def lv0_consts(key=None):
    """
    defines consts used while reading Sciamachy level 0 data
    """
    consts = {}
    consts['mph_size'] = 1247
    consts['num_aux_bcp'] = 16
    consts['num_aux_pmtc_frame'] = 5
    consts['num_pmd_packets'] = 200
    consts['channel_pixels'] = 1024

    if key is None:
        return consts
    if key in consts:
        return consts[key]

    raise KeyError('level 0 constant {} is not defined'.format(key))