Meteosat satellite

The Meteosat observes us from a distance of 36,000 km, in a geostationary orbit. At this height, the rotation period of the satellite is equal to that of the Earth, and is therefore always over the same point of the surface, which corresponds to the confluence of the Equator with the Greenwich meridian (where the Gulf of Guinea is found). With the first meteorological satellite, the American TIROS, launched in 1960, the value of photographs taken from space became obvious. The first proposal for a geostationary satellite by the French resulted in the proposition of the ESA (European Space Agency) to develop the Meteosat at a European level. The first Meteosat was launched from Florida in 1977, while Meteosat-2 was put into orbit by Ariane, which has also sent into space all the following Meteosats, up to Meteosat-7 which will be operational by the end of 2005. The M-7 is a replica of the first Meteosat and so until now has been a satellite with the technology of the 1970's. It is for this reason that the European Organisation for the Exploitation of Meteorological Satellites (Eumetsat) and the ESA have developed a second generation of Meteosat, with vastly superior features to those of its predecessors.   
 
Meteosat Second Generation

The first satellite of the new generation of Meteosat, the MSG (Meteosat Second Generation), was put into orbit on August 28th, 2002 from Kourou, in French Guiana. After a trial period, this satellite went in service renamed as Meteosat-8 and substituting M-7. The MSG programme is made up initially of 3 satellites (with the possibility of a 4th) and will provide images at least every fifteen minutes throughout the next 12 years at least.

The new features of the MSG are basically the following:

  • Twelve spectral bands (three in the previous system) which provide more exact data on atmospheric conditions.
  • The recording cycle of images is 15 minutes (30 minutes until now). The increase in the frequency of image reception will help to improve the accuracy of short-term forecasts.
  • The High Resolution Visible channel has increased to 1km. (Previously 2.5km).
  • The MSG carries the new GERB (Geostationary Earth Radiation Budget Experiment), for climatic studies and to give data about the earth radiation budget.
  • Data transmission is totally digital which means an improvement in the functioning of data reception for users.
  • The second generation satellites will have a nominal life in orbit of seven years. (two more than in the current system).
 
The sensor

The main sensor on board Meteosat-8 is a radiometer (called SEVIRI), which measures infrared and visible radiation in 12 different spectral bands. Below there is a general description of these bands arranged in spectral zones.

Visible zone: in this zone are the channels 1, 2 and 12. The first two record concrete zones of the visible spectrum (at 0.6 and 0.8 mm), while channel 12 is the high resolution channel. This High Resolution Visible channel covers the whole visible spectrum and has a spatial resolution of 1km , which is different to the rest of the channels.  

  Example of the High Resolution Visible channel: fog over the Mediterranean (February 5th, 2004).
 

Near and mid infrared rangezone: the MSG has two channels in this region of the spectrum which were not covered by the old Meteosat. Channel 3 near infrared band centred at 1.6 mm) distinguishes better between snow/ice and clouds, and it also gives information about aerosols. Channel 4 (3.9 mm) detects low clouds and fog both day and night and also smoke from forest fires amongst other things. 

  Example of a composite image (combination of 3 channels) using the two visible channels and the near infrared channel. Noticeable in the image is the fog covering a large part of central and eastern Europe and the snow in the Pyrenees and Alps.

Water vapour zone: the radiation that is read by the sensor in this channel measures the quantity of water vapour in a column of air over every point of the surface. The MSG conserves the vapour channel of the previous Meteosat and also has new vapour channel which measures the water vapour in the middletroposphere.

Thermal infrared: The MSG has three channels in this zone (channels 7, 9 and 10). They are channels which permit the measurement of surface and cloud temperature. Channel 9 corresponds to thethermal infrared of the previous Meteosat. This channel permits the monitoring of cloud dynamics 24 hours a day. The other two new channels, 7 and 10, distinguish different cloud types.  

  Example of detection of night fog by combining different channels of the MSG. In this image taken on November 11th, 2003, it is possible to see the fog (yellow) in the Po valley and central Europe. The pink areas are those without fog.
 

Other channels: The MSG has two channels which are not used for meteorological monitoring but to check atmospheric contamination. Channel 8 measures the concentrations of ozone in the lower stratosphere, while channel 11 is situated in the zone of maximum carbon dioxide absorption.
 
The Images  

Each image is taken by combining the continuous rotation of the satellite with the progressive movement of a mirror which allows the radiometer to record the entire field of view line by line. In this way the images are registered from south to north and east to west in a period of 12 minutes. In the remaining 3 minutes before the cycle begins again, an instrument calibration is made, the mirror returns to its initial position and the satellite sends the data to the ground stations.

The images are taken at two spatial resolutions. Eleven of the twelve channels of the MSG have a resolution of 3km. The remaining channel, the High Resolution Visible has a nominal resolution of 1km. This resolution corresponds to the point right under the satellite and it gets smaller (the pixel becomes bigger) to the degree that the mirror scans to the north or south. For example, in Catalonia, one pixel represents an area of 14.8 km2, while in the high resolution channel each pixel corresponds to approximately 1.6 km2.

Image processing 

The images received from Meteosat are raw data and need to be processed before they can be interpreted. What is more, a series of meteorological products are generated from the original channels, such as Cloud analysis (analysis of clouds to determine the percentage of cloud coverage and their temperature) or the Cloud Motion Winds (estimation of the winds by the analysis of cloud movement through a sequence of images), which help the interpretation of weather phenomena.

 
Meteosat image reception at SMC  

Since February 2005, the Servei Meteorològic de Catalunya (Meteorological Service of Catalonia) has had the necessary infrastructure to receive the images from the MSG. The change of generation has brought with it a renewal of the equipment for receiving and processing of images.

During 2005 the two data transmission systems worked side by side, but at the end of the year that system previously used for the M-5, M-6 and M-7 was taken out of service. The analogue format and WEFAX disappeared also. With the new digital distribution system (HRIT: High Rate Information Transmission, and LRIT: Low Rate Information Transmission), the images are sent compressed and encoded. Only the images from the synoptic hours (00, 06, 12 and 18 GMT) are sent uncoded (as was established by resolution 40 of the World Meteorological Organization) and these are available on the SMC website.

EUMETSAT have made available images for free distribution on internet which can be found at: www.eumetsat.int. These images have a resolution which has been reduced by a factor of 5 from the originals, there are 5 channels available and they are updated hourly.