Proceedings of the Symposium on Global and Environmental Monitoring: Proceedings of the Symposium on Global and Environmental Monitoring

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Persistent identifier:: 856665355

Title:: Proceedings of the Symposium on Global and Environmental Monitoring

Sub title:: techniques and impacts ; September 17 - 21, 1990, Victoria Conference Centre, Victoria, British Columbia, Canada

Year of publication:: 1990

Place of publication:: Victoria, BC

Publisher of the original:: [Verlag nicht ermittelbar]

Identifier (digital):: 856665355

Language:: English

Document type:: Multivolume work

Volume

Persistent identifier:: 856669164

Title:: Proceedings of the Symposium on Global and Environmental Monitoring

Sub title:: techniques and impacts; September 17 - 21, 1990, Victoria Conference Centre, Victoria, British Columbia, Canada

Scope:: XIV, 912 Seiten

Year of publication:: 1990

Place of publication:: Victoria, BC

Publisher of the original:: [Verlag nicht ermittelbar]

Identifier (digital):: 856669164

Illustration:: Illustrationen, Diagramme, Karten

Signature of the source:: ZS 312(28,7,1)

Language:: English

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Editor:: International Society for Photogrammetry and Remote Sensing, Commission of Photographic and Remote Sensing Data

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2016

Document type:: Volume

Collection:: Earth sciences

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Title:: [WA-2 AGRICULTURE]

Document type:: Multivolume work

Structure type:: Chapter

Chapter

Title:: CALIBRATION AND ATMOSPHERIC CORRECTION OF THE AVHRR FOR THE PURPOSES OF AGRICULTURAL MONITORING IN EUROPE. M. Sharman, R. Santer, A. Le Lerre

Document type:: Multivolume work

Structure type:: Chapter

solar zenith angle of the block of pixels being processed. Unlike channels 1 and 2 , which receive energy from the sun reflected from the earth-atmosphere complex, channels 4 and 5 are sensitive to the radiation emitted by the earth at long infra-red wavelengths. The equivalent brightness temperatures of channels 3, 4 and 5 are calculated by first applying the calibration coefficients provided by the AVHRR and then inverting the Planck function. The intensity of the response in channel 3 is partly due to reflectance of solar energy and partly due to the radiation of the earth in that portion of the spectrum. The portion due to reflectance is estimated by using the channel 4 brightness temperature. ATMOSPHERIC CORRECTION Because the atmosphere does not scatter and absorb all wavelengths equally, the radiation reaching the sensor has a spectrum different from that which would have been received in the absence of an atmosphere. The radiance measured by the orbital instrument is therefore only partially dependent on the reflectance of the target (Tanr6 e t al. 1986). As a result of atmospheric scatter a substantial portion of the radiation reaching the sensor is contributed by reflection of the sun's radiation within the atmosphere. The target is illuminated not only by direct sunlight, but also by skylight, and a non-negligible part of the energy reaching the sensor has been scattered more than once in its double passage through the atmosphere. The algorithms used in the pre-processing software will as far as possible estimate surface geophysical parameters (bi-directional reflectance factors of channels 1 and 2 and brightness temperatures of the surface) independently of atmospheric properties. The physics of the radiative transfer through the atmosphere are well documented, and form the basis of several models providing atmospheric correction. However, most such models involve recursion to account for multiple scattering or ray- or photon-tracing and are too cumbersome to incorporate into the operational preprocessing of AVHRR data, whose main advantage is its high temporal frequency, and hence the potential timeliness of the data. Most of the effects of the atmosphere can nevertheless be corrected operationally, and to this aim the LOA, under the Tecnodata contract, is to optimise its "Simulation of the Satellite Signal in the Solar Spectrum", or "5S" model (Tanre et a l . 1986) for incorporation in the pre processing software. The model, as its name suggests, was originally designed to use ground and atmospheric characteristics to model the signal received at the satellite, and not to derive ground characteristics from the satellite signal. However, it is based on a set of analytic expressions of the optical properties of the atmosphere, each of which is reversible, given the inherent symmetry and reversibility of the passage of light through any transparent medium. For the purposes of atmospheric correction of AVHRR data, the model has therefore been re organised to derive the reflectance of the surface from satellite. the signal received at the The original 5S model is described in detail (with FORTRAN listings of the code) in Tanre e t al. ( 1 986 ) . Environment correction and nature of the atmospheric Water vapour. Channel 2 suffers markedly from absorption by water vapour. This leads in particular to considerable sensitivity of the NOVI to the water vapour content of the atmosphere. In order to correct for this effect it is necessary to know or to estimate the water vapour content at the location at which the radiometric data are being collected. The European Centre for Medium Range Weather Forecasts at Reading have archives (dating from 1/1/1980) of data derived from a model of atmospheric conditions which provide an estimate of relative humidity and air temperature at midday at 7 barometric altitudes for cells in a 2.5°x2.5° grid. The European Community is covered by about 200 of these cells. At present it is not known whether the JRC will have access to these data in real-time for inclusion in the pre-processing chain, or whether it will be necessary to assume seasonal and regional mean conditions in water vapour content. Aerosols. The software will not attempt to correct the data for atmospheric aerosols, since on the scale of Western Europe, aerosol loadings vary locally and unpredictably with time. Insufficient data exist on their distribution and particle size for any kind of routine mapping, and it is therefore not possible to eliminate their effect on a continental scale and on an operational basis using present techniques. This is to be regretted since for channel 1 and 2 of the AVHRR the contribution of aerosols to noise in the signal is often more important than that of any other atmospheric source. Surface characteristics. The 5S model used to derive the algorithm for the atmospheric correction assumes that the surface is spatially non-uniform, but that it is a Lambertian reflector. This is clearly incorrect, but we lack the necessary knowledge of the bi directional reflectance of surfaces in Europe at the AVHRR scale. We hope that in the mid to long term we will possess this information as one of the results of a major collaborative effort between the JRC and the University College London. Look-up tables. The Joint Research Centre requires an algorithm that functions efficiently and rapidly to remove the major calculable effects of the atmosphere on channel 1 and channel 2 data. The need for a rapid algorithm implies the use of a formula coupled with look-up tables that are pre-loaded with the results of any calculation that can be carried out beforehand. Thus, for example, the software will set up sun angle and view angle tables which it can index when the geometry of the orbit is known . access Auxiliary files are set up to current estimates of water for rapid vapour and oxygen concentrations, 181 sags 1Ü ;SP

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