6.2. Altimetry Frequently Asked Questions
How altimetry works ?
You can find an explanation of how altimetry works, in the "Altimetry" chapter of this website.
Does sea state influence the measurement of the satellite-sea surface range ?
Sea state influence the measurement of the satellite-sea surface range because the altimeter is sensitive to sea surface elements perpendicular to the target line. These elements are more frequently in the wave trough than in the crest, so the mean height of these elements doesn't match the geometric mean height of all sea surface elements that make the mean sea level (electromagnetic bias). In this way, the altimetric-measured mean is shifted toward wave trough, and moreover if waves are high.
Are altimetry data available near Antarctica?
Using Topex/Poseidon or Jason-1 data near Antarctica is not easy, because of coasts and sea-ice. Moreover, Thier orbit passes between 66°S and 66°N (ERS-1 and 2 between 82°S and 82°N). However, ERS-1 & 2 and Envisat give data up to 82°S.
About the radiometer, I would like to know how brightness temperature is defined, and why 3 frequencies are used on some satellites.
The brigthness temperature of a surface is equal to the product of the emissivity of this surface by its physical temperature.
The radiation measured by the radiometer depends on the ocean surface emissivity, its physical temperature and water vapour and cloud absorption in the atmosphere. If you want to know precisely the atmospheric water vapour contents, you have to substract surface and cloud contribution from the signal received by the radiometer. That's why several frequencies (3) are used, each one being more sensitive than the other to one of these contributions. By combining measurements done at each frequencies, you can extract the water vapour signal.
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