Vegetation - a "green" eye for SPOT
How can we hope to understand climate change, analyse the relationship between man and his environment, monitor risk areas, follow the progress of a forest fire, pinpoint a natural disaster, forecast levels of agricultural production? Simple: by observing the planet from above and by processing data transmitted by satellite. As the first system designed specifically to observe the earth's plant cover, Vegetation offers new prospects for the study, preservation and management of the plant biosphere. Following its launch on board the SPOT 4 satellite with the aim of delivering "standard products" and with wide-ranging support from the EU, Vegetation has reaffirmed the importance of European space expertise.
On 24 March 1998 the SPOT satellite, with the Vegetation instrument on board, was launched from Kourou (Guyana). A week later, Vegetation was sending back its first images. This instrument has been specifically constructed for monitoring the plant world (in the past a global view of the earth's forests and crops was possible only by means of observation systems designed for meteorologists) and is the result of a European project drawing on the combined effort of four partners with 46% of the funding (51 million) being provided by the European Union. "The launching of such a programme has required a level of financial investment, research input, and pooling of industrial infrastructures and knowhow that could never have been fully implemented without European support," explains Michel Schouppe, a member of the scientific staff in the European Commission's Research DG. "Vegetation is part of a dynamic package of measures launched by the Commission with the aim of revitalising the earth observation market through the development of operational applications which meet the needs of users."
31.3.98. First image obtained from data supplied by Vegetation.
On a global scale
The actual design of Vegetation is dictated by the desire to deliver "/research/success/en/env/image products" that meet the requirements of the numerous players involved in the accumulation of knowledge on, and the sustainable management of, our plant heritage (1). "Vegetation's specifications were devised in line with the requirements of the basic research teams working on the functioning of major ecosystems, changes in the environment and the impact of human activities, and also in line with much more pragmatic applications - such as the control of forest fires or the forecasting of agricultural production," explains Gilbert Saint, who is responsible for the Vegetation mission at the CNES. To this end, and in order to monitor developments on a day-to-day basis, these specialists need high-quality images and a continuous flow of comparable data that are rapidly accessible from any point on the planet.
Low and high-resolution coupling
As an on-board component on SPOT, Vegetation has two strings to its bow: its own technology and the data provided by the other instrumentation on board the same satellite. Each day this satellite orbits the earth 14 times, passing directly over the same given point every 26 days. The conventional, high-resolution images provided by SPOT sample the ground across a 60 km wide band, although the photographs obtained from a scan of this nature are inevitably characterised by a significant time lag.
Observing the evolutionary cycle of plant life only makes sense if it is done on a sufficiently repetitive scale. This is why Vegetation's four low-resolution cameras have been designed to provide a "swath" on the ground of 2250 km. In a 24-hour period Vegetation is capable of sending back images of 90% of the equatorial zones, whereby the most extensive zones are overflown and the remaining 10% are automatically scanned the following day. These images, moreover, are of a remarkable precision, with very little distortion at the edges. Since the positioning of each pixel is also extremely precise - 300 metres dispersion over a year - the analysis of a series of measurements at a given geographical point, taken over a certain period of time, provides directly comparable data.
Vegetation has also been equipped with four spectral bands. Infrared and blue have been added to the two conventional bands (red and near-infrared) which are indispensable to meteorologists. The infrared, which is sensitive to the water content of soils and plants, is suitable for detecting humidity and cover structure. The blue allows better atmospheric corrections - and, incidentally, certain oceanic applications - while at the same time opening up possibilities for soil mapping and the monitoring of soil degradation.
Lastly, the ability to couple images taken simultaneously from the same platform - those from Vegetation, where each pixel represents 1km2, and those from the other instruments capable of resolving details to 10m to 20m - makes it possible to refine the analysis. "A zoom effect with high resolution and the same spectral bands makes it possible to provide a better description of virtually every pixel of Vegetation. We are therefore in a position to understand why every measurement evolves over time as a function of soil occupation diversity," explains Gilbert Saint.
Four cameras, with a ground swath of 2250 km, will enable images of 90% of the equatorial zones to be retrieved in 24 hours.
From space to earth
However, this instrument owes its efficiency to important earth-based components for the reception and preparation of the products, viz. the Vegetation Programming Centre in Toulouse (France), the data receiving station in Kiruna (Sweden) and the Vegetation Image Processing Centre (CTIV) in Mol (Belgium). These facilities are responsible for the archiving and systematic processing of all the earth's plant cover. With the help of daily updates it is possible to prepare summaries covering each decade, and the data can be processed and delivered to users at exceptionally short notice. "For the Commission," Michel Schouppe points out, "Vegetation represents an independent source of information likely to prove useful in the discharge of its specific tasks - whether in the area of support for agricultural production and restructuring, forest management, environmental monitoring, regional development, preparation of data required for drawing up international conventions, or the launching of international emergency food aid operations."
Given that satellites of the SPOT variety have a service life of about five years, a second research project, Vegetation 2, involving a large number of manufacturers, will ensure that the needs of the users of satellite data are targeted even more specifically.
(1) These needs have been identified by an international users' committee including representatives of international organisations - such as the FAO - and of international programmes - such as the International Geosphere Biosphere Programme.