An article in The Sun Monday on the Topex/Poseidon satellite gave an inaccurate description of the shape of the ocean's surface. In the absence of currents, the surface tends to follow ocean-bottom contours, bulging over submerged mountains and dipping over broad sea floors.
The Sun regrets the error.
A French rocket scheduled for launch today carries what may be the most precise instrument of its kind ever made, a radar altimeter built by Johns Hopkins University scientists to measure the subtle hills and valleys on the surface of the oceans.
If all goes as planned, the $30 million altimeter, designed and constructed by Hopkins' Applied Physics Laboratory for the National Aeronautics and Space Administration, will measure ocean topography with uncanny precision, to within 1.2 inches or less from 830 miles in space.
The space agency installed the instrument aboard the Topex/Poseidonsatellite, a joint U.S.-French spacecraft mounted on a French Ariane expendable rocket. The launch was scheduled for this morning from Kourou, French Guiana.
Over the next three to five years, scientists hope to use the altimeter to comprehensively map the ocean currents, aiding the study of how they pump heat from the tropics to the poles.
NASA says the research could eventually improve long-range climate forecasts and predict large-scale weather patterns, such the cyclic El Nino warming trend in the eastern Pacific Ocean. Data from the instrument could also help scientists understand the impact of greenhouse gases on climate.
Studies of currents by automated buoys and research vessels can measure only a relatively small stretch of water and are limited in scope, the space agency says. The $310 million Topex/Poseidon satellite, which is 18 feet long and 9 feet wide, will circle the globe every 112 minutes and begin resurveying the same stretch of water every 10 days.
The altimeter, similar to aircraft radar, bounces a radio signal off the ocean surface and measures the time it takes to return.
Similar instruments have been used to map the ocean surface on earlier spaceflights. But none of them was as precise as the device constructed by the Hopkins laboratory, which is designed to work from deeper in space.
The 70 percent of Earth's surface that is covered with water may seem flat and featureless compared with the land. But the planet's oceans actually form an intricately sculpted surface, shaped by a variety of factors.
First, gravity varies across Earth's crust because matter is more densely packed in mountain ranges and more thinly spread out in valleys. As a result, the surface above seafloor mountain ranges is lower than the surface over broad plains.
This is no trivial effect. Gravity's uneven tug makes the surface of the oceans vary as measured from the center of Earth by up to 525 feet around the globe.
There are other influences as well. The Coriolis effect, which is the influence of Earth's rotation on the motion of objects on or near the surface, and wind and wave actions also ripple the oceans, causing the surface to vary about 6 feet 8 inches globally.
The variation created by gravity is, of course, much greater.
So the National Aeronautics and Space Administration and other scientists will compare the satellite's map with another map, produced separately, of what is called Earth's gravitational topography.
To ensure the accuracy of the ocean maps, the position of the satellite must be precisely fixed while it is gathering data. NASA and the French space agency plan to follow Topex/Poseidon with three independent tracking systems, to fix its location to within 4 inches.
After launch, scientists expect to spend about 45 days positioning the satellite in orbit and checking out the instruments.