PASADENA, Calif. – The Shuttle Radar Topography Mission will rely on the principles of interferometry to create the most detailed global elevation map ever made.
Zipping around the world, the mission will map between 70 to 80 percent of the globe’s landmass -- home to 95 percent of all humans -- during the course of the 11-day mission.
To cover a city like Pasadena and its 139,000 residents, as well as the location of NASA’s Jet Propulsion Laboratory (JPL), the process will take a fleeting three seconds to complete.
To accomplish the task, the Space Shuttle Endeavour will use an antenna mounted in its cargo bay to paint a radar beam across a swath of the Earth 140 miles (225 kilometers) across.
Like a push broom, that swath will move across the Earth, gradually covering just about everything south of Saint Petersburg in Russia (60 degrees north latitude) and north of Cape Horn at the tip of South America (56 degrees south latitude).
The waves will bounce off the Earth’s surface, whether it be flat desert, deep canyon or towering peak, and travel back to the shuttle, orbiting 144 miles (233 kilometers) above. Day or night, cloudy or clear, the radar will do its job.
In the case of this mission, not one, but two radar antennas will be at the ready to receive the waves: One will reside in the cargo bay, the other at the end of a 200-foot (60-meter) mast extending into space.
"Seeing" the Earth from two viewpoints will make all the difference.
"You really see better when you look at things from two angles, rather than just from a mono-scopic view," said Diane Evans, chief scientist in the Earth Science Program at JPL.
Since the two antennas lie at minutely different distances from any one target hit by the radar beam – say, the summit of Mount Wilson above Pasadena – the time it takes the waves to bounce back to either end of the mast will vary. More to the point, the phase (the total number of cycles of the radar wave as measured from its source) of the two beams will vary.
Since scientists know the distance, or baseline, between the two antennas to within a tiny fraction of an inch (centimeter), they can exploit that phase difference to extract topographic data about whatever feature on the Earth the radar is hitting.
"Basically, what we are doing is triangulating to the surface of the ground," said Tom Farr, the deputy project scientist at JPL for the mission.
On the ground, scientists will combine the data gathered by each antenna to form what are called "interferograms." The name refers to the technique of interference, whereby two beams (radar, light or what have you) are combined or "interfered."
To illustrate the technique, hold one pebble in each hand with your arms outstretched over a large tubful of water. Drop the pebbles at the same instant, and watch the resulting waves move toward each other. Where they meet, the waves interfere.
On SRTM, scientists can calculate the elevation, or topography, or the Earth’s surface by subtracting the difference between the two waves.
"What’s driving this is analogous to the distance between your eyes," said Ed Caro, the mission’s chief engineer at JPL. "The farther apart they are, the better your ability to perceive distance and better the resolution."
In photography, the technique is somewhat akin to producing stereo images. In that case, two images of the same subject are taken from slightly different angles. When combined using special glasses, the two images appear as a single, three-dimensional photograph.
Scientists expect the technique will yield global maps that are 30 times more accurate than any other ever made, showing the topography of remote regions of the planet where cartographers have never set foot. Vertical accuracy will be to within 53 feet (16 meters).
On previous shuttle missions, scientists combined repeated passes over the same spot to create interferometric views of the Earth. However, both trees swaying in the wind and excess moisture in the atmosphere can skew those results.
"They can make mountains appear where there are none," Farr said.
Indeed, the consistency of the global atlas will likely prove to be the mission’s hallmark.
"Having this global snapshot at one time will be unprecedented," Evans said. "We’ll always be able to look back at February 2000."
The experimental mission’s success is not guaranteed. The shuttle astronauts will have to control the mast – the longest rigid structure to ever fly in space – keeping it from swaying any more than about an inch (2.5 centimeters) in order to acquire good data.
"We pull this off, I can die in peace," Caro said. "We’ve mapped the whole Earth; I’m finished."
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