URL: http://www.altimetry.info/html/alti/dataflow/processing/ref_surf/welcome_en.html
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URL: http://www.altimetry.info/html/alti/dataflow/processing/ref_surf/welcome_en.html |
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Altimetry Data flowProcessing
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Altimeter measurements | Orbit Determination | Geophysical corrections | Instrumental errors | Reference surface | Flags | Retracking | High-level processing |
Several surfaces are used as references, either for the datasets (e.g. sea surface height with respect to ..., etc), or as additional information. Most are specific to the ocean.
The reference ellipsoid is an arbitrary reference surface that is a rough approximation of the Earth's shape, which is basically a sphere 'flattened' at its poles. The length of one of the axes at the equator is chosen so that the ellipsoid coincides with mean sea level at this latitude. It is the first-order definition of the non-spherical shape of the Earth as an ellipsoid of revolution.
The marine geoid is the shape of the sea surface assuming a complete absence of perturbing forces (tides, wind, currents, etc.). The geoid reflects the Earth's gravitational field (it is an equipotential surface) and varies in height by as much as 100 metres 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 reflect the ocean bottom topography.
The marine geoid can be estimated using among other things altimetry measurements of sea surface height.
Absolute dynamic topography can be defined as sea surface height with respect to the geoid.
The Mean Sea Surface represents the sea level due to constant phenomena. It is the sum of the geoid and the Mean Dynamic Topography (that includes the permanent stationary component of ocean dynamic topography).
The Mean Sea Surface is computed from altimetry, averaging the data over several years.
Sea level anomalies (also called sea surface height anomalies) are sea surface heights with respect to the Mean Sea Surface.
The Mean Dynamic Topography is the permanent stationary component of ocean dynamic topography.
This mean circulation is not
This mean circulation is not produced directly from altimetry data, which rather provide the mean sea surface, consisting of the marine geoid plus the sea elevation due to the ocean mean. We therefore have to combine altimetry data with other data (in-situ, gravimetric satellites, etc), to determine the geoid precisely, and by subtracting it, compute the mean circulation.
Bathymetry is the measurement of the ocean depths. Although is has no direct use in altimetry data processing, it can be useful in shallow water (since ocean altimetry is not as effective there), or for comparing with ocean features such as currents.
Bathymetry can be computed using among other things altimetry sea surface height measurements (via the geoid).