Artist concept of Seasat

Major geologic features on the ocean floor are shown in images recently produced from data collected by the Seasat oceanographic satellite, flown by NASA in 1978.

Each image represents global snapshot of sea floor characteristics never before available, and at least one previously unknown feature is revealed.

The images were made by measuring the topography of the ocean surface with the satellite's altimeter.

The global image is comprised of more than 50 million physical measurements including 10 corrections for atmospheric and other interferences. They were produced from the same data, but each was processed differently to emphasize unique set of features.

The images supply new, detailed bathymetric (water depth) and geologic information for wide areas of the world's seas. This is especially true in the southern oceans, which have been poorly surveyed. Existing bathymetric charts show the Louisville Ridge as discontinuous chain of mountains running southeast of the Tonga-Kermadec Trench. The Seasat image of this region clearly shows nearly continuous chain of features.

Dr. Michael Parke, of NASA's Jet Propulsion Laboratory, Pasadena, Calif., produced the data base used to construct the maps from the Seasat altimeter. The images were produced by Parke, Dr. Timothy Dixon, JPL marine geologist, and Kevin Hussey of JPL's Image Processing Labratory. Scientific analysis of the images was conducted by Parke and Dixon.

Seasat collected 70 days of oceanographic data over 100-day period. The radar altimeter measured the distance from the spacecraft to the ocean surface. By calculating the satellite's position, and correcting for passage of the radar beam through the atmosphere, the height of the ocean surface relative to reference ellipsoid was determined.

The resulting maps, which show the ocean surface at one-half degree resolution, were created through computer processing of the Seasat altimeter data. They are designed to emphasize features on the ocean floor ranging in size from 50 to 500 kilometers (30 to 300 miles).

Mapping the sea floor by measuring the sea surface topography is possible because of the relationship between gravity, the sea floor and the ocean. Gravity over the earth is not constant and it varies depending on the local thickness, density, age and geology of the crust.

The ocean conforms to variations in this uneven gravity field because it is fluid. Sea surface topography dominantly conforms to the sea floor topography. mountainous formation on the sea floor, for example, causes peak on the ocean surface detectable by satellite altimeter.

NASA is considering follow-on mission called TOPEX (Ocean Topography Experiment) to study the ocean's circulation with high-resolution satellite altimeter.

Seasat is managed for NASA by JPL. Production and analysis of the altimeter maps was sponsored by the Oceanic Processes Branch in NASA's Office of Space Science and Applications.

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