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  • ACC: Antarctic Circumpolar Current
    Cold current encircling the Antarctic, driven by westerly winds between 40° and 50° South. The flow of the ACC is not checked by the continents. It thus links the Atlantic, Indian and Pacific Oceans.
  • ADT: Absolute Dynamic Topography. Ocean topography with respect to the geoid
  • AGC: Automatic Gain Control
  • Along-trackAlong-track data are data chronologically ordered, following the satellite “ground track”, i.e. the virtual track left by the radar beam on the ground.
  • Altimetry: Technique for measuring height. Satellite altimetry measures the time taken by a radar pulse to travel from the satellite antenna to the surface and back to the satellite receiver. Combined with precise satellite location data, altimetry measurements yield sea-surface heights.
  • ASCII: American Standard Code for Information Interchange
  • ASIC: Application Specific Integrated Circuit
  • ATSRAlong-Track Scanning Radiometer instrument measuring sea-surface temperatures (ESA)
  • Aviso: Archiving, Validation and Interpretation of Satellite Oceanographic data. French data distribution and archiving centre for altimetry satellites and Doris data. Further information:-  Website

  • Calval: CALibration – VALidation
  • CCICentre de Contrôle des Instruments. Instruments control centre (Jason-1)
  • CD RomCompact Disk Read Only Memory
  • CersatCentre ERS d’Archivage et de Traitement French Processing and Archiving Facility Website
  • CHAMPCHAllenging Minisatellite Payload Website
  • ClivarClimate Variability and Predictability (WCRP) Website
  • CLSCollecte, Localisation, Satellites Website
  • CMACentre Multimissions Altimétrique CNES altimetry multi-mission ground segment
  • CNESCentre National d’études Spatiales French Space Agency Website
  • CryosatESA’s altimetry mission, to determine variations in the thickness of the Earth’s continental ice sheets and marine ice cover Website
  • CTRSConventional Terrestrial Reference System
  • Cycle: Satellite repetitivity, or repeat orbit

  • DASData Assimilation System
  • Data assimilation: Use of data as initial input to a model and during computation to yield results that fit/approximate reality as closely as possible. For example, assimilation of readings from meteorology stations into weather forecasting models generates more reliable results.
  • Data processing: Antennas on the ground receive raw telemetry from satellites. These raw data must be processed by applying corrections and combining them with complementary data before they are usable. Data are processed to different levels:
    Level 0: raw telemetry
    Level 1: time-tagged data located and corrected for perturbing effects (level 1, 1.5 and 1b data fall into this category)
    Level 2: geophysical data (GDR-M) time-tagged, precisely located and corrected for environment effects. Data at this level are used by specialists with a close knowledge of their subject.
    Level 3: data ready for immediate use in applications, crrected and/or inter-calibrated.
    Level 4: gridded or model output data.
    Depending on the mission, retracking is performed at level 1 or 2.
  • DEOSDelft Institute for Earth-Oriented Space research Website
  • DHUData Handling Unit
  • DIODEDétermination Immédiate d’Orbite par DORIS Embarqué
    DORIS onboard navigator, processing orbit in real time.
  • Doppler effect (Doppler-Fizeau effect): The pitch of a sound emitted by a moving object appears to be higher the faster it approaches, and lower the faster it moves away. A good analogy is a stream into which leaves are thrown at regular intervals. As we move upstream towards the source, leaves will flow past us more often. The faster we go, the more leaves we will see. Conversely, as we move downstream away from the source leaves will flow past less often-to the point where we would only see a single leaf when moving at the same speed as the current. High-pitched sounds have high frequencies, which means we “meet” the sound wave often-as if we were moving towards the source (or it was approaching us); low-pitched sounds have lower frequencies-as if we were moving away. The same principle applies to light rays, which shift towards the longer wavelengths as they move away, and towards the shorter wavelengths as they approach. (Adapted from Evry Schatzmann, Les Enfants d’Uranie)
    We can thus determine the velocity of a moving object emitting sound or light waves by measuring the shift between the transmitted and received frequencies. The DORIS system achieves precise orbit determination and location by measuring the Doppler shift in this way.
  • DORISDoppler Orbitography and Radiopositioning Integrated by Satellite. Precise orbit determination and location system using Doppler shift measurement techniques. A global network of orbitography beacons has been deployed. DORIS was developed by CNES, the French space agency, and is operated by CLS.
  • Dry tropospheric correction: Permanent gases in the atmosphere (oxygen, nitrogen) slow the radar pulse, generating an error on altimetry measurements of the order of 2.30 metres. The value of the correction is determined from atmospheric pressure data supplied by a meteorological model.
  • DTDelayed Time: data used to compute the product were processed using precise orbit
  • DUACSData Unification and Altimeter Combination System (as part of the SSALTO processing system) (previously Developing Use of Altimetry for Climate Studies, in the EU project frame)
  • DVD RomDigital Versatile Disk Read Only Memory.
  • Dynamic topography: Sea level driven by thermodynamic processes in the ocean.

  • ECMWFEuropean Centre for Medium-range Weather Forecasting Website
  • Eddies: Hydrodynamic instabilities in the oceans that form in the wake of currents or are generated by winds. Eddies may persist for weeks or months, and may be tens to hundreds of kilometres across. They penetrate quite deep below the ocean surface and transport heat, salts and nutrients. Electromagnetic Bias (EMB)Perturbation of the altimeter radar pulse due to the fact that wave troughs reflect more energy than wave crests.
  • EMElectromagnetic
  • ENSOEl Niño – Southern OscillationEl Niño Southern Oscillation Climatic phenomenon occurring in the Tropics that involves transports of warm (El Niño) or cold (La Niña) water masses from west to east across the Pacific basin. This displacement of water is accompanied by a shift of atmospheric cells, and therefore of winds and monsoons across the Tropics. This oscillation, which has been occurring at irregular intervals for thousands of years, has a global effect on climate. The phenomenon is named “El Niño” (meaning the Christ Child or the Little Boy in Spanish) because warm waters often reach the coast of Peru around Christmas time, often severely disrupting local fisheries.
  • Environmental corrections: The radar pulse used to measure altimetry is subjected to a number of disturbances as it passes through the atmosphere and when it is reflected by the sea surface.
  • EnvisatENVIronmental SATellite Earth-observing satellite (ESAWebsite
  • ERMExact Repeat Mission (Geosat)Phase of the Geosat mission during which the satellite was put into a 17-day repeat orbit to study the oceans.
  • ERSEuropean Remote Sensing satellites (ERS-1, ERS-2) Earth-observing satellites (ESA)
  • ESA: European space Agency Website
  • ESOCEuropean Space Operations Centre Website
  • ESRIN: ESA Centre for Earth Observation Website
  • ESTECEuropean Space Research and Technology Centre Website
  • EUEuropean Union
  • EumetsatEuropean Organisation for the Exploitation of METeorological SATellites Website

  • F-DAC: French-Distribution and Archiving Centre (F-DAC)
  • F-PAC: French-Processing and Archiving Centre (F-PAC)
  • FDP:Fast Delivery Product
  • FMFrequency Modulation 
  • FNMOCFleet Numerical Meteorological & Oceanographic Center (US Navy)
  • FNOCFleet Numerical Oceanographic Center
  • Full-derampThe full-deramp technique concept consists in mixing this incoming signal with a replica of the transmitted chirp, slightly shifted in frequency. This circonvents problems of power necessary for the emitted pulse, and of the very short duration that would be needed to obtain requested accuracy.

  • GDR: Geophysical Data Record(s) GeodesyThe science of measuring the Earth, its shape and gravitational field.
  • Geoid: The shape of the sea surface assuming a complete absence of perturbing forces (tides, wind, currents, etc.). The geoid reflects the Earth’s gravitational field (it is an equipotential surface) and varies in height by as much as 100 metres over distances of several thousand kilometres due to uneven mass distribution within the planet’s crust, mantle and core. Other, less pronounced, irregularities are also visible over smaller distances. These reflect the ocean’s bottom topography.
  • Geophysical corrections: The radar pulse used to measure altimetry is subjected to a number of disturbances as it passes through the atmosphere and when it is reflected by the sea surface.
  • GEOS 3Geodynamics Experimental Ocean Satellite Altimetric satellite (NASA)
  • Geosat: Geodetic & Oceanographic SATellite Altimetric satellite (US Navy)
  • Geostrophic circulation: Ocean circulation generated by the balance between the horizontal pressure gradient forces exerted by water masses and the effect of acceleration due to the Earth’s rotation.
  • GFO: Geosat Follow-On Altimetric Satellite (US Navy).
  • GMGeodetic Mission (Geosat). Phase of the GEOSAT mission during which the satellite’s orbit was designed to study the geoid.
  • GOCEGravity field and steady-state Ocean Circulation Website
  • GODAEGlobal Ocean Data Assimilation Experiment. Website
  • GPS: Global positionning System.
  • GPSDRGlobal Positioning System Demonstration Receiver. One of three positioning systems on Topex/Poseidon that uses the GPS to determine the satellite’s position.
  • GRACEGravity Recovery and Climate Experiment Website
  • Greenhouse effect: A good proportion of the heat penetrating the Earth’s atmosphere is not reflected back into space but is “trapped” by clouds and water vapor. This heat is thus returned to the surface, maintaining an average temperature of 15°C on Earth. Without it, the temperature would be nearer -18°C. Increasing quantities of certain gases in the atmosphere-called greenhouse gases-such as carbon dioxide are thought to amplify this phenomenon, leading to an increase in temperature on the surface of the globe.
  • Ground Segment: The teams, hardware and software involved in controlling a satellite and in retrieving and processing its data.
  • Gulf Stream: Western boundary current of the subtropical gyre. The Gulf Stream is a strong current that transports warm waters from the Gulf of Mexico up the south coast of the United States to the Mid-Atlantic Ocean.

  • IATInternational Atomic Time
  • IDSInternational DORIS Service Website
  • IERSInternational Earth Rotation Service Website
  • IGDRInterim Geophysical Data Record(s) or rapid-delivery products (48 hours after acquisition)
  • IMInstrument Module
  • Inverted Barometer (IB): Correction applied to allow for atmospheric forcing of the ocean surface. The sea level is lower when atmospheric pressure is high, and higher when atmospheric pressure is low.
  • Ionosphere: Layer of the upper atmosphere where electron and ionisation activity is particularly high.Electromagnetic waves are subjected to a number of perturbing effects as they pass through the ionosphere.
  • Ionospheric correction: The altimeter radar signal is delayed as it travels through the ionosphere. Free electrons slow the radar pulse, causing an error on altimetry measurements of the order of 0 to 30 centimetres. The value of this correction is determined by combining range measurements acquired at two different frequencies (Topex and Poseidon-2 altimeters), as it is inversely proportional to the square of the frequency. For mono-frequency altimeters such as that used on the ERS satellites, this value is determined using a statistical model of the ionosphere.
  • ISROIndian Space Research organisation Website
  • ITRFInternational Terrestrial Reference Frame.

  • Kuroshio Current: Western boundary current of the subtropical gyre. The Kuroshio Current is a strong current that transports warm waters from the China Sea and the Philippines up past Japan to the Mid-Pacific Ocean. It is the counterpart of the Gulf Stream in the North Atlantic.

  • La Niña: See ENSO
  • LEOLow Earth Orbit
  • LODLength-Of-Day
  • LRALaser Retroreflector Array; One of three positioning systems on Topex/Poseidon and Jason. The LRA uses a laser beam to determine the satellite’s position by measuring the round-trip time between the satellite and Earth to calculate the range.
  • LRRLaser RetroReflector; One of three positioning systems on ERS-1 and 2, Envisat and Cryosat. The LRR uses a laser beam to determine the satellite’s position by measuring the round-trip time between the satellite and Earth to calculate the range.

  • MADTMaps of Absolute Dynamic Topography
  • MMSMultimission Modular Spacecraft
  • Modelling: Modelling a phenomenon involves identifying its main characteristics and expressing them mathematically to better understand and, above all, predict how the phenomenon is likely to evolve.
  • MOEMedium Orbit Ephemeris
  • Mean Sea Level (MSL): The sea surface height averaged across all the oceans of the globe. An increase in the mean sea level is an indicator of a possible global warming. MSLAMaps of Sea Level Anomalies
  • MSS: Mean Sea Surface. Permanent component of the sea surface height. The mean sea surface comprises a geoid contribution (~100 m) and a permanent circulation contribution (~1 m).
  • MWRMicroWave Radiometer (onboard Envisat) MWSMicroWave Radiometer Sounder (onboard ERS-1 and 2).

  • nadir: point of the celestial sphere representing the downward vertical direction, in one place (as opposed to the zenith)
  • NASANational Aeronautics and Space Administration  Website
  • NAONorth Atlantic Oscillation
  • NAVOCEANONAVal OCEANographic Office
  • Niña (La): See ENSO
  • Niño (El): See ENSO
  • NOAANational Oceanic and Atmospheric Administration Website
  • NoiseStatistical quantity used to estimate the inherent error on measurements induced by the instrument itself.
  • NPOESSNational Polar-orbiting Operational Environmental Satellite System
  • NRANASA Radar Altimeterother name for the Topex altimeter
  • NRCSNormalised Radar Cross Section NRTNear-Real Time: data using preliminary orbit (produced in 2 or 3 days)

  • Ocean-atmosphere couplingThe ocean and atmosphere are mutually interdependent. In particular, water that evaporates from the ocean causes the atmosphere to become hotter and more humid. This phenomenon affects atmospheric movements (winds), which in turn affect ocean circulation and so on: the temperature of air masses in contact with the ocean is modified and, conversely, ocean currents are driven by dominant winds. These ocean-atmosphere interactions are a major factor in weather forecasting and a key element in our understanding of climate.
  • Orbitography: The study of satellite orbits. Very precise determination of a satellite’s exact position is a key factor in altimetry. Depending on the satellite, two or three orbit determination systems are used.
  • OSDR: Operational Sensor Data Record(s) (Jason-1)

  • PAC: Processing and archiving centres (ESA)
  • PCU: Processing and Control Unit
  • PDS: Payload Data Segment, responsible for exploiting the Envisat instrument data
  • Permanent circulation: Steady state of ocean currents in an idealised unchanging atmospheric system.
  • POCCProject Operation Control Centre (Jason-1)
  • PO-DAACPhysical Oceanography – Distributed Active Archive Center (NASA/JPL)Website
  • PODPrecise Orbit Determination
  • POEPrecise Orbit Ephemeris
  • Poseidon: One of the two altimeters onboard Topex/Poseidon (Cnes); Poseidon-2 is Jason-1 altimeter.
  • PRAREPrecise Range and Range-rate Equipement (ERS-1 & 2)
  • PRFPulse Repetition Frequency
  • PROTEUSPlate-forme Réutilisable pour l’Observation, les Telecommunications et les Usages Scientifiques. Minisatellite bus developed in France for Jason-1 and other missions.

  • Radar Altimeter: RA is the altimeter onboard ERS-1 and 2; RA-2 is onboard Envisat.
  • Radiometer:  Passive instrument that detects and measures radiant energy, usually in the microwave, infrared and near-infrared wavebands. Radiometers flown on altimetry satellites help us to measure water vapor and liquid water content in the atmosphere. These measurements are used to apply corrections to altimetry data. The radiometer’s antenna collects radiation reflected by the ocean. The amount measured depends on surface winds, ocean temperature, salinity, foam, absorption by water vapor and clouds, and other factors. To determine atmospheric water vapor content accurately, the surface and cloud contributions must be filtered out from the signal received by the radiometer. For this reason, different signal frequencies are used to increase sensitivity to each of these parameters. By combining measurements acquired at both frequencies, we can extract the water vapor signal.
  • Range: the altimeter satellite-to-surface distance, deduced from the return echo time delay.
  • Reference ellipsoid: Arbitrary reference surface that is a raw approximation of the Earth’s shape, which is basically a sphere “flattened” at its poles. The length of one of the axes at the Equator is chosen so that the ellipsoid coincides at this latitude with the mean sea level. For example, the ellipsoid used by the Topex/Poseidon mission has a radius of 6378.1363 km and a flattening of 1/298.257.
  • Retracking: Retracking altimetry data is done by computing the departure of the waveform’s leading edge from the altimeter tracking gate and correcting the satellite range measurement (and surface elevation) accordingly.
  • RFRadio Frequency
  • RFURadio Frequency Unit
  • RMSRoot Mean Square.

  • SARSynthetic Aperture Radar
  • SFDUStandard Formatted data Unit
  • SGDR: Sensor Geophysical Data Record(s)
  • Sigma 0: or sigma-naught, or backscatter coefficient, reflexivity coefficient of the radar wave on the surface.
  • Significant Wave Height (SWH): Symbol Hs or H1/3. The significant wave height is obtained by analyzing the shape and intensity of the altimeter radar beam reflected from the sea surface (radar echo). A long time delay in the return signal indicates that waves are high and, conversely, a short delay indicates that the sea surface is calm.
  • SIRAL: altimeter onboard Cryosat.
  • SLASea Level Anomalies or SSHA (Sea Surface Height Anomalies)
    Difference between the observed sea surface height and the mean sea level. The SLA allows us to monitor ocean variability due to seasonal variations and climatic phenomena such as El Niño. Sea level variability (the standard deviation over time) is somewhere between 2-3 cm and 60 cm, depending on energy levels in different parts of the ocean. For altimetry data, these anomalies are not usually computed with respect to a seasonal mean, but to a multi-annual mean.
  • SLHSea Level Height
  • SLRSatellite Laser Ranging
  • SSALT: Solid State radar ALTimeter
  • SSALTOSegment Sol multimissions d’ALTimétrie, d’Orbitographie et de localisation précise. CNES multimission ground segment
  • SSB:Sea State BiasBias due to the sea-surface state, which consists of two components: electromagnetic bias and tracker bias.
  • SSDP:Segment Sol DORIS/PoseidonDORIS/Poseidon ground segment
  • SSDTSea Surface Dynamic Topography
  • SSH:Sea Surface HeightHeight measured by altimetry. Not the sea’s depth (i.e., the distance from the surface to the ocean floor), but a height measured with respect to an arbitrary reference level, called the reference ellipsoid. The sea surface height includes the geoid and the dynamic topography, which is the height due to ocean circulation. This dynamic topography includes a permanent component (permanent circulation) and a highly variable component driven by variations in currents, winds, tides, surface temperatures, and so on.
  • SSHASea Surface Height Anomalies see Sea Level Anomalies (SLA)
  • SSSSea Surface Salinity
  • SSTSea Surface Temperature Water temperature at the ocean surface. The SST can be measured by satellite-based infrared radiometers.

  • TDRSTracking and Data Relay Satellite System
  • TGSTopex Ground Segment
  • Thermocline: Zone between the colder water of the ocean depths and warmer surface water. Large temperature variations occur with depth in the thermocline zone. Near the surface, however, where water is always in motion, and in the ocean depths where there is no source of heat, the water temperature is fairly uniform.
  • Thermohaline circulation: Large-scale global ocean circulation driven by variations in the temperature and salinity of water masses. Cooled and saline waters downwell at high latitudes (off the coast of Norway and Greenland). Waters warmed in the Tropics upwell to the surface, where they are cooled, and so on. It is estimated that a single water molecule takes about 1,000 years to complete the circuit.
  • Tide: Variation in sea level due to the gravitational attraction of the Sun and Moon. Tides can be higher than seven metres in certain harbours. As well as ocean tides, Earth tides describe variations in the solid Earth caused by these gravitational forces.
  • TMRTopex Microwave RadiometerRadiometer onboard Topex/Poseidon
  • Topex: Ocean TOPography Experiment. One of the 2 altimeters (Nasa) onboard Topex/Poseidon.
  • Topex/Poseidon: Altimetric satellite (Nasa/Cnes)
  • T/PTopex/Poseidon
  • TRSR: Turbo Rogue Space Receiver. One of three positioning systems on Jason-1 that uses the GPS to determine the satellite’s position.

  • UTCUniversal Time Coordinated. Timekeeping system that relies on atomic clocks to provide accurate measurements of the second, while remaining coordinated with the Earth’s rotation, which is much more irregular. To stay synchronised, UTC has to be adjusted every so often by adding one second to the day, called a leap second, usually between June 30 and July 1, or between December 31 and January 1. This is achieved by counting 23h59’59”, 23h59’60” then 00h00’00”. This correction means that the Sun is always at its zenith at noon exactly (accurate to the second)
  • UCLUniversity College London Website

  • Waveform: The magnitude and shape of the radar altimetry return echoes
  • Wet tropospheric correction: Water vapor in the atmosphere slows the radar pulse. This effect can generate mean errors of the order of 15 centimetres on altimetry measurements. The value of the correction applied is determined using measurements by radiometer on the satellite.
  • WITTEXWater Inclination Topography and Technology Experiment  Website
  • WOCEWOrld Ocean Circulation Experiment
  • WSOAWide-Swath Ocean Altimeter

  • ZWD: Zenith Wet Delay

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