Illegal option --
usage: /usr/bin/geographiclib-get-gravity [-p parentdir] [-f] [-d] [-h] gravitymodel...

This program downloads and installs the datasets used by the
GeographicLib::GravityModel class and the Gravity tool to compute
gravity fields.  gravitymodel is one of more of the names from this
table:

                       size (kB)
  name     degree    tar.bz2  disk
  egm84      18       27      26
  egm96      360     2100    2100
  egm2008   2190    76000   75000
  wgs84      20        1       1
  grs80      20        1       1

The size columns give the download and installed sizes of the datasets.
In addition you can specify

  all = all of the supported gravity models
  minimal = egm96 wgs84

-p parentdir (default /usr/share/GeographicLib) specifies where the
datasets should be stored.  The "Default gravity path" listed when running

  Gravity -h

should be parentdir/gravity.  This script must be run by a user with
write access to this directory.

Normally only datasets which are not already in parentdir are
downloaded.  You can force the download and reinstallation with -f.
The -f flag also let you download new models (not yet in the set
defined by "all").

If -d is provided, the temporary directory which holds the downloads,
$TMPDIR/gravity-XXXXXXXX or /tmp/gravity-XXXXXXXX,
will be saved.  -h prints this help.

For more information on the gravity datasets, visit

  https://geographiclib.sourceforge.io/C++/doc/gravity.html