It is not yet in the direct, but in a context of crisis we can respond to events within twenty-four hours, estimating the damage of a volcanic eruption, assessing a situation of conflict, monitor maritime pollution... "According to Eric Boussarie, project leader at the national Centre for space studies, the generation of the Pleiades satellites under construction will changer in the observation of the Earth. Suffice it to 3 seconds to acquire an image high definition color near infrared. 70 Cm resolution claimed by civil and military users has led to extreme miniaturize instrumentation optics that delivered Alcatel Alenia Space to EADS Astrium, charge of the satellite constellation. "This new generation of tools has been built to easily turn around its centre of gravity." "This agility will expand the possibilities of taking", said Eric Boussarie.
Compared to Spot, sweeping a relatively broad geographic scope with a resolution of 3 metres, the generation Pleiades will deliver images of 20 kilometres apart from a Sun-synchronous orbit over the poles to 694 kilometers following the longitudes.
Has reason to orbit every 90 minutes travelled at the speed of 7 km/s, two satellites separated from 180 degrees balaieront the world in a day. Turning easily on its axis, Pleiades may acquire command on a mosaic of images on several adjacent terrestrial bands forming in the end a square of side 120 kilometres. "It is a gain of productivity by a factor of 3 compared to the older generation of satellite." Combining the effect multispot allows for multiple shots of the same surface in a single day, we hope to enter the operational loop of large organizations such as the Global Monitoring Environment and Security of the European Commission or civil security. "The CNES will also provide a"layer"radar whose development has been entrusted to the Italian Space Agency to cover the needs of information even when cloud cover or at night.
The interest of flexibility of Pleiades is the possibility of taking stereo and tristéréo in a pass. To obtain this result with Spot, operators are overlay several images sometimes within a few months apart. By programming a cyclometer oscillation, can photograph the same subject from several angles in a single overview of the area. This capability has been integrated as the major constraint upon the design of the spacecraft. "A real technological challenge", summarizes Bruno Bailly, head of project at Alcatel Alenia Space. To be agile, Pleiades must be more compact than its predecessors: a tonne, from three to five different Spot satellites, including 200 kg only for instrumentation. To achieve this balance, the Cannes company has not only saved on materials. "We have new technical options, in particular as regards the telescope", says the engineer. No lenses on Pleiades: to gain in compactness, scientists have integrated system optical Korsch, consisting of only three mirrors in spacer. A technology jump was conducted including in Siu to succeed polishing of curved reflector that focuses the beam detectors.
To provide optimal view taken, the instrument also incorporates an original device of thermal refocusing. This lack of complex mechanism was combined with a skeleton in carbon at very low coefficient of expansion, which ensures the stability of the alignment of the mirrors to micron near. "Submicron gap leads to a shift in the image of 70 micron", says Bruno Bailly. Technical options resulted in effects on the entire system. Instead of the actuators which usually fly satellites, engineers for example preferred a gyroscopic system similar to the one aboard the international space station, less énergétivore. To improve stability catches of sight, all flexible elements have been rigidifiés, including the solar generators.
Another compromise to the level of the embedded computer memories: better definition being heavier image files, should be logically more storage capacity. Denied. "We therefore designed a new way of coding to densify the wireless transmission of data over receiving stations", said Eric Boussarie. During the ten minutes of overview of based antennas to Toulouse and Kiruna (Sweden) for the civil part, satellites will be able to transfer each 450Mbits of data per second, or about 450 images every day, three times more than the generation Spot. Consequence: Electronics has been reduced to a single integrated block. Less than three meters of cables were required against more than seventy on the Spot, and the total weight was divided by five despite an increased complexity will in particular help compartmentalize the military and civilian applications.