Chaired by André Morel (LOV, France) and James Mueller (CHORS, USA)
Currently, two different solar flux spectra are used within the ocean-colour community. There are small, but significant, differences between the determinations of the solar spectral irradiance at the top of the atmosphere, F0(λ), as measured by Neckel and Labs (1984) and that of Thullier et al. (1998). Maximal differences (3-5%) occur in the blue part of the spectrum (445-510 nm), and various agencies use either the old values or the new ones (see Minutes, IOCCG-7). The consequences of these differences are that:
- The top of the atmosphere radiances recorded by several instruments are not directly comparable (they are calibrated with reference to the moon or a diffusing plate, and then by assuming a F0(λ) spectrum).
- This problem can be solved if you know the F0(λ) values adopted by the various agencies (by accounting for the ratio between the two F0(λ) spectra; the comparison is always valid in terms of reflectance)./li>
- The normalised water-leaving radiance produced from sensor measurements are independent of the choice of F0(λ) adopted (by virtue of ratios, which cancels the spectrum).
- For in situ measurements, the situation is different. Top of atmosphere radiances cannot be directly compared since the results are dependent on the choice of F0(λ).
It is important that a single, common standard solar flux spectrum be used in every aspect of research and validation, since it enters into the normalisation of water leaving radiance, calibration of atmospheric radiation measurements and atmospheric correction algorithms.
Recommendation from the Working Group
An ideal solution to the problem would be to adopt a common international standard scale for the spectrum of extraterrestrial solar irradiance. At the 17th CEOS Plenary (18-20 November 2003, Colorado Springs) the Working Group on Calibration and Validation recommended that all Agencies converge to the recently refined solar reference spectrum of Thullier et al. (2003) and encouraged its use at the highest possible spectral resolution. This is an ideal solution, but may take a while to implement as it requires significant changes in the software used for operational processing and validation analysis. Therefore, as an intermediate solution, IOCCG recommended that all agencies publish (on their web-site) the F0(λ) spectrum adopted by each satellite ocean-colour sensor project, as well as the convolution of this spectrum, with the spectral profile of each channel of their sensor. The solar spectra adopted by various ocean-colour missions are listed below.
| Mission | Launch | Applied Solar Spectrum | Spectral Range of the Solar Spectrum | References | Data |
|---|---|---|---|---|---|
| MOS-IRS | 1996 Mar | Neckel & Labs (1984) | 400-1250 nm | Sumnich (1998) [Abstract] | see oceancolor.gsfc.nasa.gov |
| SeaWiFS | 1997 Aug | Thuillier et al. (2003) | 400-1250 nm | Barnes & Zalewski (2003) | see oceancolor.gsfc.nasa.gov and oceancolor.gsfc.nasa.gov |
| MODIS Terra | 1999 Dec | Thuillier et al. (2003) | 350-800 nm 800-1100 nm 1100-2500nm |
Xiong et al. (2003) | see oceancolor.gsfc.nasa.gov |
| MODIS Aqua | 2002 May | Thuillier et al. (2003) | 350-800 nm 800-1100 nm 1100-2500nm |
Xiong et al. (2003) | see oceancolor.gsfc.nasa.gov |
| MERIS | 2002 | Thuillier et al. (2003) | 199-2493 nm | Thuillier et al. (2003) | MERIS_Wavelengths.xls NB read README file! |
| GLI | 2002 | Thuillier et al. (2001a) | 199-2493 nm | Nieke & Fukushima |
References:
Barnes, R.A. and Zalewski, E.F. (2003). Reflectance-based calibration of SeaWiFS. II. Conversion to radiance. Appl. Optics, 42(9): 1648-1660.
Mueller, J.L., R.W. Austin., A. Morel, G.S. Fargion, and C.R. McClain (2003). Ocean optics protocols for satellite ocean color sensor validation, Revision 4, Volume 1: Introduction, background and conventions. NASA/TM-2003-211621, Natioanl Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland.
Neckel, H., and D. Labs (1984). The solar radiation between 3300 and 12500. Solar Phys., 205-258.
Sumnich, K-H. (1998). In-flight calibration of the Modular Optoelectronic Scanner (MOS). Int. J. Rem. Sens.,19(17): 3237-3259.
Thuillier, G., M. Herse, P. C. Simon, D. Labs, H. Mandel, D. Gillotay, and T. Foujols (2003). The solar spectral irradiance from 200 to 2400 nm as measured by the SOLSPEC spectrometer from the ATLAS 1-2-3 and EURECA missions. Sol. Phys., 214: 1-22.
Thuillier, G., M. Herse, P.S. Simon, D. Labs, H. Mandel, D. Gillotay, and T. Foujols (1998a). The visible solar spectral irradiance from 350 to 850 nm as measured by the SOLSPEC spectrometer during the Atlas 1 mission. Solar Phys., 177: 41-61.
Thuillier, G., M. Herse, P.S. Simon, D. Labs, H. Mandel, and D. Gillotay (1998b). Observation of the solar spectral irradiance from 200 to 870 nm during Atlas 1 and Atlas 2 missions by the SOLSPEC spectrometer. Metrologia, 35:689-695.
Xiong, X., Chiang K, Esposito, Guenther, B., and Barnes, B. (2003). MODIS On-orbit Calibration and Characterization. Metrologia, 40: S89-S92.
The sixth International Ocean Colour Science (IOCS) meeting will take place in Darmstadt, Germany from 1 – 4 December 2025, hosted by EUMETSAT and ESA with support from other agencies.