GSC Sentinel-2 PDGS OCD - Emits - ESA

GSC Sentinel-2 PDGS OCD
Issue 1 Revision 2 (draft) - 25.07.2010
GMES-GSEG-EOPG-TN-09-0008
page 155 of 350
○ On-board sun diffuser images acquired by the instrument to determine radiometric
equalization parameters for inter-pixel calibration (on-board and on-ground).
This operation will be data-driven on reception of Absolute Calibration data with CSM
in sun-diffuser position that will be commanded every 3 repeat cycles (30 days).
On-board equalization parameters (NUC table) with associated on-ground processing
parameters (GIPP) will be derived and the NUC parameters will be checked against
their current values automatically. In case of minor changes, the NUC will not be
updated and the calibration will only be performed on the on-ground processors
through the activation of a newly generated GIPP. Otherwise a coordinated update of
the NUC (on-board) and the GIPP (on-ground) will be triggered after validation by an
expert Cal/Val team. In this case, the Level-0/1A/1B and 1C of the calibration dataset
will be provided to the expert Cal/Val team to perform the validation of the precomputed
coefficients..
Calibration using the on-board sun-diffuser will be performed at specific orbits of the
repeat cycle such that (1) the angle between the +x-axis of the MSI and the spacecraft-
Sun direction is minimum and (2) the impact on the nominal acquisition plan, if any, is
minimised while taking into account the required 10 minutes waiting-times with no
possibility of nominal acquisition due to the MSI shutter movement.
As per the simulations presented in [RD-19], those conditions can be optimised at one
specific orbit distinct between summer and winter repeat-cycles. Sun-calibration
operations will hence be pre-defined and commanded once every 3 repeat-cycles in a
deterministic manner.
○ Images acquired over ground uniform areas (e.g. Greenland, Dome-C).
In principle this calibration operation will be triggered only during Phase-E1 for onboard
diffuser validation, and on contingency during Phase E2. It will not be automated
and be carried out when required by an expert Cal/Val team after appropriate
commanding of specific image acquisitions in Nominal mode.
4.5.4.10.2.3 Determination of Absolute Radiometric Calibration Parameters
Absolute radiometric calibration parameters will be determined through the following
operations:
o Calibration using on-board sun diffuser images.
This operation will be data-driven on reception of Absolute Calibration data that will be
performed on a 30-days basis reusing the pre-defined planning outlined for the
detector relative sensitivities determination (cf. previous section).
Although generated automatically, updated calibration parameters will be validated and
accepted by an expert Cal/Val team prior to triggering their feedback in the PDGS
processing chains.
o Vicarious calibration and validation based on image acquisitions over specific areas
such as snow sites (e.g. Dome-C in Antarctica), instrumented Cal/Val sites from the
CEOS network (e.g. La Crau), or desert sites for cross calibration with other sensors
and monitoring of temporal stability.
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