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Atmospheric correction in polar and subpolar regions: evaluation and improvements of the 6S program
2007 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

This masters thesis is a part of the nordic collaboration project NorSEN, whose goal is to improve the environmental surveillance in the regions north of the Arctic circle. This implies the build-up of a data-acquisition network that can be used for tracing environmental changes over the years. The data from these sites can also be used as a reference for satellite image validation and to improve the atmospheric correction procedures of satellite imagery. Atmospheric correction is the process where an image taken at satellite level is recreated as it would have looked like on the ground. The signal is distorted along its way through the atmosphere by the means of absorption and scattering. Swedens contribution to this project is a raft with on board spectrometers placed on the lake Torneträsk near Abisko in the summer of 2005. The lake is surrounded by mountains which are often covered with clouds that changes the light conditions. This poses a problem for the atmospheric correction since these types of situations are more difficult to model. The lake is also often exposed to rough weather with strong winds, leading to high waves. When this occurs, it is much harder to calculate the reflectance from the data. Severe rainfalls can also lead to sediments in the lake which changes the lakes reflectance. One part of the project is therefore to evaluate if Torneträsk is a good reference surface. This part will however not be covered by the report. The masters thesis has been devoted to the evaluation and changing of the satellite signal simulation program 6S, which is also able to apply atmospheric correction to known satellite signals. Pictures from the instrument MERIS on board ESAs Earth observation satellite ENVISAT has been available within the project. Some assumptions in 6S that has proved to be a problem is that of horizontal homogeneity and the plane-parallel assumption. Since the clouds and mountains need to be modeled, an inhomogeneous model must be implemented. This can be done by introducing a horizontal x-scale which adds a parameter to all the equations governing the radiative transfer in the atmosphere. Since the x-scale cant go on forever, there must however be a combination of the homogeneous and the inhomogeneous model where the homogeneous assumption is used at the boundary layer of the inhomogeneous model. Tests have been made with a horizontal scale of 300 kilometers. The grid has to be about this wide since the atmosphere is modeled with an altitude of 150 kilometers and it is desired that as many beams as possible, they are traveling with different inclinations, reaches the top of the atmosphere. This however slows down the program significantly. The computation is therefore made with less accuracy the further you get from the lake, while an area of 10 kilometers where the raft is situated is modeled with high accuracy. Up to date it is hard to draw any substantial conclusions about whether the changes made to 6S will lead to any significant improvement. This is because the evaluation of the program has taken so long time, leaving very little time to test the new program. However, it looks like we have made some progress.

Place, publisher, year, edition, pages
Keyword [en]
Technology, atmospheric correction, atmospheric science, NorSEN
Keyword [sv]
URN: urn:nbn:se:ltu:diva-47286ISRN: LTU-EX--07/142--SELocal ID: 4d808257-96ef-4a46-ad19-56c769f8e85cOAI: diva2:1020606
Subject / course
Student thesis, at least 30 credits
Educational program
Space Engineering, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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