code: def avhrr_gac(scan_times, scan_points,
                scan_angle=55.37, frequency=0.5):
      
      try:
          offset = np.array([(t - scan_times[0]).seconds +
                             (t - scan_times[0]).microseconds / 1000000.0 for t in scan_times])
      except TypeError:
          offset = np.arange(scan_times) * frequency
      scans_nb = len(offset)
  
      avhrr_inst = np.vstack(((scan_points / 1023.5 - 1)
                              * np.deg2rad(-scan_angle),
                              np.zeros((len(scan_points),))))
  
      avhrr_inst = np.tile(
          avhrr_inst[:, np.newaxis, :], [1, np.int(scans_nb), 1])
      # building the corresponding times array
      times = (np.tile(scan_points * 0.000025, [scans_nb, 1])
               + np.expand_dims(offset, 1))
      return ScanGeometry(avhrr_inst, times)
docs: Definition of the avhrr instrument, gac version
  
      Source: NOAA KLM User's Guide, Appendix J
      http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/j/app-j.htm