Operational Calibration of the Imagers and Sounders
on the GOES-8 and -9 Satellites, Page 6
Michael Weinreb, Michael Jamieson, Nancy Fulton, Yen Chen, Joy Xie Johnson,
James Bremer, Carl Smith, and Jeanette Baucom
4. REVISION FOR VARIATION IN SCAN-MIRROR EMISSIVITY
Shortly after the GOES-8 instruments began operating in orbit, it was
observed that the outputs of the imager and sounder channels at wavelengths
between 6 and 14 µm varied with the east-west position of the field of view.
This was most apparent when the instruments viewed space. As an example, Fig. 1
shows raw imager data from east-west scans of space above the north pole at
three different times of day. Although the scans extend approximately 19o in the
east-west direction (which slightly exceeds the 17.4o width of the Earth), they
correspond to a mechanical scan-mirror rotation of half that, or 9.5o. (In Fig.
1, the abscissa is mechanical scan mirror angle.) At a scan-mirror angle of 45o,
the imager views the meridian directly below the satellite; at angles of 40o and
50o, the imager views space beyond the west and east sides of the Earth,
respectively. The differences in output between the east and west sides of the
scans correspond to an artificial temperature difference of approximately 150K.
More to the point, there were also radiometric errors--as large as several
degrees Kelvin in brightness temperature--in observations of the Earth, although
they were masked by the variability in the scene. Similar angle dependences and
radiometric errors were also observed later in data from the GOES-9 instruments.
The cause of this behavior is believed to be an absorption feature of the
silicon oxide coatings of the scan mirrors, which causes the mirrors'
emissivities to vary with the incidence angle of the incoming radiation.
Processing changes8 (described below) to eliminate these radiometric errors were
made operational in April 1995 and August 1995 for the GOES-8 and GOES-9
imagers, respectively. For the GOES-8 and GOES-9 sounders, the corrections
became operational in June and April 1996, respectively.
Fig. 1. Raw output (counts) of channel 5 (12 µm)
of GOES-9 Imager from east-west scan of space.
The data are labeled by the time of day in GMT. The abscissa is
mechanical scan angle. Since the imager was constructed so that increasing
counts correspond to decreasing brightness temperature, this figure makes it
appear as if the temperature of space were increasing from west to east.
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Contact Michael P. Weinreb at michael.weinreb@noaa.gov
Latest Revision: July 9, 1997
