def _get_onc_sog_adcp_matfile(ftp, filepath, dest):
    """Download the ADCP :kbd:`.mat` file to the directory given by dest.

    For use by :py:func:`~salishsea_tools.data_tools_get_onc_sog_adcp_mat`.

    :rtype: :py:class:`pathlib.Path`
    """
    dest_path = dest / filepath.name
    ftp.retrbinary("RETR {}".format(filepath), dest_path.open("wb").write)
    return dest_path