The Moon - Natural Standard for Calibration of the Visible and Infrared Images of the Earth

The method where the Moon is used as a standard of calibration of spectrophotometric under-satellite observations is presented. Observations are produced by the satellite (GOMS), which was placed on geostationary orbit in October 1994 for meteorological forecasting and monitoring of the Earth's environment [I]. A calibration is based on measurements made by Saary and Shorthill [2] by means of scanning an illuminated lunar disk in the visible (0.45 microns) and in the infrared (1 012 microns) wavelengths during a lunation. The intensity scattering of the lunar surface in visible and infrared spectrum is constant and it is not change in space during time. Radiative lunar surface can be easily discribed in the form of analytical dependences, that allow to transform brightness and temperature values to any angle parameters of observation and illumination. Procedure of the calibration is based on the comparison between of signal voltages of the scan image and computer data base, which includes spectrophotometric measurements of the photometric brightness and infrared brightness temperature of a large amount of separate spots on the lunar surface. The computer data base is constructed in the form of logical structure of the relational type and includes the information about 3000 lunar sites, brightness and temperature surface which were measured during lunation for 23 phase angles. The image of the Moon in the visible and IR spectrum on the IBM PC screen may be reproduced by an automatic programs for the calibration and normalization of the photometrical mesurements of terrestrial natural objects. The algorithm of the programs systematizes the information by specific regional parameter, it is necessary for estimation the shortage of the informative numerical data in the comparative matrix "parameter object". Numerical models of visible and infrared radiation of the lunar surface were investigated to transform brightness and temperature measurements to any angle parameters. The Akimov's theoretical model of radiation scattering of lunar surface [3] was chosen for calibration of the observed signal in the visible region of spectrum. The relationship for current brightness is following: B = kAb(G, W,L), where A is brightness of surface for phase angle -20 , constant k is calculated a least squares for approximating phase curve to values of the data base, b(G, W, L) is photometric function, which is a function of phase angle G, luminance latitude W, and luminance longitude L. The photometric function may be written in the general form: b(G, W, L) = cos[rc (L-G/2)/(rc-G) ][cos(G/2)/cos(L) ] Observed signals in the infrared region of spectrum are calibrated by method appoximation of the dependence between brightness temperature and angle parameters. The least-squares fit of the measurement data to phase function brightness temperature was accomplished by trigonometric polynomial: T = T(A,r)[qcos(i) +pcos(i)sin(G-18) +dcos(G+I 0) +262], where T(A,r) is temperature of the subsolar point, which depends on the albedo A, r is distance between the Sun and the Moon, G is phase angle, i is incidence angle, constants q, p, d are the best fit values to phase function temperature for each site of the lunar surface.