SPOT 4 is a new generation system which has significantly improved the fulfilment of the SPOT programme purposes thanks to the use of new technologies, the enhancement of high resolution instruments of the previous generation, and the implementation of a complementary payload, the VEGETATION system and its associated ground segment. This evolution is described in the present paper.
As MEDIAS Newsletter readers well know, the SPOT system has been designed by the CNES and implemented by France, in partnership with Belgium and Sweden. It is a system both operational and commercial, which products are distributed by the SPOT IMAGE company. Till the operational starting of SPOT 4, the system had been supplying monoscopic or stereoscopic observations at a resolution of 10 metres in panchromatic mode and of 20 metres in multispectral mode, in three spectral bands in the visible and the near infrared domain.
The SPOT programme is based on two essential principles:
The SPOT 4 programme is a new generation system which has significantly improved the fulfilment of its purposes thanks to the use of new technologies, the enhancement of high resolution instruments of the previous generation, and complementary payloads. The following main improvements are worth mentioning:
This programme included 39 pilot projects managed by expert groups including scientists and companies from various horizons. The results have been summarised in synthesis reports (available in CNES), which can be summed up as follows:
In the field of environment and agriculture, the sensitivity of the short wave infrared band to variations in canopy or soil water content has the effect of creating a strong contrast between soil and vegetation reflectances; it will thus be easier to identify variations in the canopy structure, especially when these are not very great. The interest of the SWIR is significant for the discrimination of wet areas, the identification of water stress in plants, the classification of some crops and vegetation areas.
In particular, the SWIR enables to obtain a better characterisation of arid and semi-arid areas. Areas with low vegetation cover are better differentiated; soil characterisation is certainly improved.
As for hydrology, the SWIR enables to obtain a better mapping of infiltrating and run-off soils, of snow layers and irrigated crops.
The SWIR is also useful to study forest areas. In particular, it provides increased precision for the mapping of forests types and is of great interest for areas which have been poorly mapped (tropical environment), the detection of clearcuts and thinned areas, the estimation of cover rate and the relative humidity of plant communities.
Picture of the satellite SPOT 4
As a complement to the two HRVIR instruments, SPOT 4 satellite takes on board the VEGETATION instrument. These two kinds of instruments on board a same satellite will enable observations in the same radiometric and geometric conditions, thus leading the way to scientific and commercial projects which will exploit the high and low resolution or high and medium repetitivity synergy.
The VEGETATION system is the result of a co-operation between the European Union, France, Sweden, Belgium and Italy. It aims at ensuring a regional and global continuous monitoring of the continental biosphere and of crops. It observes the Earth at a resolution of 1.15 km, quite invariable in the swath width of nearly 2250 km. This gives almost daily access to any point on the earth surface. Taking into account the measurements which have to be discarded due to cloud coverage or bad atmospheric conditions, this feature mzimases the probabilty to get one useful measurement per ten-day periods, the multitemporal spatial resolution being of about 1 km. These characteristics suit the observation and study of seasonal evolutions in the biosphere and its processes. Moreover, SPOT enables to lead these studies and observations in a multi-scale context, as the spectral bands and geometrical references of the VEGETATION instrument are the same for the two HRVIR instruments, the three instruments being inter-calibrated.
The acquired data are archived on board, then transmitted to a main station (Kiruna, Sweden) and processed through a ground segment which delivers controlled high quality products. It is also possible to transmit regional data to ground stations in L band, within their coverage.
The central system supplies two kinds of products, all of them supplied according to map projections previously defined. The products known as VGT-P products, more intended for a scientific use, correspond to images of viewing segments without atmospheric corrections, which enable to compute the reflectance at the top of the atmosphere.
The products known as VGT-S products, more intended for operational projects, are geocoded products with atmospheric corrections, and for which daily syntheses (S1) or 10-day syntheses (S10) are established. The normalised vegetation index (NDVI) is systematically included in these products.
As stated above, its characteristics enable the system to perform an accurate follow-up of the seasonal evolution in the continental biosphere and to create consistent temporal series. It therefore enables to lead applications such as decision helping for agriculture, early warning systems, follow-up of deforestation and forest degradation, various environmental studies (vegetation cover dynamics, follow-up of large ecosystems, management of natural resources). In the scientific field, it particularly enables to develop models necessary to a better understanding of the interactions between biosphere and climate. One of these scientific applications, well-known to MEDIAS Newsletter readers, concerns the studies on the evolution of the greenhouse effect particularly caused by the accumulation of carbon dioxide in the atmosphere, and the part played in this field by the biosphere (photosynthesis, decomposition) and its evolution (deforestation, desertification). The recent attempts at forecasting climate changes in relation with the evolution of the vegetation cover lead to conflicting scenarios.
The researched magnitude is 2 billion tons out of a total of approximately 120 billion tons, the human contribution being approximately of 6 billion tons. VEGETATION is consequently an essential tool to quantify these modifications and help forecasting the resulting evolutions in the climate. These studies will be essential for the implementation of the international convention on climate change and the Kyoto procotol.
A VEGETATION preparatory programme has been implemented under the supervision of the International Users Committee (IUC) in order to promote the development of methodologies for the use of spatial remote sensing data of medium resolution and high repetitivity, in order to carry out studies on vegetation and also to prepare the integration of the VEGETATION databases into existing or under development databases.
This programme is mainly based on investigations selected through two calls for tenders, in July 1994 (24 proposals selected) and July 1996 (8 proposals selected).
Three specific domains of priority have been defined by the IUC for these investigations:
Updated information regarding the SPOT system, and particularly VEGETATION, may be found on the CNES server (http://www.cnes.fr/) and the SPOT IMAGE server (http://www.spotimage.fr/). Information regarding namely the VEGETATION preparatory programme may be found at the following address: http://sirius-ci.cst.cnes.fr:8080/
CNES - 18avenue Edouard Belin - 31401 TOULOUSE Cedex 4
A. PODAIRE - Responsable Programme VEGETATION
Tél. 33 (5) 61 27 44 18
Fax 33 (5) 62 27 40 13
E-mail : [email protected]
G. SAINT- Responsable scientifique VEGETATION
Tél. 33 (5) 61 27 36 54
Fax 33 (5) 61 27 38 05
E-mail : [email protected]