Nadir - Forward Shifts in ATSR Multi-mission Level 1b archive v2.1

This summarises some results from Owen Embury looking at whether, with the latest ATSR multi-mission archive, there is still a need (as in Phase 1) to shift the forward view to better line up with the nadir view for the ATSRs. We always knew that our empirical shifts in Phase 1 were only approximate and hoped that the v2.1 archive would have resolved the issue completely. Bottom line: it is an improvement, but not perfect.

Here is a scene with the v2.0 nadir-forward collocation. Note that in the Nad-Fwd differences there are red and blue (warm and cold) differences along SST fronts (wavy lines) in the thermal (middle panel) and the island have a bright edge and dark shadow. Both these effects imply the two views are not perfectly collocated.

Here is the equivalent from v2.1. The effects are much less, but the opposite effects can just be discerned.

Here is the same scene with a shift of a pixel. The fronts are less obvious and the shadowing of the land is not really much different. For SST this is suggests an improvement.

Owen did a further analysis, minimising nadir-forward variances across many small scene extracts, to see if the offsetting effect is constant in time. It appears not to be, as per the example below:

At the beginning of the AATSR mission, the best shift of the forward view is -1 pixel along track and 0 across track on the left side of the swath -- but from 2007, it appears better to shift across track by +1. For other sensors, the equivalent curves suggest the instrument geometries are even less stable.

However, the good news is that pretty much all of the required shifts are no greater than 1 pixel, which is an improvement over v2.0, where shifts up to 3 pixels were required.