Meteosat Design

Meteosat satellites are spin-stabilised with instruments designed to provide permanent visible and infrared imaging of the Earth.

Meteosat Instruments (tn)

When operating in geostationary orbit, 36,000 km above the equator, the satellites spins counter-clockwise at 100 rpm around their longitudinal axis, which is aligned with the Earth's rotational axis.

Meteosat Second Generation spacecraft

The MSG body is a cylindrical-shaped solar drum, 3.2 m in diameter and 2.4 m high.

The satellite itself is built in a modular way around three main sub-assemblies:

  • Spinning Enhanced Visible and Infrared Imager (SEVIRI) instrument in the central compartment.
  • GERB scanning radiometer in the central compartment.
  • Mission Communication Payload (MCP), including antennas and transponders, in the upper compartment.
  • The platform support sub-systems, in the lower compartment.
Meteosat Second Generation spacecraft technical details
Orbit time Repeat cycle Mass in Orbit Imager Mass Imager Ground Sampling Distance Power capability
24 hour (geostationary) 15-mins (full disc)
Rapid Scan 5-mins (Europe)
1,200 kg 260 kg 1 km Ch 12 (HRV)
3 km Ch 1–11 (VIS, IR, WV)
600 W average

For its initial boost into geostationary orbit, as well as for station keeping, the satellite uses a bi-propellant system. This includes small thrusters, which are also used for alttitude control. The MSG solar array, built from eight curved panels, is wrapped around the satellite body.

The MSG payload is designed in accordance with the MSG Mission objectives, in order to accomplish the following functions:

  • Permanent visible and infrared imaging of the Earth's disc, with a baseline repeat cycle of 15 minutes.
  • High-resolution visible HRV imaging of half of the Earth's disc.
  • Transmission of raw data, and other information, from the satellite to the PGS.
  • Transmission of Data Collection Platforms (DCP) information, via the satellite, to the PGS.
  • Accommodation of a scientific payload.
  • Geostationary Search and Rescue (GEOSAR).

Detailed information on the MSG instruments is provided below. See also the Animation on Meteosat instruments

 

Spinning Enhanced Visible and Infrared Imager (SEVIRI)

Illustration of the inside of the SEVIRI instrument

The MSG system provides accurate weather monitoring data through its primary instrument — the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) — which has the capacity to observe the Earth in 12 spectral channels.

SEVIRI has twelve spectral channels, as opposed to three on the previous system. These provide more precise data throughout the atmosphere, giving improved quality to the starting conditions for Numerical Weather Prediction models.

Eight of the channels are in the thermal infrared, providing, among other information, permanent data about the temperatures of clouds, land and sea surfaces. One of the channels is called the High Resolution Visible (HRV) channel, and has a sampling distance at nadir of 1 km, as opposed to the 3 km resolution of the other visible channels.

Using channels that absorb ozone, water vapour and carbon dioxide, MSG satellites allows meteorologists to analyse the characteristics of atmospheric air masses and reconstruct a three-dimensional view of the atmosphere.

The improved horizontal image resolution for the visible light spectral channel (1 km as opposed to 2.5 km) also helps weather forecasters in detecting and predicting the onset or end of severe weather.

For details on the data ad products available from SEVIRI see our 0 Degree Service page.

Known anomalies in SEVIRI images

Geostationary Earth Radiation Budget (GERB)

The Geostationary Earth Radiation Budget (GERB) instrument is a visible-infrared radiometer for Earth radiation budget studies. It makes accurate measurements of the short wave (SW) and long wave (LW) components of the radiation budget at the top of the atmosphere. GERB provides valuable data on reflected solar radiation and thermal radiation emitted by the Earth and atmosphere.

A GERB International Science Team (GIST) had been established and tasked to, among other things, define the science requirements, products and processing algorithms, and to implement science and validation activities.

Grey image of the Earth, taken by the GERB instrument

The consortium that developed and is responsible for operating the GERB system includes the Rutherford Appleton Laboratory (RAL), UK; Imperial College of Science, Technology and Medicine (ICSTM), UK; Met Office Hadley Centre, UK; Leicester University, UK; Koninklijk Meteorologisch Instituut van België (KMI); Advanced Mechanical and Optical Systems (AMOS), Belgium, and SELEX Galileo, Italy.

The GERB instrument has two broadband channels — one covering the solar spectrum (0.32 to 4.0 µm), the other covering a wider portion of the electromagnetic spectrum (0.32 to 30 µm). Together these channels are used to derive the thermal radiation emitted by the Earth in the spectral range 4.0 to 30 µm.The GERB broadband channels span the 12 much narrower channels measured by the MSG’s other instrument — the Spinning Enhanced Visible and Infrared Imager (SEVIRI). GERB fills in the gaps in the thermal radiation spectrum missed by the SEVIRI channels, but measures the thermal radiation at a coarser spatial resolution.

Data are calibrated on board, in order to support the retrieval of radiative fluxes of reflected solar radiation and emitted thermal radiation at the top of the atmosphere, with an accuracy of 1%. The radiation budget represents the balance between incoming energy from the Sun and outgoing thermal (long-wave) and reflected (short-wave) energy from the Earth. Back on the ground, RMI scientists use the finer spatial resolution of the SEVIRI data to improve the spatial resolution of the GERB images.

Search and Rescue transponder

The transponder or Search and Rescue signal repeater (SARR), allows a continuous monitoring of the earth disc and immediate alerting of distress signals.

As part of the Cospas-Sarsat programme it relays distress signals from 406 MHz beacons within the MSG coverage zone in Europe, Africa and the Atlantic Ocean.

The signals are sent to Geostationary Earth Orbit Local User Terminals (GEOLUTs) and eventually passed on to Rescue Coordination Centres (RCC) for quick organisation of rescue activities.

Geostationary satellites, such os MSG, continually view large areas of the Earth and can provide near instantaneous alerting from a 406-MHz beacon. However, they cannot determine a beacon's location using Doppler shift processing ideally, a LEOSAR or Sarsat satellite (a specially-equipped polar-orbiting satellite) will fly over the the beacon within the next hour and calculate the beacon's location.

Mission Communication Payload (MCP) Subsystem

MCP contains all antennas and transponders necessary to meet the demanding communication needs of the MSG mission. This includes the acquisition and transmission of SEVIRI and GERB raw data, as well as the delivery of House Keeping Telemetry (HKTM).

The dissemination of processed images and meteorological products to the users by transponding from S-Band to L-Band two separate links: the High Rate Image Transmission (HRIT and the Low Rate Image Transmission (LRIT). The relay of messages from Data Collection Platforms (DCPs) and Search and Rescue (S&R) beacons.

Telemetry, Tracking and Command (TT&C) Subsystem

The TT&C S/S provides the MSG spacecraft with the capability for receiving and demodulating telecommands, transmitting status and data in the form of spacecraft telemetry, and transponding ranging signals from and to the MSG Ground Stations.

Spectral Responses

The spectral response of an instrument is a measure of the instrument's response to radiation at specific wavelengths. Spectral response characterisation is the most crucial aspect of satellite calibration. The responses are non-linear and may change over the lifetime over an instrument, making it necessary to correct for these changes when producing images from the system.

The accuracy of pre-launch spectral response characterisation, and how well the on-orbit changes are understood, directly affects calibration accuracy and the quality of the data products. Even within the same satellite series, spectral response varies by instrument and even by detector on the same instrument. Spectral responses are derived for all 12 channels of the SEVIRI instrument.

Spectral responses for MSG (ZIP, 300 KB)

Modulation Transfer Function (MFT) characterisations for MSG-1, MSG-2 and MSG-3 (ZIP, 1 MB)

Note: SEVIRI PFM is onboard Meteosat-8, SEVIRI PFM2 is onboard Meteosat-9, SEVIRI FM3 is onboard MSG-3, and SEVIRI FM4 is onboard MSG-4.

Decontaminating satellite instruments

MFG Technical details

Meteosat First Generation spacecraft technical details
Cycle Mass in Orbit Resolution
30 mins 282 kg 2.5 km VIS
5 km IR, WV

Decontaminating satellite instruments

The instruments on our satellites are routinely checked every year to see if decontamination is needed. The best time to do one is just after the winter solstice. However, as satellites are most heavily contaminated when new, there is often a need to decontaminate more frequently. This is because the contamination is water vapour which is trapped in the coatings used in the satellite manufacture and, so, is launched with the satellite.

In July 2012 Meteosat-10 was launched, carrying its prime instrument SEVIRI (the Spinning Enhanced Visible and Infrared Imager). A few days after launch the SEVIRI instrument needed to be decontaminated and again five months and 12 months after launch.

MSG Spacecraft Operations Manager Paolo Pili explains, these decontaminations do not mean there is a problem with the newest SEVIRI instrument.

"The instruments on all our satellites are routinely checked to see if decontamination is needed, especially for the optical parts and/or detectors working at low temperature. As satellites are most heavily contaminated when new, there is often a need to decontaminate more frequently."

Why decontaminate?

The scope of decontamination is to remove ice and other contaminants from the cold parts in the optics. Condensation of particle or chemical contamination on the cold optics of the SEVIRI affects the instrument’s radiometric performance and the bias of the instrument’s final calibrated radiances.

Also, the passive cooling system is impacted by contamination, so that it loses its effectiveness and, in an extreme case, may not be able to maintain the instrument's operating temperature. The contaminants migrate to the cold parts from the rest of the spacecraft over time, caused by things such as condensation of outgassing material from the spacecraft. This rate is higher at beginning of life in orbit as the satellite releases moisture and contaminants when exposed to the deep vacuum of outer space.

For this reason the frequency of decontaminations is higher at beginning of life in orbit and it tends to become very low or null at end of life, as there is no more significant release of moisture and contaminants from the satellite.

The decontamination process

The decontamination operation consists in warming up the optical cold parts up to a temperature that allows both ice and other contaminants to evaporate in space, after which the instrument is allowed to cool down back to 95 K. This is better achieved in the winter season when the solar input helps to warm up the cold parts. During decontamination the instrument is not at its normal operating condition and, therefore, either its performance is degraded or the instrument is non-operational.

Because EUMETSAT has more than one Meteosat Second Generation satellite in orbit, during the decontamination period the data, which would have been provided by SEVIRI on Meteosat-10, comes from the same instrument on Meteosat-8. Similar decontamination procedures are conducted for the IASI instrument on Metop, although the build-up of ice only increases its radiometric noise because of its channels’ high spectral resolution.

Meteosat First Generation spacecraft (Meteosat-7)

The overall size of the satellite is 2.1 metres in diameter and 3.195 metres long. MFG Technical details

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.

Meteosat-7'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.

Meteosat-7 currently provides the Indian Ocean Data Coverage (IODC) service (in parallel with Meteosat-8)

Details on previous Meteosat First Generation satellites can be found in the Past Satellites section.

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 daylight. This band corresponds to peak solar irradiance. 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.