Calibration of satellite radiation instruments

J. T. Houghton

In 1973 COSPAR and the Radiation Commission set up an ad hoc committee to consider problems of cali-bration and intercomparison of radiation measuring instruments mounted on rockets or satellites. Thiscommittee organized the Grenoble symposium in 1975. This short paper introduces some of the papers pre-sented there and summarizes certain recommendations made at Grenoble.

The author is with University of Oxford, Clarendon Laboratory,Department of Atmospheric Physics, Oxford OX1 3PU, U.K.

Received 13 August 1976.

Radiation processes are fundamental to atmospher-ic science. The basic source for the atmospheric engineis solar radiation and the basic sink longwave radiationemitted by the atmosphere and the surface out to space.Careful measurements of both solar and terrestrial ra-diation have been made from stations on the surface formany years. A review of the instrumentation and themeans of standardization of such measurements is givenby various authors in ADVANCES IN GEOPHYSICS,A. J. Drummond, Ed. (Academic, New York, 1970), Vol.14. During the last 15 years it has become possible tomake observations of components of the earth's radia-tion budget from above the atmosphere by instrumentsmounted on artificial satellites. The great advantageof the satellite as a platform for radiometric instru-mentation is that an approximation to truly globalcoverage in space and continuous coverage in time canbe achieved. Because the distribution in space andtime of the components of the radiation budget outsidethe atmosphere is closely related to the climate, thereis a great deal of importance attached to making suchmeasurements with high accuracy. With this in mind,in 1973 an ad hoc international committee was set upjointly by the Committee on Space Research (COSPAR)and the Radiation Commission of the InternationalUnion of Geodesy and Geophysics (IUGG) to considerthe problems of calibration and intercomparison ofradiation measuring instruments mounted on rocketsor satellites. The membership of the committee is givenin Table I. The committee organized a symposium onthe subject at the meeting of the IUGG in Grenoble in1975. Some of the papers presented there are collectedin this issue of Applied Optics. These papers indicatethe demands for extremely high accuracy of radiationmeasurement being made by the atmospheric sciencecommunity and the state of the art of current instru-mentation.

February 1977 / Vol. 16, No. 2 / APPLIED OPTICS 295

Table I. Membership of Committee

H. -J. Bolle, Germany. W. A. Hovis, U.S.A.A. A. Buznikov, U.S.S.R. M. Kano, JapanD. Crommelynck, Belgium A. A. Kmito, U.S.S.R.E. J. Gillham, U.K. K. Sekihara, JapanR. A. Hanel, U.S.A. G. F. Sitnik, U.S.S.R.D. F. Heath, U.S.A. K. H. Stewart, U.K.J. T. Houghton, U.K. M. P. Thekaekara, U.S.A.

(Chairman) H. Yates, U.S.A.

The committee at its meeting in Grenoble made anumber of recommendations which may be summarizedas follows:

(1) In view of the high accuracy (of the order of 0.1%)needed for continuing measurements of the componentsof the radiation budget required for research into cli-matic variation, the group recommends that great carebe taken into the calibration and intercomparison ofradiation instruments.

(2) The group recommends that radiometers mea-suring total solar radiation or longwave radiation beindividually calibrated in terms of absolute electricalstandards rather than against standard instruments andthat careful intercomparison of different absolute ra-diometers be made.

(3) That in regard to standards of spectral irradiance,particularly in the visible and uv regions, laboratoriesin different countries holding primary standards (e.g.,synchrotron radiation sources, plasma arcs) and sec-ondary standards (e.g., filament lamps) be encouragedto arrange for intercomparison of these standards. Anaccurate knowledge of the spectral distribution of solar

radiation is required and because of the differenceswhich are thought to exist between different standards,these intercomparisons should be pursued as a matterof urgency.

(4) In regard to the important role of radiometersmounted on geostationary satellites in studies of theatmospheric radiation budget and of the difficulty incalibrating instruments of large aperture and field ofview in terms of absolute standards, agencies flying orintending to fly radiometers or photometers observingreflected solar radiation should establish good com-munication between themselves regarding the detailedcalibration procedures employed.

(5) That emphasis be given to the important rolewhich can be played by SPACELAB in providing a labo-ratory in which radiation instruments can be calibratedand with which continued checks could be made ofperformance of radiometers mounted on free flyingsatellites.

296 APPLIED OPTICS / Vol. 16, No. 2 / February 1977

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