SEVIRI L1.5 native reader does not support files not including 0100 in the file name

[adriaat]

Describe the bug
The SEVIRI L1.5 native reader (seviri_l1b_native) cannot read files that do not contain 0100 in the file name, as it expects the pattern {satid:4s}-{instr:4s}-MSG{product_level:2d}-0100-NA-{end_time:%Y%m%d%H%M%S.%f}000Z. SEVIRI L1.5 native files can have at least another number, which is 0201. These files can be downloaded from EUMETSAT Data Store.

To Reproduce

from satpy.utils import debug_on; debug_on()
import satpy

seviri_file = "/Users/adria/Downloads/MSG1-SEVI-MSG15-0201-NA-20060615135740.240000000Z-NA/MSG1-SEVI-MSG15-0201-NA-20060615135740.240000000Z-NA.nat"

scn = satpy.Scene(filenames=[seviri_file], reader='seviri_l1b_native')

Expected behavior
One should be able to read all SEVIRI L1.5 native files without errors without having to modify the reader YAML definition, as a workaround is to change the 0100 in the file patterns for {dummy:4d}.

Actual results

[DEBUG: 2021-03-20 21:19:47 : satpy.readers.yaml_reader] Reading ('/Users/adria/miniconda3/envs/myenv/lib/python3.8/site-packages/satpy/etc/readers/seviri_l1b_native.yaml',)
[WARNING: 2021-03-20 21:19:47 : satpy.readers.yaml_reader] No filenames found for reader: seviri_l1b_native
[WARNING: 2021-03-20 21:19:47 : satpy.readers] Don't know how to open the following files: {'/Users/adria/Downloads/MSG1-SEVI-MSG15-0201-NA-20060615135740.240000000Z-NA/MSG1-SEVI-MSG15-0201-NA-20060615135740.240000000Z-NA.nat'}
Traceback (most recent call last):
  File "sandbox.py", line 6, in <module>
    scn = satpy.Scene(filenames=[seviri_file], reader='seviri_l1b_native')
  File "/Users/adria/miniconda3/envs/myenv/lib/python3.8/site-packages/satpy/scene.py", line 108, in __init__
    self._readers = self._create_reader_instances(filenames=filenames,
  File "/Users/adria/miniconda3/envs/myenv/lib/python3.8/site-packages/satpy/scene.py", line 157, in _create_reader_instances
    return load_readers(filenames=filenames,
  File "/Users/adria/miniconda3/envs/myenv/lib/python3.8/site-packages/satpy/readers/__init__.py", line 495, in load_readers
    raise ValueError("No supported files found")
ValueError: No supported files found

Environment Info:

• OS: macOS 10.15.7
• Satpy Version: 0.26.1.dev5+g2286d243
• PyResample Version: 1.18.0
• Readers and writers dependencies (when relevant): when from satpy.utils import check_satpy; check_satpy() it lists seviri_l1b_native: ok

[TAlonglong]

In the HRIT specification page 35 this field is labeled DisseminationID CHARACTERSTRING SIZE (3) Value between '000' and 999 I guess this would be similar here. But I don't know if the disseminationID affects whats in the data itself.

[sfinkens]

I think 0201 are data before 2008/05 that were reprocessed with the new radiance definition:

Note: Original SEVIRI Level 1.5 image processed before May 2008 used spectral black-body radiances, which were defined in a different way from the effective radiances used in data processed later and for reprocessed data now available from the Data Centre before this change. The differences between these radiance definitions, together with other processing changes, and their impacts are described in the document On Differences in Effective and Spectral Radiance MSG Level.

(https://www.eumetsat.int/msg-calibration)

A colleague of mine also ran into this problem. I can provide a fix this week.

[sfinkens]

For the record: The Native format user guide doesn't explain this filename component. But if you order in the data centre and display the record details, it says Base Algorithm Version.