This glossary provides short explanations of important terms and acronyms related to remote sensing that are used in this Atlas:
absorption – The process in which radiant energy is retained by a substance.
active system (active sensor) – A remote-sensing system that transmits its own radiation to detect an object or area for observation and receives the reflected or transmitted radiation. Example: Radar.
aerosol – A colloid of solid or liquid particles suspended in a gas (e.g. smoke, fog).
albedo – The ratio of the outgoing solar radiation reflected by an object to the incoming solar radiation incident upon it.
altimeter – An active instrument used to measure the altitude of a satellite above a fixed level. Coupled with exact orbit knowledge this enables determination of the topography.
amplitude – The magnitude of the displacement of a wave from a mean value. For a simple harmonic wave, it is the maximum displacement from the mean.
apogee – Point of the elliptic orbit of a satellite where its distance from the Earth is maximal.
Ariane – Launch vehicles developed for ESA by a subsidiary of the European Aeronautic Defence and Space Company (EADS) and launched from the Kourou Space Centre in French Guiana.
ASAR – Advanced Synthetic Aperture Radar – a radar sensor on the satellite ENVISAT operating in C-band.
ascending node – The point in an orbit (longitude) at which a satellite crosses the equatorial plane in south-north direction.
ASTER – Advanced Spaceborne Thermal Emission and Reflection Radiometer – An imaging instrument on NASA’s Terra. ASTER is used to obtain detailed maps of land surface temperature, emissivity, reflectance, and elevation.
atmospheric windows – The range of wavelengths at which the atmospheric gases only slightly absorb radiation. Atmospheric windows allow the Earth’s radiation to escape into space unless clouds absorb the radiation.
attenuation – The decrease in the power of a signal in transmission. Attenuation may be expressed in decibels, and can be caused by interferences such as rain, clouds, or radio frequency signals.
AVHRR – Advanced Very High Resolution Radiometer – A five-channel scanning instrument that quantitatively measures electromagnetic radiation on NOAA environmental satellites. Determines cloud cover and surface temperature. Visible and infrared detectors observe vegetation, clouds, lakes, shorelines, snow, and ice.
azimuth – The direction, in degrees referenced to true north, that an antenna must be pointed to receive a satellite signal. The angular distance is measured in a clockwise direction.
backscatter – Process by which up to 25% of radiant energy from the sun is reflected or scattered away from the surface by clouds.
band – A region of the electromagnetic spectrum to which a remote sensor responds; a multispectral sensor makes measurements in a number of spectral bands.
bandwidth – The total range of frequency required to pass a specific modulated signal without distortion or loss of data.
bathymetry – Mapping the topography of the ocean floor.
Biomass – Part of the Earth Explorer Mission of ESA, created to observe and analyse the world’s forests using radar technology.
calibration – Act of comparing an instrument’s measuring accuracy to a known standard.
C-Band – part of the electromagnetic microwave spectrum between 500 MHz and 1000 MHz.
climate model – Computer model to calculate and project the climate for a specific period.
Copernicus – former name GMES (Global Monitoring for Environment and Security), the EU’s Copernicus Earth observation programme.
CryoSat – ESA research satellite mapping the Earth’s cryosphere and collecting data in particular on the volume of the ice sheets in the Arctic and Antarctic (since 2010).
culmination – The point at which a satellite reaches its highest position or elevation in the sky, relative to an observer.
declination – The angular distance from the equator to the satellite.
descending node – The point in a satellite’s orbit at which it crosses the equatorial plane from north to south.
digital elevation model (DEM) – A representation of the topography of the Earth in digital format, i.e. by coordinates and numerical descriptions of altitude.
DMSP – Defense Meteorological Satellite Program – U.S. Air Force meteorological satellite program with satellites circling in sun-synchronous orbit. Imagery is collected in the visible- to near-infrared band (0.4 to 1.1 μm) and in the thermal-infrared band (about 8 to 13 μm) at a resolution of about 3 km.
Dobson Unit (DU) – The standard unit for ozone amounts in the atmosphere. One DU is 2.7 x 1016 ozone molecules per cm² and refers to a layer of ozone that would be 0.001 cm thick under conditions of standard temperature (0°C) and pressure.
EAC – European Astronaut Centre, Cologne; central ESA facility for training astronauts.
EarthCARE – Scheduled space mission to research aerosols and clouds and their impact on radiation in the Earth’s atmosphere, as part of the Living Planet Programme of the ESA. Launch date: 2024.
Earth Explorer – An ESA mission and part of the Living Planet Programme, which comprises the satellite missions providing new observation data on the Earth.
Earth observation satellite – Satellites observing the Earth from space.
eccentricity – Describes the shape of an orbit. The satellite orbit is an ellipse, with eccentricity defining the shape of the ellipse. When e=0, the ellipse is a circle. When e is very near 1, the ellipse is very long and skinny.
electromagnetic radiation – Energy propagated as time-varying electric and magnetic fields at the speed of light. Light and radar are examples of electromagnetic radiation differing only in their wavelengths (or frequency).
electromagnetic spectrum – The entire range of radiant energies or wave frequencies from the longest to the shortest wavelengths. The spectrum usually is divided into seven sections: radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma-ray radiation.
elliptical orbits – Bodies in space orbit in elliptical rather than circular orbits because of factors such as gravity and drag. The point where the orbiting satellite is closest to Earth is the perigee, the point where the satellite is farthest from Earth is called apogee.
emissivity – The ratio of the radiation emitted by a surface to that emitted by a black body at the same temperature.
Enhanced Thematic Mapper Plus (ETM +) – An eight-band multispectral scanning radiometer onboard the Landsat 7 satellite that provides high-resolution imaging information of the Earth’s surface.
Envisat – Environmental Satellite mission, 2002-2012; large platform monitoring the environmental changes on earth and in the atmosphere, scientific mission to study Earth observation applications.
ERS – European Remote Sensing Satellite; two radar satellites operated by ESA, 1991-2011, precursors to Sentinel-1.
ESA – European Space Agency
ESOC – European Space Operations Centre
ESRIN – European Space Research Institute
ESTEC – European Space Research and Technology Centre
false colour – A colour imaging process which produces an image of a colour that does not correspond to the true or natural colour of the scene (as seen by our eyes).
far infrared – Electromagnetic radiation, longer than the thermal infrared, with wavelengths between about 25 and 1000 micrometres.
field of view – The range of angles that are scanned or sensed by a system or instrument, measured in degrees of arc.
Fraction of Photosynthetically Active Radiation (FPAR) – Radiation between 400 and 700 nm used by the green canopy in the photosynthetic process.
frequency (F) – Number of cycles and parts of cycles completed per second. F=1/T, where T is the length of one cycle in seconds.
Galileo – European satellite navigation system, consisting of 30 satellites in 3 orbits.
gamma ray – A high energy photon, especially as emitted by a nucleus in a transition between two energy levels.
geocoding – One element of georeferencing, where data with no georeference are translated into the desired reference system.
Geographic Information System (GIS) – A system for archiving, retrieving, and manipulating data that has been stored and indexed according to the geographic coordinates of its elements. The system generally can utilize a variety of data types, such as imagery, maps, table, etc.
geostationary – Describes an orbit in which a satellite is always in the same position with respect to the rotating Earth. The satellite travels around the Earth in the same direction, at an altitude of approximately 35,790 km because that produces an orbital period equal to the period of rotation of the Earth. Used for weather satellites and most commercial telecommunications satellites.
GOES – Geostationary Operational Environmental Satellite – Observes the U.S. and adjacent ocean areas from vantage points 35,790 km above the equator at 75 degrees west and 135 degrees west. GOES satellites have an equatorial, Earth-synchronous orbit with a 24-hour period, a visible resolution of 1 km, and an IR resolution of 4 km.
geosynchronous – Synchronous with respect to the rotation of the Earth.
GPS – global positioning system – A system consisting of 25 satellites in 6 orbital planes at 20,000 km altitude with 12 hr periods, used to provide highly precise position, velocity and time information.
GOME – Global Ozone Monitoring Experiment – Instrument aboard ERS.
greenhouse gas – Gaseous substances in the atmosphere affecting radiation and contributing to the warming of the Earth’s atmosphere, known as the greenhouse effect.
Ground resolution – Describes the length of the side of a single square pixel in an image. The smaller this value is, the more precise and detailed the image.
ground track – The inclination of a satellite, together with its orbital altitude and the period of its orbit, creates a track defined by an imaginary line connecting the satellite and the Earth’s centre.
inclination – Inclination is the angle between a satellite orbit and the equator.
infrared radiation (IR) – Infrared is electromagnetic radiation with wavelengths from about 0.7 to 1000 μm (between visible and microwave radiation), subdivided into visible and near-infrared, mid-infrared and far infrared.
interferometry – data analysis method exploiting the phase difference between two optical beams; used e.g. to derive elevation differences or changes.
ISS – International Space Station, a joint project between 16 countries designed as a scientific laboratory in space.
Ku-band – Radar and microwave band in which the wavelengths vary from 1.67-2.4 cm.
land cover – The characteristics of a land surface as determined by its spectral signature (the unique way in which a given type of land cover reflects and absorbs light).
Landsat – Land Remote-Sensing Satellite, operated by the U.S. Earth Observation Satellite Company (EOSAT). Commercialized under the Land Remote-Sensing Commercialization Act of 1984, Landsat is a series of satellites (formerly called ERTS) designed to gather data on the Earth’s resources in a regular and systematic manner.
land use – The characteristics of a land surface as determined by its use (the unique way in which a given type of land is – or is not – exploited by man).
MERIS – Medium Resolution Imaging Spectrometer – Instrument aboard Envisat for the observation of the ocean and vegetation.
METEOSAT – METEOrological SATellite – Europe’s geostationary weather satellite, launched by the European Space Agency and operated by Eumetsat.
microwave – Electromagnetic radiation with wavelengths between about 1000 μm and 1 m.
middle infrared – Electromagnetic radiation between the near infrared and the thermal infrared, about 2-5 μm.
MODIS – Moderate-resolution Imaging Spectroradiometer – Sensor flying aboard Terra and viewing the entire surface of the Earth every 1-2 days, making observations in 36 spectral bands, at moderate resolution (0.25 – 1 km), of land and ocean surface temperature, primary productivity, land surface cover, clouds, aerosols, water vapor, temperature profiles, and fires.
multispectral – Comprising data from different spectral bands.
Multispectral Scanner (MSS) – A line-scanning instrument flown on Landsat satellites that continually scans the Earth in a 185 km swath. On Landsat 1, 2, 4, and 5, the MSS had four spectral bands in the visible and near-infrared with an IFOV of 80 meters. Landsat-3 had a fifth band in the thermal infrared with an IFOV of 240 meters.
multitemporal – Comprising data from different times.
NASA – National Aeronautics and Space Administration – NASA, established in 1958 with Headquarters in Washington D.C., is the agency responsible for the public space program of the USA. Its mission is to plan, direct, and conduct aeronautical and space activities.
NDVI – normalized difference vegetation index – A model for converting satellite-based measurements into surface vegetation types. The NDVI uses a complex ratio of reflectance in the red and near-infrared portions of the spectrum to accomplish this. It is a quantity that measures greenness and vigour of vegetation.
near infrared – Electromagnetic radiation with wavelengths from just longer than the visible (about 0.7 μm) to about 2 μm.
NOAA – National Oceanic and Atmospheric Administration – NOAA was established in 1970 within the U.S. Department of Commerce. It is focused on the conditions of the oceans and the atmosphere. NOAA has two main components: the National Weather Service (NWS), and the National Environmental Satellite, Data, and Information Service (NESDIS).
ocean colour – The ability of phytoplankton to appear as different colours in certain bands of the electromagnetic spectrum because of their chlorophyll concentrations.
OLI – Operational Land Imager, multispectral imaging sensor on board Landsat 8 and 9.
orbit – The path described by a heavenly body in its periodic revolution. Earth satellite orbits with inclinations near 0° are called equatorial orbits because the satellite stays nearly over the equator. Orbits with inclinations near 90° are called polar orbits because the satellite crosses nearly over the north and south poles.
orbital plane – An imaginary gigantic flat plane containing an Earth satellite’s orbit. The orbital plane passes through the centre of the Earth.
panchromatic – Sensitive to all or most of the visible spectrum.
passive microwave radiometer – A system sensing only microwave radiation emitted by the object being viewed or reflected by the object from a source other than the system.
passive system – A system sensing only radiation emitted by the object being viewed or reflected by the object from a source other than the system.
payload – The instruments that are accommodated on a spacecraft.
perigee – On an elliptical orbit path, the point where a satellite is closest to the Earth.
period – Time required for a satellite to make one complete orbit.
pixel – ‘Picture element’, ground area corresponding to a single element of a digital image data set.
platform – A satellite that can carry instruments.
polar orbit – An orbit with an orbital inclination of near 90° where the satellite ground track will cross both polar regions once during each orbit.
PROBA – Project for On-Board Autonomy – small scale technology demonstration satellite of ESA, carrying several earth observation instruments with ground resolutions down to 20 m (CHRIS, multispectral) and 5 m (HRV, panchromatic).
radar interferometry – The study of interference patterns caused by radar signals; a technique that enables scientists to generate three dimensional images of the Earth’s surface.
radiation – Energy transfer in the form of electromagnetic waves or particles that release energy when absorbed by an object.
radiation budget – A measure of inputs and outputs of radiative energy relative to a system, such as Earth.
reflectance – The proportion of irradiated electromagnetic radiation reflected by a surface.
reflection – The return of light or sound waves from a surface. If a reflecting surface is plane, the angle of reflection of a light ray is the same as the angle of incidence.
remote sensing – The technology of acquiring data and information about an object or phenomena by a device without physical contact. Earth remote sensing refers to gathering information about the Earth and its environment from a distance.
repetition rate (orbital period) – Time required for a satellite to complete one orbit.
resolution – In the case of imagery, it describes the area represented by each pixel of an image. The smaller the area represented by a pixel, the more accurate and detailed the image.
SAR – synthetic aperture radar – A high-resolution ground-mapping technique that effectively synthesizes a large receiving antenna by processing the phase of the reflected radar return.
satellite – A free-flying object orbiting the Earth, another planet, or the sun.
Satellite image map – Map created on the basis of satellite image data.
S-band – Frequency range from 4 to 2 GHz (7 to 20 cm wavelength) within the microwave (radar) portion of the electromagnetic spectrum. S-band radars are used for medium-range meteorological applications, e.g. rainfall measurements.
scanning radiometer – An imaging system consisting of lenses, moving mirrors, and solid-state image sensors used to obtain observations of the Earth and its atmosphere.
scattering – The process by which electromagnetic radiation interacts with and is redirected by the molecules of the atmosphere, ocean, or land surface.
scatterometer – A high-frequency radar instrument that transmits pulses of energy towards the ocean and measures the backscatter from the ocean surface. It detects wind speed and direction over the oceans by analysing the backscatter from the small wind-induced ripples on the surface of the water.
scene – Object space illuminated by a sensor.
sea surface temperature (SST) – The temperature of the layer of seawater (approx. 0.5 m deep) near the surface.
sensor – Device producing an output in response to incident radiation. Sensors aboard satellites obtain information about features and objects on Earth by detecting radiation reflected or emitted in different bands of the electromagnetic spectrum.
Sentinel – Series of Earth Observation satellites under responsibility of ESA in the Copernicus programme.
shortwave radiation – The radiation received from the sun and emitted in the spectral wavelengths less than 4 μm. It is also called solar radiation.
SMOS – Soil Moisture and Ocean Salinity Mission, the ESA’s second Earth Explorer Opportunity mission to gather global observation data for the modelling of weather, climate and ocean currents.
SRTM – Shuttle Radar Topography Mission – A Space Shuttle mission that used C-band and X-band interferometric synthetic aperture radars (IFSARs) to acquire topographic data over 80% of Earth’s land mass (between 60°N and 56°S) in February 2000.
solar constant – The constant expressing the amount of solar radiation reaching the Earth from the sun, approximately 1370 W/m².
solar radiation – Energy received from the sun. The energy comes in many forms, such as visible light. Other forms of radiation include radio waves, infrared, ultraviolet waves, and x-rays.
spectral band – A finite segment of wavelengths in the electromagnetic spectrum.
spectral signature – This refers to the particular form or shape evinced by the power spectrum calculated from the data comprising the time series of a process.
SPOT – Systeme Pour l’Observation de la Terre – French, polar-orbiting Earth observation satellites with ground resolution of 10 m. SPOT images are available commercially and are intended for such purposes as environmental research and monitoring or ecology management.
sun-synchronous – A sun synchronous orbit is a nearly polar orbit. Every time it crosses the Equator, it does it at the same local time.
Swarm – Earth Explorer Mission of the ESA, to observe the magnetic field of the Earth.
swath – The area observed by a satellite as it orbits the Earth.
terrestrial radiation – The total infrared radiation emitted by the Earth and its atmosphere in the temperature range of approximately 200-300 K.
Thematic Mapper – Multispectral imaging sensor with 7 spectral bands on board of Landsat-4 and Landsat-5, acquiring data in the visible and infrared parts of the electromagnetic spectrum. The ground resolution of the visible and short-wave infrared bands is 30 m/pixel, the thermal band 120 m/pixel.
thermal infrared – Electromagnetic radiation with wavelengths between 3 and 25 µm.
TRMM – Tropical Rainfall Measuring Mission, Satellite programme launched by NASA and NASDA in 1997 to acquire data on tropical precipitation.
ultraviolet radiation – Part of the electromagnetic spectrum with wavelengths below the violet region; it contains about 5% of the energy radiated by the Sun and is the main source of energy in the stratosphere and in the mesosphere.
USGS – United States Geological Survey, US American office responsible for the acquisition and distribution of geoinformation.
VHR – Very High Resolution, Earth observation data with a ground resolution of 1 m and better.
VISSR – Visible/Infrared Spin Scan Radiometer, Multispectral imagining system with high resolution, deployed onboard of GOES weather satellites (until GOES-7).
visible light – The part of the electromagnetic spectrum visible to the human eye (0.4 μm to 0.7 μm).
water vapour – water in its gaseous state; in the atmosphere the most important factor of the natural greenhouse effect.
wave – Moving changes of a parameter, periodical in space and time, e.g. electromagnetic radiation.
wavelength – Distance between two maxima of a wave.
weather satellite – Earth observation satellite that observes meteorological processes.
X band – Radar frequency band between 12.5 and 8 GHz (wavelength 2,4-3,75 cm).