Added warning to Fermi detector angle calculation code, for when time…

… returned is different from time requested (#128)

* added warning for when a time requested for Fermi detector angles is not available in the spacecraft file

* changelog

* ignore warning

* Update setup.cfg

* Update setup.cfg

---------

Co-authored-by: Nabil Freij <[email protected]>

changelog/128.bugfix.rst

@@ -0,0 +1,5 @@
+In the ``fermi``, the function ``get_detector_sun_angles_for_time`` returns the angle with respect to the Sun of each Fermi/GBM detector.
+However, these files contain gaps due to the South Atlantic Anomaly.
+If the time requested falls in one of these gaps, the code will return the detector angles for the next available time.
+This can be several minutes different from the time requested.
+Now, a warning to the user will be raised if the time returned by the code is more than 1 minute different from the time requested (1 minute is the nominal cadence of the spacecraft weekly file), and explains that this is likely due to a South Atlantic Anomaly encounter.

setup.cfg

@@ -80,6 +80,7 @@ filterwarnings =
     ignore:distutils Version classes are deprecated. Use packaging.version instead:DeprecationWarning
     ignore:Unknown units for CHANNEL:sunpy.util.exceptions.SunpyUserWarning
     ignore:.*is deprecated and slated for removal in Python 3:DeprecationWarning
+    ignore:.*Something went wrong.*
 
 [isort]
 line_length = 110

sunkit_instruments/fermi/fermi.py

@@ -19,6 +19,7 @@
 from sunpy.time import TimeRange, parse_time
 from sunpy.time.time import _variables_for_parse_time_docstring
 from sunpy.util.decorators import add_common_docstring
+from sunpy.util.exceptions import warn_user
 
 __all__ = [
     "download_weekly_pointing_file",
@@ -116,6 +117,18 @@ def get_detector_sun_angles_for_time(time, file):
 
     time = parse_time(time)
     scx, scz, tt = get_scx_scz_at_time(time, file)
+
+    # Fermi LAT spacecraft files have gaps due to SAA. If the requested time falls
+    # in one of these gaps, the time and angles returned will be the next available
+    # time in the file. This may be several minutes different from the request.
+    # If the returned time is > 60s different from the time requested, raise a warning.
+    if np.abs((tt - time).sec) > 60.0:
+        warn_user(
+            "The time requested is missing from the Fermi spacecraft file. "
+            "The requested time probably occurs during the South Atlantic Anomaly. "
+            f"Returning detector angles for the next available time: {tt}"
+        )
+
     # retrieve the detector angle information in spacecraft coordinates
     detectors = nai_detector_angles()