Fix negative channel 3A slope calibration coefficient in AVHRR reader

[ninahakansson]

Handle negative slopes for 3A in calibration the same way as for channel 1

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

Merging #2123 (4d88d41) into main (8b5d493) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2123      +/-   ##
==========================================
- Coverage   93.88%   93.88%   -0.01%     
==========================================
  Files         283      283              
  Lines       43093    43092       -1     
==========================================
- Hits        40458    40457       -1     
  Misses       2635     2635              

Flag	Coverage Δ
behaviourtests	4.78% <0.00%> (+<0.01%)	⬆️
unittests	94.53% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/readers/aapp_l1b.py	89.69% <100.00%> (-0.04%)	⬇️

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[coveralls]

Coverage decreased (-0.0001%) to 94.48% when pulling 4d88d41 on ninahakansson:fix_aapp_calibration into 8b5d493 on pytroll:main.

[djhoese]

So the original fix was for Channel 2 (at index 1) and this is for 3a (at index 2). Given that it took so long to discover the issue with Channel 2 and that it took an email from Nigel Atkinson, do you @ninahakansson have any idea if any other channels (channel 1?) suffer from this?

[ninahakansson]

I think is could in principle also happen to channel 1. So maybe we should just apply it to all visible channels?

[adybbroe]

I think is could in principle also happen to channel 1. So maybe we should just apply it to all visible channels?

Yes, please do that, also just agreed via mail by Nigel Atkinson (NWPSAF)

[djhoese]

Looks good to me. One question, is it OK for IR bands slopes to be negative?

[gerritholl]

Is this ready to merge? It seems the codecov failure is due to the total number of lines of code being reduced?