MetOp Glossary

A - B - C


Property of an optical system that causes an image to have certain easily recognisable flaws. Aberrations are caused by geometrical factors such as the shapes of surfaces, their spacing, and alignments. Image problems caused by factors such as scratches or contamination are not called aberrations.

Absolute zero
The lowest temperature ever reachable in the Universe: 0 Kelvin (0K), equivalent to minus 273 degrees Celsius (-273ºC). In laboratories on Earth physicists can get very close to that temperature, but it is impossible to achieve the absolute zero.

The reflecting power of a non-luminous body. This is defined as the proportion of incident total energy that is reflected in all directions.

Dimension parallel to the path of the vehicle carrying the radar, sometimes called the cross range or azimuth direction for side-looking radars.

Height in space of an object or point relative to sea level or ground level.

Continuous signal, transmitting information. The amplitude or frequency of the signal varies in direct proportion to intensity.

Antenna (high gain, low gain)
An aerial for receiving or transmitting radio signals. A high gain antenna is highly focused, whereas a low gain antenna receives or transmits over a wide angle.

Opening that allows light to fall onto an instrument's optics.

A facility providing long storage and preservation of data sets and associated documentation.

Layer of gases surrounding a star or planet.

Orientation of the spacecraft's axes relative to Earth.

The relative position of an object within the field of view of an antenna in the plane intersecting the moving radar's line of flight. The term commonly is used to indicate linear distance or image scale in the along-track direction.


The (microwave) signal reflected by elements of an illuminated scene back in the direction of the radar. The term 'backscatter' was chosen to make a clear distinction between energy scattered in arbitrary directions, and energy which returns to the radar and thus is received and recorded by the sensor.

A measure, according to a standard definition (see width), of the span of frequencies available in a signal or other distribution, or of the frequency limiting stages in the system. Typical bandwidths in the range channel of a SAR are on the order of 20 MHz, and in the azimuth channel are on the order of 1 kHz. The azimuth frequency domain is also known as the Doppler domain. Bandwidth is a fundamental parameter of any imaging system, and determines the ultimate resolution available.

A measure, according to a standard definition, of the width of the radiation pattern of an antenna. For SAR applications, both the vertical beamwidth (affecting the width of the illuminated swath) and the horizontal or azimuth pattern (which determines, indirectly, the azimuth resolution) are frequently used concepts. Beamwidth may be measured in the one-way or two-way form, and in either voltage or power.

A platform at, or below the sea surface for carrying instruments to measure oceanographical and meteorological parameters.


Process whereby one may relate the digital numbers describing an image to physical quantities such as reflectivity, geometry (position or size), or phase.

Cassegrain telescope
Popular design for large, two-mirror reflecting telescopes in which the primary mirror has a concave parabolic shape and the secondary mirror has a convex hyperbolic shape. A hole in the primary allows the image plane to be located behind the large mirror.

Celsius (ºC)
Scale of temperature for which water freezes at 0 degrees and boils at 100 degrees (under standard conditions).

Layer of the Sun's atmosphere located above the photosphere and below the corona. It is about 10,000 km deep and consists mainly of ionised hydrogen, helium and calcium. Its temperature ranges from 6000°C at the photosphere boundary to 100,000 °C at the boundary with the corona. Its low density means it is usually only visible during a total solar eclipse.

The study of climate - the prevailing atmospheric conditions of humidity, temperature, winds etc.

Cold target
A cold reference data set is generated by sampling data when viewing deep space (approx 4 K). Data from this Calibration target is used during ground processing of the data, to correct and compensate for errors in Earth view samples. Also called Space View Target.

The outermost part of the Sun's atmosphere. It extends outwards for several times the Sun's radius and consists of extremely hot plasma which may exceed one million degrees Celsius. Normally only visible as a white halo during a total solar eclipse. Usually seen as streamers or filaments flowing away from the Sun, but sometimes it almost disappears in regions known as coronal holes.

The interval of time between exact repeats of an orbit (e.g., in the 35-day cycle, orbits repeat exactly every 35 days).

D - E - F


Facts (in the form of values, quantities given by an instrument) from which other information may be inferred. Plural term commonly treated as singular.

Data product
1.) a homogeneous set of information resulting from raw data processing: it includes annotated, transcribed or decommuted raw data
2.) general term to indicate raw data, validation data, auxiliary data, fast delivery, regenerated, or precision products
3.) a uniformly processed and formatted data set, portion of a data set, or transformed representation of data (e.g., plot, photograph); may be produced by, or for, a project or by a data centre.

Mass per unit of volume.

Frequency splitting.

Dichroic filter
A filter that is coated rather than coloured so that it does not transmit infrared.

Dichroic mirror
A mirror that reflects a particular range of spectral energies and absorbs all others.

Diffraction grating
Grating device that splits light into a spectrum of the component wavelengths.

Doppler, Doppler frequency
Shift in frequency caused by relative motion along the line of sight between the sensor and the observed scene. In SAR, it is more formally the first derivative of the signal phase in the azimuth direction. The span of useful Doppler frequencies illuminated by the antenna must be smaller than the azimuth pulse repetition frequency (PRF), otherwise false image features (azimuth ambiguities) will occur.

Doppler shift
The change in observed frequency due to relative motion between source and observer.


The third of the four terrestrial planets counting out from the Sun. The distance between the Earth and the Sun is 1nbsp;astronomical unit (about 150 million km).

Eclipse, solar
When the Earth enters the Moon's shadow as the Moon moves wholly (total eclipse) or partially (partial eclipse) in front of the Sun as seen from Earth.

Electromagnetic radiation
Electromagnetic radiation, or light, can be considered to be composed of particles (photons) or waves. Its properties depend on its wavelength: longer waves are less energetic than shorter waves - photons with long wavelength have less energy than short-wavelength photons. Electromagnetic radiation is usually described as bands of radiation of similar wavelength, e.g., infrared, radio waves, microwaves, gamma rays, X-rays... (These bands of radiation roughly correspond to the range of wavelengths which can be detected by different instruments.) Only a small fraction of the entire range of electromagnetic radiation can be detected by the human eye: visible light, or what in everyday-life is referred to simply as light. The human eye cannot detect wavelengths longer than those of the visible light, such as those of infrared light, microwaves (wavelengths of centimetres), or radio waves (wavelengths of metres). Wavelengths shorter than visible light cannot be seen either: ultraviolet light, X-rays, gamma rays (the most energetic). Electromagnetic radiation can be described in terms of wavelength (L), measured in metres (m), or frequency (f), measured in hertz (Hz). The relationship between these two is given by: f = L/c where c = speed of light.

Electronvolt (eV)
Unit of energy defined as the energy acquired by an electron in falling through a potential difference of one volt. Electronvolts are used as a measure of the energy of cosmic rays and high-energy photons. For example, X-rays can have energies of 1000 eV (1 keV) or more.

The ratio of emitted flux from a body to the emitted flux from a black body at the same temperature per unit area.


Fahrenheit (ºF)
Temperature scale on which water freezes at 32ºF and water boils at 212ºF. The conversion between the Fahrenheit temperature scale (F) and the Celsius temperature scale (C) is: F = 32 + 1.8 x C

Field of view (FOV)
The full angular extent of the scene being viewed by an instrument.

Focal plane
Axis or geometric plane where incoming light is focused by the telescope.

Rate of oscillation of a wave: the number of full cycles performed by the wave in a second. In the microwave region, frequencies are on the order of 1 GHz to 100 GHz. For electromagnetic waves, the product of wavelength and frequency is equal to the speed of propagation, which, in free space, is the speed of light.

G - H - I


Gamma ray
The most powerful form of electromagnetic radiation. A typical gamma ray is a photon with an energy greater than 100 keV.

Grating (transmission, reflection)
Optical device which has a fine regular pattern used to disperse electromagnetic radiation into a spectrum. A transmission grating consists of a large number of narrow, closely spaced bars. A reflection grating consists of narrowly spaced saw-teeth or steps ruled on a polished surface such as glass or metal.

The Global Ozone Monitoring Experiment. This instrument is a nadir-viewing spectrometer onboard MetOp measuring a range of atmospheric trace constituents and in particular ozone.

Ground segment
All the facilities and systems required on Earth to control and operate a space mission.


Hot target
A hot target reference data set is generated by sampling data when viewing a warm target inside the instrument. Data from this Calibration target, usually in conjunction with data from a cold target (see previous), is used during ground processing of the data, to correct and compensate for errors in Earth view samples. Errors are due to variations in the analogue electronics, primarily due to temperature effects, but also due to any initial offsets, and long term effects of ageing, radiation and so on. Also called On-Board Calibration Target (OBCT).

Housekeeping data
Engineering data used exclusively for managing the operation, health and safety of a spacecraft, platform, instrument or equivalent.


Instantaneous Field of View

Infrared light
Infrared radiation (or infrared light) is invisible to the human eye, but can be sensed as 'heat', or thermal radiation. Even cold objects emit infrared radiation. It has a wavelength between 7000 Angstroms (less than a micron) and several hundred microns. (See also Electromagnetic radiation.) Only a small fraction of the infrared light coming from astronomical objects can go through the Earth's atmosphere: to detect the full range of infrared wavelengths a space telescope is needed. Cold and dusty astronomical objects - such as planets, asteroids or star forming regions - are best observed with infrared telescopes.


1.) apparatus capable of registering information with a precise objective. A science spacecraft can carry several instruments.
2.) a hardware integrated collection of one or more sensors and associated hardware/software controls contributing data to an investigation.

Instrument data
Data produced and transmitted by the science and engineering sensors of an instrument.

Instrument engineering data
Data produced by an instrument's engineering sensor(s) (e.g., instrument temperature).

Instrument science data
Data produced by an instrument's science sensor(s).

Interference fringes
The alternate light and dark bands that are formed when two beams of monochromatic light having a constant phase relation overlap and illuminate the same portion of the screen. Fringes can be produced by division of wavefront or by division of amplitude.

Instrument that forms an acoustic, optical, or microwave interference pattern of fringes which is used to make precision measurements, usually of wavelengths.

Rate of flow of radiated energy through unit area of surface normal to the beam.

J - K - L



The Kelvin (K) is the unit of the absolute temperature scale, in which the temperature of the triple point of water (the temperature at which water can exist simultaneously in solid, liquid and gaseous form) assumes the value of 273.16 K. Kelvin can be converted to degrees Celsius by subtracting 273.15.

Kilohertz (1000 Hz)


Powered vehicle used to carry one or more satellites into space.

All electromagnetic radiation can be called light. However, the term 'light' is commonly used for the electromagnetic radiation that the human eye can detect, that is, the 'visible' or 'optical' light.

Line (absorption)
Absorption spectra are formed when continuous spectra from a star shine through a gas that absorbs only certain colours of light. The absorption spectra, therefore, look like continuous spectra with dark bands (absorption lines) at discrete wavelengths. These lines characterise the chemical composition of the gas which surrounds the star.

M - N - O


Light, or electromagnetic radiation, whose wavelength ranges from millimetres to almost one metre. The cosmic microwave background radiation emits strongly in microwaves.

An optical element that reflects electromagnetic waves (such as visible light, infrared, gamma or X-rays..) towards a camera or detector.

Module (satellite)
Part of a satellite that has been designed and built and often tested as an entity.


Facing the Earth.

Near-real time data
Data from the source that are transmitted with propagation delays and minimal delays due to buffering.


Complete or partial eclipsing of one astronomical object by another.

On-Board Calibration Target (OBCT)
A hot target reference data set is generated by sampling data when viewing a warm target spot usually inside the instrument. Data from this Calibration target is used during ground processing of the data, to correct and compensate for errors in Earth view samples. Errors are due to variations in the analogue electronics, primarily due to temperature effects, but also due to any initial offsets, and long term effects of ageing, radiation and so on. Also called Hot target.

The path through space of one celestial body or spacecraft about another.

Spontaneous liberation of gas from a material in a low pressure environment e.g. space.

Trioxygen (O3). A colourless gas formed by the action of ultraviolet radiation or electrical corona discharge on oxygen or air. It protects life on Earth by absorbing harmful ultraviolet radiation in the atmosphere.

Ozone layer
A layer in the Earth's atmosphere at 15-30 km altitude in which ozone is at higher concentration than at lower or higher altitudes. The ozone is created by a series of processes beginning with the splitting up of the oxygen molecule to single oxygen atoms. The ozone layer protects the Earth from UV radiation harmful to life.

P - Q - R


Parabolic reflector
A curved reflector formed in the shape of a parabola. Made of a rigid material such as metal, glass or a composite material, with a surface finish (smoothness) compatible with the frequency of the radiation that is to be reflected. When a radiation source is positioned at the focal point, a parallel beam is produced, and vice versa. Used to collect the radiation over a large area (the width of the beam) and concentrate it onto the smaller area of the detector, ie acting like an amplifier. A parabolic reflector is also used in optical telescopes.

A particle of light, unit of electromagnetic energy.

Restricting the vibrations of waves, particularly light, to move in one direction, along one plane.

Water condensing from the clouds and falling as rain, hail, snow, sleet. This precipitation does not necessarily reach the Earth's surface as a certain amount evaporates into the atmosphere as it falls.

Primary mirror
Large mirror in a reflecting telescope the size of which determines the light-gathering power of the prism.

Device that breaks light into its composite wavelength spectrum.

Final result from the application of data processing algorithms by computer to the raw data acquired by the satellite. The user can select from a range of products. The product media consists of hard copy (e.g., photographic), telecommunication links and various forms of computer compatible media (e.g., CCT).



Used as a synonym for electromagnetic radiation.

The luminous flux radiated per unit area.

Raw data
1.) instrument data or housekeeping data in the same format as transmitted from the spacecraft or collected on a storage medium on the carrier (e.g., tape recorder, optical disk).
2.) the data received from the satellite prior to the application of any on-ground data processing algorithms. The raw data media will be as for products, i.e. hard copy and various forms of computer compatible media.

Real time data
Data from the source that are transmitted with only propagation delays.

A telescope that uses a mirror - instead of a lens - to collect and focus the light coming from astronomical objects. The term 'reflector' is also used for the mirror itself.

Commonly used term for a launch vehicle.

S - T - U


1.) a small, natural celestial body (such as our Moon) revolving around a larger one.
2.) a man-made object (such as a spacecraft) placed in orbit around the Earth, another planet or the Sun.

An instrument that measures the light scattered from a surface. On MetOp, this is used to measure the height and movement of the waves on the sea.

Secondary mirror
In a two-mirror reflecting telescope, the secondary mirror sits in front of the larger primary mirror and reflects light to the point at which it will be detected and recorded by an instrument. In simple telescopes, the secondary mirror is flat and bounces the light out the side of the tube to an eyepiece. In more complex and larger telescopes, it is convex and reflects light through a hole in the primary mirror.

Service module
Part of a satellite which contains the housekeeping equipment, i.e. power generation, conditioning and control, stabilisation and ground-satellite link.

Sigma (s)
The conventional measure of the strength of a radar signal reflected from a geometric object (natural or manufactured) such as a corner reflector. Sigma specifies the strength of reflection in terms of the geometric cross section of a conducting sphere that would give rise to the same level of reflectivity (units of area, such as meters squared). See also radar cross section.

Sigma naught (so)
The conventional measure of the strength of radar signals reflected by a distributed scatterer, usually expressed in dB. It is a normalized dimensionless number, comparing the strength observed to that expected from an area of one square meter. Sigma naught is defined with respect to the nominally horizontal plane, and in general has a significant variation with incidence angle, wavelength, and polarization, as well as with properties of the scattering surface itself (see speckle, and statistics).

Generalized terminology used to signify a mathematical description of a wave, pulse or other sequence of interest. It often suggests the ensemble of data corresponding to observed scattering from the scene, either before reception, within the radar or processor, or in the image file. Normally there is a distinction between 'signal' and noise.

Soyuz (-Fregat) rocket
Russian rocket which has been launched 1500 times since 1963. It is 43.5 m high and can bring a payload of up to 6 tonnes to 400 km. A manned version carries crews to space stations such as Mir, while an unmanned version is used to launch satellites and Progress cargo craft. A fourth stage called Fregat may also be added to the standard three-stage booster.

Artificial satellite. Term often used before a satellite is placed in orbit around the Earth, when it is transporting something or when it is being sent into deep space.

Spectral lines
The lines that appear in the spectrum of an astronomical object. They are an indicator of the chemical elements present in the object, as well as of its physical conditions. See also Spectrograph

An instrument used to disperse or separate the light into all its wavelengths which allows quantitative measurements of intensity to be made. As an instrument on a telescope, the spectrograph obtains the spectrum of the light from the astronomical objects. Each spectrum contains a wealth of information. For instance, each chemical element has a distinctive spectrum, which varies with temperature. By analyzing the spectrum of a star astronomers can learn about its chemical composition, its temperature.

An instrument used for the measurement of wavelength or energy distribution in a heterogeneous beam of radiation. It measures the position of spectral lines.

Electromagnetic radiation whose beam is dispersed like a natural rainbow so that components with different wavelengths are separated in space in order of increasing or decreasing wavelength. There are three kinds of spectra that interest astronomers.

  • Continuous Spectra: The surface of a star is heated to such an extent that it glows with a particular colour. Red for cool stars, bluish-white for very hot stars. Because the light emitted at the surface has been absorbed and transmitted by many atoms by the time it reaches the surface, the discrete colours of the emission spectra of the atoms have been evened out to form a continuous spectra.
  • Absorption Spectra: See Line (absorption).
  • Emission Spectra: These usually arise from gas that is in the outer regions of stars, where the light is not absorbed and emitted many times before being transmitted to space. An emission spectrum consists of sharp peaks in the spectrum corresponding to the wavelengths of the emitted light.

Giant ball of gas in space which produces vast amounts of energy through nuclear reactions in its core. There are many different types of stars, which are classified according to their temperatures, colours, ages and compositions.

The layer of the atmosphere stretching from the troposphere to the mesosphere (an altitude of apporximately 50km). In the stratosphere temperature generally increases with height.

Structural and thermal model
Satellite prototype used to test the space performance of the final flight model.

Our nearest star and the central object in the Solar System. Compared with other stars it is fairly average in terms of size and temperature. It seems to have formed from a cloud of dust and gas about 5 billion years ago. A giant ball of gas, mainly hydrogen and helium, it contains 745 times as much mass as all of the planets put together. Energy is generated through nuclear fusion in its core. The temperature of the core is about 15 million degrees Celsius, while the temperature of its visible surface (the photosphere) is 5700°C. Above the photosphere are the chromosphere and the corona, where the temperature exceeds one million degrees. The energy generated in the core takes 30,000 years to reach the surface, when it is mostly emitted as light and infrared (heat) radiation.

The swath, or ground track, is the area of the Earth's surface that is 'viewed' by a particular instrument, as the satelite passes over the Earth's surface.


Data and commands sent from the spacecraft to ground stations.

Instrument used to focus electromagnetic radiation (light, X-rays...) into an image.

Physical parameter characterising the thermal state of a body. Measured in units of degrees Celsius (ºC), Fahrenheit (ºF) or Kelvin (K).

Ground facilities employed to follow the progress and to communicate with a satellite.

The Lowest layer of the Earth's atmosphere ranging in thickness from about 9km at the poles to 17km at the equator. In the troposhere temperature generally decreases with height.


Ultraviolet light
Region of the electromagnetic spectrum spanning wavelengths from 91.2 nm to 350 nm, wavelengths largely blocked by the Earth's atmosphere.

Ultraviolet radiation
Electromagnetic radiation in a wavelength range 10 - 400 nm. This lies between the visible and X-ray regions of the spectrum.

V - W - X



The distance between two peaks of a wave. See also Electromagnetic radiation and Frequency.


Electromagnetic radiation with wavelengths between those of ultraviolet and gamma rays, approximately 0.01 - 10 nm. At these short wavelengths, it is more common to talk in terms of photon energies. These energies range from 0.1 - 100 keV.

Y - Z



Last update: 21 December 2005

Copyright 2000 - 2014 © European Space Agency. All rights reserved.