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# f98u2le3 - Lesson III TOPEX/Poseidon Altimetry The goal of...

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Lesson III. TOPEX /Poseidon Altimetry The goal of this unit is to explain how altimeters on the TOPEX/Poseidon satellite obtain global sea level measurements. Keywords: Altimeter, remote sensing, radiometer measurements, and ocean topography Measurement System The TOPEX/Poseidon uses altimeter to provide levels of precision previously unobtainable in global sea level measurements. The measurement system is very complicated so we're just going to focus on how the satellite's measurement is optimized. To do this, we need to know: 1) where the satellite is in its orbit and 2) the distance from the satellite to the ocean. Satellite position is known relative to the center of the Earth and so we measure sea height relative to the Earth's center. The position of the TOPEX/Poseidon satellite is determined using three methods. TOPEX/Poseidon has two altimeters that measure the distance from the satellite to the ocean. These altimeters send radar signals straight down which "bounce off" the ocean surface. The time it takes for the radar signal to return to the satellite tells us how far the satellite is from the ocean's surface. To improve the altimeter measurement, we measure the water content of the atmosphere. This is because water along the path of the signal changes the speed at which the altimeter signal travels. TOPEX/Poseidon makes very precise measurements of sea surface height. Let's discuss the meaning of the term "precise." You are given two rulers, each a foot long, but one has tick marks to indicate only whole inches and the other with tick marks every 1/16th of an inch...which would provide a more precise measurement of the length of your thumb? The more precise TOPEX/Poseidon's

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measurement is, the smaller the ocean feature we can measure. Later, we'll focus on the relative heights of certain ocean features that scientists want to measure. Right now, TOPEX/Poseidon's measurement precision for sea surface height is 4.3 cm (1.7 inches). Because the satellite flies 1330 km (830 miles) above the Earth's surface, that's comparable to knowing the sea surface height to much less than the thickness of a dime while flying in a jet at 35,000 feet altitude! Source: http://dutlru8.lr.tudelft.nl/pages/altim.ht ml Satellite Radar Altimetry Satellite altimetry offers the possibility to measure sea surface heights globally. Sea surface height is determined by measuring the distance between a satellite and the sea surface using radar. After computing the location of the satellite during the measurement and a number of atmospheric corrections are made, the sea surface height is known. Movement of water in the sea on spatial scales exceeding ~30 km and temporal scales exceeding ~1 day are manifested by deflections of the sea surface; that is, by changes in mean sea level associated with the strength and direction of the flow. This change in topography varies from the 1 meter increase in mean sea level in 100 km over the Gulf Stream to a 10 cm change over 1000 km over an El Niño event in the tropical Pacific. Given the
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f98u2le3 - Lesson III TOPEX/Poseidon Altimetry The goal of...

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