Sentinel-1 is one element in the GMES (Global Monitoring for
Environment and Security) programme, an initiative by the European
Commission and ESA to set up a sustainable European network for
recording and analysing environmental data. Sentinel-1 will help to
monitor and analyse environmental events round the globe.
Astrium will supply the C-band radar for the Sentinel-1 satellite.
Weighing some 2.2 metric tons, from 2011 the satellite will orbit the
Earth at an altitude of 700 kilometres. Its orbital path will take it
across the poles on each orbit, enabling the radar instrument to scan
the Earth in 'swathes' as it rotates beneath the satellite. Designed as
a successor to the present satellites ERS and Envisat to assure
continuity in radar-based Earth observation, Sentinel-1 will observe
the Earth from orbit for at least seven years. In contrast to its
predecessors ERS and Envisat, however, the radar instrument carried by
Sentinel-1 will include improvements such as enhanced antenna
performance, which in turn will ensure better data quality.
The Astrium-built C-band SAR (Synthetic Aperture Radar) instrument,
which delivers radar images of the Earth’s surface, is the core element
of the mission. By taking advantage of the satellite’s flight motion,
SAR is able to simulate a considerably larger antenna than is actually
present, thus significantly increasing the image resolution. The radar
on Sentinel-1 operates in the C band frequency range. The C-band radar
beam, which has a wavelength of six centimetres, penetrates forests and
scrub to reach the ground, and the C band radar registers any movements
or changes on the Earth’s surface to within a centimetre. Sentinel-1
will be equipped with an active antenna made of up 280 individual
antennas. The active antenna can be electronically directed towards a
new observation terrain without having to be physically moved. Thanks
to this technology, several adjacent swathes can be scanned on each
overflight and later combined to form a larger overall picture. The
radar instrument can operate in four different observation modes which
differ primarily in the width of the scanned corridor and the
resolution of the radar images. This enables Sentinel-1 to respond to a
wide range of varying requirements. In strip-map mode, strips of
terrain 80 kilometres wide are scanned at a resolution of 5x5 metres.
The interferometric wide-swathe mode registers corridors 250 kilometres
wide at a pixel size of 5x20 metres. The extra-wide swathe mode
provides a more extensive overview of larger expanses of terrain. In
this mode, a corridor 400 kilometres wide is mapped at a resolution of
100x25 metres. The fourth mode, known as wave mode, scans 20x20
kilometre spots at a resolution of 20x5 metres.
