SC 2.5: Satellite Altimetry

Chair: Xiaoli Deng (Australia)
Vice-Chair: C.K. Shum (USA)

Terms of Reference

Satellite altimetry missions (e.g., Geosat, TOPEX/Poseidon, ERS-1/2, Envisat and Jason-1/2/3) have been providing vital measurements of global ocean surface topography since 1991. The latest altimetry missions (e.g., HY2a/2b, Ka-band altimetry SARAL/Altika, SAR and SARIn altimetry CryoSat-2 and Sentinel-3A/B, and laser altimetry ICEsat-2) are providing higher resolution observations. The upcoming Jason-CS/Sentinel-6 mission includes two identical satellites scheduled to launch in 2020 (satellite A) and 2025 (satellite B), which will continue measuring the sea level for at least a decade. The future Surface Water and Ocean Topography (SWOT) satellite mission equipped with radar interferometry, due to launch in 2021, will substantially improve measurements of sea surface heights and surface water hydrology at finer scales that has not been possible before. In addition, the in situ GNSS reflectometry (GNSS-R) and the NASA CYGNSS 8-satellite constellation have been providing water/sea level, landcover, water/snow extents, wave and wind measurements. 

Altimetry observations cover the global oceans, cryosphere, sea-ice, ice-covered oceans and inland water bodies, providing invaluable geodetic and climatic information for studying the Earth and ocean dynamics (e.g., sea level, ocean wave and wind speed, ocean surface topography, tides, soil moisture, snow depth, ice sheet, ice caps, mountain glacier, inland water and solid Earth deformation), and geophysical features (e.g., marine gravity field, mean sea surface and bathymetry). 

The growing altimetry datasets are driving technological leaps forward for satellite geodesy and oceanography. At the same time, they will bridge an observational gap on a spatial-temporal domain critical for solving interdisciplinary problems of considerable societal benefit. The purpose of this IAG sub-commission is to promote innovative research using historic and future altimeter observations to study local, regional, and global geophysical processes, with emphasis on emerging cross-disciplinary applications using satellite altimetry, and in combination with other in situ data sets and techniques including hydrography data, GNSS-R, CYGNSS, SAR/InSAR and GRACE/GOCE.


Sub-Commission 2.5 will:

  • Establish a close link between this sub-commission and the International Altimeter Service (IAS) and data product providers, in order to (1) establish scientific forums to discuss new results, (2) bring new algorithms from expert research into data production, and (3) encourage development of data products that more directly facilitate cross-disciplinary applications using satellite altimetry;
  • Promote innovative applications of satellite altimetry, including evaluations and cross-disciplinary applications of future satellite altimetry;
  • Continue developing techniques to improve altimeter data quality, aiming towards the development of new data products across the coastal zones including the coastal ocean, estuaries and inland water bodies;
  • Focus on capabilities of the very high spatial resolution from SAR and SARAL altimeters, as well as upcoming SWOT, for precisely modelling the marine gravity field, mean sea surface, bathymetry and ocean mean dynamic topography, as well as temporal variations induced by solid Earth processes and the global terrestrial water cycle; and
  • Promote cross-disciplinary research on the shapes and temporal variations of land/ice/ocean surfaces, such as studies of long-term ocean variability, regional and global sea level changes, mountain glaciers/ice-sheet ablations/accumulations, permafrost degradation, coastal and ice-shelf ocean tides, vertical displacements at major tectonic-active zone, land subsidence and other geophysical processes.

Program of Activities 

This sub-commission will 

  • Organize independent workshops or special sessions in major meetings to promote altimetric applications in interdisciplinary earth sciences, and to increase the visibility of IAG in altimetric science; and
  • Provide independent forums for potentially improved altimetry data processing and data product access, to encourage innovative and interdisciplinary scientific research and applications of satellite altimetry.

Study groups

SG2.5.1: High-resolution altimetry for geodetic, oceanographic, cryosphere and hydrology studies (HRA)

Chair: Luciana Fenoglio-Marc (Germany)
Vice-Chair: Ole Baltazar Andersen (Denmark)

Terms of References 

The mapping of the surface water elevation (SWE) at high resolution in space and time has been a goal of geodetic, oceanic and hydrologic scientific communities. Satellite altimetry global observations of high spatial and temporal resolution are required to understand the climate-related oceanic and hydrologic dynamics, which involve small-scale processes and their interaction with larger-scale dynamics. In the ocean, small-scale processes are related to meso- and sub-mesoscale ocean eddies, to internal tides and internal waves, to the cross-shelf interaction between coastal regions, open ocean dynamics and ecosystem. The dynamical processes in coastal oceans and estuaries are responsible for the along-shore distribution of nutrients and pollutions. On land, the small-scale processes relate to river dynamics and river discharge of water and nutrients from land into oceans, to extreme hydrological events and to climate change in inland waters. In the cryosphere, small-scale processes relate to dynamics of land ice and sea ice change. High-resolution altimetry (HRA) also provides improvement of bathymetry and geoid at small scales.  

New technologies, like Ka-band in pulse-limited altimetry and Delay Doppler (DD), with one or two antennas in Synthetic Aperture Radar (SAR) and SAR-interferometric (SARin) modes, with un-focused and fully focused (FF-) SAR processing for CroySat-2, Sentinel-3 and Sentinel-6 missions, provide today the highest accuracy and along-track resolution. Moreover, the Surface Water and Ocean Topography Mission (SWOT), to be launched in 2022, will offer a unique contribution by providing observational evidence of currently unobserved wavelengths between 100 km and 15 km. 

In light of the above rationale, HRA is proposed to investigate the development allowed by high-resolution altimetry in 1-D and 2-D fields. Mechanism of operations will be under IAG structure, with an elected International Governing Board and coordinated peer-reviewed projects.


HRA is a scientific and independent entity organized by the international scientific community. HRA will host and support peer-reviewed community proposed ‘projects’ with the aims to:

  • Provide a forum for scientific exchange on enhanced usage of past and future altimetry data to improve the understanding of small-scale signals.
  • Support discussions on innovative interdisciplinary scientific research and applications. 
  • Provide training courses for scientific users during IAG and IUGG general assemblies. 

An initial list of possible “projects”, may include, but is not limited to the following: 

  • Enhanced processing of high-resolution altimetry along-track in SAR mode and comparison of available techniques (e.g., FF-SAR, LRMC, Unfocused SAR and Reduced SAR). 
  • Understanding of the SWOT signal with pre-launch simulation of future swath-like observations from model outputs and realistic errors, and post launch calibration/validation of the observations.
  • High resolution altimetry in open seas to study eddy dynamics and related vertical processed with exchanges of heat and carbon between the ocean and the atmosphere. The assimilation in high-resolution ocean models is recommended.
  • High resolution altimetry in coastal zones and estuaries to study dynamics of exchanges in the river-estuary and open ocean continuum. The assimilation in high-resolution hydrodynamic models is recommended.
  • High resolution altimetry in rivers to study river dynamics and river discharge. The assimilation in hydrodynamic model is recommended.
  • High resolution altimetry in lakes and wetland to study water mass change on land.
  • High resolution altimetry on polar regions to study ice mass change and sea ice variation, ocean-cryosphere and solid Earth-cryosphere interactions.
  • High resolution altimetry for determination of bathymetry and geoid.
  • Innovative merging of products for interdisciplinary science and applications.
  • Support activities for inter-comparisons of processing algorithms and evaluations of data products.


Lifeng Bao (China)
Xiaoli Deng (Australia) 
Taoyong Jin (China)
Cheinway Hwang (China-Taipei)
Armin Agha Karimi (Sweden)
Chungyen Kuo (China-Taipei)
Jürgen Kusche (Germany)
Hyongki Lee (USA)
Karina Nielsen (Denmark)
Fukai Peng (China)
David Sandwell (USA)
Walter Smith (USA)
C.K. Shum (USA) 
Xiaoli Su (USA)
Christopher Watson (Australia)

SG2.5.2: Synergistic Applications of Satellite Altimetry with Other Satellite Sensors and Physical Models (SASA)

Chair: Hyongki Lee (USA)
Vice-Chair: Chungyen Kuo (China-Taipei)

Terms of References

With advances in waveform retracking and new technologies, such as altimeters operating in Synthetic Aperture Radar (SAR, or Delay-Doppler) and SAR-interferometric (SARin) modes, satellite radar altimetry is a mature geodetic technique successfully providing surface elevation changes over, not only oceans, but also inland water bodies, ice-sheets/glaciers and topographic lands. In particular, satellite altimetry has been used to augment existing monitoring network or to derive new estimates with data from other satellite sensors or a physical model, especially over terrestrial surfaces. By adopting daily water levels created by integrating altimetry data and a hydrological model, daily-inundated areas can be generated with a data decomposition technique such as empirical orthogonal function analysis for finding correlation between altimetry-derived water levels and satellite imagery-derived inundated areas. The fusion of satellite laser altimetry (e.g., ICESat-2), optical imageries and models can be used to derive shallow water bathymetry (depth <40m) and to reconstruct topography of a tidal flats, producing a seamless ocean-land topography. Altimetry in synergy with GRACE and GRACE-FO has also played a major role in monitoring mass changes of the ice sheets and glaciers of the world. 

Another class of sensors, which is potentially able to sense high temporal frequency signals, is the spaceborne Global Navigation Satellite System Reflectometry (GNSS-R). It measures the Earth surface from different constellations for sensors already in place (e.g., GNSS). The NASA CYGNSS and the UK TechDemoSat-1 satellites both carry the GNSS-R receiver. The application of GNSS-R for altimetry offers a unique opportunity to monitor the ice sheets, sea ice, land cover, soil moisture, polar and coastal oceans. 

It is therefore important to promote innovative usage of altimetry data synergistically integrated with data obtained from other satellite sensors (e.g., optical/SAR imaging sensors, laser altimetry, GRACE/GRACE-FO and GNSS-R) and physical models in order to advance scientific studies and real world applications. 


To support synergistic applications of altimetry data with data from other satellite sensors and physical models, SASA aims to:

  • Support collaborations among scientific users of altimetry and other satellite sensors – working groups from different sub-commissions;
  • Merge altimetry-derived water levels with imaging sensor-derived river widths/inundated areas, in situ data, model outputs and GRACE/GRACE-FO data for river discharge estimation, reservoir monitoring and inundation mapping;
  • Integrate altimetry data with hydrologic models to reduce uncertainties in model-derived streamflow;
  • Use altimetry-derived water level based on flow correlation for evaluation of hydrological model for ungauged basins;
  • Study geophysical processes, merging multi-mission radar altimetry and laser altimetry (ICESat, ICESat-2), other geodetic data and SWOT interferometric altimetry in the future;
  • Combine altimetry with GRACE and GRACE-FO data to estimate ice-sheet/glacier firn density, and to isolate contributions from Glacial Isostatic Adjustment (GIA);
  • Map land ice and mountain glacier elevations using multiple radar altimetry missions; and 
  • Use in situ/SNR and spaceborne GNSS-R altimetry/radiometry for monitoring coastal sea levels, inland water bodies, soil moisture, snow elevation changes and land/water classifications. 


Lifeng Bao (China)
Xiaoli Deng (Australia) 
Luciana Fenoglio-Marc (Germany)
Cheinway Hwang (China-Taipei)
Yuanyuan Jia (USA)
Taoyong Jin (China)
Hahn Chul Jung (Korea)
Per Knudsen (Denmark)
Jürgen Kusche (Germany)
Wenhau Lan (China-Taipei)
Karina Nielsen (Denmark)
Fukai Peng (China)
David Sandwell (USA)
C.K. Shum (USA) 
Walter Smith (USA)
Xiaoli Su (USA)
Kuo-Hsin Tseng (China-Taipei)
Christopher Watson (Australia)

SC2.5.3: High Resolution Mean Sea Surface (MSS)

Chair: David Sandwell (USA)
Vice-Chairs: Ole Andersen (Denmark) and Philippe Schaeffer (France)

Terms of References

Satellite radar altimeter measurements of sea surface heights have been continuously collected since the early 1990’s for two main purposes. The physical oceanography (PO) community has nearly 30 years of measurements along the 10-day repeat ground tracks of TOPEX/Poseidon, Jason-1/2/3, and Sentinel-6 missions. In addition, there has been a similar 30-years of data collected along the 35-day repeat ground tracks of ERS-1/2, Envisat and SARAL, and 3–5 years of 27-day repeat ground tracks of Sentinel-3A/-3B interleave orbits. These two tracks have been used to monitor global and regional sea level rise and provide a highly accurate framework for time variations in the mean sea surface (MSS) (https://www.aviso.altimetry.fr/en/data/products/auxiliary-products/mss.html, https://www.space.dtu.dk/english/research/scientific_data_and_models/global_mean_sea_surface).  

Higher spatially sampled altimetry data have been also collected along orbits having much longer repeat times greater than 180 days from Geosat, ERS-1, Cryosat-2, SARAL/Altika, and Jason-1/2 (extension of life). While these data have poor temporal resolution, the high spatial resolution provides new information on the marine gravity field (https://topex.ucsd.edu/grav_outreach/).  

These two kinds of altimetric missions are fundamentally complementary. Exact Repeat Missions (ERM) allow to access to an accurate determination of the steady state of the ocean, and Geodetic Missions (GM) provide the knowledge of the shortest topographic structures to a few kilometer. In addition, new altimeter technologies such as the Surface Water and Ocean Topography (SWOT) mission will collect data at both high spatial and high temporal resolution. Our study group is developing a MSS to provide a validation surface for these new measurements.

This group listed below has been collaborating for the past three years on the development of this MSS by comparing largely independent developments. There are a number of important challenges including: the definition of the averaging time for the MSS; the methods of combining the short wavelength information from the high spatial density measurements with the sparse framework provided by the PO missions; and extending the MSS into the Arctic where the spatial and temporal coverage is less than optimal due to sea ice cover.


To provide a long-term reference sea surface for the physical oceanographic and geodetic communities as well as for CAL/VAL of new altimeter missions, the MSS study group aims to:

  • Support discussions and collaborations in the international scientific community on the development of a high resolution MSS.
  • Develop a consensus global MSS by combining the long-term framework from exact repeat altimeter missions including: TOPEX/Jason, Envisat/SARAL and Sentinel-3 with the high spatial resolution data provided by Geosat, Cryosat-2, SARAL and Jason-1/2/3 (extension of life).
  • Hold regular meetings to compare and contrast MSS models developed at CLS, DTU and SIO.
  • Distribute the consensus MSS model(s) to the physical oceanography, geodetic, offshore industry and altimetry communities.
  • Focus, in particular, on the development of a MSS for the upcoming SWOT mission to provide calibration and validation early in its mission.


Adili Abulaitijiang (Germany)
Xiaoli Deng (Australia)
Bruce Haines (USA)
Cheinway Hwang (China-Taipei)
Per Knudsen (Denmark)
Eric Leuliette (USA)
Yuanyuan Jia (USA)
Isabelle Pujol (France)
Walter Smith (USA)
C.K. Shum (USA) 
Jinbo Wang  (USA)
Daocheng Yu (China-Taipei)
Shengjun Zhang (China)

SC25.4: The International Altimeter Service (IAS) Planning Group

Chair: C K Shum (USA)
Co-Chair: Xiaoli Deng (Australia) 

Terms of References 

Satellite altimetry has evolved to a unique and operational geodetic remote sensing measurement system with multi-missions and multi-satellite constellations generating an unprecedented climate data record since 1991, for over three decades and in the decades to come. Satellite altimetry has demonstrated seminal research in interdisciplinary Earth sciences, including general ocean circulation, sea-level science, global gravity field models (EGMs), bathymetry and marine geophysics, ocean tides, ice reservoir mass change, polar and coastal and oceanography, hydrology and water cycles, low altitude total electronic contents, land and ice digital surface models, near-surface soil moisture, and land surface deformation. Satellite altimetry is deemed to be operational, and its applications so far include meteorological and ocean circulation forecasting, vertical datum realization, ocean wind and wave forecasting, flood and water resources management, potential monitoring of land subsidence, lake seiche or meteotsunamis, cyclone and coastal storm surge, seismic-induced tsunamis, and ocean rescue services.  The major geodetic and geophysical contribution of satellite altimetry has been and will be the refinement of the global high-resolution gravity field and seafloor bathymetric models.

At present and since late 1970’s, multiple national and international space, defense, oceanic, atmospheric and meteorological agencies, Universities, industries, and other organizations have collectively launched satellite radar and laser altimetry missions. These constellations of altimeter missions enable the generation of multi-decadal, continuous, and uniform geophysical and climate data records at unprecedented spatiotemporal resolution and accuracy.  Innovative instrumentation advanced from pulse-limited to Delay-Doppler or SAR, to wide-Swath (SWOT mission), to multi-beam photo counting laser altimetry (ATLAS), and to the more recent exploitation of SoOP (Signals of Opportunity) satellite sources in bistatic radar enabled satellite altimetry, including L-band GNSS-Reflectometry enabled altimetry. Current pulse-limited and SAR altimetry missions are operational, with other contemporary scientific missions poised to be operational.

The significant opportunities below potentially could afford, more than ever, a need for the establishment of the International Altimeter Service (IAS).  If so, it would be the overachieving goal of this IAS planning group to recommend a strategically plan and eventually its establishment under the International Association of Geodesy.  They include, but not limited to, the following: 

  • Satellite radar altimetry has been declared mature and operational, with priorities of generating near-real data products for assimilative coastal ocean and wind/wave forecasting, Lake meteorological forecasting, and to potentially support disaster response;
  • High quality satellite altimetry missions have been generating global climate and geophysical data records since the 1980’s (Geosat), currently flying and planned “direct” altimetry missions are abundant in the decades to come.  They include HY2A/2B/2C/2D, Sentinel 3A/-3B, ICESat-2, Sentinel Michael Freilich -6A/-6B, and SWOT.  To extend the multi-altimetry climate record, it necessitates collaborations beyond a single space agency for calibration and validation of multiple altimetry instrument,  and orbits, media and geophysical corrections, to assess potential inter-mission instrument biases, their drifts, and overall consistency of the interdisciplinary data products, including multi-decadal elevation evolutions of ice-sheets, mountain glaciers/ice caps, ice-shelves, and sea-/Lake-ice, Lake water and river levels, and solid Earth displacements; 
  • Multi-mission, multi-decadal radar altimetry in repeat and geodetic orbital phases of the missions contribute significantly to the success of global high-resolution to the estimates of Earth Gravity Models (EGMs), and the dynamic topography models. The new suite of altimetry mission data will continue to improve the next EGM and seafloor bathymetry development, and innovative geophysical studies.  It is anticipated that improved methodology resulting from new and additional data, will produce various disciplinary and interdisciplinary data products;
  • Altimetry measurement technology has evolved from pulse-limited Ku-/C-/S-Band altimeters to Ka-Band and with higher sampling (1–40 Hz), to Delay-Doppler or synthetic aperture radar (SAR) and SAR Interferometric altimetry (SARIn), to multi-beam photon counting laser altimetry IALTLAS), to the planned wide-swath Ka-band radar interferometric (KaRIn) altimeter, to the L-band GNSS-Reflectometry bistatic altimetry from in situ GNSS receivers or from current and future constellations of low Earth orbiters (CyGNSS, Spire, Triton, HydroGNSS, PRETTY, others), other bistatic altimetry from different frequency-band(s) and/or different Signal of Opportunity (SoOP) sources (e.g., SNoOPI).  In particular, GNSS-R bistatic forward scattering radar signals have been advancing studies of cyclone science and land processes. It is likely that interdisciplinary and unique GNSS-R and other SoOP data products will be developed.

Some or all of the above rationales articulated to promote the establishment of IAS initiated over a decade ago. Here, based on lessons-learnt, a renewed strategy, new innovative technologies, and availability of increased numbers of contemporary satellite altimetry constellations, we will deliberate the feasibility of potential establishment of an International Altimeter Service (IAS) under this planning group.

Specific Objectives 

IAS as a dedicated planning group aims to:

  • Convene annual workshops and conducts dialogs with scientists and key technical curators of existing altimeter data product services, to assess the rationales on establishing a potential IAS would be mutually beneficial to them and to their sponsoring agencies;
  • Continuing to confirm a list of key members at the end of the IAS Planning Group activity period, towards a potential next phase of IAS activities;
  • Seek and disseminate ideas from the Planning Group and the community to identify potential new and useful altimetry data products, with the associated data retrieval and visualization tools;
  • Pursue new potential altimetry mission data product processing and validation collaborations with, for example, Chinese satellite altimeter missions, such as HY2A historic data re-processing, and processing strategies for HY2B/2C/2D in the future; as well as potential GNSS-R altimetry data products from existing and future LEOs (CyGNSS, Triton, HydroGNSS, PRETTY, and possible commercial LEOs, including Spire constellation);
  • Seek a consensus among the IAS Planning Group members on the feasibility assessment to establish the IAS during the next phase.


Mohammad Al-Khaldi (USA)
Ole Andersen (Denmark)
Lifeng Bao (China)
Jean François Crétaux (France)
Philip Chu (USA) 
Xiaoli Deng (Australia), Co-Chair
Luciana Fenoglio-Marc (Germany)
James Garrison (USA)
Mostafa Hoseini (Norway)
Cheinway Hwang (China-Taipei)
Yongjun Jia (China)
Yuanyuan Jia (USA)
Gholamreza Joodaki (Norway)
Chungyen Kuo (China-Taipei)
Weiqiang Li (Spain)
Xiaopeng Li (USA)
Hossein Nahavandchi (Norway)
Jason Otero Torres (USA)
Fukai Peng (China)
Richard Salman (USA)
David Sandwell (USA)
Christian Schawtke (Germany)
C K Shum (USA), Chair
Stefano Vignudelli (Italy)
Yixin Xiao (USA)

Members to be Confirmed

Michaël Ablain (France) Tentative
Brian D. Beckley (USA) Tentative
Jérôme Benveniste (Italy) Tentative
Scott Gleason (USA) Tentative
Vinca Rosmorduc (France) Tentative
Walter Smith (USA) Tentative
Josh Willis (USA) Tentative

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