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EADS Astrium’s ground-breaking SEVIRI instrument provides unrivalled weather forecasting accuracy
Events over the last year have again highlighted the critical importance of precise weather forecasting.
MSG-2, launched on 21 December 2005 on an Ariane 5 from Kourou, is the second of three satellites of the Meteosat Second Generation (MSG) programme, a joint project between the European Space Agency and Eumetsat, the 18-member European Organisation for the Exploitation of Meteorological Satellites. MSG is the most advanced weather satellite system, and its most important on-board instrument, which contributes most to the programme’s exceptional performance, is the EADS Astrium-built Spinning Enhanced Visible and Infra-Red Imager SEVIRI.
The eye of the MSG satellites: SEVIRI
SEVIRI is the latest in a long line of state-of-the-art meteorological instruments developed and built by EADS Astrium. With more than three decades of experience in the construction of meteorological measuring instruments, EADS Astrium was in an excellent position to develop a measuring instrument for MSG which opened new perspectives to meteorologists and climate researchers; the SEVIRI project began in August 1995, and met its challenging design and development schedule, with the delivery of the first instrument for the MSG-1satellite in October 1999.
SEVIRI is a radiometer, a ‘telescope’ which measures the radiation from the Earth in various ranges of the electromagnetic spectrum. It is more powerful and more advanced than the predecessor model aboard the first generation Meteosat satellites, also supplied by EADS Astrium, and it incorporates many new features designed to respond to the MSG mission objectives, including unprecedented pointing accuracy and simultaneous precise multi-spectral radiometry. The resolution of the SEVIRI telescope is considerably higher (1 km compared to 2.5 km for the first generation), and whereas the first-generation radiometers recorded three spectral channels, (visible light, infrared and water vapour), SEVIRI records radiation in 12 different channels of the electromagnetic spectrum, allowing comprehensive observation of many parameters, such as cloud, land and sea temperatures, land and sea surfaces, as well as the composition of air masses in the atmosphere, offering meteorologists a much more differentiated database for numeric weather forecasting models.
Data recorded by SEVIRI is not only more precise but also more frequent. Compared to the telescopes of the first Meteosat generation, which produced a satellite image every 30 minutes, SEVIRI supplies a multi-spectral image of the Earth’s cloud systems and surface-pattern development every 15 minutes, enhancing short-term weather evaluation. ‘Nowcasting’, short-term weather forecasting, is the most immediate beneficiary of the new capabilities of the instrument, allowing more precise predictions of extreme and dangerous weather phenomena, such as thunderstorms or heavy rainfall. In the event of fog, for example, airports can be warned a mere one to two hours in advance. In addition to the high-resolution image itself, data recorded by SEVIRI is of great use for mathematical meteorological models and for atmospheric and environmental research.
In technical terms, SEVIRI is a light-weight, compact telescope and scan assembly, 2.43 m in height and 1.5 m in diameter. The telescope element is equipped with three mirrors made of Zerodur, an extremely light glass ceramic material. The scan assembly consists of an additional adjustable Zerodur mirror positioned in front of the telescope. Its operating principle is based on target-area radiation collection via a bi-dimensional Earth scan, combining a rapid scan, tracking from East to West using the satellite’s spin motion at 100 rpm around its axis perpendicular to the geostationary orbit plane, and a slow North–South scan by SEVIRI’s mirror rotation mechanism. The telescope focuses the collected radiation onto detectors where it is divided into the 12 different electromagnetic spectrum channels and transferred to 42 sensors. Electronic processing of the signals follows. Recorded data is sent to the Functional Control Unit, the interface with the MSG satellite data transmission system.
The compact structure of the telescope and the scan assembly enables the use of a large passive cooler which improves the performance of the infrared measuring systems by lowering their operating temperature to –188 °C. The total weight of the SEVIRI telescope is no more than 260 kg and its power consumption is approximately 150 Watts, equal to around that of two medium-power light bulbs.
EADS Astrium: 30 years in the service of meteorology
For 30 years, EADS Astrium has been the prime contractor and/or supplier of key instruments for the European meteorological systems Meteosat Second generation (MSG) and Metop. Meteosat images are broadcast every day by many television news bulletins around the world. Between 1977 and 2005, eight satellites were placed in geostationary orbit, all equipped with radiometers, the principal measuring instruments, designed and supplied by EADS Astrium.
Following the success story of the seven first-generation Meteosat satellites (three of which are still in orbit today), a new era began in August 2002 when the first of the three Meteosat Second Generation (MSG) satellites was launched. The second satellite, MSG-2, was launched on 21 December 2005. All MSG satellites use the on-board instrument SEVIRI (Spinning Enhanced Visible and Infrared Imager) designed and built by EADS Astrium.
From 2006, these European weather satellites in geostationary orbit (at an altitude of 36,000 km above the equator) will be supplemented by low-Earth monitoring satellites. Europe will thus possess its own fleet of polar-orbiting Meteosat satellites, Metop. With their numerous instruments for probing the atmosphere, these satellites will measure meteorological parameters extremely accurately at low altitude, providing data of particular benefit to medium- and long-term weather forecasting and climatology research. The three identical Metop satellites, which will be launched at four-year intervals (like the MSG fleet), will all carry the EADS Astrium measuring instruments Ascat (Advanced Scatterometer), GRAS (GNSS Receiver for Atmospheric Sounding) and MHS (Microwave Humidity Sounder). Ascat is a radar that measures the speed and direction of winds above the oceans. It also helps to monitor the distribution of snow and ice on land and sea. MHS is a measuring sensor for atmospheric humidity and its distribution, and also registers cloud and precipitation parameters. MHS is currently showing its mettle in orbit on the US NOAA weather satellite, supplying data to several meteorological institutions around the world (the UK Met Office, Météo France and the US National Oceanic and Atmospheric Administration, NOAA).
As of 2007, one of the last gaps in global weather and climate research will be closed with the ESA’s AEOLUS system (the name is taken from the god of the winds in Greek mythology), for which EADS Astrium is again prime contractor. AEOLUS will make it possible to study three-dimensional wind fields on a planetary scale. Its objective is to refine existing knowledge and enhance today’s methods of modelling and analysing the Earth’s atmosphere in order to satisfy the needs of weather forecasting and climatology research. The ALADIN (Atmospheric Laser Doppler Instrument) on-board AEOLUS will measure wind speed and direction with unprecedented accuracy. Aladin is basically a direct detection lidar incorporating Mie imaging (pulses reflected by aerosols and clouds) and Rayleigh imaging (molecular reflection). It works by sending brief, harmless laser pulses into the atmosphere and analysing the Doppler shift of the laser beam reflected back by each atmospheric cell.
These powerful instruments and the new fleets of European satellites will lead to a significant improvement in weather surveillance and a more accurate prediction of dangerous weather conditions such as storms or hurricanes.