def _get_odim_sweep_mode(nch, grp):
    """Retrieve sweep mode

    Parameters
    ----------
    nch : netCDF4.Dataset handle
    grp : dict
        Dictionary of dataset hdf5 groups ('how', 'what', 'where')

    Returns
    -------
    out : str
        'azimuth_surveillance' or 'rhi'

    """
    odim_mode = grp['what'].attrs[nch._swmode]
    return ('rhi' if odim_mode == 'RHI' else 'azimuth_surveillance')