users provided some evidence that show the improvements to retrievals as a
result of the calibration corrections being applied to the data. This could simply be some
before and after trends with some accompanying text (e.g. Aerosol trends). This is to
support our service in maintaining the AATSR visible calibration tables on behalf of ESA and
will ultimately go towards improving the quality of the calibration corrections for AATSR.
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Methodology
An example of how to apply the corrections is given in the IDL function
AATSR_APPLY_CORRECTIONS.PRO.
It is important to implement the corrections in the right sequence as described below.
1) Apply the non-linearity correction to the 1.6um channel data if not already done so
using the function AATSR_CORRECT_V16_NONLINEARITY.PRO with the call
IDL> V16_CORRECTED = AATSR_CORRECT_V16_NONLINEARITY
(GC1_FILENAME, V16_UNCORRECTED)
Where:
GC1_FILENAME - String containing name of GC1 filename associated with L1B product DSD32 in product
V16_UNCORRECTED - Variable containing 1.6um reflectance values from L1B product.
The value(s) must be in the range 0-1
2) In order to implement the ‘improved’ long term drift correction it is necessary to remove
any previous corrections where they had been applied by the processor. This is done
by using the IDL function AATSR_REMOVE_DRIFT_CORRECTION.PRO with the call
IDL> UNCORRECTED_REFLETCANCE =
AATSR_REMOVE_DRIFT_CORRECTION(VC1_FILENAME, ACQ_TIME, ICH,
REFLECTANCE)
Where:
VC1_FILENAME - String containing name of VC1 filename associated with L1B product DSD31 in product
ACQ_TIME - String containing acquistion time of product - format = DD-MMM-YYYY
HH:MM:SS
ICH - Integer Variable Containing Channel Number to be processed
0 = 555nm
1 = 659nm
2 = 870nm
3 = 1600nm
REFLECTANCE - Variable containing reflectance reading from L1B product
3) Use drift correction look up table to obtain the drift measurement for a given channel
and acquisition using the function AATSR_APPLY_DRIFT_CORRECTION.PRO with the
call
IDL> CORRECTED_REFLECTANCE =
AATSR_APPLY_DRIFT_CORRECTION(ACQ_TIME,ICH,UNCORRECTED_REFLECTA
NCE,DRIFT_TABLE=DRIFT_TABLE)
Where:
ACQ_TIME - String containing acquistion time of product - format = DD-MMM-YYYY
HH:MM:SS
ICH - Integer Variable Containing Channel Number to be processed
0 = 555nm
1 = 659nm
2 = 870nm
3 = 1600nm
UNCORRECTED_REFLECTANCE - Variable containing reflectance reading
DRIFT_TABLE - If specified, a structure containing the drift look up table
The drift correction table can be read into an IDL structure using the function
AATSR_READ_DRIFT_TABLE.PRO with the call
IDL> NTAB = AATSR_READ_DRIFT_TABLE(DRIFT_TABLE)
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Additional Functions
In addition to the drift correction procedure described above, there are some additional
functions that allow the reflectance data to be rescaled to a different reference, e.g. AATSR
to MERIS.
SCALE_ATSR2_TO_AATSR.PRO
Scales ATSR2 reflectance values to AATSR using biases measured by intercomparisons over
deserts This routine applies an offset correction and assumes a normalised radiance NOT
scaled to cos(sza) in the range 0-1. The call is
IDL> CORRECTED_REFLECTANCE =
SCALE_ATSR2_TO_AATSR(ICH,UNCORRECTED_REFLECTANCE)
Where:
ICH - Integer Variable Containing Channel Number to be processed
0 = 555nm
1 = 659nm
2 = 870nm
3 = 1600nm
UNCORRECTED_REFLECTANCE - Variable containing reflectance reading
SCALE_AATSR_TO_MERIS.PRO
Scales AATSR (or ATSR-2) reflectance values to MERIS using biases measured by
intercomparisons over deserts (Smith et al 2008) using the call
IDL> CORRECTED_REFLECTANCE =
SCALE_AATSR_TO_MERIS(ICH,UNCORRECTED_REFLECTANCE)
Where:
ICH - Integer Variable Containing Channel Number to be processed
0 = 555nm
1 = 659nm
2 = 870nm
UNCORRECTED_REFLECTANCE - Variable containing reflectance reading
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References
Smith, D. L., C. T. Mutlow, and C. R. N. Rao, (2002). “Calibration monitoring of the visible and
near-infrared channels of the Along-Track Scanning Radiometer-2 by use of stable terrestrial
sites.” Appl. Opt., 41, p515—523.
Smith D.L. and Poulsen C.A., 2008, “Calibration Status of the AATSR Reflectance Channels”,
Proceedings of MERIS/AATSR Workshop
Smith, D.L., 2008, PO-RP-RAL-AT-0599 , “Final Report on Multi-Mission Calibration Study”,
Envisat project document.
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Acknowledgements
This works has been funded by ESA under the Multi-Mission Vicarious Calibration -Contract
No. 23011-09-I-OL