Simulation of SPECTRA like scenes using coupled leaf, canopy and atmospheric radiative transfer models

Heike Bach, W. Verhoef

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Future remote sensing satellite missions, like SPECTRA (Surface Processes and Ecosystem Change Through Response Analysis), that has been nominated as a candidate Earth Explorer Mission by the European Space Agency, will feature advanced hyperspectral and directional optical imaging instruments. Given the complex nature of the data to be expected from these missions, a thorough preparation of their use is essential and this can be accomplished by realistic simulation of the expected imagery data years before the actual launch. Based on given spectral and directional capabilities of the planned SPECTRA instrument, and in combination with biophysical land surface properties obtained from existing imagery, the spectral and directional responses of several types of vegetation and bare soil were simulated pixel by pixel for a real scene using radiative transfer models.

Canopy reflectances were modelled using GeoSAIL, a two-layer version of the SAIL model which incorporates a sub-model for the effect of soil moisture on the soil's background spectrum. In the GeoSAIL model two spectrally different leaf types (e.g. green and brown leaves) are divided over both layers in different proportions, so as to mimic the vertical leaf colour gradient often seen in crops like wheat. The spectra of green and brown leaves were modelled using PROSPECT. By combining the models PROSPECT and GeoSAIL, the major aspects related to optical observation of vegetation and realistic modelling of directional canopy reflectance spectra were covered. However, the result represents the reflectance spectrum as it would have been measured on the ground, not what a sensor would observe from space. Therefore, the coupling of the surface reflectance model to an atmospheric radiative transfer model also needs to be addressed. For this, the model MODTRAN4 was selected, for computing of absorption and scattering in the terrestrial atmosphere at high spectral resolution over the solar-reflective and thermal windows. In this respect, MODTRAN4 was treated as a black box and an "interrogation" technique was applied in order to derive six effective atmospheric parameters from the outputs of three different MODTRAN runs.

In this paper the modelling methodologies for image simulation in the solar-reflective spectral window are described, as well as the study area, the set-up of the simulations and the results. As test site Barrax in Spain, where already a lot of land surface processes research was carried out, was selected for the simulation of imagery to be expected from the SPECTRA mission.
Original languageEnglish
Title of host publicationProceedings of the First International Symposium on Recent Advances in Quantitative Remote Sensing, RAQRS- I
Subtitle of host publication16-20 September, 2002, Torrent, Spain
EditorsJ.A. Sobrino
Place of PublicationValencia, Spain
PublisherUniversity of Valencia
ISBN (Print)84-370-5515-6
Publication statusPublished - 2002


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