Meteosat's primary instrument is the Meteosat Visible and InfraRed Imager (MVIRI) — a high resolution radiometer with three spectral bands. With a mass of nearly 63 kg and a total height of 1.35 m, it constitutes the main payload of Meteosat. It provides the basic data of the Meteosat system, in the form of radiances from the visible and infrared parts of the electromagnetic spectrum. The instrument allows continuous imaging of the Earth.

The MVIRI acquires radiance data from the full earth disc during a 25-minute period. This is followed by a five-minute retrace and stabilisation interval, so that one complete set of full earth disc images is available every half-hour. The radiation is gathered by a reflecting telescope, with a primary mirror diameter of 400 mm.

Spectral Channels

The radiometer operates in three spectral bands, chosen in accordance with Meteosat's primary task of mapping the distribution of clouds and water vapour.

The Visible (VIS) band (0.45 to 1.0 µm) is used for imaging during dayight. This band corresponds to peak solar irradiance; furthermore the atmospheric gases are fairly transparent to incoming and outgoing (reflected) solar radiation in this band.

The Water Vapour (WV) absorption band (5.7 to 7.1 µm) is used in determining the amount of water vapour in the upper troposphere. It takes advantage of the strong absorption of emitted terrestrial radiation by atmospheric water vapour. In this spectral region, the atmosphere is very opaque if water vapour is present, but transparent if the air is very dry.

The Thermal Infrared (IR) band is used for imaging by day and night, and also for determining the temperature of cloud tops and the ocean's surface. This band corresponds to peak re-emission of radiation from the Earth's surface and atmosphere, according to their temperature. As with the VIS band, the atmospheric gases are fairly transparent in this region.

Architecture

The Meteosat satellite system is an example of a very successful European endeavour. First designed in the early 1970s, the first model was launched in 1977, and the same design was in use until the end of 2005. A few relatively minor design changes were introduced after Meteosat-3, and it is this updated satellite specification which is summarised below.

Meteosat First Generation spacecraft architecture

• The overall size of the satellite is 2.1 metres in diameter and 3.195 metres long.
• Its initial mass in orbit is 282 kg. The hydrazine propellant used for station-keeping, adds another 40 kg at the beginning of its life.
• In orbit, the satellite spins at 100 rpm around its main axis, which is aligned nearly parallel to the Earth's north-south axis.

Meteosat is composed of a main cylindrical body, on top of which a drum-shaped section (diameter 1.3 m). Two further cylinders are stacked concentrically. The main cylindrical body contains most of the satellite subsystems, including the radiometer. Its surface is made up of six panels covered with the solar cells, which provide the electrical power. The panels also have cut-outs for sensors, thrusters and umbilical connectors.

The cylindrical surface of the smaller drum-shaped section, mounted on top of the S/UHF platform, is covered with an array of radiating dipole antenna elements. Electronics within the drum activate the individual elements in sequence, in reverse order to the satellite spin sense. This subsystem constitutes an electronically-despun antenna whose function is to ensure that the main transmissions in S-band are always directed towards the Earth. The two cylinders mounted on top of the drum are toroidal pattern antennas for S-band and low UHF respectively.

An apogee boost motor containing solid propellant is initially attached to the bottom of the satellite at launch. This is used to boost it from its post-launch, highly elliptical orbit into the required circular equatorial orbit. Following this burn, the apogee boost motor is jettisoned, leaving an opening to give a clear field of view for the radiative cooler that cools the radiometer infrared detectors.