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The Sentinel satellites – Astrium is playing a key role in environment and security for Europe

GMES will give Europe a unique global environmental information system that European citizens can be proud of.

“GMES will give Europe a unique global environmental information system that European citizens can be proud of. This programme is already regarded by other countries as exemplary in terms of utilising existing operational Earth observation systems to provide critical information for decision-makers and ordinary citizens,” says Volker Liebig, ESA’s Director of Earth Observation Programmes.

ESA, which is responsible for the overall management and coordination of the activities associated with the GMES space component in Europe, has now pushed ahead with the satellites in the Sentinel series by entering Phase 2 of their development. This initially comprises the manufacture of three satellites (Sentinel-1, -2 and -3) and the construction of the associated ground segment.

Until the launch of these satellites, which is slated for 2011/12, ESA will also co-ordinate the provision of the Earth observation data needed for the services currently operated by the European Community. In this way, GMES will advance step by step from the pre-operational stage through to full operational readiness. This stage will have been reached when the Sentinel satellites are in position on their respective orbits.


Sentinel-1 is an all-weather, day/night radar imaging satellite mission for land and ocean services. Weighing roughly 2.2 metric tonnes, from 2011 the satellite will orbit the Earth at an altitude of 700 km. Its orbital path will always take it across the Earth’s poles, enabling the radar instrument to scan the Earth in swaths as the planet rotates beneath it. Sentinel-1 will observe the Earth for at least seven years.

The C-band SAR (synthetic aperture radar) instrument built by Astrium is the core element of the mission. The C-band radar beam, which has a wavelength of six centimetres, penetrates forests and bushes to reach the soil beneath, so that any movements and changes to the Earth’s surface can be registered to within a few centimetres.

Synthetic aperture radar makes use of the satellite’s flight motion to simulate a considerably larger antenna than is actually present, thereby significantly improving image resolution. Sentinel-1 will actually be equipped with an active antenna consisting of 280 individual antennas. It will be possible to electronically direct the antenna at a new imaging area without having to be moved for the purpose. SAR technology thus enables several contiguous swaths to be scanned as the satellite flies over them, and these images can later be assembled to form a larger overall picture.

A further notable feature is that the radar instrument is capable of operating in four different modes, which differ primarily in terms of the corridor width covered and the resolution of the radar images – which means that Sentinel-1 can cater for many different requirements, from images of 80km-wide swaths, down to 20×20km spots.

Sentinel-1, primed by Thales Alenia Space, is specially designed to perform a broad range of tasks in the areas of environment and security. These include observing environmental phenomena, deployment for coverage of crisis areas or humanitarian relief projects following natural disasters, and support for agricultural projects – to name just a few examples.

ESA Sentinel-2 is a mission to provide high-resolution, multi-spectral images for land-based services. It will deliver sophisticated optical images in a large number of spectral ranges. Starting in 2012, the roughly 1.1-metric-tonne satellite will orbit the Earth on a sun-synchronous, polar orbit at an altitude of 786 km, providing complete global coverage in just ten days. Sentinel-2 is intended to image the Earth’s land masses from orbit for at least seven years, ensuring continuity in Europe’s optical Earth observation capabilities.

Astrium was appointed prime contractor for the design, development and integration of the platform and satellite – and now heads a European industrial consortium whose core team members are as follows: Astrium SAS is responsible for the payload instrument and system support activities; Boostec, France, is supplying the silicon-carbide telescope with three mirrors; Astrium Madrid-Barajas is in charge of structural and thermal work on the satellite; Jena-Optronik, Germany, is responsible for the electrical instrument architecture, including video signal processing and data compression; and Sener, Spain, is supplying the mechanism for calibrating and the protection of the instruments. As prime contractor, Astrium Friedrichshafen will also perform the entire satellite integration and associated testing. Astrium Toulouse will supply the multi-spectral instrument which will deliver optical images in 13 spectral channels in the visible and short-wave infrared range down to a resolution of 10m with an image width of 290km.

It is planned to extend the overall Sentinel-2 system by a second satellite later on so that, by operating the two satellites simultaneously, it will be possible to provide global coverage every five days.

Sentinel-3 is a satellite designed to deliver infrared radiometry, i.e. high-precision temperature readings, altimetry (altitude measurements) via radar, and multi-spectral images with a ground resolution of 500–1,000m. This combination of measurements is vital for coastal surveillance, forecasting of ocean currents and waves, and for environmental data on land. It will thus be used for global monitoring of oceans and land areas. The first Sentinel-3 satellite is scheduled for launch in 2012. Astrium is bidding to deliver important, flight-proven instruments for this satellite. Astrium Espana has already been selected to prime the microwave radiometer. Thales Alenia Space is prime contractor for the satellite.

The gradual implementation of GMES has been made possible through joint activities and investments by ESA and the EU member states. It thus constitutes the second flagship of European space policy after the Galileo navigation system. By its very nature, Earth observation is first and foremost a huge adventure for science and technology. But over the last few years it has also become an important part of the economy – in the first instance through operation of the satellites, controlling them in orbit, and recording and distribution of the data acquired. Seen as a whole, however, the marketing of this information today adds real value to this knowledge.