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page 73
all areas of its organisation, not just into space segment policy, during the late 1980s and early 1990s. If EUMETSAT had not undergone this process, it would not have been in a position to make a sustainable cooperative agreement with ESA. It very likely would not have been in a position to develop MSG at all.
Meteosat First and Second Generations compared
The imager on the MSG satellites is called a Spinning Enhanced Visible and Infrared Imager (SEVIRI). It detects upwelling radiation from the Earth's surface and atmosphere in twelve different frequency bands (15).
By contrast, the Meteosat satellites monitor only three frequency bands: one in the visible and two in the infrared regions of the electromagnetic spectrum. The visible radiation gives an image of the Earth as seen by the eye. One of the infrared bands gives the horizontal temperature distribution and the other the horizontal water vapour distribution.
Given that SEVIRI is monitoring 12 rather than three frequency channels, more images showing the horizontal distribution of radiation can clearly be created with data from the MSG satellites than from Meteosats-I to 7. Each radiation band allows different information about the Earth's atmosphere and surface to be deduced, so an increase in the number of frequency bands gives improved performance. There is also a high resolution visible channel, which should improve
localised weather forecasting. Interestingly, too, by carefully selecting the frequency bands, the SEVIRI can give information about conditions at different altitudes in the atmosphere. Thus there is a limited sounding capability.
Being in geostationary orbit, the Meteosats have a constant field of view. Once every 30 minutes the imaging radiometer on the first generation of Meteosats scans the Earth's disc, building images line by line that correspond to the radiation returns in the three frequency bands. This represents the raw satellite data, of which 48 sets are collected and processed each day for transmission to users via the Primary Ground Station of the Meteosat ground segment and the satellite's on-board communications system.
The specification for the MSG satellites calls for images to be scanned in 15 minutes. The raw data are processed and relayed continuously to the user rather than being sent only when the whole image has been received, as is the case with the first generation of Meteosats. Such realtime transmission of more frequently scanned data should markedly improve very short-range forecasting.
The sounding data, limited though it is, and improved cloud track winds (possible because of the increase in frequency bands and more frequent scans) should also lead to improvements in the output of computer models that predict the weather for up to 14 days in advance.
15 - The visible and infrared radiation that satellite instrumentation detects is part of the electromagnetic spectrum and is defined by its wavelength and frequency. Visible light has a range of frequencies that are higher than the infrared part of the spectrum. Radio waves, too, are part of the electromagnetic spectrum. Just as a radio can be tuned to detect different frequency bands, so the instrument on the Meteosat Second Generation satellites can detect different frequency bands in the visible and infrared. Unlike a radio, the satellite instrument is designed to sense all 12 frequency bands at the same time. |