Weather Observation satellites - Glossary
Albedo
Albedo - the amount (usually quoted as a percentage %) of incident light that is reflected from a surface. Different surfaces have different reflectances - snow has a very high reflectance (around 80%) and so appears white, whilst the sea can absorb almost all light falling onto it and thus appears dark with a very low (3%) reflectance.
AVHRR
AVHRR - the Advanced Very High Resolution Radiometer. An instrument capable of detecting small changes in emitted radiation over very small areas. AVHRR can detect very small temperature variations over large areas (such as variations in sea surface temperatures of a little as 0.5 C) or small areas of high variations (such as forest fires or farmers illegally burning corn stubble in their fields!). AVHRR senses radiation in seven visible and infra-red bands. See also Radiometer.
ESA
ESA - European Space Agency - an intergovernmental European agency developing space related programmes. ESA's work includes the operation of Meteosat as well as contributions to the Hubble Space Telescope program.
ESOC
European Space Operations Centre (ESOC) at Darmstadt, Germany, controls the Meteosat satellite and processes the primary data to create the secondary WEFAX transmissions, uploaded and retransmitted from the Meteosat platform.
Geostationary/Geosynchronous orbit
Satellites launched into an orbit at an altitude of 36,000Km have a period of 23 hours and 56 minutes. This is exactly the same time as the Earth takes to complete one revolution (i.e. the length of a day). If the satellite is launched into an equatorial orbit (orbiting above the Equator and passing around the Earth in an anti-clockise direction) then it will appear to be stationary above one point on the Earth's surface. This is very useful - it is easy to transmit and receive information from the satellite by pointing fixed aerials at the satellite, much much easier than trying to follow a moving object! Meteosat 5 is in orbit above the intersection of the Equator and the Greenwich Meridian (0° Latitude, 0° Longitude), Meteosat 3 is above the Equator at 75° West, GOES is at 135° West, Insat (the Indian satellite) is at 75° East and GMS (Japanese) is at 140° East. For communications purposes, the geostationary orbit is extremely important, providing uninterrupted television and telephone links around the world. 'Sky' satellite TV transmissions come from the Astra group of satellites in orbit above the Equator at 19.2° East.
GMS
Geosynchronous Meteorological Satellite operated by the Japanese at 140° East
GOES
Geostationary Operational Environmental Satellite operated by the USA (NOAA) at 135° West.
GOES page at NASA's Goddard Space Flight Center.
Ground station
Receiving equipment (typically a small dish antenna, radio receiver and computer equipment) to detect the signals transmitted by the satellite and decode the data for display on a screen.
Infra Red
Electromagnetic radiation beyond the limit of human visibility - with a longer wavelength (and lower frequency) than the visible range. Infra red typically begins at a wavelength of 1µm (1 micron - 1 millionth of a metre) and extends to the region of 100 µm. All warm objects radiate infra-red, the hotter they are then the shorter the wavelength of the radiated IR (if they are very hot, they radiate waves short enough to become visible - the principle of the electric light bulb!). Some short wave (but invisible) infra-red is emitted by electronic devices - typically used for the remote control of television, video recorders etc.
ITCZ
Inter Tropical Convergence Zone - a region near the equator where air streams converge. Surface air in the northern hemisphere moves south and meets air moving north from the southern hemisphere. The air converges and is forced upwards creating a region of low pressure. The upward moving air takes water vapour with it and this condenses to form a highly visible line of clouds. The ITCZ moves with the season, moving north of the equator during the northern summer and drifting towards the south in the northern winter.
NOAA
NOAA National Oceanographic and Atmospheric Administration - United States governmental agency dealing with weather and sea conditions
Orbit
Path of a satellite around the earth. Low orbits at around 200 miles (320Km) altitude are just above the upper layers of the atmosphere. At these altitudes satellites must move quickly to stay in orbit and take around 90 minutes to complete one revolution. Satellites at these altitudes can provide very high resolution images of surface features (including buildings!). The angle of inclination of the orbit can take the satellite right over the Poles, or limit it to flying over a maximum northerly latitude. Orbits can be near circular, or elliptical, coming closest to the earth at perigee and being furthest at apogee. Orbits with a greater altitude have a longer period. At an altitude of 36000 Km, the period of the satellite equals the rotational period of the Earth (23 hours and 56 minutes) - and the satellite can appear to hang in space, always above one point on the earth's surface.
Period
Time taken to complete one revolution of the satellite orbit.
Polar orbit
Satellites in Polar orbits pass over the North and South poles. This is a commonly used track for low altitude observational satellites - as the satellite flies along its orbital track the earth rotates beneath it. Each flyby takes the satellite over a different part of the earth - eventually (after several orbits) the satellite will have flown above all of the earth's surface. This is very useful for military purposes! NOAA polar satellites fly over the UK and western Europe three or four times a day, providing regularly updated images of the weather systems below them.
PDUS
Primary data user station - typically a large dish with more sophisticated receiving equipment capable of receiving and displaying the high resolution (HRI) data transmissions from Meteosat.
Radiometer
An instrument for detecting and measuring electromagnetic radiation. Satellite radiometers detect the radiation (both visible and infra red) reflected from the Earth below. As Meteosat spins (at 100 rpm), its radiometer telescope scans the surface of the earth and clouds, building up an electronic picture of the surface. See also AVHRR.
SDUS
Secondary data user station - typically a smaller station receiving the secondary WEFAX transmissions. SDUS stations are the most common in schools and colleges.
Solar Radiation
Solar panels convert sunlight into electrical energy to power a satellite's systems. The solar panels are typically used to charge batteries whilst the satellite is in sunlight. When the satellite passes to the far side of the Earth (into eclipse) the satellite systems will continue to run powered by the on-board batteries. Individually, solar panels provide little power, but large numbers are connected together covering the outside of the satellite or arranged on long arms to increase the surface area that is available to 'catch' sunlight and therefore to provide more electrical energy.
Visible Light
Electro-magnetic radiation in the visible range - i.e. that which can be detected by the human eye. Typically, the range of visible light extends from around 0.7µm at the red end to 0.35µm at the blue end (one octave, a region in which the frequency doubles from one end to the other). Beyond the red end (and invisible) is Infra Red, beyond the blue end (and thus with a shorter wavelength) is ultra-violet. Visible light is divided into a series of colours - the colours of the spectrum from Red, through Orange, Yellow, Green, Blue and Indigo, to Purple.
WEFAX
WEFAX is the automatic picture transmission (APT) scheme developed for direct broadcast from the early and subsequent US polar orbiting weather satellites. WEFAX is an analogue transmission system (as distinct from more modern digital systems) that is now used widely by all operators of weather satellite systems. WEFAX signals from Meteosat consist of 800 lines transmitted at a rate of 4 a second on a carrier frequency (Channel 1) of 1691.0 MHz.