III. Calibration of satellite instruments

between observed solar constant variations and Zurich sunspot number discussed at the symposium. Finally, comparisons were made of the results derived from various climate models (Hansen et ai., Molnar and Mika) as applied to the real climate system. Computed variations from different dimensional models were used to estimate the effect of various forcing functions, including solar variability. The models were tested for their sensitivity in deriving a global surface temperature response to + 1y/, variation in the solar constant (Molnar and Mika). III. Calibration of Satellite Instruments The session on Calibration of Satellite Instruments highlighted three important points : (1) the growing importance of calibration due to the increased demand for quantitative data from satellites coupled with a recognition of the serious problems attending the maintenance of rehable calibrations on unmanned spacecraft over a period of years; (2) the concomitant growing interest in what has come to be called “vicarious” methods; and (3) the continuing difficulties of providing reliable calibrations in the ultraviolet region of the electromagnetic spectrum. Under the first item, Dr. C. Frohlich, V. N. Kaduskar and Dr. W. Hovis outlined the status of radiometric standards, traced their transfer procedures to individual satellite instruments, and discussed the limitations inherent in the resulting accuracy. For applications such as the earth radiation balance measurement, where accuracy of the order of 0.57; or better is considered essential, it is again concluded that the requirements cannot be met by conventional techniques. These satellite instruments can provide useful regional radiation balance data using instruments which measure directly the ratio of incoming solar radiation and reflected and emitted radiation, but are not considered to be capable of providing global energy balance to the absolute accuracy necessary for climate studies. The current technology of earth radiation balance measuring instruments was discussed by Dr. D. Crommelynck and Dr. B. R. Barkstrom. The “vicarious” technique, in which a portion of the scene viewed by the satellite instrument serves as a calibration source, was discussed by Dr. P. Koepke, Dr. N. Berriot, Dr. S. Campbell, Mr. M. Reynold and Dr. W. Hovis. Two methods are recognized: (a) The portion of the scene serving as a calibration source is measured locally by ground or airborne instruments and thereby established as a quantitative source. Examples are the dunes area of the White Sands National Monument of the US to be used as a calibration for visible channels; and the Red Sea which has been used as a calibration source for the infrared channels on Meteosat. (b) The intensity and spectral characteristics of the scene are established by a separate spacecraft instrument whose calibration is thereby transferred to the instrument to be calibrated. An example of this is the use of the TIROS-N HIRS infrared instrument to establish the brightness of 30

scene features which are then used to calibrate infrared radiometers aboard the geostationary satellites. Ultraviolet instruments, important for the measurement of ozone by the backscatter technique and for measurements of aerosols, represent the most difficult calibration problem. There is no satisfactory method of providing a calibration vicariously and therefore direct methods must be used. The quantitative accuracy of most conventional laboratory sources such as arc or tungsten filament lamps is poor - 207, is considered excellent - and the synchrotron is considered the only truly quantitative and reliable source, The speakers stressed the need for the radiometric community to have continued access to a reliably maintained synchrotron source for calibration and standardization of satellite instruments. The continued availability of the synchrotron at the US Bureau of Standards has been threatened by current budgetary restrictions. The speakers emphasized the importance of the loss of this facility estimating that in its absence quantitative ultraviolet calibrations using alternative sources would probably be no better than k 25’4, which would render the instruments essentially useless for most quantitative applications.

1.5. REPORT ON THE SEVENTH CANADIAN SENSING’*’

SYMPOSIUM

ON REMOTE

Winnipeg, Manitoba, Canada, 9--11 September 1981 The Seventh Canadian Symposium on Remote Sensing was held from 9-l 1 September 1981, in Winnipeg, Manitoba. It was organized by the Manitoba Branch of the Canadian Institute of Surveying, and sponsored by the Canadian Remote Sensing Society of the Canadian Aeronautics and Space Institute. The theme of the Symposium being “Down to Earth Management”, most of the papers dealt with actual applications of remote sensing (aerial photography, Landsat, radar) in the management of Canadian resources. The sessions were organized under four sub-themes: Land use applications, Earth science applications, Ecological applications, and Data management and acquisition. Approximately one quarter of the papers were related to research being carried out in the Western provinces, which allowed the many Eastern Canadian participants to get a better grasp of the implications of remote sensing for Western Canada.

Q) Extract

from Remote Sensing in Canada,

Vol. 9, No. 1, December 1981.

31