Using the space shuttle orbiter Endeavour as a research platform, astronauts used a special radar array in Earth orbit to map the surface of the planet in greater detail than ever before. The entire shuttle mission was dedicated to the collection of these data.
On February 11, 2000, at 12:43
Part of the Shuttle Radar Topography Mission (SRTM) hardware as seen from space shuttle Endeavour’s flight deck windows during the eleven-day SRTM mission in 2000.
The primary cargo that nearly filled Endeavour’s payload bay on STS-99 was an interferometric synthetic aperture radar system. Indeed, this shuttle flight was so dedicated to the use of the radar system that it was identified as the Shuttle Radar Topography Mission (SRTM). Endeavour’s SRTM payload weighed 11,793 kilograms (about 25,944 pounds). SRTM represented a collaboration between NIMA, NASA, and the DLR.
Using imaging radar on shuttle flights originated as early as the second mission in the program, STS-2 carried the Shuttle Imaging Radar-A
What made the SRTM system unique was its ability to perform interferometric measurements as a result of having detectors both in the shuttle’s payload bay and on a sixty-meter-long boom that extended over the side of the vehicle. On that boom near the end of its mast was the outboard antenna structure with both C-band and X-band antennae, a pair of Global Positioning System (GPS) antennae, light-emitting diode targets, and a corner-cube reflector. Radar signals were transmitted by the main antenna in the payload bay, and the antennae on the boom then detected signals reflected off the Earth’s surface. Through accurate measurement of the position of the boom-mounted antenna relative to the main antenna, even more sophisticated data were obtained than if a reflected signal were picked up directly by the same antennae that had transmitted the original signal. Data collection required an enormous number of orbiter maneuvers, more than on any previous mission. Having the long boom hanging over the edge of the payload bay added to the difficulty of those maneuvers, as bending motions of the boom were not desirable. However, the end result was a three-dimensional map of the Earth’s surface of unprecedented detail and accuracy.
After Endeavour was launched, it entered an orbit inclined 57 degrees to the equator. As early as possible on the first day of flight, the crew deployed the SRTM imaging radar system into its operational configuration. Data collection commenced from an altitude of 223 kilometers (about 139 miles) once the sixty-meter-long boom and its radar antenna were set up over the side of Endeavour’s payload bay. To accomplish the enormity of the planned SRTM data collection, the STS-99 crew was divided into two teams, designated Red (astronauts Kevin R. Kregel, Janet L. Kavandi, and Gerhard P. J. Thiele) and Blue (astronauts Dominic L. Gorie, Janice E. Voss, and Mamoru Mohri), so that round-the-clock operations could be conducted over the entire eleven days of this ambitious mission.
The first major job for the Red Team to conduct after boom deployment was performing a series of thruster firings to test procedures for maneuvering the orbiter and the ability of dampers to preclude boom movements that would take the inboard and outboard antennae out of alignment during data collection. The Blue Team began radar mapping operations, with initial images being taken on a pass from south Asia to the eastern coast of mainland Asia out into the northern Pacific Ocean. The first X-band image was taken on the second day. The target of interest was the White Sands, New Mexico, area.
Mapping operations required Endeavour to fly in a relatively low altitude orbit. Thus, drag with the upper atmosphere resulted in changes in orbital parameters that had to be compensated for throughout the flight. For mapping operations, Endeavour flew in a “tail-forward” attitude; as a result, the crew could see out the forward windows where the shuttle had been as it continued around the Earth in its orbit. For orbital adjustments, a special procedure called the flycast maneuver had been developed. First, the orbiter was reoriented to a nose-forward attitude with the long SRTM boom sticking upward. Then a burst from small thrusters would set the boom into oscillation. When the boom returned to the vertical position, a larger thruster firing was conducted to simultaneously damp out boom oscillations and increase Endeavour’s speed, thereby increasing the shuttle’s orbital altitude and negating the orbital drag on the vehicle. After the flycast maneuver, the orbiter was returned to the tail-forward attitude for continued data collection.
The final SRTM pass was conducted on February 21 over Australia. The main antenna inside Endeavour’s payload bay was latched into position for the return to Earth, and the long boom was carefully collapsed back into its payload canister, less than three meters in height. In the end, data covering 120 million square kilometers (about 46.3 million square miles) had been collected during the intense mission.
The STS-99/SRTM ended on February 22 at 23:43 coordinated universal time when Endeavour touched down at the Kennedy Space Center, concluding a 6.5-million-kilometer (4-million-mile) journey over the course of 11.2 days. On board Endeavour were sufficient SRTM radar data and imagery to fill twenty thousand compact discs (CDs), enough to take years to analyze and to create the best high-resolution map of the majority of the Earth’s surface ever made.
Data gathered on the SRTM/STS-99 mission produced the most highly resolved images of the Earth’s surface between the latitudes of 60 degrees north and 56 degrees south. In this enormous region resides nearly 95 percent of the world’s population. The result of this 222-hour-long radar mapping effort was sufficient data to generate a map of Earth with a resolution of approximately thirty meters. Prior to the STS-99 mission, some of the areas included in this huge data set had never before been mapped accurately. This unprecedented data set was available to the researcher and average person alike. Much of the data was placed on official Web sites, but data could also be requested in CD form.
Prior to STS-99, space-based investigations of other worlds, such as Venus, had produced more highly resolved maps of planetary surfaces other than Earth. STS-99’s SRTM work gave scientists and geographers their best map of Earth, one superior in quality to any produced by a spacecraft at any other planet.
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Kramer, Herbert J. Observation of the Earth and Its Environment: Survey of Missions and Sensors. New York: Springer, 2001. Described by some as an essential reference for anyone delving into Earth resources studies. Includes a CD-ROM database. -
Qu, John J., et al. Earth Science Satellite Remote Sensing. New York: Springer, 2007. Technical work describes how remote sensing is accomplished, the manner in which data are collected, and how that data set is interpreted. For the informed reader or a college audience. -
Richards, Mark A. Fundamentals of Radar Signal Processing. New York: McGraw-Hill, 2005. Covers in-depth radar signal processing technology. Though a technical work, it is useful to the person unfamiliar with that technology, as it begins with first principles before proceeding to become very technical. -
Zelon, Helen. The Endeavour SRTM: Mapping the Earth. New York: PowerKids Press, 2002. Provides a description of the SRTM shuttle flight and its scientific mission. Meant for a younger audience. Illustrations outline both the specific mission and how shuttle flights are carried out.
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