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1.1.3 Principles Of Measurement

The Radar Altimeter measures the transit time of a radar pulse reflected from the Earth's surface back to the instrument. If the transit time is measured with great accuracy then the range from the instrument to the surface can be determined with great accuracy. Precise orbit determination computes the altitude of the satellite above the reference ellipsoid to cm accuracy. Subtracting the measured range from the satellite altitude leave the height of the surface above the reference ellipsoid.


Surface Height = Satellite Altitude - (Measured Range + Corrections)

In practice the measurement must be corrected for a number of effects to provide an accurate range. The instrument must be regularly calibrated to determine the delay times of the signal within the instrument electronics. These instrument delays vary with the heating/cooling cycle of the satellite around the orbit. Although Radar travels at the speed of light in vacuous it is delayed when travelling through the Ionosphere and Atmosphere. A range of geophysical corrections are needed to allow correction for for these delays. Finally the tides at the Earth's surface must be accounted for to determine an instantaneous surface height.

Usually the corrections to be added to the measured range are as follows:

Geophysical Corrections = Inverse Barometer + Sea State Bias + Ionospheric correction + Ocean Tide + Polar Tide + Earth Tide + Wet Tropospheric correction + Dry Tropospheric correction

Note that the Instrumental Corrections are nominally already applied to the altimetric range

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry