OVERVIEW - Ocean Surface Topography
Topography is the shape of a surface, including its relief. The topography of the sea surface
is influenced by both gravity and ocean circulation.
The actual shape of the Earth is not a perfect sphere--it's not even a perfect ellipsoid
(which would account for the effect of its rotation). If you measure the strength of the
gravity field across the Earth's surface, you will find local variations caused by
irregularities in the crust, which produce an irregular figure called the geoid--the
actual shape of the Earth. To find the surface caused by ocean circulation, we
mathematically remove the sea height caused by gravity (the geoid) and create maps
of ocean topography. We then calculate the speed and direction of ocean currents
similar to how meteorologists use atmospheric pressure maps to calculate winds.
Altimetry is the measurement of the height of the sea surface above (or below) some
reference level. Every 10 days, Jason-1 measures heights of more than 90%
of the world's ice-free oceans with the radar altimeter and completes 127 revolutions,
or orbits, around the Earth.
To measure sea surface height we need the satellite's position in its orbit and the
height between the satellite and the ocean's surface. We may also have to correct
for variable amounts of water vapor in the lower atmosphere and free electrons in
the upper atmosphere. Both of these can delay microwave pulses.
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