Weather satellite

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A weather satellite is an earth observation satellite that is used to observe meteorological processes, i.e. physical and chemical processes in the earth's atmosphere . Data from weather satellites are indispensable, especially in areas where on-site observation is not possible or very expensive (e.g. oceans ). A branch of meteorology , satellite meteorology , deals with the evaluation and use of weather satellites . It uses the data primarily for weather forecasting and climatology , the two best-known branches of meteorology.

History of meteorological earth observation

Geostationary weather satellites

Geostationary satellites fly at an altitude of 35,800 km above the equator. Since they rotate around the earth with the same angular speed as the earth rotates around itself (" earth rotation "), they stand at a fixed point above the earth. The Meteosat satellites also have to rotate around their own axis in order to stabilize .

Geostationary satellites have the advantage of a high temporal resolution; you receive a new picture every 5 to 30 minutes and can thus assess the development of weather systems over time. Another great advantage is that the same image section is captured with every exposure. You can make satellite films, so-called loops ; these show the recorded by the satellite in time lapse. All (weather) satellite films known from the media come from geostationary satellites. The spatial resolution is in the kilometer range (approx. 1 to 5 km at the sub- satellite point, i.e. at the point on the earth's surface vertically below the satellite). A geostationary satellite sees about 2/5 of the earth's surface. Mathematically, you can almost completely observe the earth with three satellites. The resolution gets worse and worse towards all four edges of the image (the Arctic can be seen at the upper edge of the image; the Antarctic at the lower edge) because there is no longer a vertical view through the satellite.

At the beginning of 2017 the following geostationary satellites were in use:

  • two Meteosat satellites from EUMETSAT (Meteosat-9 at 0 ° west longitude and Meteosat-7 at 57 ° east longitude)
  • three GOES satellites of the American weather agency NOAA (GOES-13 at 75 °, GOES-14 at 105 ° and GOES-15 at 135 °)
  • a third satellite of the Meteosat series (Meteosat-8) operates in the 'Rapid Scan Service' (RSS) at 9.5 ° east longitude and also serves as a reserve for Meteosat-9
  • In addition, satellites from the Japanese MSAS , the Chinese Fengyun and the Indian Insat series are in use for meteorological purposes
  • the GOES-16, a satellite that went into operation in 2016, observes the weather with a high-resolution camera.

Polar orbiting weather satellites

NOAA-M

Polar orbiting weather satellites fly on a polar, sun-synchronous orbit at an altitude of approx. 800 km (see also sun-synchronous orbit , SSE). One cycle takes about 100 minutes. Thus, the earth's surface is completely scanned once every 12 hours. The disadvantage of the low refresh rate is offset by the advantage of the good spatial resolution (100 to 1000 m, also in the area of ​​the earth's poles).

Reception system for polar orbiting weather satellites in the 137 MHz range " Rohde & Schwarz " - rotating stand (year of construction approx. 1965)

Together with the geostationary satellites, the earth can be observed seamlessly.

Polar orbiting weather satellites are operated by EUMETSAT ( MetOp satellite), the USA (NOAA type), China ( Fengyun ) and Russia ( Meteor ).

Tasks of weather satellites

  • Analysis of the current weather situation ( synoptic meteorology ), especially in inaccessible or difficult to access or sparsely populated areas, so that meteorologists get a precise overview of the weather- related events ( i.e. pressure areas and cloud shapes ); so the spatial resolution of the images must be sufficiently large with a small temporal resolution.
  • Use as input in weather forecast models (assimilation) and check the accuracy of weather forecasts
  • Determination of vertical gradients of various sizes, for example the temperature , especially over oceans and other areas with little or no ground measurements
  • also atmospheric research ( meteorology , climatology , aerology ), because less and less money is available for our own systems

Functionality and data evaluation

Weather satellites carry image-recording sensors ( radiometers ) as their payload . These measure the radiation in different spectral bands (the channels), mainly in the visible and infrared range, and occasionally also in the microwave range . For the correct interpretation of the data one has to apply the radiation laws of physics . In the infrared range, the earth radiates with an average temperature of 15 degrees Celsius .

Most weather satellites can also measure electromagnetic radiation from the earth's surface and atmosphere . If the radiation is only detected, one speaks of passive instruments, in contrast to active instruments in which radar or laser beams are emitted and the reflected signal is measured.

The visible (solar) channel of the weather satellite (abbreviated to Vis for visible) only measures the solar radiation reflected from the earth and the atmosphere . Since clouds made of water droplets reflect particularly strongly, they appear very bright in the Vis channels, in contrast to clouds made of ice crystals , which absorb the most in the near infrared and therefore appear dark in these channels. Thus, the different types of clouds can be distinguished. The combination of the data from the different infrared channels allows conclusions to be drawn about the different vertical cloud layers. The wind direction can be determined from the shifts of clouds in successive images .

Since the reflectivity of the earth's surface depends on the type of soil (the so-called albedo ), the subsurface can be identified by comparing the spectra of different Vis channels; the distinction between the different vegetations works particularly well , which is what the newer Meteosat generation (MSG ) is used. If there are no disturbing clouds, blackbody radiation can be used to determine the temperature of the ground or the sea surface, which is also important for making weather forecasts.

Since transmission energy emitted by satellites is scattered by water waves and the frequency of the echo signal changes due to the Doppler effect , wave movements can also be measured with weather satellites. Another possibility is to use long-wave radar devices to measure the size of the waves as a function of the frequency through the Bragg scattering .

The weather forecast for the shortest possible period (one to three hours), the so-called nowcasting , is obtained directly from the satellite images . For further predictions, a time series is created from images recorded one after the other and the development is expanded into the future. Since nowcasting makes predictions that are clearly reliable, the network of data must be very close-knit for a reliable forecast, which is why high-resolution weather satellites are used for this.

Furthermore, facilities for communication on board the weather satellites, z. B. to receive weather reports from automatic weather stations and to broadcast the recorded satellite images ( weather service via satellites ).

literature

Web links

Wiktionary: Weather satellite  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ The first images from the new weather satellite just arrived, and they're absolutely incredible , This new weather satellite isn't just good for the US - it's good for the world (November 19, 2016)