January 25, 2000
Mapping the Earth, Swath by Swath
By WARREN E. LEARY
WASHINGTON, Jan. 24 -- In an ambitious move to better understand the ups and downs of Earth, the space shuttle Endeavour is to make a detailed radar map of the planet's surface that could be a boon to everyone from military planners to weekend backpackers.
Endeavour and a six-member crew are to be launched next Monday on an 11-day mission to sweep most of Earth's land masses with radar signals and create the most complete, three-dimensional map of the planet's surface ever assembled.
The mission is to gather so much data about the hills, plains, valleys and other surface details of planet topography that it will take more than a year of analysis to produce the global map. To retrieve information needed for the three-dimensional views, the shuttle will have to extend a 197-foot metal and plastic antenna mast, the longest rigid structure ever deployed in space.
Scientists say comprehensive information about height variations on land will aid studies of erosion, flooding, earthquakes, volcanoes, landslides and climate change. In addition, it will improve maps used for land and forest management, recreation and the selection of sites for development, including the placement of communications equipment like wireless telephone towers.
Topographical information from ground and air surveys, and from radar data gathered by aircraft and space satellites, is available for most parts of the world in varying degrees of detail. But because information gathered at different times from disparate sources is often hard to use in research, scientists say the shuttle data will be invaluable because it will be consistent for the whole world. Spacecraft sent to Venus and Mars have produced better topographical maps of those worlds than exist for Earth.
In addition to topographical maps showing the elevations and depths of the surface, the radar scans are to produce black and white images of most of the planet's surface area that are unobscured by clouds, fog or darkness. Scientists say this picture mosaic covering almost all of the populated area of Earth will serve as an unprecedented reference point from which to study future changes on the planet's surface.
"It will be a snapshot of the planet in one 10-day period," said Dr. Michael Kobrick of NASA's Jet Propulsion Laboratory, the chief scientist of the project. "There is nothing else like it."
Endeavour is to fly in an orbit between 60 degrees North and 56 degrees South that allows it to map all the land from the southern edge of Greenland to the southern tip of South America, an area encompassing 80 percent of Earth's land mass and 95 percent of its population.
Originally set to fly last September, the mission was delayed while NASA inspected the entire four-shuttle fleet for electrical wiring problems. Inspections and subsequent repairs were ordered after the shuttle Columbia suffered an electrical short that knocked out two engine computers during its launching in July.
The radar mapping flight was delayed again when NASA decided to launch the shuttle Discovery ahead of it in December to repair the Hubble Space Telescope.
Researchers involved with the mapping flight were surprised, and some were disturbed, by a decision last week by mission managers to reduce the number of days of radar mapping to 9 from 10.
Paul Dye of NASA's Johnson Space Center in Houston, the lead flight director for the mission, said officials had decided to cut data gathering short to improve the chances of retracting and saving the long antenna mast in case problems developed at the end of the mission. Earlier plans called for severing the expandable mast from the shuttle and discarding it in space if problems developed in retrieving it. The new schedule gives astronauts time to perform an emergency spacewalk to help reel in the mast.
Although there are currently no plans to fly the radar equipment and mast again, Mr. Dye said managers wanted to keep future options open. The mast is also very similar to long trusses that will be deployed from the International Space Station to support its solar power arrays, and some experts said NASA might wish to re-examine one that had been used in space.
"We decided it was a good idea to preserve the mast, to keep the capability to bring the mast in," Mr. Dye said. "This is a device which we've never deployed before."
Dr. Kobrick, disappointed that scientists would lose 10 percent of their expected data, said, "I wish it were otherwise." He said the research team was looking into alternatives for getting the lost data, including the somewhat unlikely possibility of extending the mission for a day by conserving power aboard the shuttle. The radar equipment uses so much power that some officials believe this is improbable.
Another issue raised by the mission is who will have access to the information it gathers.
A major co-sponsor of the mission, contributing $200 million toward its cost, is the Defense Department's National Imagery and Mapping Agency, which provides maps and pictures to military and civilian intelligence agencies. The military agency said it would restrict availability to the mission's most detailed mapping data for national security reasons.
Military applications for this information include improved weapons targeting, better aircraft navigation and flight simulator training, and enhanced battlefield management and tactical planning.
The radar mission will produce highly detailed data in so-called 30-meter resolution, about three times more detailed than 90-meter data commonly available for most parts of the world. This resolution will provide three-dimensional information in an area about 100 feet on each side that has a vertical height accuracy of at least 16 meters (about 50 feet) and down to 6 meters (20 feet) in the best case.
The military agency said it would make worldwide 90-meter data available to anyone who wanted it, as well as 30-meter data for the United States, where information of this detail is already commonly accessible. However, it said, 30-meter information for the rest of the world would be restricted and available to civilian researchers only on a case-by-case basis. Agency spokesmen have questioned whether civilian users need the detailed map data.
Dr. Victor R. Baker, a professor of hydrology and water resources at the University of Arizona, said his research on flooding and the flow of rocks and other debris along waterways was improved by having the most detailed topographical information available. "We can do a better job if we have 30-meter data, particularly if it's digital and can be combined with other kinds of data, such as imagery," he said, "But even having 90-meter data helps if it is from the same source."
Dr. Baker said most of his research concerned the United States, where 30-meter data from the shuttle mission was expected to be available. "But having the same data available for places elsewhere would help us compare similar features and enhance our work," he said.
Dr. Kobrick of NASA said he saw little difficulty working under his agency's arrangement with the Defense Department about the data. "The 30-meter data will be available, but I will have to ask for it," he said, "Their only request is that I don't publish the raw data itself with my results. I'm perfectly happy with this arrangement we have."
Thomas A. Hennig, the military agency's program manager for the project, pointed out that only about 60 to 70 percent of the world was now mapped at 90-meter resolution and in some places, even this is restricted information. Releasing data of this resolution for the world, along with the radar image mosaic, amounts to a huge increase in topographical information benefiting people worldwide, he said.
John E. Pike, a space and military policy expert with the Federation of American Scientists, said the military agency had legitimate concerns about releasing the detailed data. "This information is potentially useful to other militaries, also," he said, "The Defense Department wants to see that the data doesn't fall into hostile hands."
Space shuttles have flown radar-mapping instruments on five previous occasions, culminating in two flights in 1994 that demonstrated three-dimensional mapping by flying over the area twice to take slightly offset images that were later combined into pictures showing elevation differences. Dr. Kobrick and Edward Caro, an engineer also with the Jet Propulsion Laboratory, came up with the idea of getting three-dimensional topographical images from the shuttle in a single pass by putting additional radar receivers on a boom extending from the spacecraft.
Once Endeavour reaches orbit, it is to deploy the boom almost immediately from a canister in its cargo bay.
Flying with modifications to the same equipment used in 1994, Endeavour is to orbit 145 miles above Earth, making radar scans in swaths 140 miles wide. The international mission features radar equipment made by the German Space Agency and the Italian Space Agency.
The shuttle, flying upside-down and tail first, will sweep the path with C-Band and X-Band signals transmitted by antennas that are part of its 29,000-pound cargo of radar equipment. The signals will bounce off Earth and back to the shuttle, where they will be picked up by receiving antennas in the cargo bay and at the end of the 640-pound mast almost 200 feet away.
Using a technique called interferometry, each antenna will record almost identical data to form images, but the offset distance will cause slight phase variations in the signal that allow creation of three-dimensional images when the data is combined. NASA said the scans would produce so much data that it would take more than 13,500 standard CD-ROM disks to contain it.