Glossary of terminology, key words and acronyms used on this site.

This section contains comprehensive definitions for many of the general and technical terms used by EUMETSAT.

The Image glossary provides definitions of the keywords used in the searchable Image Library.

The Product Navigator glossary provides definitions for items which are in the product catalogue.




Instrument used to measure the altitude of an object above sea level.


The point in a satellite's orbit furthest away from Earth.


An arc of the horizon measured between a fixed point (as true north) and the vertical circle passing through the center of an object.


The portion of power scattered back in the incident direction.


A hypothetic body that completely absorbs all wavelengths of thermal radiation incident on it. Such bodies do not reflect light, and therefore appear black if their temperatures are low enough so as not to be self-luminous.

Infra-red radiation (IR) and Infrared satellite imagery

IR is the region of the electromagnetic spectrum adjacent to that of visible light, but with longer wavelengths (0.65-10 micrometers, typically). IR satellite imagery senses surface and cloud top temperatures by measuring the wavelength of electromagnetic radiation emitted from these objects.

In plane manoeuvre

Operation implementing, by activation of thrusters, a change of the orbital velocity of a satellite in the in-flight direction (and consequently a change of the semi-major-axis of the orbit). These manoeuvres are used to compensate for the orbital perturbations caused by the drag (for Low Earth Orbit satellites, leading to orbital decay) or the gravity field asymmetry (for geostationary satellites, leading to longitude drift); as the eccentricity of the orbit is also modified at the same time, these manoeuvre can be used to compensate for the orbital perturbations caused by the solar radiation pressure (leading to a change of the orbital eccentricity).


The path of a body in space, generally under the influence of gravity.

Out-of-plane manoeuvre

Operation implementing, by activation of thrusters, a change of the orbital velocity of a satellite in the direction normal to the orbital plane (and consequently a change of the inclination of the orbit). These manoeuvres are used to compensate for the orbital perturbations caused by the sun and moon gravity (leading to a change of the orbital inclination). For LEO satellites, as the rate of drift of the mean local solar time of the orbit is linked with the orbital inclination, these manoeuvre can also be used to implement a change of that rate.


The carrying capacity of a spacecraft, including cargo, scientific instruments or fuel.


The point of a satellite's orbit closest to Earth.


Radar systems that infer wind speed by measuring the backscattering cross section. Scatterometers can measure the received power of surface backscattering reflected from the surface of objects.


Instruments that acquire multispectral measurements from which vertical profiles of atmospheric temperature and humidity can be derived to produce particular measurements of depth of water below an instrument (at the surface or at some moored depth), which is computed from the travel time of the acoustic pulse emitted by this sounder.


The method of stabilizing a satellite by means of spin.


Extra rays of light hitting SEVIRI's optical path and degrading images.


In a sun-synchronous orbit, though, the satellite passes over the same part of the Earth at roughly the same local time each day.

Three-axis stabilized

Satellite platform type. Involves the use of three gyroscopes — one for each axis — to keep satellites correctly oriented in space.



The portion of the electromagnetic spectrum ranging from 4-8 GHz.

Climate Monitoring

Monitoring changes in climate trends in satellite data.

Coordinated Universal Time (UTC)

By international agreement, the local time at the prime meridian, which passes through Greenwich, England (previously know as GMT). The time used by EUMETSAT on data/products and satellite operational events.


Data occurring over a 10-year period.


Daily data


EUMETSAT's Data Distribution System, which broadcasts data to users.


The scientific study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water on or below the land surface.


The portion of the electromagnetic spectrum ranging from 12-18 GHz.

Near real-time

Data which is available after a short time delay, typically less than three hours.

Soil Moisture

Water contained in the upper part of the soil mantle.


The shape of the ocean and land. It is used in the context of Jason-3 service.



The fraction of radiation striking a surface that is reflected by that surface.

El Niño

Term for climatic events originating in the tropical Pacific that recur every few years as part of a naturally-occurring cycle. Specifically for the anomalous sustained warming of sea surface temperature that occurs every few years, typically concentrated in the central-east equatorial Pacific.


A statement of prediction, e.g weather forecasts.

Jet stream

The core of fast air flow associated with the westerlies, occurring close to the top of the troposphere.

La Niña

La Niña refers to the extensive cooling of the central and eastern Pacific Ocean, often associated with increased probability of wetter conditions.


The region of the atmosphere extending from the top of the troposphere to the base of the mesosphere.


The measure of the internal energy that a substance contains. This is the most measured quantity in the atmosphere.

Urban Heat Island

The increased air temperatures in urban areas in contrast to cooler surrounding rural areas.


The state of the atmosphere with respect to wind, temperature, cloudiness, moisture, pressure, etc. Weather refers to these conditions at a given point in time.

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