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 The position of the TOPEX/Poseidon to provide levels of precision satellite is determined using three previously unobtainable in global sea methods. TOPEX/Poseidon has two level measurements. The altimeters that measure the distance measurement system is very from the satellite to the ocean. These complicated so we're just going to altimeters send radar signals straight focus on how the satellite's down which "bounce off" the ocean measurement is optimized. To do this, surface. The time it takes for the radar we need to know: 1) where the signal to return to the satellite tells us satellite is in its orbit and 2) the how far the satellite is from the distance from the satellite to the ocean's surface. To improve the ocean. Satellite position is known altimeter measurement, we measure relative to the center of the Earth and the water content of the atmosphere. so we measure sea height relative to This is because water along the path the Earth's center. of the signal changes the speed at which the altimeter signal travels. TOPEX/Poseidon makes very precise and the other with tick marks every measurements of sea surface height. 1/16th of an inch...which would Let's discuss the meaning of the term provide a more precise measurement "precise." You are given two rulers, of the length of your thumb? The each a foot long, but one has tick more precise TOPEX/Poseidon's marks to indicate only whole inches measurement is, the smaller the ocean above the Earth's surface, that's feature we can measure. comparable to knowing the sea surface height to much less than the Later, we'll focus on the relative thickness of a dime while flying in a heights of certain ocean features that jet at 35,000 feet altitude! scientists want to measure. Right Source: now, TOPEX/Poseidon's http://dutlru8.lr.tudelft.nl/pages/altim.ht measurement precision for sea surface ml height is 4.3 cm (1.7 inches). Because the satellite flies 1330 km (830 miles) Satellite Radar Altimetry Satellite altimetry offers the Oceanography. Since currents possibility to measure sea surface are detectable as slopes in the sea heights globally. Sea surface height is surface, the world's ocean currents determined by measuring the distance can be detected and monitored. Small- between a satellite and the sea surface scale features are visible as well, like using radar. After computing the eddies, which are generated by the location of the satellite large-scale currents (by the Gulf during the measurement and a number Stream, for example). Altimeter data of atmospheric corrections are made, is also used for tide modeling. the sea surface height is known. Geophysics. Density differences Movement of water in the sea on in the Earth's crust cause local spatial scales exceeding ~30 km and differences in gravity (gravity temporal scales exceeding ~1 day are anomalies). These affect the manifested by deflections of the sea topography of the sea surface. The sea surface; that is, by changes in mean surface is always perpendicular to the sea level associated with the strength (local) gravity so a "mountain" in and direction of the flow. This change local gravity shows up as a "hill" in in topography varies from the 1 meter the sea surface (see Figure below). increase in mean sea level in 100 km This "mountain" can be both a real over the Gulf Stream to a 10 cm subsurface seamount or island, or it change over 1000 km over an El Niño may be a local increase in density in event in the tropical Pacific. Given the the Earth's crust. An accurate strength of the sea surface topography determination of the constant part of signature, one can infer the magnitude the sea surface (as opposed to the and direction of the oceanic water time-dependant part, mostly due to movement. oceanographic influences) is made by averaging as much data as possible Satellite altimetry is used for: from as many satellites as possible. Graphic Source Unknown So far, eight altimeters have flown on put into hibernation a year after its satellites. The first experiment was successor ERS-2 was launched and flown on board SKYLAB and carried over its operational duties. provided many useful engineering Simultaneously, the dedicated data on which the following altimeters altimeter satellite TOPEX/Poseidon were further developed. The first has been providing the most precise scientific altimeter mission was the information to date and carries two one on GEOS-3. It demonstrated how altimeters: a French solid-state flat land and ocean surfaces could be altimeter and a US profiled, but lacked sufficient dual-frequency altimeter, charting the accuracy and coverage to do serious time at a ratio of 1:9. By combining science. SEASAT carried a better the data sets of the last four satellites, altimeter and offered global coverage; a data set with an excellent coverage unfortunately the mission failed after can be obtained. The fact the two only some three months. The next satellites are now carrying altimeters altimeter satellite, GEOSAT, worked in orbit simultaneously greatly for several years. The GEOSAT data improves the coverage for set has proven the usefulness of the oceanographic studies both temporal altimeter. In the 1990's, already four and spatial. altimeters have been launched: two Source:http://dutlru8.lr.tudelft.nl/page/alt identical altimeters are carried on the im.html multi-disciplinary remote-sensing satellites ERS-1 and ERS-2. Although ERS-1 is still in good health, it was Measurement Process TOPEX/Poseidon’s primary signal. A second instrument called a instrument for measuring ocean radiometer is used to correct for the topography is an altimeter. The influence of water in the atmosphere. altimeter bounces a microwave pulse off the surface of the ocean. By Altimeter and radiometer measuring how long it takes the signal measurements, in conjunction with to return, the altimeter determines the precise orbit determination, are used distance between the satellite and the to obtain a detailed map of the ocean surface. ocean’s topography. From this, scientists calculate ocean circulation Air, water vapor, clouds and rain slow and tides. down the return of the microwave Altimeter Antenna The altimeter antenna is used by aboard the spacecraft. It works by the NASA /CNES radar altimeters sending radio pulses at 13.6 Ghz and carried in TOPEX/Poseidon’s 5.3 Ghz toward the Earth and instrument module (IM). The antenna measuring the characteristics of the is a 1.5 meter diameter parabolic dish echo. By combining this and is fixed on the nadir face of the measurement with data from the IM surrounded by the laser microwave radiometer and with retroreflector array. other information from the spacecraft and ground, scientists can calculate The dual frequency NASA radar the height of the sea surface to within altimeter is the primary instrument 4.3 centimeters. Altimeter Verification TOPEX/Poseidon altimeter measures of obtaining the altimeter-to-ocean the distance from the spacecraft to distance. the ocean surface. To verify the accuracy of the measurement made This measurement comes from sea by TOPEX/Poseidon, the Precision level monitoring instruments placed Orbit Determination and Verification on Texaco’s Platform Harvest, an oil Team has a second, Earth-based way production platform located off the coast of California. The site was would not corrupt the altimeter return chosen so that land interference signal. Measurement Systems The TOPEX/Poseidon spacecraft approximately 10 days, uses a high precision radar altimeter TOPEX/Poseidon completes 127 to take measurements of sea surface orbits, or a cycle. In that time, it height over 90% of the world’s ice- records sea level measurements for free oceans. The satellite orbits at an the entire globe. TOPEX/Poseidon altitude of 1336 km above the Earth measures the distance from the and at an inclination of 66 degrees. satellite to the sea surface to within One revolution around the planet is approx. 2.5 centimeters, about the referred to as an orbit. In diameter of a quarter. Data Products Scientists and engineers process perceptible water vapor and total measurements gathered with the electron content. radar altimeter on board TOPEX/Poseidon to produce six These data sets enable scientists to separate data sets focusing on: ocean meet the project objective of a better temperature, sea surface variability, understanding of global ocean significant wave height, wind speed, circulation and its effect on the world’s climate. Science Data TOPEX/Poseidon’s data flow is the members of the Science Working process by which data are transferred Team in 10 different countries. from the spacecraft to the Earth and converted into useful science Information pertaining to and products such as maps and images. samples of the different data sets created by these scientists are The flow takes the data from the currently available to science spacecraft’s instruments to its tape investigators, educators, students and recorders, to processing centers on the general public through the two continents, and finally to TOPEX/Poseidon Educational Outreach program. Data Flow Data from the altimeter are stored in data used to produce ocean on-board tape recorders and played topography are corrected for back daily to control and processing instrument biases, tropospheric centers at the Jet Propulsion delay, ionospheric delay, Laboratory in California and in electromagnetic bias and tides. The Toulouse, France.