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 88 of 350
step. This method is commonly used to correct potential noise (jitter and/or drift) on the time
stamp delivered by the on board clock and inserted in the image data. It consists in an
adjustment of the time stamp of each image line (thanks to root mean square method)
computed over the overall imaging orbit. This processing option will be activated during
commissioning phase to validate the above figures in-flight by GPP.
4.3.9.2.2 Band Co-Registration Constraints
As described in section 3.5.2.2, the instrument focal plane staggered configuration induces an
inter-band and inter detector parallax (also referred to as the B/H stereoscopic effect).
Similarly to the radiometric corrections, fore and aft processing margins hereafter referred to
as the “geometric processing margin” will be required to reconstruct the inter-band coregistration.
Depending on the geometric processing performed, the fore and aft processing
margins will be different:
○ As applied to one single detector processing, the processing margin will correspond to
the maximum inter-band parallax equal to 14km along-track hence corresponding to
about 2/3 rd of a scene.
○ If applied to all detectors at once for the entire MSI swath reconstruction across-track,
the processing margin would correspond to the maximum inter-detector parallax equal
to 46km along-track (maximised for band B9) hence corresponding to two MSI
scenes.
4.3.9.2.3 Spatial Registration
The stringent requirements of multi temporal registration (less than 0.3 pixels for 10m bands,
cf. section 4.4.4.1) lead to improving the location accuracy though the refinement of the
geometric model based on automatic registration with ground control points (GCPs) using an
reference image.
This geometric refinement process will correct perturbations which are not detected by the
AOCS sensors (star trackers, GPS, gyros) by estimating the perturbations on the image itself
and modelling polynomial corrections derived from the measured GCPs.
To be optimised, this refinement shall be applied on the largest possible number of GCPs,
ideally extracted from the complete imaging orbit. Expected perturbations of this correction
possibly induced by refining the geometry using smaller image portions is reported in [RD-30].
In particular, it was concluded that spatial registration will be achieved with sufficient accuracy
if applied on a continuous segment of image data of no less than 60km along-track over the
full swath.
In addition, it is anticipated that although spatially co-registered over the same image
reference, the stitching of two images registered independently using different sets of GCPs
could introduce visual discontinuities at the sewing boundary (at sub-pixel level).
Whereas the geolocation error budgets of the spacecrafts theoretically allow for a polynomial
based refinement model as currently specified, it is anticipated that potential thermo-elastic
effects of the MSI could add local perturbations to the measurements which could in turn
induce small discontinuities between the geometric models constrained independently.
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