Update angle generation to prefer "actual" satellite position

[djhoese]

As discussed in #2012 and #2031 and #2030, some data like AHI HSD have inconsistent satellite position data between bands and even segments. This PR updates sensor angle generation so it requests the "actual" satellite position instead of the "nadir" satellite position. This allows AHI HSD true_color to be generated in 3 minutes instead of 4+.

However, now that I re-read the documentation for the nadir position I see it says that nadir should be used for computing viewing angles. This makes me wonder if I should have rounded the nadir position instead of the "actual" position. Thoughts @sfinkens @mraspaud @pnuu @simonrp84?

  * ``nadir_longitude/latitude``: Intersection of the instrument's Nadir with the surface of the
    earth. May differ from the actual satellite position, if the instrument is pointing slightly
    off the axis (satellite, earth-center). If available, this should be used to compute viewing
    angles etc. Otherwise, use the actual satellite position.

Additionally, @mraspaud do you want me to define a configuration option for this preference?

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2034 (7f2a59c) into main (c4f484d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2034   +/-   ##
=======================================
  Coverage   93.65%   93.66%           
=======================================
  Files         282      282           
  Lines       41954    42005   +51     
=======================================
+ Hits        39293    39344   +51     
  Misses       2661     2661           

Flag	Coverage Δ
behaviourtests	4.73% <0.00%> (-0.01%)	⬇️
unittests	94.20% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
satpy/_config.py	98.59% <ø> (ø)
satpy/modifiers/angles.py	96.70% <100.00%> (+0.01%)	⬆️
satpy/tests/test_modifiers.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_seadas_l2.py	98.66% <0.00%> (+0.22%)	⬆️
satpy/readers/seadas_l2.py	98.11% <0.00%> (+0.28%)	⬆️

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[coveralls]

Coverage increased (+0.005%) to 94.145% when pulling 7f2a59c on djhoese:feature-angle-gen-actual-pos into 0a5acaa on pytroll:main.

[mraspaud]

Yes, I would like this to be configurable

[djhoese]

@mraspaud Config option has been adding.

[sfinkens]

Yes in general the Nadir position would be more accurate, but we agreed that it doesn't make a big difference. Quoting @simonrp84's reply from the slack discussion:

If so, it's probably fine to use the "actual" position, as sfinkens says the two quantities don't differ much for Himawari (or any of the other geosats) as the sensor typically points nadir.

[mraspaud]

I suppose attitude offsets could be a problem here, but it seems geostationary satellites aren't really affected by these, right?

[mraspaud]

LGTM

[sfinkens]

I suppose attitude offsets could be a problem here, but it seems geostationary satellites aren't really affected by these, right?

To be honest I don't know and I'm relying on Simon's experience here :) To my knowledge the Nadir position is only provided by AHI, so at least it doesn't seem to be a common problem.

[sfinkens]

And I don't want to create even more work for @djhoese if it doesn't make a big difference 🙂

[mraspaud]

Agreed, merging.

[simonrp84]

Only just saw this. Off nadir pointing can be a problem (Met-8 is off nadir quite often) but as most of the satellites don't provide any information about their actual attitude there's not much we can do about it!
For Himawari (which does provide) it's hardly ever an issue, Him-8 is well stabilised.