The first application of the Sentinel-3 altimetry mission is the study of ocean topography including mean sea level, wave height, wind speed over the surface, sea-ice, ocean currents, Kelvin and Rossby waves, eddies and tides. Other application is the study of land topography including land ice, sea ice, land and inland waters.
The Sentinel-3 mission improves measurement quality in coastal zones, inland waters, sea ice and land ice areas compared to conventionnal LRM altimetry, thanks to its improved along-track resolution.
Examples of these applications and further descriptions can be found in the blue portlet on the right.
The main applications related to oceanography are:
large scale ocean circulation: studying the ocean currents at the global sea surface ("Mean Dynamic Topography" maps show oceanic relief corresponding to permanent ocean circulation)
mesoscale circulation: studying the sea currents and eddies at spatial scales of approximately 10 - 300 km
tides: due to the combined attraction of the moon and sun, accounting for some of the most significant variations in sea level.
The Sentinel-3 topography mission is able to provide 100 km resolution maps (large scale) every 10 days with an accuracy of 1 - 2 cm, and 25 km resolution maps (mesoscale) every 7 days with an accuracy of 2 cm.
Figure 1: Large-scale ocean circulation. Mean Dynamic Topography in cm (i.e. oceanic relief corresponding to permanent ocean circulation). Arrows are proportional to current speed. [Credits: CLS]
Media 1: Ierapetra gyre at South-East of Crete September-December 2010.
The Ierapetra gyre is a Mediterranean oceanic feature that occurs regularly. This anticyclonic, warm, gyre (turning clockwise, since it is in the Northern hemisphere) forms as a result of northerly wind interactions with the relief on Crete and of ocean circulation in the Kasos strait. The mountains cause the wind to change course, generating swirling movements, which the wind transfers to the sea to form what is known as the Ierapetra gyre.
The strength of this gyre depends on the speed and direction of the winds, with the gyre sometimes visible and sometimes not. Variations from year to year are thought to be due to the winds and the mid-Mediterranean jet along the coast of North Africa. In September-December 2010 (see animation), it clearly appears in the sea surface temperature data. It is also tracked by an Argo float trajectory (platform 6900679) that matches the contours of the highest temperatures in the centre of the gyre. (Credit: MyOcean)
Figure 2: Comparison of semi-diurnal and diurnal tides in the world. The blue/violet areas correspond to dominant, diurnal tides, while the yellow/red areas are semi-diurnal. [Credits: CLS/Legos]
The need to globally understand fresh water resources has increased the use of radar altimetry data for measuring river and lake water levels. The advantages of altimetry measurements are the continuity, global coverage and accuracy of the measurements available.
The Sentinel-3 improved along-track resolution (approximately 300 m) in SAR mode facilitates the measurement of narrow rivers and small lakes. The Level-2 SRAL/MWR products have a field (surface type) indicating whether the records are for enclosed seas or lakes.
New disseminated Level-2 thematic products (S3x_SR_2_LAN_HY_) allow improved water level retrieval through improved and customised land contamination and retracking techniques over inland waters.
Figure 3: Monitoring of the Issyk-kul lake Water Surface Height
The cryosphere plays an important role in moderating the global climate, and as such, the consequences of receding ice cover due to global warming are far-reaching and complex. While evidence suggests that ice sheets are relatively stable, there are indications that rapid changes are occurring around their margins, where the ice reaches the sea: changes that could weaken the ice sheet. Altimetry is one of the most powerful tools for observing sea ice and ice sheets.
The Sentinel-3 improved along track resolution in SAR mode (approximately 300 m) facilitates measurement over sea-ice and land-ice. The SRAL instrument operates in Closed Loop mode over the polar ice sheets, and in Open Loop mode over sea-ice areas.New disseminated Level-2 thematic products (S3x_SR_2_LAN_LI_ and S3x_SR_2_LAN_SI_) allow improved measurements over the ice sheets and sea ice areas, through improved and customised delay Doppler processing for these surfaces.
The main Sentinel-3 STM measurement over Sea Ice is the radar freeboard, defined as “the height of the radar scattering horizon from sea ice floes, above the local sea surface”. Radar freeboard is an important parameter for climate studies as, when combined with models of snow thickness and snow and ice densities, it can be further processed into Sea Ice Thickness.
Thanks to delay-Doppler processing capabilities, Sentinel-3 can better sample and discriminate the sea ice leads in comparison to conventionnal LRM. With the Thematic Products, the freeboard derived from the Sentinel-3 mission is in good agreement with CryoSat-2, as it can be noticed in the figure below.
Figure 4: Radar freeboard estimated in January 2019 by the Sentinel-3 mission, with the ESA Thematic Products (left) and CryoSat-2 mission, ESA Baseline-D (right)
Over Land Ice, Sentinel-3 main application is the estimation of the polar ice sheets topography, to monitor the ice sheet mass balance over the time. Compared to convetionnal altimetry, the enhanced along-track resolution (~300 m) of Sentinel-3 brings several improvements. They can be summarized as follow:
The SAR mode measure is not, or very weakly, sensitive to the along-track surface slope. This facilitates the measurement relocation, currently the largest source of error for land ice altimetry
Because the on-ground footprints of the successive 20 Hz measurements are discriminated (no overlap), there is a constant posting rate of ~300 meters along the satellite track. This gives access to fine scales of the along-track surface topography. In contrast (P)LRM measurements tend to oversample the surface peaks and undersample the surface troughs.
The SAR waveform leading edge is less sensitive to volume scattering compared to P-LRM. The measurements are theoretically less impacted by spatial-temporal variations of snow parameters (Aublanc et al., 2018)
Figure 5: Rates of Antarctic surface elevation change derived from Sentinel-3A Delay-Doppler altimetry acquired between May 2016 and June 2018 (McMillan et al., 2019)
Altimetry over open ocean is a mature discipline, useful for process studies and operational forecasting. In coastal zones (the strip of land within a few tens of kilometres from a coast) data are often discarded because it is not known how to interpret or model land contamination effects on altimetric waveforms.
There are intrinsic difficulties in making corrections especially in the wet tropospheric component, high-frequency atmospheric and oceanic signals and tides.
The Sentinel-3 SRAL improved along-track resolution (approximately 300 m) in SAR mode facilitates sea surface height measurement close to the coast. The Level-2 SRAL/MWR products have a parameter indicating the distance to the coast as well as dedicated coastal geophysical corrections (e.g. Composite wet tropospheric correction).
New disseminated Level-1 products (Level-1A, Level-1B-s and Level-1B) allow improving sea surface height measurement over the coast through the possibility of improving and customising land contamination and retracking techniques.
Figure 6: Modification of the shape waveforms (in red) when a satellite altimeter approaches the coast entering the radar footprint, making the estimate of range and other derived quantities more difficult. [Credits: COASTALT]
The main application of Sentinel-3 in relation to climate is in determining mean sea level rise due to global warming. As the ocean warms in response to global warming, sea waters expand and, as a result, sea level rises. When mountain glaciers melt in response to increasing air temperature, sea level rises because more freshwater glacial run-off discharges into the oceans. Similarly, ice mass loss from the ice sheets causes sea level to rise. The increased amount of freshwater flowing into the oceans reduces salinity, decreasing density and affecting ocean circulation patterns, which in turn affects sea level spatial variability.
The global mean level of the oceans is an indicator of climate change. It incorporates the reactions from several different components of the climate system. Precise monitoring of changes in the mean level of the oceans is vitally important for understanding not just the climate, but also the socio-economic consequences of any rise in sea level.
Global mean sea level is an average, over all the oceans, of sea surface height, with respect to a reference. However, what is really sought, is the regional variation in sea level over time.
Figure 7: Regional Mean Sea Level Trends from October 1992 to April 2012 [Credits: CNES/LEGOS/CLS]
Figure 8: Global Mean Sea Level Trend from 1992 to 2012 Derived from SSH [Credits: Altimetrics LLC]
Media 2: El Nino event March 2009 - February 2010. Cyclones have increased in the Pacific, whereas the 2009-2010 Atlantic hurricane season showed a decrease in events. El Nino, the ocean/climate phenomenon, is back. [Credits: MyOcean]
The Sentinel-3 topography mission could be useful for the study of the Earth's shape and size, gravitational anomalies or sea floor relief.
Satellite altimeter measurements, in combination with sparse measurements of sea floor depth, can be used to construct a uniform resolution map of the sea floor topography. These maps do not have sufficient accuracy and resolution to be used to assess navigational hazards but are useful for diverse applications such as locating obstructions or constrictions to the major ocean currents, and locating shallow seamounts where sea life is abundant.
Geodesy is the science of the Earth's shape and size. Altimetry makes it possible to compute Mean Sea Surface. Such a surface includes the geoid, i.e. the shape of the sea surface, assuming a complete absence of any perturbing forces (e.g. tides, winds, currents). The geoid reflects the Earth's gravitational field. It varies in height by as much as 100 m over distances of several thousand kilometres due to uneven mass distribution within the planet's crust, mantle and core. Other less pronounced irregularities are also visible over smaller distances. These mostly reflect the bathymetry (ocean bottom topography).
Figure 9: Geophysical Information Extracted from Altimetry [Credits: University of Calgary]
Detailed and accurate Digital Terrain Models (DTM) are widely used in earth sciences. DTM data derived from in situ and aircraft measurements have historically been available on a regional scale.
Satellite altimeter measurements can be used to derive DTM products and will be of use to Copernicus services. Satellite radar altimetry height estimation, over land targets, requires careful classification of each waveform according to shape, followed by the application of re-tracking algorithms to obtain the best range to surface measurement.
Thefirst altimeter-informed Global Digital Terrain Model (GDTM), Altimeter Corrected Elevations (ACE), was created by fusing altimeter derived heights produced using a system of multiple altimeter re-trackers, with ground-truth from a range of publicly available datasets to create an enhanced GDTM. A new ACE2 dataset was created by synergistically merging the Shuttle Radar Topography Mission (SRTM) dataset together with satellite radar altimetry.
Figure 10: Monitoring of the Ladoga lake Water Surface Height - Zawadzki et al, 2009