Ocean Surface Topography from Space-Glossary

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Glossary

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A TOP

ACC: Antarctic Circumpolar Current

ACS: Attitude Control System

ADEOS-1: Advanced Earth Observation Satellite

Advanced Earth Observation Satellite-1 (ADEOS-1 or Midori): The Japanese polar orbiting earth-observing platform launched on 17 August 1996 from Tanegashima, failed 30 June 1997, due to structural damage in its solar array. Carried US NSCAT and TOMS.

Aerodynamic drag: a retarding force acting on a body moving through a gas parallel to the motion of the body.

AG: Alaska Gyre

Air pressure: the force exerted by the weight of a column of air above a surface; measured in pounds per square inch (14.7 lb/in², standard), the height of a column of mercury (760 mm or 29.9 inches), or millibars (1013 mb, standard).

Altimeter: an instrument that measures altimetry. Two basic types in use: aneroid altimeters measure air pressure; radar or laser altimeters time returning microwaves or light.

Altimetry: determination of the altitude of an object or surface with respect to a fixed level, like sea level. See altimeter.

Antarctic Circumpolar Current (ACC): Cold current encircling the Antarctic, driven by westerly winds between 40° and 50° S. Because the flow of the ACC is not checked by the continents, it links the Atlantic, Indian, and Pacific Oceans.

Anti-cyclone: an area of high pressure with circulation clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. (See also cyclone, Coriolis force, atmospheric pressure)

ARGO: Array for Real-time Geostrophic Oceanography

Ariane: French-made family of launch vehicles used by ESA.

Array for Real-time Geostrophic Oceanography (Argo): a global array of 3,000 free-drifting profiling floats to continuously monitor the temperature and salinity of the upper 2000 m of the ocean. The ARGO array is part of GCOS/GOOS, CLIVAR, and GODAE.

Attitude Control System (ACS): The spacecraft system that determines and maintains spacecraft orientation in space.

AVISO: Archivage, Validation et Interprétation des données des Satellites Océanographiques; CNES' system for archiving, validation, and interpretation of data from Satellites.

AXBT: Airborne Expendable Bathythermograph

B TOP

Betaprime: the angle between the orbital plane and the sunline; a key parameter governing a spacecraft's attitude control. See Attitude Control System.

BlackJack GPS receiver: an advanced TRSR, "GPS-on-a-chip" receiver from JPL that can pinpoint the position of its host satellite continuously with an accuracy of 2 to 3 centimeters--three orders of magnitude better than older models.

C TOP

California Current: the cold ocean current flowing southward along the western coast of North America from the Gulf of Alaska to Baja California.

California Land-falling Jets Experiment (CALJET): a NOAA-sponsored study to predict landfall of heavy-rain-bearing cyclones on the northern California coast by observing the behavior of prefrontal, low-level, offshore jets that precede the front.

CALJET: California Land-falling Jets Experiment

C-band: radio-frequency band between 3.9 and 6.2 GHz. See microwave, radar.

CCAR: Colorado Center for Astrodynamics Research

Center for Space Research (CSR): was established in 1981 under the direction of Dr. Byron D. Tapley at the University of Texas at Austin. The Center conducts research in orbit determination, space geodesy, the Earth and its environment, exploration of the solar system, as well as expands the scientific applications of space systems data.

Challenging Minisatellite Payload (CHAMP): a German scientific satellite carrying a BlackJack GPS flight receiver launched July 15, 2000 from Plesetsk in northern Russia. The three antennas of the instrument (1) determine the satellite's orbit precisely in order to improve our knowledge of Earth's gravity field, (2) make precise measurements of atmospheric temperature, pressure and moisture for studies of climate change, and (3) test the possibility of using reflected GPS data to acquire information about ocean height and sea-surface winds.

CHAMP: Challenging Minisatellite Payload

Climate: The long-term state of the atmosphere at a specific site; average weather over time.
Refers to long term-trends

Climate Model: mathematical analog of real climate with heuristic and predictive value.

Climate Variability and Predictability Experiment (CLIVAR): an international research programme, part of the wider WCRP, addressing many issues of natural climate variability and anthropogenic climate change.

CLIVAR: Climate Variability and Predictability Experiment

Closure: the independent estimate of the satellite-to-sea-level distance compared to the orbiter's own altimeter measurement.

CNES: (Centre National d'Etudes Spatiales) French Space Agency

Coastal tide: effect of lunar and solar gravity on coastal water.

Colorado Center for Astrodynamics Research (CCAR)--: a multidisciplinary group involving faculty, staff and students from the Departments of Aerospace Engineering Sciences and Electrical and Computer Engineering at the University of Colorado at Boulder. Its research program emphasizes astrodynamics, satellite meteorology, oceanography, geodesy, and terrestrial vegetation studies.

Condensation: the change of water vapor into a liquid. In order to condense water vapor, the air must be at or near saturation in the presence of condensation nuclei.

Condensation nucleus: a particle, liquid or solid, upon which condensation of water vapor begins in the air.

Continental climate: characterizes the interior of a large land-mass, marked by large annual, day-to-day, or day/night temperature ranges; low relative humidity; and moderate to low irregular rainfall. Annual temperature extremes occur soon after the solstices. See maritime climate

Coriolis force: the deflection of moving objects (air and water currents) due to the rotation of the Earth--to the right in the northern hemisphere, and to the left in the southern. (See also anticyclone, cyclone, gyre, eddy)

Corrected sea-surface height (CORSSH): data include various corrections and geophysical data (tides, geoid, etc.) in altimetry measurements.

Corsica: the CNES verification site is on this French island in the Mediterranean Sea.

CORSSH: corrected sea-surface height

CPCA: Complex Principal Component Analysis

Cross calibration: coordinated calibration of two instruments to ensure uniform output.

CSR: Center for Space Research

Cyclone: an area of low pressure, with circulation counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. (See also anticyclone, Coriolis force, atmospheric pressure)

D TOP

Data product: numerical end result of processing original, raw data; designed for scientific research or for use by end users or service providers.

Decommissioning: Removal of a spacecraft from active status; usually caused by exhaustion of critical resource, degradation of critical systems, or loss of funding; usually accompanied by deorbiting.

Delta: Boeing-made family of launch vehicles used by NASA.

Developing Use of Altimetry for Climate Studies (DUACS)

a shared cost project that processes altimeter data in near real time for climate simulations and seasonal climate forecasts. The products are widely used, covering a large spectrum of operational oceanography needs, from mesoscale to climate applications.

Homogeneous historical (HH) altimeter products include T/P, ERS-1 and ERS-2 data, and cover the period October 1992 through June 2000. They include along-track CORSSH files, along-track SLA files, and MSLAs.

The near-real-time (NRT) products cover the period 1998/01/14 through 2001/06/13, using both T/P and ERS-2 data. Products are along-track SLA files and MSLAs.

Doldrums: The narrow, low-pressure belt centered on the equator, characterized by light, variable winds, rising air currents, and heavy rainfall.

Dew point: the temperature to which air must be cooled to bring it to saturation; depends the amount of water vapor in the air.

DMI: Dipole Mode Index

DUACS: Developing Use of Altimetry for Climate Studies

E TOP

EBC: Eastern Boundary Currents

ECMWF: European Centre for Medium-range Weather Forecasting

Eddy: A rotating water current that runs contrary to the main current. Eddies can average up to a few hundred kilometers in diameter, contain up to 5,000 cubic kilometers of water, and last several years. They can be cyclonic or anticyclonic, and their rotation is determined by the Coriolis force. (See also gyre)

El Niño: warm-water conditions off the western tropical coasts of the Americas, occurring irregularly but usually around Christmastime, caused by weakening trade winds and causing depleted fisheries, heavier-than-normal rain in the central and eastern Pacific, and drought in the western Pacific.

El Niño, Southern Oscillation (ENSO): an interannual see-saw in tropical sea-level pressure between the eastern and western hemispheres. During El Niño, unusually high atmospheric sea-level pressures develop in the western tropical Pacific and Indian Ocean regions, and unusually low sea-level pressures develop in the southeastern tropical Pacific. So tendencies for unusually low pressures west of the date line and high pressures east of the date line have also been linked to periods of anomalously cold equatorial Pacific sea-surface temperatures sometimes referred to as La Niña.

ENSO: El Niño/Southern Oscillation

ENVISAT: Environmental satellite; the successor to the ESA Remote Sensing Satellites ERS-1/2.

EOF: Empirical Orthogonal Function

Equinox: the date when the sun is directly overhead at the equator (0° latitude). On this day, at all places on the globe, night and day are of equal length (12 hours each). The spring or vernal equinox (for the northern hemisphere) occurs on about March 21. The fall or autumnal equinox (for the northern heisphere) occurs on or about September 23. See also solstice.

European Centre for Medium-Range Weather Forecasts (ECMWF): an international organization established in 1973 and supported by 21 European States, the ECMWF has been producing operational medium-range weather forecasts since 1979.

F TOP

Flight Team: The coordinated group of engineering teams involved in the flight operation of a spacecraft, including the Navigation Team, the Spacecraft Performance Analysis Team, the Mission Planning and Sequencing Team, the Flight Control Team, and the Precision Orbit Determination and Verification Team.

Flight Control Team: the engineering team responsible for sending approved sequences and real-time command instructions to the satellite and for receiving data from the satellite.

G TOP

GCOS: Global Climate Observing System

Geodetic nadir: nadir defined as a line normal to the geoid surface, the shortest path to the ocean (rather than a line through the center of the planet). See groundtrack

Geoid: the baseline figure of the Earth, considered as a sea-level surface including local gravitational effects, without accounting for topographic features, and extended over the entire Earth's surface.

Geosat: US Navy satellite, launched in 1985, to measure sea surface heights to within 5 cm.

Geostrophic velocity vectors: geostrophic currents flow along contours of constant pressure, rather than from high to low pressure. The pressure gradient force (at right angles to the geostrophic velocity vector) is balanced by the Coriolis force due to the Earth's rotation; the Coriolis force is at right angles to the velocity vector, and equal and opposite to the pressure gradient force. Contours of constant pressure are streamlines for the geostrophic flow.

Global Climate Observing System (GCOS): established in 1992, ensures that the observations and information needed to address climate-related issues are obtained and made available to all potential users. A user-driven operational system, it provides the comprehensive observations required for monitoring climate, for detecting and attributing climate change, for assessing the impacts of climate variability and change, and for supporting research toward improved understanding, modeling and prediction of the climate system. It addresses the total climate system including physical, chemical and biological properties, and atmospheric, oceanic, hydrologic, cryospheric and terrestrial processes.

Global Ocean Data Assimilation Experiment (GODAE): demonstrates the practicality and feasibility of routine, real-time global ocean data assimilation and prediction. GODAE integrates remote and direct data streams, and uses models and data assimilation to draw maximum benefit from the observations. GODAE provides regular, complete depictions of the ocean circulation, at high temporal and spatial resolution, and, consistent with a suite of space and direct measurements and appropriate dynamical and physical constraints, delivers useful products in a timely manner.

Global Ocean Observing System (GOOS): a permanent, global system for observations, modeling, and analysis of marine and ocean variables to support operational ocean services worldwide. GOOS provides accurate descriptions of the present state of the oceans, including living resources; continuous forecasts of the future conditions of the sea for as far ahead as possible; and the basis for forecasts of climate change.

GODAE: Global Ocean Data Assimilation Experiment

GOOS: Global Ocean Observing System

GPSDR: (Global Positioning System Demonstration receiver)--early model GPS receiver test-flown on TOPEX/Poseidon with unprecedented success. See BlackJack, TRSR

GRACE: (Gravity Recovery and Climate Experiment)--joint US/German Earth-orbiting mission, with launch planned for November 2001. The mission's two spacecraft will fly in tandem to precisely measure Earth's gravitational field and enable a better understanding of ocean surface currents and ocean heat transport.

Ground System: that portion of a space mission--hardware, software, and staff--located on the ground that supports telemetry routing and processing, spacecraft commanding and navigation, data simulation, and data management.

Groundtrack: the theoretical mark laid on the surface of the earth from an orbiting body passing directly overhead. See nadir.

Gulf Stream: the warm ocean current flowing northward off the eastern coast of North America from the Bahamas to the north Atlantic.

Gyre: a great, circular motion of water in each of the major ocean basins centered on subtropical high-pressure region, with circulation clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. (See also coriolis force, eddy)

H TOP

Hadley Cell: A thermally driven, global band of rotating air approximately 30° latitude in width, in both the northern and southern hemispheres, that drives weather systems and controls climate.

Hawaii CoastWatch Satellite Data Resources: remotely sensed environmental data of interest to marine researchers, resource and crisis managers and the public at large; products of the Hawaii Regional CoastWatch Node

Hawaii Regional CoastWatch Node: is funded by a contract with NOAA/NESDIS, located at the Honolulu Laboratory, and serves as a regional focal point for the acquisition, processing, and distribution of remotely sensed environmental data, Hawaii CoastWatch Satellite Data Resources.

Heat-sink: a region or structure that can absorb and hold large quantities of heat.

Heat transport: the bulk movement of heat through the ocean.

Horse latitudes: The narrow, high-pressure belts, centered around 30°N and S latitude, characterized by calm and relatively dry air. So-called because 16th-century seafarers, becalmed in these latitudes, threw their horses overboard to conserve drinking water.

Humboldt Current: See Peru Current.

Humidity: the presence of water vapor in air.

I TOP

Intertropical Convergence Zone (ITCZ): nearly solid ring of thunderstorms surrounding the globe in the tropics as easterly trades of both hemispheres converge at equator

Ionosphere: that portion of the upper atmosphere sufficiently ionized by solar ultraviolet radiation that the concentration of free electrons affects the propagation of radio waves. Its base begins at about 70 or 80 kilometers (40 or 50 miles) and extends upwards indefinitely.

ITCZ: Intertropical Convergence Zone

J TOP

Japan Current: See Kuroshio.

Jason-1: First follow-on mission to TOPEX/Poseidon.

Jason Microwave Radiometer (JMR): improved microwave radiometer from NASA to be flown on the Jason-1 mission

JGM-3: Joint Gravity Model-3

JMR: Jason Microwave Radiometer

Joint Gravity Model-3 (JGM-3)-: an improved model of the geoid applied to GDRs that span both the Geodetic Mission (3/85 - 9/86) and Exact Repeat Mission (11/86 - 12/89) of Geosat; computed by the NASA Goddard Space Flight Center and incorporating a new 10-cm satellite orbit.

K TOP

KE: Kuroshio Extention

Kelvin wave: an eastward-moving ocean wave that crosses the equatorial Pacific in about 2 months.

Kourou: French-operated launch site in French Guiana. Location at 5° N latitude provides fuel-efficient, near equatorial launches.

Ku-band: radio-frequency band between 10 and 26.5 GHz. See microwave, radar.

Kuroshio: the warm ocean current flowing northeastward from Taiwan, along the eastern coast of Japan, to the north Pacific. Also called the Japan Current.

L TOP

La Niña: cold-water conditions off the western tropical coasts of the Americas, occurring irregularly and occasionally following El Niño conditions, caused by stiffening trade winds and regenerated fisheries, drought in the central and eastern Pacific, and rain in the western Pacific.

Laser light: visible light of a single wavelength emitted in a tight, coherent beam.

M TOP

Maritime climate: characterizes oceanic islands or coastal regions of continents, marked by small annual, day-to-day, or day/night temperature ranges; high relative humidity; and regular rainfall. Annual temperature extremes lag after the solstices. See continental climate.

Master Control Station: the control segment station responsible for operating the GPS, located near Colorado Springs, Colorado, that receives satellite information from a worldwide network of monitoring stations, updates the "navigational message," and uploads it back to the satellites.

Mediterranean Climate: mid-latitude climate found on the western coasts of continents, characterized by mild, rainy winters and dry summers.

Microwave: Electromagnetic radiation with a wavelength of between 0.3 and 30 centimeters (between infrared and radio), corresponding to frequency of 1 to 100 gigahertz. Favored for radar because of its cloud-penetrating properties
radio-frequency band between 1 and 100 GHz, favored by radar systems. See C-band, Ku-band.

Mission Operations: the hardware, software, and staff that supports telemetry processing, spacecraft commanding, and data simulation.

Mission Planning and Sequence Team (MPST): the engineering team that writes sequences, develops and maintains software, and schedules communications to ensure that information flows smoothly between the ground system and the satellite.

MSLA: Maps of Sea Level Anomalies

N TOP

Nadir: the point on the celestial sphere vertically below the observer, 180° from zenith; down. See geodetic nadir, groundtrack.

NASA Scatterometer (NSCAT): an active microwave radar, part of the ADEOS instrument payload, to measure winds over the oceans by transmitting Ku-band microwave pulse (13.995 GHz) and receiving backscatter powers from the ocean surface.

NESDIS: National Environmental Satellite, Data and Information Service

National Environmental Satellite, Data and Information Service (NESDIS): NOAA service that provides and ensures timely access to global environmental data from satellites and other sources to promote, protect, and enhance the Nation's economy, security, environment, and quality of life. To fulfill its responsibilities NESDIS acquires and manages the Nation's operational environmental satellites, provides data and information services, and conducts related research.

Naval Research Laboratory (NRL): the US Navy's corporate laboratory, which conducts a broadly-based multidisciplinary program of scientific research and advanced technological development directed toward maritime applications of new and improved materials, techniques, equipment, system, and ocean, atmospheric, and space sciences and related technologies.

Navigation and Precision Orbit Determination Team (NAV): the engineering team that keeps the satellite "on track" and provides very precise measurements of where the satellite is in space.

NECC: North Equatorial Counter Current

NLOM: Navel Research Laboratory Ocean Model

NOAA: National Oceanic and Atmospheric Administration.

Nodal crossing: the time when a satellites passes through its orbit's nodes.

Nodes: points where a satellite's orbit crosses the plane of the Earth's equator.

NPC: North Pacific Current

NRCS: Normalized Radar Cross Section

NRL: Naval Research Laboratory

NSCAT: NASA Scatterometer

O TOP

Ocean Altimetry and Climate group: based in Toulouse, France, designs, produces and operates satellite altimeter systems and their ground segments and supports altimetry research, applications, and services.

Ocean circulation: large-scale horizontal water circulation within an ocean.

Ocean current: transport of masses of water along definable paths through the ocean measured by changes in sea-level height.

Ocean surface topography: Topography of the ocean surface

Ocean circulation model: mathematical analog of real ocean circulation with heuristic and predictive value.

Ocean season: seasonal change in sea-level height caused by change in heat content and prevailing winds.

Ocean tide: effect of lunar and solar gravity on mid-ocean water.

Ocean weather: see eddy.

Orbital decay: reduction in the size (and period) of a satellite's orbit due to gravitational effects or aerodynamic drag, eventually causing it to reenter the atmosphere. See orbit maintenance.

Orbit maintenance: keeping a spacecraft's orbit within a given set of orbital parameters, the reference orbit, over time. See orbit maintenance maneuvers, orbital decay.

Orbit maintenance maneuvers: changes in a spacecraft's velocity, usually through thruster firings, designed for orbital maintenance.

Orbital plane: the plane, passing through a planet's center of gravity, in which a satellite orbits.

Orbitography: determination of the orbital parameters of an orbiting body.

P TOP

Pacific Decadal Oscillation (PDO): long-term (20 to 30 years) fluctuation in sea-surface heights/ocean temperature along eastern/western coasts of the Pacific Ocean.

PACJET: Pacific Landfalling Jets Experiment; see CALJET

PDO: see Pacific Decadal Oscillation.

Peru Current: the cold ocean current flowing northward along the western coast of South America from southern Chile to Ecuador. Also called Humboldt Current.

Physical Oceanography Distributed Active Archive Center (PODAAC): an element of the Earth Observing System Data Information System at JPL responsible for archiving and distributing data relevant to the physical state of the ocean.

Platform Harvest: a Texaco oil rig in the Santa Barbara Channel, off the coast of southern California, that hosts the NASA verification site. See Corsica.

POD: Precision Orbit Determination

PODAAC: Physical Oceanography Distributed Active Archive Center.

Polar front: The narrow, semicontinuous belt, centered around 60°N and S latitude, separating cold polar and warm temperate air masses, and characterized by frequent cyclonic storms.

POSEIDON: CNES's radar altimeter, originally slated as a SPOT payload, and precursor to TOPEX/Poseidon.

Poseidon-2 instrument: the CNES dual-band (C- and Ku-band), radar altimeter, to be flown on the Jason-1 mission

Poseidon instrument: the CNES single-band (13.65 GHz) radar altimeter, flown on TOPEX/Poseidon.

Precision Orbit Determination and Verification Team: the engineering team (and part of the Flight Team; sometimes part of the Navigation Team) that studies orbit maintenance maneuvers and tracking and ranging data to determine how close to the reference orbit an orbit is maintained.

Prevailing Westerlies: Highly variable westerly winds that blow from the horse latitudes to the polar front.

Project Manager: the primary leader for a project, with specific programmatic, personnel, discipline, technical, and institutional responsibilities.

Project Operations Control Center (POCC): the site that houses Mission Control and the Project Ground System.

Project Scientist: co-leader for a project, responsible for the scientific integrity of the mission; representing the science investigators of the mission to the project, to NASA, and to CNES; and serving as scientific spokesperson for the project and for the investigators.

Q TOP

QuikSCAT: NASA/JPL mission launched in 1999 to study remote sensing of ocean winds with the SeaWinds scatterometer.

R TOP

Radar: a system using beamed and reflected radio-frequency radiation, usually microwave, to detect objects and measure ranges.

Radar Altimeter(s): Altimeters that use radar.

Reaction wheel: a spinning flywheel that stores and releases angular momentum, thus controlling a spacecraft's attitude in space.

Reference ellipsoid: the reference surface (which takes the flattening effect of the earth's rotation into account) that represents the size and shape of the earth for cartography. See geoid.

Reference orbit: The precisely maintained, base-line orbit from which instrument readings and camera and radar images are taken.

Repeat period: The time period in the orbital operation of a mapping satellite within which the spacecraft completes total (global) coverage of its target.

Relative humidity: the ratio of how much water vapor a quantity of air at a given temperature actually holds compared to the amount it can hold expressed as a percentage.

Rossby waves: an extraordinarily slow westward-moving ocean wave of low amplitude (10 to 20 centimeters) and great width (hundreds of kilometers) that crosses the Pacific over several decades.

S TOP

Satellite Performance Analysis Team (SPAT): the engineering team that makes sure that the satellite and all its instruments are healthy and working well.

Saturation: for air, holding as much water vapor as is physically possible; being at the dew point.

Scatterometer: a microwave (radar) sensor that scans the surface of the earth from an aircraft or satellite and reads the reflection or scattering coefficient of the return pulse to measure surface roughness and derive wind speed and direction.

Science Data Team (SDT): the engineering team that takes the science telemetry--raw data--received from the satellite and turns it into science data products.

Sea level: the level of the surface of the ocean; especially the mean level halfway between high and low tide, used as a standard in reckoning land elevations or sea depths.

Sea level anomaly: the difference between the actual, measured sea level height and a mean sea level based on a mathematical reference. See geoid, reference ellipsoid, reverse barometer.

Sea level height: the actual, measured height of sea level against a standard reference. See geoid, reference ellipsoid, sea level anomaly.

Seasat: JPL-designed Earth-orbital mission, launched in 1978, to flight-test five instruments(a synthetic aperture radar, a radar altimeter, a scatterometer, a scanning multichannel microwave radiometer, and a visible and IR radiometer) to study the ocean surface; important legacy for many later Earth-orbiting instruments developed at JPL.

Sea surface height: Sea surface height is defined as the distance of the sea surface above the reference ellipsoid. The sea surface height is computed from altimeter range and satellite altitude above the reference ellipsoid. The "reference ellipsoid" is the first-order definition of the non-spherical shape of the Earth as an ellipsoid of revolution with equatorial radius of 6378.1363 kilometers and a flattening coefficient of 1/298.257. Sea surface height is often shown as a sea-surface anomaly or sea-surface deviation, this is the difference between the sea surface height at the time of measurement and the average sea surface height for that region and time of year.

Sea surface height: the variable height of the sea surface above or below the geoid.

Sinusoidally: back-and-forth variation in time, as a sine wave.

SLA: Sea Level Anomalies

SLR: Satellite Laser Ranging

SPOT: (Systeme pour l'Observation de la Terre)--French-government-sponsored civil Earth observation program, with support from Belgium and Sweden.

Solar constant: the amount of solar radiation incident on a plane oriented perpendicular to the solar beam outside the Earth's atmosphere--1.98 cal/cm2. Because some 30% is reflected back into space by clouds and ice in the atmosphere, and another 20% is absorbed by moisture and dust, only some 50% actually reaches the surface.

Solstice: the date when the sun reaches its highest (summer solstice) or lowest (winter solstice) noon-time point in the sky. On the summer solstice in the northern hemispheres (on or about June 22), the subsolar point is 23.5°N (Tropic of Cancer). On the summer solstice in the southern hemisphere (on or about December 22) the subsolar point is 23.5°S (Tropic of Capricorn). See also equinox.

SSB: Sea State Bias

SSH: Sea Surface Height

SSS: Sea Surface Salinity

Star tracker: a device that continuously and automatically records the position and heading of selected stars for spacecraft attitude control. See Attitude Control System.

Sunline: the line-of-sight to the sun, against which angular measurements are made.

Subsidence: for air, sinking, usually over a broad area, with associated increase in air pressure and rise in temperature.

T TOP

Tandem mission: Two or more spacecraft flying the same orbit, and acquiring the same or complementary data.
A single mission carried out by two spacecraft flying in close formation.

TAO: Tropical Atmosphere Ocean Array

Temperate climate: Characterizes mid-latitude regions with hot summers and cold winters.

Temperate zone: The mid-latitude climatic zone stretching from the tropic of Cancer to the arctic circle or from the tropic of Capricorn to the Antarctic circle and characterized by hot summers and cold winters.

Thruster: a control jet in a spacecraft, usually using a monopropellant like hydrogen peroxide or hydrazine.

TMR: TOPEX Microwave Radiometer

TOGA: Tropical Ocean-Global Atmosphere

TOPEX: (TOPography EXperiment for Ocean Circulation); unflown NASA precursor mission to TOPEX/Poseidon.

TOPEX instrument: the NASA dual-band (C- and Ku-band) radar altimeter, flown on TOPEX/Poseidon.

TOPEX/Poseidon: Joint US-French orbital mission, launched in 1992 to track changes in sea-level height with radar altimeters.

Topography: The shape of a surface, including its relief and the relative position of features.
the general configuration of a surface, including its relief.

Torque bar: a set of magnetized bars that provide damping action to prevent excessive rotation from the reaction-wheels.

TPEM: TOPEX/Poseidon Extended Mission

Trade winds: Strong, dependable easterly winds that blow from the horse latitudes to the doldrums.

TRMM: Tropical Rainfall Measuring Mission

Tropical Ocean-Global Atmosphere (TOGA): a major component of the WCRP aimed specifically at the prediction of climate phenomena on time scales of months to years. TOGA depends on the premise that the dynamic adjustment of the ocean in the tropics is far more rapid than at higher latitudes. Thus disturbances emanating from the western Pacific Ocean (like El Niño) may propagate across the basin on time scales of weeks compared to years for corresponding basin-wide propagation at higher latitudes. The significance of shorter dynamic times scales near the equator is that they are similar to those of highly energetic atmospheric modes. This similarity allows the formation of coupled modes between the ocean and the atmosphere.

Tropical zone: The low-latitude climatic zone centered on the equator, extending between the tropics of Cancer and Capricorn, and characterized by year-round hot weather.

TRSR: (TurboRogue Space Receiver)--a family of GPS receiver, first flown in 1995, and now featuring the advanced BlackJack receivers. See also GPSDR

Turbo Rogue: GPS receiver on TOPEX/Poseidon.

V TOP

Vandenberg: USAF-operated launch site on the southern California coast. Location at 121° W longitude provides easy polar and high-latitude launches over open ocean.

Verification site: a benchmark of known altitude against which altimeters can be calibrated. See Corsica and Platform Harvest.

W TOP

WAM: wave model

Water vapor: the gaseous phase of water.

wave height: the average crest-to-trough difference in the height of the surface of the open ocean, obtained by analyzing the shape and intensity of the altimeter radar beam reflected from the sea surface (radar echo).
otherwise referred to as significant wave height Significant wave height is calculated from altimeter data based on the shape of a radar pulse after it bounces off the sea surface. A calm sea with low waves returns a short, sharply defined pulse whereas a rough sea with high waves returns a stretched pulse because the energy begins to come back from the tops of the waves and continues until the pulse hits the valley of the wave. The significant wave height is the average height of the highest one-third of all waves in a particular time period.

WBC: Western Boundary Current

WCRP: World Climate Research Programme

Weather: The short-term state of the atmosphere at a specific site with respect to temperature, humidity, wind speed and direction, clarity, and cloud cover.
Refers to short-term events

wind speed: average speed of the air moving over the ocean surface, obtained by analyzing the strength of returning radar pulses
Wind speed as measured from an altimeter is an indirect measurement based on the small-scale roughness of the sea surface. Wind speed is measured from the strength of the returned signal in the time following the first return from the surface. A windless sea is smooth on small , say a few centimeters, even if it is rough on the scale of swell; such a sea will have a returned signal that is short in duration because it comes only from the center of the transmitted pulse. If there is a wind, the sea will be rough on the small scales, and there will be signal returned from the parts of the ocean not directly beneath the satellite; that is, as the outer part of the incident pulse strikes the ocean well away from the point of first encounter, some signal is scattered, by the small scale wavelets, back to the satellite, and this energy arrives after the first returns. Consequently the strength of the signal following the main return is a measure of the windspeed at the sea surface. There is a reliable relationship between the energy scattered from near-nadir and the wind speed.

WOCE: World Ocean Circulation Experiment

WOD98: World Ocean Database 1998

World Climate Research Programme (WCRP): develops and supports studies to determine how much people have influenced climate, and how much climate can be predicted. Its program encompasses studies of the global atmosphere, oceans, sea and land ice, and the land surface that together constitute the Earth's physical climate system.

World Ocean Circulation Experiment (WOCE): a component of the WCRP and the most ambitious oceanographic experiment undertaken to-date. In addition to global observations furnished by satellites, conventional in-situ physical and chemical observations have been made by nearly 30 nations in four of the world's oceans at the time global numerical ocean models are being developed to assimilate these measurements.

WSG: Western Subarctic Gyre