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The radar altimeter is designed to make accurate measurements of the satellite's height above the sea surface which is then converted to the sea surface's height above a reference ellipsoid. When the altimeter takes a height measurement it is measuring a height contributed to by many different types of phenomenon, from the underlying marine geoid , through the large scale general circulation of the oceans, to mesoscale eddies 100km across. In addition to highly precise height measurements, the altimeter makes measurements of the heights of waves that appear in its footprint, and of surface wind speed.


Measuring the marine geoid

The height of the ocean surface relative to the centre of the Earth is not only a function of ocean currents, but also of the Earth's composition - changes in composition have an effect on the Earth's gravity field, and this is reflected in the relief of the sea surface.

Information has been extracted from altimeter data, particularly that provided by the high resolution dedicated Geodetic Mission of ERS-1, to provide maps of average sea surface topography - the marine geoid. The geoid is the fundamental reference surface of geodesy. The surface of the geoid is expressed in terms of its departure from a simple mathematical figure, known as the reference ellipsoid. It represents the dynamic zero level, to which all differences in height and gravity potential, both on land and at sea, are referred.

Through its use in geoid determination, altimetry aids in revealing the location of ocean floor features such as faults, trenches, spreading zones, sea mounts and hot spots. Information may also be gained on the age, structure and dynamics of the lithosphere, particularly in the area of subduction zones, leading to a better understanding of the relationship between the lithosphere and the mantle, and of mantle convection. Additional, commercially valuable information can be derived on potential locations of oil-bearing structures using the effect that low density deposits (such as crude oil) have on the shape of the gravity field. This information has been derived not only over oceans, but also in the Arctic Ocean, using altimetry over sea ice.

Measuring sea state

The radar altimeter also measures the heights of waves that appear within its 'footprint', and the wind speed at the sea surface. Near real time measurements of Significant Wave Height (SWH) by the ERS altimeter are assimilated operationally into wave models to provide wave forecasts, essential for optimisation of a range of marine operations. An archive of these measurements, and others from similar instruments on other polar orbiting satellites, are being used to derive the climatology of marine areas worldwide, allowing changes in climate to detected and monitored. This information is also useful for a wide range of marine industries to be used in designing offshore structures, for example, or in pre-project planning.

Measuring the topography of the oceans

Worldwide sea level varies significantly in space and time. Regional variation in sea level occurs as a result of pressure differentials within the ocean resulting from momentum and heat flux exchange with the atmosphere. The resultant differences in sea level are thus directly related to ocean currents. Ocean topography can be measured directly and monitored for change using the ERS radar altimeter. Along with data from other similar instruments, the information can be assimilated into ocean circulation models which transform satellite surface information into three dimensional descriptions of ocean currents and transports.

An important fluctuation in the ocean-atmosphere system is the El Nino Southern Oscillation Phenomenon (ENSO), which causes an increase in ocean temperatures throughout the central and tropical Pacific which can produce dramatic changes in climate on the timescale of months to years. The events associated with ENSO can be measured in sea surface topography by the ERS altimeter, and in sea surface temperature by the ERS ATSR.

ERS is contributing to the following areas of ocean dynamics research:

  • Mapping ocean topography from the mesoscale to the global scale
  • Monitoring large scale features, such as El Nino events and associated Rossby waves
  • Modelling tides