SPOT4 (Take5) : Simulation of Sentinel-2 Time Series on 42 Large Sites
Hagolle, Olivier1; Sylvander, Sylvia2; Huc, Mireille3; Clesse, Dominique4; Houpert, Laurence2; Daniaud, Frédéric5; Leroy, Marc2; Dedieu, Gérard1; Koetz, Benjamin6; Paganini, Marc6; Seifert, Frank Martin6; Pinnock, Simon6; Hoersch, Bianca6; Bartholomé, Etienne7; Achard, Frédéric7; Mayaux, Philippe7; Masek, Jeffrey8; Claverie, Martin9; Vermote, Eric8; Fernandes, Richard10

This paper presents the SPOT4 (Take 5) experiment, aimed at providing time series of optical images simulating the repetitivity and large area of Sentinel-2 images, in order to help users set up and test their applications and methods, before the mission is launched.

In 2016, when Sentinel-2 constellation is complete, and for at least seven years, users will have access to high resolution time series of images acquired every 5 days, anywhere among the Earth land surfaces. This new dataset will drastically change and enhance the way land surfaces are monitored using remote sensing. Sentinel-2 frequent revisit will assure that a given surface will be observed at least once a month, except in the most cloudy periods and regions, enabling to develop operational applications.

New methods and algorithms will have to be developed, in order to handle time series covering very large areas. The methods will need to be robust to data gaps due to clouds that will be present on most Sentinel-2 290*290 km images. Given the number of images to handle, the methods will have to be automatic and the large area covered will push users to develop methods that can handle large territories with variations in climate and land cover. At the Sentinel-2 preparatory symposium in 2012, the user community voiced a high interest to develop such new methods and applications well in advance before the launch of Sentinel-2, enabling a timely start of operational applications as soon as the data becomes available. The SPOT4 (Take5) experiment is providing the users with time series of observations close to those of the Sentinel-2 mission in terms of temporal revisit and spatial resolution, which will help them to prepare for the exploitation of Sentinel-2 data.

Several simulations of Sentinel-2 images with all its 13 spectral bands have been created from hyperspectral images, but these images lack the repetitivity and large area of Sentinel-2 data. Other simulations of Sentinel-2 data have been obtained using Landsat and RapidEye, providing images over large areas, but here again, the repetitivity is much lower than that of Sentinel-2. CESBIO distributed time series of Formosat-2 images with a very high repetitivity, but they only covered small sites (24*24km2).

When CNES offered to use SPOT4 for technical experiments, at the end of its commercial life, CESBIO proposed to change SPOT4 orbit, in order to place it on a 5 days repeat cycle orbit. CNES started this experiment on the 31st of January 2013, and it will last until the end of May 2013.

Time series of SPOT4 images will be acquired every 5th day, over 42 sites scattered in nearly all continents, and covering very diverse applications (land cover and land use, agriculture, phenology, hydrology, snow monitoring, coasts monitoring, habitats characterisation and biodiversity...).

Several space and research agencies (ESA, NASA, CCRS, JRC) also contribute to the mission by funding the cost of edition of the products acquired on the sites they selected. CNES and CESBIO preprocessing methods will be used to deliver ortho-rectified images of surface reflectance after atmospheric results, and delivered with a high quality cloud and cloud shadow mask. At the date of the conference, the produced time series should have been made available to anyone, through the new French Land Data Center.

This paper presents the preprocessing methods and their performances validated against in-situ measurements. The clouds and aerosol detection methods are based on a combination of multitemporal and multispectral methods already applied to Landsat and Formosat-2 images, but here, these methods have to be applied to a much greater number of sites and must also handle the lack of a blue band on SPOT4. Finally, the paper also presents a few application results obtained from SPOT4(Take 5) experiment.

One of the first SPOT4 (Take 5) images over Provence, made from a Mosaic of 4 SPOT4 images acquired simultaneously and ortho rectified, and thus covering 120*120km2. This site, and the 41 other sites will be observed every 5th day during 4 months.