This web site provides satellite-based information relating to the current and historical status of the global oceans, lakes & reservoirs, and rivers & wetlands. Here, “status” is primarily centered on variations in surface elevation as provided by a suite of satellite-based radar altimeters. These instruments are onboard various space-agency (NASA/CNES/ESA/ISRO) missions with data set and auxiliary information sourced from NOAA, NASA/GSFC, and RADS. Variations in surface elevation enables changing sea level to be monitored and reveals the seasonal and inter-annual fluctuations in continental waters. The products integrate responses to regional and global climate variations as well as anthropogenic influences. Over land, short- and long-term drought, as well as approaching potential flood conditions, can be highlighted. The data sets span multiple decades and are available in near real time, they thus serve both science and applied science programs. Applications include water resources management, estimating irrigation potential for crop analysis, hazard monitoring, hydro-electric power stability, and regional security.
A satellite radar altimeter is a nadir-pointing instrument continuously recording average surface heights directly below the satellite, as it transverses over the Earth's surface. Operating at Ku or Ka band frequencies, each altimeter emits a series of microwave pulses towards the surface. By noting the two-way time delay between pulse emission and echo reception, the surface height can be deduced. Each returned height value is an average of all surface heights found within the footprint of the altimeter. The diameter of the footprint depends on the surface roughness, but can typically range between 200m (for open pools of water in calm conditions) to a few kilometers (open water with surface waves). Each satellite is placed in a specific repeat orbit, so after a certain number of days the same point (to within 1km), on the Earth's surface is revisited. In this way, time series of surface height changes can be constructed for a particular location along the satellite ground track during the lifetime of the mission. Elevation measurements can be retrieved over ocean surfaces, ice sheets, sea ice, and continental waters such as lakes, reservoirs, river reaches, and wetland regions. Data interpretation techniques are mature, well-validated, and published, and mission continuity is assured to at least to 2030. Ongoing research continually strives to make improvements in both the quality and quantity of available elevation measurements.
Satellite laser altimeters are also nadir-pointing instruments though they have the capability to off-point to specific targets of interest. Operating at green or infrared wavelengths these missions are primarily focused on mapping ice-dominated regions, mapping canopy or vegetation cover, and studying the atmosphere. While GEDI (laser waveform data, mid to low latitudes) resides on the International Space Station, ICESat-1 (laser waveform data) and ICESat-2 (laser photon data) operate from their own orbits and record surface elevations to high geographical latitudes.
There have been a number of altimetric satellite missions to date and follow-on missions can be expected (see Table below).
| Satellite | Operation | Repeat Period |
|---|---|---|
| Topex/Poseidon | 1992-2002 | 10 days |
| Jason-1 | 2002-2008 | 10 days |
| Jason-2/OSTM | 2008-2016 | 10 days |
| Jason-3 | 2016-2022 | 10 days |
| Sentinel-6A MF | 2020-current | 10 days |
| HY-2A | 2011-2023 | 14 days |
| HY-2B | 2018-current | 14 days |
| HY-2C | 2020-current | 14 days |
| HY-2D | 2021-current | 14 days |
| Seasat | 1978 | 17 days |
| Geosat | 1985-1989 | 17 days |
| GFO | 2000-2008 | 17 days |
| SWOT-nadir altimeter | 2022-current | 21 days |
| Sentinel-3A | 2016-current | 27 days |
| Sentinel-3B | 2018-current | 27 days |
| ERS-1 | 1992-1993, 1995-1996 | 35 days |
| ERS-2 | 1996-2003 | 35 days |
| ENVISAT | 2002-2010 | 35 days |
| SARAL | 2013-2016 | 35 days |
| ICESat-1 (laser) | 2003-2009 | 90 days |
| ICESat-2 (laser) | 2018-current | 91 days at high latitudes |
| Cryosat-2 | 2010-current | 365 days |
| SWOT-KaRIn swath altimeter | 2022-current | variable |
| GEDI (laser) | 2018-2023, 2024-current | variable |
For general information relating to the satellite products, contact
Dr. Charon Birkett
Charon.M.Birkett@nasa.gov
Tel: 301-614-6464
For technical issue relating to the web site and product access, contact
Dr. Martina Ricko
Martina.Ricko@nasa.gov
The following programs and funding sources are acknowledged,
Lake/Reservoir elevation products can also be found at the USDA/FAS CropExplorer and the NASA PO.DAAC portals . Global mean sea level and gridded sea level products can also be found at the NASA PO.DAAC portals.
The project team acknowledges NASA/PODAAC, NASA/GSFC, NOAA, AVISO, ESA, and the TU-Delft/NOAA-NESDID/STAR “RADS” database, for the provision of altimetric datasets and associated physical parameters. The team also gratefully acknowledges Google Earth.
Many elements of this program reside in the research domain. The project management, support staff, and associated science investigators accept no responsibility for the accuracy and application of the surface elevation products