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Nr. 19 - June-July '98
As the first system specifically designed to observe the Earth's vegetation cover, VEGETATION opens up new opportunities to study, protect, and manage the vegetal biosphere. Benefiting from substantial EU funding, the VEGETATION sensor was launched aboard the SPOT 4 satellite with the mission to send back "standard products" in line with users' demand. The programme reaffirms the importance of European space expertise.
The SPOT 4 satellite was launched on 24 March 1998
in Kourou, French Guyana, with the Vegetation sensor on board. A week
later, Vegetation was sending back its first images. Previously, US satellites
(1) with observation systems designed for meteorologists had been the only ones
to provide a global view of the Earth's forests and crops. With the implementation
of the Vegetation programme, Europeans have developed an instrument specifically
designed to monitor the Earth's plant life by making full use of the many possibilities
offered by the orbiting SPOT platform.
The 46% (ECU 51 million) Commission financing for Vegetation demonstrates Europe's commitment to Earth observation - which received funding of ECU 275 million between 1995 and 1998 - and its desire to develop its role as a catalyst in this field further. Vegetation reinforces the European expertise developed during projects such as Fuego and Megafires (fire detection and risk zone studies in the Mediterranean region), FIRS and TREES (cartography of coniferous and tropical forests) and MARS (operational monitoring of European crops and the decline of harvests), with the support of the Space Applications Institute (SAI) at the Commission's Joint Research Centre at Ispra.
"The Vegetation specifications were drawn up on the basis of the needs of fundamental research teams working on the functioning of major ecosystems, environmental change and the impact of human activities, and much more pragmatic applications, such as controlling forest fires or forecasting agricultural production," explains Gilbert Saint, head of the Vegetation mission at the CNES. "Although they are all studying the same biophysical phenomena, the scientists can generally use less-operational data than are required by bodies interested in the status or forecasting of stocks. Also, as such data are intended for global users, the specifications had to conform as much as possible to existing international image archives."
A Preparatory programme for the use of Vegetation products has been set up, following a call for proposals, in order to improve the processing algorithms for the Vegetation data, define innovative products, identify potential fields of application and stimulate the interest of the widest possible community of users.
The CTIV is able to process and deliver to users in under three days the products ordered by subscription. A European consortium formed by Nuova Telespazio (I), Satellitbild (S), CLEO (B) and SPOTImage (F) is responsible for the worldwide marketing of the Vegetation data. "Access to the data overviews is also very rapid," points out Gilbert Saint. "Whereas in the United States the USGS gathers data received by about 40 stations for processing purposes - which is going to take some time because in 1998 the summaries of data acquired in 1996 are only just becoming available."
The information provided by Vegetation also guarantees European autonomy. "For the Commission, Vegetation represents an independent information source which can support its sectoral policies - whether in assisting agricultural production and reform, forest management, environmental monitoring, regional development, preparing the data necessary for drawing up international agreements, or implementing international emergency food aid," explains Michel Schouppe.
Worth noting is the fact is that the European Commission's financial contribution to Vegetation 2 - which will be developed with financial participation of industry - will be much reduced. This reflects industry's growing interest in very high-level scientific and technological investments in an up-and-coming field offering the prospect of a fast-growing market for applications.
(1) Of the AVHRR (Advanced Very High-Resolution Radiometer) type fitted to NOAA (National Oceanic and Atmospheric Administration) satellites.
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Spatial resolution - The present state of detector technology, with
the detectors arranged in a line on a small bar, makes it possible to reduce
the geometric distortions observable at the edges of the image considerably
and to maintain the spatial resolution of 1.15 km (resolution variations ranging
from 1 km to more than 6 km are observable in AVHRR images). The localisation
of each pixel is very precise - 300 metres dispersion in the course of one year.
The analysis of a series of measurements of a given geographical point, taken
over a specific period of time, is therefore based on reliable spatial data.
Four spectral bands - In addition to the two traditional spectral bands of red and near infrared (which give indications of photosynthetic activity), medium infrared and blue are now included.
The radiometric resolution has been adapted in order to measure variations linked to ground use - and not only the major contrasts which meteorologists need. Medium infrared, which is sensitive to the water content of soils and plants, is able to detect humidity and cover structure. The blue was mainly added in order to obtain better atmospheric corrections - and secondarily for certain oceanic applications. This band also provides opportunities for soil cartography and the monitoring of desertification.
The linking of resolutions - Vegetation works at two levels. Its low-resolution data (each pixel represents 1x1km2) can be combined with those of other high-resolution SPOT 4 instruments (HRVIR - High Resolution Visible Infrared) which are able to observe details of 10 to 20 cm. This linking of low- and high-resolution images taken simultaneously from the same platform permits a finer analysis. "A zoom effect with the high resolution and the same spectral bands makes it possible to describe better virtually every Vegetation pixel. We can therefore understand why such a measurement changes over time depending on the diversity of the soil use," explains Gilbert Saint. Conversely, Vegetation makes it possible to make up for the low frequency of the high resolution which only permits a few images a year - between three and five in Europe.
Vegetation supplies its data in two forms: primary products
(VGT-P) and synthesised products (VGT-S). The former relate to a measurement
taken when the instrument passes a certain point. The latter select, within
a given period (one day or 10 days), the best measurement from those available
at a given geographical point.
"The method currently used for synthesised products results from tests using traditional AVHRR data," explains Gilbert Saint. "We are waiting to receive sufficient Vegetation data to validate new methods using blue and medium-infrared bands in particular."
The 10-day overview seems sufficient to determine the development of vegetation. But in order to obtain a quality measurement every 10 days, it is necessary to continue to gather data every day.
The traditional high-resolution images transmitted by
SPOT satellites scan a strip of ground 60 km wide. This relatively narrow and
necessarily intermittent scanning of the Earth's surface only permits pictures
obtained at quite widely spaced time intervals. But the observation of the evolutionary
cycle of vegetation at any given site is only of any meaning if it is repeated
with sufficient frequency. This is exactly what the four low-resolution cameras
which function as Vegetation's eyes are able to do.
They can photograph an area 2,250 km wide in the course of SPOT's 14 daily orbits (1), a field of view which allows them to obtain excellent-quality data on any point on the Earth's surface on an almost daily basis. In the space of 24 hours, Vegetation is able to transmit pictures of 90% of the world's equatorial zones, the remaining 10% being scanned the next day without fail. For latitudes above 30°, the frequency of passage over a given area increases progressively. At the Kiruna receiving station in Sweden, for example, data is transmitted five or six times daily, each time the satellite passes overhead.
(1) This scanning shifts slightly every day, with SPOT 4 returning above any given point every 26 days, after 369 orbits.