How to use the ephemeris.GET_FIELD_U function in ephemeris

def gen_nav_data(msg, nav_frame_buffer):
  # TODO this stuff needs to be parsed and published.
  # refer to https://www.u-blox.com/sites/default/files/products/documents/u-blox8-M8_ReceiverDescrProtSpec_%28UBX-13003221%29.pdf
  # section 9.1
  msg_meta_data, measurements = msg.unpack()

  # parse GPS ephem
  gnssId = msg_meta_data['gnssId']
  if gnssId  == 0:
    svId =  msg_meta_data['svid']
    subframeId =  GET_FIELD_U(measurements[1]['dwrd'], 3, 8)
    words = []
    for m in measurements:
      words.append(m['dwrd'])

    # parse from
    if subframeId == 1:
      nav_frame_buffer[gnssId][svId] = {}
      nav_frame_buffer[gnssId][svId][subframeId] = words
    elif subframeId-1 in nav_frame_buffer[gnssId][svId]:
      nav_frame_buffer[gnssId][svId][subframeId] = words
    if len(nav_frame_buffer[gnssId][svId]) == 5:
      ephem_data = EphemerisData(svId, nav_frame_buffer[gnssId][svId])
      return gen_ephemeris(ephem_data)