Vicarious satellite calibration in the solar spectral range by means of calculated radiances and its application to Meteosat

Peter Koepke

doi:10.1364/AO.21.002845

Applied Optics
Vol. 21,
Issue 15,
pp. 2845-2854
(1982)
•https://doi.org/10.1364/AO.21.002845

Vicarious satellite calibration in the solar spectral range by means of calculated radiances and its application to Meteosat

Peter Koepke

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Peter Koepke, "Vicarious satellite calibration in the solar spectral range by means of calculated radiances and its application to Meteosat," Appl. Opt. 21, 2845-2854 (1982)

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Abstract

The method of vicarious calibration by means of calculated radiances allows absolute calibration of satellite radiometers in orbit. It works by comparing counts from the radiometer to be calibrated with corresponding absolute radiances, calculated from actual values of the relevant optically acting parameters of the atmosphere and the earth’s surface. The method is applied to the VIS-channel (it measures in the visible and near IR) of the European geostationary satellite Meteosat-1. To minimize uncertainties, the procedure is carried out over different surfaces, at different atmospheric conditions, and at different sun and satellite angles. The ratio between the effective radiances (the radiances at the satellite weighted with its spectral response) and the measured 6-bit counts of the Meteosat-1-VIS-channel is the calibration constant c_sat = 2.66 W · m⁻² · sr⁻¹/count. The accuracy of the calibration is ±6%. The inaccuracy is mainly due to the broad digitization steps of the channel. Conversion factors are presented which allow one to calculate from the effective radiance the radiance at the satellite (the radiance leaving the atmosphere).

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E_⊙	Solar irradiance at the top of the atmosphere
L_rec	Upward emerging radiance from the top of the atmosphere, the radiance at the satellite, integrated over the wavelength between 0.4 and 1.1 μm [with rectangular spectral response equal to 1, Eq. (7)]
L_sat	Upward emerging radiance from the top of the atmosphere measured by Meteosat-VIS-channel, weighted with its spectral response [Eq. (2)], the effective radiance
L_sol	Upward emerging radiance from the top of the atmosphere, the radiance at the satellite, integrated over the total solar wavelength region [Eq. (5)]
L_T	Upward emerging radiance from the top of the atmosphere, the radiance at the satellite
T	Linke turbidity factor (total optical depth in multiples of Rayleigh optical depth)
c_sat	Calibration constant for the Meteosat-VIS-channels [Eqs. (3), (4), (10), and (11)]
f_rec	Conversion factor for calculating L_rec from L_sat
f_sol	Conversion factor for calculating L_sol from L_sat
h_⊙,h_sat	Elevation angle of sun, satellite
p	Scattering phase function
γ_r	Bidirectional reflection function
λ	Wavelength; converts a quantity into spectral quantity when used as a subscript
σ_a	Gas absorption coefficient
τ_sat	Relative spectral response of Meteosat-channels-VIS-1 and VIS-2
φ	Azimuth angle of satellite relative to sun’s azimuth
$\tilde{ω}$	Single-scattering albedo

	This study
Target type	Conversion factors (f_rec)	Calibration factors (f_rec*c_sat) [see Eq. (9)] (%)	Kriebel5 calibration factors (u_c) (%)
Land surface	1.6–1.8	4.26–4.79 ± 6	5.16 ± 5
Stratocumulus	1.9–2.0	5.05–5.32 ± 6	5.40 ± 5
Atlantic Ocean	2.9–3.0	7.71–7.98 ± 6	9.60 ± 20
Mediterranean Sea	2.5–2.8	6.65–7.45 ± 6	8.00 ± 20

E_⊙	Solar irradiance at the top of the atmosphere
L_rec	Upward emerging radiance from the top of the atmosphere, the radiance at the satellite, integrated over the wavelength between 0.4 and 1.1 μm [with rectangular spectral response equal to 1, Eq. (7)]
L_sat	Upward emerging radiance from the top of the atmosphere measured by Meteosat-VIS-channel, weighted with its spectral response [Eq. (2)], the effective radiance
L_sol	Upward emerging radiance from the top of the atmosphere, the radiance at the satellite, integrated over the total solar wavelength region [Eq. (5)]
L_T	Upward emerging radiance from the top of the atmosphere, the radiance at the satellite
T	Linke turbidity factor (total optical depth in multiples of Rayleigh optical depth)
c_sat	Calibration constant for the Meteosat-VIS-channels [Eqs. (3), (4), (10), and (11)]
f_rec	Conversion factor for calculating L_rec from L_sat
f_sol	Conversion factor for calculating L_sol from L_sat
h_⊙,h_sat	Elevation angle of sun, satellite
p	Scattering phase function
γ_r	Bidirectional reflection function
λ	Wavelength; converts a quantity into spectral quantity when used as a subscript
σ_a	Gas absorption coefficient
τ_sat	Relative spectral response of Meteosat-channels-VIS-1 and VIS-2
φ	Azimuth angle of satellite relative to sun’s azimuth
$\tilde{ω}$	Single-scattering albedo

	This study
Target type	Conversion factors (f_rec)	Calibration factors (f_rec*c_sat) [see Eq. (9)] (%)	Kriebel5 calibration factors (u_c) (%)
Land surface	1.6–1.8	4.26–4.79 ± 6	5.16 ± 5
Stratocumulus	1.9–2.0	5.05–5.32 ± 6	5.40 ± 5
Atlantic Ocean	2.9–3.0	7.71–7.98 ± 6	9.60 ± 20
Mediterranean Sea	2.5–2.8	6.65–7.45 ± 6	8.00 ± 20

Abstract

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Equations (12)

Applied Optics