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| satellite altimetry | |
|---|---|
| Name | Satellite altimetry |
| Type | Remote sensing technique |
| First used | 1970s |
satellite altimetry is a spaceborne remote sensing technique that measures the distance between an orbiting spacecraft and the surface of Earth, yielding precise sea level and surface elevation data used across oceanography, glaciology, hydrology, and geodesy. Developed through collaborations among agencies such as NASA, CNES, ESA, NOAA, and JAXA, the method underpins climate monitoring, coastal management, and geophysical research by providing continuous, global measurements. Instruments flown on missions like TOPEX/Poseidon, Jason-1, CryoSat-2, and ICESat-2 have produced long-term records that inform assessments by bodies such as the Intergovernmental Panel on Climate Change and initiatives like the Global Climate Observing System.
Satellite altimetry exploits radar and lidar systems aboard spacecraft to determine surface heights relative to a reference ellipsoid or geoid, enabling derivation of sea surface height, glacier mass balance, inland water storage, and crustal deformation. Operational and research missions integrate data streams from TOPEX/Poseidon, Jason-2, Sentinel-3, Envisat, ERS-1, and ICESat to create multi-decadal records used by NOAA, NASA, ESA, and academic institutions including Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, and University of Southampton. The datasets support global observing systems coordinated by organizations like the World Meteorological Organization and the Committee on Earth Observation Satellites.
Altimeters emit electromagnetic pulses and record round-trip travel time to infer height, employing radar altimeters (Ku-band, Ka-band) and spaceborne laser altimeters (photon-counting and full-waveform). Key instrument architectures have been implemented on missions led by NASA and CNES partnerships such as TOPEX/Poseidon and Jason-3, on ESA platforms like Envisat and CryoSat-2, and on ISRO and JAXA missions. Supporting subsystems include precise orbit determination using Doppler Orbitography and Radiopositioning Integrated by Satellite techniques, microwave radiometers for wet-tropospheric path delay correction (used on TOPEX/Poseidon, Jason-1), and laser retroreflectors for satellite laser ranging with networks maintained by International Laser Ranging Service and stations such as Yarragadee and Herstmonceux. Altimeter footprints, pulse repetition frequencies, antenna designs, and waveform processors vary across platforms like Geosat, ERS-2, Sentinel-6 Michael Freilich, and ICESat-2.
Raw altimetric range measurements undergo precise corrections: ionospheric delay (dual-frequency or model-based), wet and dry tropospheric delay using microwave radiometers and reanalysis products from ECMWF and NOAA/NCEP, sea state bias corrections derived from wind and wave models such as WaveWatch III, and tidal corrections referencing global tide models like FES2014 and TPXO. Orbit determination integrates tracking from DORIS, GPS, SLR, and gridded gravity field solutions from missions like GRACE and GOCE to reference heights to a geocentric frame defined by standards from IERS. Waveform retracking algorithms (Brown model, adaptive retrackers) and parameter estimations from missions including CryoSat and ICESat-2 are applied for ice-sheet and inland-water measurements. Quality control and intercalibration use crossover analysis, colocated in situ tide gauges such as Tide Gauge Network sites, and tandem mission strategies demonstrated by Jason-1 and Jason-2.
Altimetric products support ocean circulation and climate studies by mapping geostrophic currents, mesoscale eddies, and sea level rise assessed by IPCC reports and operational centers like AVISO. In the cryosphere, radar altimetry from CryoSat-2 and laser altimetry from ICESat-2 quantify ice-sheet elevation change for Greenland and Antarctica mass balance studies contributing to Sea level rise projections used by UNFCCC. Hydrological applications exploit repeat-track altimetry on rivers and lakes (Amazon, Nile, Mekong) integrated with missions from ESA and CNES-ISRO to monitor storage changes alongside GRACE mass anomalies. In geodesy, altimetry constrains mean sea surface and dynamic topography, informing global vertical datums defined by institutions like IAG and national agencies such as NOAA and Ordnance Survey for tide-gauge datum unification.
Sources of error include atmospheric path delays, surface roughness and wave-induced sea state bias, land contamination in coastal and inland-water returns, insufficient spatial sampling for small-scale features, and electromagnetic penetration in snow and firn affecting ice elevation retrievals. Instrumental biases, drift, and orbit errors propagate into long-term trends absent careful cross-calibration between missions (for example, cross-calibration exercises among Jason series, TOPEX/Poseidon, and Sentinel-3). Geophysical processes such as Glacial Isostatic Adjustment—modeled using ice history reconstructions from Peltier and others—and regional ocean dynamics require auxiliary datasets (gravimetry from GRACE-FO) and models from CESM or MITgcm to separate signals. Coastal altimetry improvements such as SAR altimetry modes on CryoSat-2 and Sentinel-3 mitigate some limitations but require enhanced retrackers and validation with coastal tide gauges at sites like Honolulu and Vega Bay.
Pioneering experiments include altimetry on Skylab and the operational breakthrough with SEASAT and GEOS-3, followed by landmark missions TOPEX/Poseidon (joint CNES/NASA), Jason-1, Jason-2/OSTM, Jason-3, and the Sentinel-3 series from ESA in partnership with EUMETSAT. Cryospheric-focused missions such as ICESat (NASA) and CryoSat (ESA) advanced laser and SAR altimetry, while dedicated inland and coastal initiatives include HY-2 (China) and SARAL/AltiKa (ISRO/CNES). Complementary gravimetry missions GRACE and GRACE-FO and gravity mission GOCE provided essential geoid and mass-change context. Community coordination through the CEOS and operational continuity ensured by programs like Jason Continuity of Service sustained multi-decadal records relied upon by IPCC assessments and national agencies including NASA and NOAA.
Future advances emphasize higher-resolution SAR altimetry, Ka-band and multi-frequency radar, photon-counting lidar evolution as demonstrated by ICESat-2, constellation approaches exemplified by concepts from Spire and Planet Labs collaborations, and synergy with gravimetry and GNSS reflectometry. Planned and proposed missions aim to improve coastal and inland-water sampling, real-time operational uses for services by Copernicus and NOAA, and integration with Earth system models like CMIP6 ensembles. Emerging technologies include onboard AI for waveform processing, deployable small-satellite constellations, and enhanced inter-agency partnerships among NASA, ESA, CNES, ISRO, and commercial providers to maintain and extend the sea-level and surface-elevation records crucial for global change science.