Ocean Surface Topography
Ocean Surface Topography (OST) represents variations in sea surface height (SSH) against the geoid, which is the shape the ocean surface would assume with no currents or tides. OST measurements can provide long-term information about ocean circulation, mass and heat content, weather, tides and other dynamic ocean phenomena – all of which are critical factors in the global climate. Practical applications include ship routing, offshore oil operations, fisheries, search and rescue, oil spill remediation and maritime operations. 1) 2)
Changes in OST can be measured by satellite altimeters, which are non-imaging radar sensors that use precise ranging measurements to derive the satellite’s height above the ocean. Altimeters obtain this ranging information by measuring the time interval between the transmission and reception of microwave pulses. Higher resolution data can be obtained through spaceborne synthetic aperture radar (SAR[e]) devices, like the European Space Agency (ESA) mission CryoSat-2, which has helped explain the connection between polar ice sheet melting and the rise of sea levels. The internationally cooperated altimetry missions, such as Jason-1, Jason-2/OSTM, Jason-3 and Sentinel-6, are capable of determining global mean sea level with a precision of several millimetres, which is achieved by echoing microwave pulses from the ocean surface to obtain altimetric range, then subtracting from the satellites’ orbital heights for OST measurements accurate to 3.3 cm relative to the centre of the Earth. Global mean sea level can be determined to such accuracy after averaging a myriad of OST measurements over their 10-day revisit period. 1) 3) 4) 5)
Large-scale OST measurements performed by satellite altimeters can track global ocean circulation, like during El Niño events, where the westward trade winds weaken and the tropical Pacific ocean is flooded with warm, nutrient poor water. On the contrary, La Niña events strengthen the trade winds and allow cold, nutrient rich water to enter the tropical Pacific. The trade winds are prevailing Easterly winds that circle the Earth around the equator, and El Niño and La Niña are opposing climate patterns that affect the trade winds’ global upwelling. 6) 7) 8)
On a local scale, topographic information gathered by satellites can be used for offshore resource exploration, oil spill detection, and seafloor pipeline routing. Ocean circulation information including current and frontal boundaries can be obtained by spaceborne SARs or VNIR (Visible and Near Infrared) imagers looking at changes in ocean temperature or colour. Ocean altimetry is invaluable for measuring sea level rise, something essential to monitoring global warming, coastal cities, ecosystems and other human features. 6)
To measure OST and SSH, the method used involves the following steps:
- First the satellite’s orbit is determined with great accuracy using GPS satellites, satellite laser ranging (SLR) stations and Doppler Orbitography and Radio-positioning (DORIS) stations. From this information, the satellite’s altitude above a global reference ellipsoid can be derived.
- A radar altimeter onboard the satellite echoes signals from the Earth’s surface, which are returned to the satellite receiver, and their travel times are measured.
- The satellite’s altimetric range – the distance between the satellite and the Earth’s surface – is determined as approximately equalling half the pulse travel time multiplied by the pulse speed (speed of light).
- The height delta between satellite altitude and altimetric range gives the sea surface height (SSH). 9)
- Subtracting SSH from the known shape of the geoid provides OST.
In order to calculate SSH, knowledge of the ‘ellipsoid’ is required - an idealised mathematical model that smoothly approximates the shape of the Earth as an oblate spheroid. The ellipsoid is used in geodesy and satellite positioning systems like GPS to derive the shape and size of the Earth. Meanwhile, the geoid is a surface of constant gravitational potential that approximates global mean sea level, taking into account variations in the planet’s mass distribution. It is used to measure deviations in SSH, useful for oceanography, geodesy and geophysics. 6)
OST is caused primarily by tides and currents; if they didn’t exist, the ocean surface would take the shape of the equipotential surface of Earth’s gravitational field, (the geoid). Tides are long-period waves created by the gravitational forces of Moon and Sun that originate in the ocean and move toward coastlines where they appear as the ocean surface regularly rising and falling. Ocean currents are continuous streams of moving water driven by wind, gravity, and water density that can create OST variations. Transporting large amounts of energy from the tropics to the poles, ocean currents moderate climates at higher latitudes, establishing a connection to the global climate. OST also has contributions from changes in temperature and salinity - known as ‘steric’ signals, eddies, and propagating planetary waves. Propagating planetary waves are generated by the rotational motion of the Earth, where one patch of air meets another with different rotational speeds and the Coriolis force pushes it back. Gravity waves (not to be confused with gravitational waves, which are ripples in spacetime) propagate due to gravity’s restoring force acting on vertical perturbations in the atmosphere. 1) 6) 9) 10) 11) 12)
The Copernicus Marine Environment Monitoring Service (CMEMS) and the Copernicus Climate Change Service (C3S) provide a wide range of satellite ocean data products at varying processing levels and areas.
Sea Level Anomaly (SLA) and Sea Surface Height Anomaly (SSHA)
SSHA is the difference between the SSH and mean sea surface height (MSS), typically measured with long-term regional or global measurements. SLA is the difference between sea level and mean sea level, (the average height of the ocean’s surface relative to the Earth’s centre), and tends to represent measurements made over small locales and durations. These types of OST measurements have applications in monitoring climate change and sea level rise, weather forecasting including El Niño and La Niña monitoring, and studying ocean circulation patterns. 13)
Near-Real-Time Altimetry Data
Following the improvement and streamlining of orbit determination and geophysical correction models, the time to receive altimetry data products has decreased, resulting in high quality, near-real-time (NRT) data. 15)
Ocean Surface Currents (OCS)
Satellite altimeters are capable of deriving detailed information into the Earth’s global ocean circulation, with one influence being currents. Ocean currents can be measured through variations in SSH, sea surface winds and temperature, and doppler shifted measurements of sea surface velocity. At all scales, currents are responsible for water exchange between different parts of the ocean. OSC play important roles in large-scale phenomena dynamics, marine life migration, shipping and fishing, search and rescue, and pollutant dispersal. OSC created the ‘Great Garbage Patch’, an enormous floating island of plastic debris formed in the North Pacific Ocean by converging currents. 16) 17)
Figure 5: Global OSC animation (NASA Goddard Space Flight Centre, 18))
Figure 5 consists of OSC measured from June 2005 to December 2007, by the NASA project named Estimating the Circulation and Climate of the Ocean (ECCO), a collaboration between NASA Jet Propulsion Laboratory (JPL) and the Massachusetts Institute of Technology (MIT). The MIT numerical ocean model was combined with observations from NASA satellite missions.
Satellite measurements used in the project included: SSH from NASA’s Topex/Poseidon, Jason-1 and Jason-2/OSTM satellite altimeters; gravity from the NASA/German Aerospace Centre (DLR) Gravity Recovery and Climate Experiment (GRACE) mission; surface wind stress from NASA’s QuikScat mission; sea surface temperature from the NASA/JAXA Advanced Microwave Scanning Radiometer for EOS (AMSR-E); sea ice from passive microwave radiometers and other in-situ measurements. 19)
Mean Dynamic Topography (MDT)
MDT, or Dynamic Ocean Topography (DOT), is the difference between MSS height and the geoid. MSS is measured by spaceborne altimeters like the Indian Space Agency (ISRO) and French Space Agency (CNES) (Satellite with ARgos and ALtiKa) SARAL minisatellite mission. The geoid’s shape is derived through gravity measurements made by geodetic missions like ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE).
Figure 6 shows variations in MDT that span over 3 metres, with red colours depicting the ocean level above the geoid and blue colours below. MDT and data from GOCE improves our understanding of ocean circulation, sea level, ice dynamics and the Earth’s interior. 20)
Sea Level Rise
Through long-term continuous OST measurements satellite altimeters can map global sea level change, and it’s on the rise. The National Oceanic and Atmospheric Administration (NOAA) created a map showing the global rise in sea level in less than 20 years. According to the U.N. Atlas of the Oceans, 8 of the world’s largest cities are near the coast. In urban settings around the world, rising sea levels threaten almost all infrastructure within its reach. 21)
Jason (Joint Altimetry Satellite Oceanography Network) Missions
Jason is a joint French space agency (CNES) and NASA constellation of three oceanography missions with the objective to monitor global ocean circulation, understand the connection between oceans and atmosphere, improve climate predictions, and monitor events like El Niños and ocean eddies. Jason-2, also known as the Ocean Surface Topography Mission (OSTM), provided continuity to Jason-1 and Posiedon/TOPEX, as well as measuring time-averaged ocean circulation, global sea-level change and improved ocean tide models. The missions carried suites of radar altimeters, radiometers, and DORIS instruments, for multitudes of OST-related applications. Collaboration from the National Oceanic and Atmospheric Administration (NOAA) and the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) was introduced for the Jason-2 and Jason-3 missions.
SWOT (Surface Water and Ocean Topography)
SWOT is a collaborative mission between NASA, CNES, the Canadian Space Agency (CSA), and the UK Space Agency (UKSA), launched in December 2022. SWOT provides global altimetry measurement continuity and follows on from the Jason missions. With the Ka-band Radar Instrument (KaRIn), SWOT will perform the first global survey of the Earth’s surface water resources in SAR-interferometry, providing input for models of fine-scale currents, the geoid, bathymetry, ocean and internal tides, SSH, sea-ice, and biodiversity structure.
Sentinel-6 is a successor mission to the Jason-3 altimetry mission developed by ESA, NASA, NOAA and EUMETSAT. Sentinel-6 Micheal Freilich (formerly Jason Continuity of Service Mission) is the new Reference Altimetry Mission for the worldwide altimetry constellation, providing continuity for previous OST missions. Launched in 2020, the satellite is an essential observing system for operational oceanography and seasonal forecasting. With its dual frequency SAR altimeter - Poseidon-4, and two microwave radiometers - the Advanced Microwave Radiometer for Climate (AMR-C) and the High-Resolution Microwave Radiometer (HRMR), Sentinel-6 can provide measurements of SSH, significant wave height and wind speed with high accuracy and precision. The mission’s OST products and services are operated by EUMETSAT. Sentinel-6 Micheal Freilich will be joined by Sentinel-6B in 2025 to provide SSH measurement continuity.
Envisat (Environmental Satellite)
Envisat was an ESA research mission launched in March 2002 with the aim to study and monitor the Earth’s environment on various scales. The satellite studied many disciplines including OST, with its Advanced SAR (ASAR), which measured sea-state conditions, ice-sheet dynamics and distribution; and Radar Altimeter-2 (RA-2), which provided high-precision topography measurements from the ocean, ice, and land. Envisat operated for 10 years, providing science with a wealth of data on our planet’s health and climate change.
GOCE (Gravity field and steady-state Ocean Circulation Explorer)
GOCE was an ESA geodynamics and geodetics mission launched in March 2009, with the objective to map the Earth's gravity field and produce a high-accuracy global height reference system - the geoid. The advanced gradiometer aboard GOCE measured gravity variations, facilitating a global reference for OST. GOCE measurements contributed to continuous and comprehensive monitoring of ocean dynamics, marine resources, and climate-related phenomena. Ending its mission in November 2013, GOCE played a crucial role in advancing our understanding of Earth's gravity field, providing valuable data for geophysical studies and solid Earth processes.
TOPEX (Ocean Topography Experiment) / Poseidon
TOPEX / Poseidon (T/P for short) was the first spacecraft that provided highly accurate global sea-level measurements. The NASA JPL / CNES collaboration was launched in 1992 and for over 15 years, mapped the oceans and their changing surface topography. The groundbreaking mission drastically improved our understanding of oceanography, climate patterns, and sea level changes. T/P housed numerous altimetry instruments including the NASA Radar Altimeter (NRA), the TOPEX Microwave Radiometer (TMR), DORIS, and the Single-Frequency Solid State Altimeter (SSALT). T/P was succeeded by the three Jason missions after its mission ended in January 2006.
Sentinel-3 is an OST mission in ESA’s Copernicus program operated by EUMETSAT, consisting of two identical SAR satellites - Sentinel-3A, launched in February 2016, and Sentinel-3B, launched in April 2018.
HY-2 (Haiyang-2) / Ocean-2
HY-2 is a constellation of ocean observation satellites, operated by the China National Space Administration (CNSA) and the Chinese Academy of Space Technology (CAST), first launched from August 2011.
ERS-1 (European Remote-Sensing Satellite-1)
ERS-1 was ESA’s first environmental monitoring satellite that conducted a variety of observations including advancing OST, from July 1991 to March 2000.
- Ocean Surface Topography (OST) - the difference between sea surface height (SSH) and the geoid. A comprehensive term that encompasses many related measurements and observations.
- Sea Surface Height (SSH) - the height of the sea surface above the ellipsoid, derived with satellite altimetry. Calculated as the difference between the satellite’s altitude and altimetric range.
- Sea level - the average height of the ocean surface relative to the centre of the Earth.
- Geoid - a reference model of constant gravitational equipotential that approximates global mean sea level, the shape the ocean would take with no OST.
- Ellipsoid - a mathematical reference model that approximates the earth as an oblate spheroid.
- Sea Surface Height Anomaly (SSHA) - the difference between the instantaneous SSH and mean SSH.
- Sea Level Anomaly (SLA) - the difference between sea level and mean sea level.
- Mean Dynamic Topography (MDT) - also known as Dynamic Ocean Topography (DOT), is the difference between Mean Sea Surface (MSS) height and the geoid.
1) “Ocean Surface Topography,” NASA Science, 22 July 2023, URL: https://science.nasa.gov/earth-science/oceanography/physical-ocean/ocean-surface-topography
2) “On the topic of topography,” ESA, 27 February 2012, URL: https://www.esa.int/Applications/Observing_the_Earth/On_the_topic_of_topography
3) CEOS, “Radar Altimeters,” The Earth Observation Handbook, 2012, URL: http://www.eohandbook.com/eohb2012/sat_earth_obs_radar_altimeters.html
4) earth online, “CryoSat,” The European Space Agency, URL: https://earth.esa.int/eogateway/missions/cryosat
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6) CEOS, “Ocean Topography/Currents,” Earth Observation Plans: By Measurement, December 2013, URL: http://www.eohandbook.com/eohb2014/earth_observation_plans_ocean.html
7) NOAA, “What are El Niño and La Niña?,” National Ocean Service, 2 February 2023, URL: https://oceanservice.noaa.gov/facts/ninonina.html
8) “El Niño,” ESA, Space for our climate, URL: https://www.esa.int/Applications/Observing_the_Earth/Space_for_our_climate/El_Nino
9) “DOT - Dynamic Ocean Topography Models,” Global Geodetic Observing System, URL: https://ggos.org/item/ocean-topography-models/
10) NOAA, “What are tides?,” National Ocean Service, 20 January 2023, URL: https://oceanservice.noaa.gov/facts/tides.html
11) Anna Scott, “Atmospheric Waves Awareness: An Explainer,” The Planetary Society, 20 April 2016, URL: https://www.planetary.org/articles/atmospheric-waves-awareness
12) “Sea Surface Heights,” Global Geodetic Observing System, URL: https://ggos.org/item/sea-surface-heights/
13) The Climate Change Coastal sea level team: “ESA Sea Level Climate Change Initiative: Collection of datasets of altimeter along-track high resolution sea level anomalies and associated trends in some coastal regions,” v1.1. NERC EDS Centre for Environmental Data Analysis, September 2023. URL: http://catalogue.ceda.ac.uk/uuid/eaec37f29d234843bfd50accee2de0d0
14) K. E. Case, A. W. Bingham, R. W. Berwin, E. M. Rigor and R. G. Raskin, "Ocean surface topography data products and tools," IGARSS 2004. 2004 IEEE International Geoscience and Remote Sensing Symposium, Anchorage, AK, USA, 2004, pp. 662, URL: https://doi.org/10.1109/IGARSS.2004.1369115.
15) “Altimetry,” ESA Sentinel Online, URL: https://sentinels.copernicus.eu/web/sentinel/missions/sentinel-3/data-products/altimetry
16) Dohan, K., and N. Maximenko, “Monitoring ocean currents with satellite sensors”, Oceanography 23(4):94–103, 2010, URL: https://doi.org/10.5670/oceanog.2010.08.
17) “Sea Surface Temperature Services,” EUMETSAT, URL: https://www.eumetsat.int/sea-surface-temperature-services
18) “NASA Views Our Perpetual Ocean”, NASA, 7 August 2017, URL: https://www.nasa.gov/topics/earth/features/perpetual-ocean.html
19) “An ‘experiment’ 28 years ago proves its worth,” EUMETSAT, 10 August 2020, URL: https://www.eumetsat.int/experiment-28-years-ago-proves-its-worth
20) “Understanding the ‘OC’ in GOCE,” ESA, 25 November 2014, URL: https://www.esa.int/Applications/Observing_the_Earth/FutureEO/GOCE/Understanding_the_OC_in_GOCE
21) Rebecca Lindsey, “Climate Change: Global Sea Level,” NOAA Climate.gov, 19 April 2022, URL: https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level
22) Aviso+. “Timeline of modern radar altimetry missions”. https://doi.org/10.24400/527896/A02-2022.001 version 2023/09
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).