Fix get_observer_look for satellite exactly at nadir #77
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When the satellite is exactly above the observer the satellite elevation angle is 90 degrees the azimuth angle is undefined. Set asimuth angle to 0 when satellite elevation is 90 degrees. Also handle the case when top_z / rg_ is larger than 1.0 due to rounding.
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+ Coverage 85.31% 85.67% +0.35%
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This happens when satellite is exactly at observer nadir. The problem depends on rounding and varies between array types: numpy, dask, and xarray. Removed the check for sat_at_nadir as is not needed any longer.
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While slightly out-of-scope for this PR, do you know if this problem that you've fixed here would also affect solar angles? |
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Good question! It should not happen for solar azimuth angles as pyorbital in astronomy get_alt_az uses np.artan2 that handles special values For the sun zenith angle the formulas are different so I don't think the arcsine for values larger than 1 could happen. |
pyorbital/orbital.py
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| top_z_divided_by_rg_ = top_z / rg_ | ||
| el_ = np.arcsin(top_z_divided_by_rg_) |
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Using clip might help avoid the extra filtering later:
| top_z_divided_by_rg_ = top_z / rg_ | |
| el_ = np.arcsin(top_z_divided_by_rg_) | |
| top_z_divided_by_rg_ = top_z / rg_ | |
| top_z_divided_by_rg_ = top_z_divided_by_rg_.clip(max=1) | |
| el_ = np.arcsin(top_z_divided_by_rg_) |
pyorbital/orbital.py
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| # And azimuth undefined when elevation is 90 degrees | ||
| if has_dask and isinstance(az_data, da.Array): | ||
| el_data = da.where(top_z_divided_by_rg_ > 1.0, np.pi/2, el_data) | ||
| az_data = da.where(el_data == np.pi / 2, 0, az_data) |
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Instead of this filtering and the undet_azi higher up, would np.nan_to_num be helpfull here? or do we still want some azimuths to be nans?
When the satellite is exactly above the observer the satellite
elevation angle should be 90 degrees and the azimuth angle is undefined.
Set azimuth angle to 180 (initially 0) when satellite elevation is 90 degrees.
Also handle the case when top_z / rg_ is larger than 1.0 due to rounding.
Previously nans were often returned for satellite elevation 90 degrees.
The Orbital class also has a get_observer_look function which does not handle
dask and xarray. This function has not been updated.
flake8 pyorbital