def update_iers(save_name='iers_frozen.ecsv', num_avg=1000):
"""Update the IERS table used by astropy time, coordinates.
Downloads the current IERS-A table, replaces the last entry (which is
repeated for future times) with the average of the last ``num_avg``
entries, and saves the table in ECSV format.
This should only be called every few months, *e.g.*, with major releases.
The saved file should then be copied to this package's data/ directory
and committed to the git repository.
Requires a network connection in order to download the current IERS-A table.
Prints information about the update process.
The :func:`desiutil.plots.plot_iers` function is useful for inspecting
IERS tables and how they are extrapolated to DESI survey dates.
Parameters
----------
save_name : :class:`str`, optional
Name where frozen IERS table should be saved. Must end with the
.ecsv extension.
num_avg : :class:`int`, optional
Number of rows from the end of the current table to average and
use for calculating UT1-UTC offsets and polar motion at times
beyond the table.
"""
log = get_logger()
# Validate the save_name extension.
_, ext = os.path.splitext(save_name)
if ext != '.ecsv':
raise ValueError('Expected .ecsv extension for {0}.'.format(save_name))
# Download the latest IERS_A table
if astropy.utils.iers.conf.iers_auto_url == 'frozen':
raise ValueError("Attempting to update a frozen IERS A table!")
iers = astropy.utils.iers.IERS_A.open(
astropy.utils.iers.conf.iers_auto_url)
last = Time(iers['MJD'][-1], format='mjd').datetime
log.info('Updating to current IERS-A table with coverage up to %s.',
last.date())
# Loop over the columns used by the astropy IERS routines.
for name in 'UT1_UTC', 'PM_x', 'PM_y':
# Replace the last entry with the mean of recent samples.
mean_value = np.mean(iers[name][-num_avg:].value)
unit = iers[name].unit
iers[name][-1] = mean_value * unit
log.info('Future %7s = %.3f', name, mean_value * unit)
# Strip the original table metadata since ECSV cannot handle it.
# We only need a single keyword that is checked by IERS_Auto.open().
iers.meta = dict(data_url='frozen')
# Save the table. The IERS-B table provided with astropy uses the
# ascii.cds format but astropy cannot write this format.
iers.write(save_name, format='ascii.ecsv', overwrite=True)
log.info('Wrote updated table to %s.', save_name)
