Mapping the One of the Space Shuttle’s enduring science legacies is the near-global topographic mapping of the Earth with innovative radar remote sensing Earth: Radars technologies. The shuttle also served as an important engineering test and Topography bed for developing the radar-based mapping technologies that have ushered in a quiet revolution in mapping sciences. The Shuttle Radar Topography Mission data set, in particular, has had an enormous Michael Kobrick Kamlesh Lulla impact on countless scientific endeavors and continues to find new applications that impact lives. This mission helped create the first-ever global high-resolution data for Earth topography—a data set for the ages. On average, one Shuttle Radar Topography Mission-derived topographic data set is downloaded from the US Geological Survey’s servers every second of every day—a truly impressive record. Experts believe the mission achieved what conventional human mapmaking was unable to accomplish—the ability to generate uniform resolution, uniform accuracy elevation information for most of the Earth’s surface. In all, the development of imaging radars using the shuttle demonstrated, in dramatic fashion, the synergy possible between human and robotic space operations. Radar remote sensing technology advanced by leaps and bounds, thanks to the five shuttle flights, while producing spectacular science results. 360 Major Scientific Discoveries Why Do We Need “Seeing” Through the Clouds Accurate Topographic Maps of the Earth? What Is If you have ever used a global Imaging Radar? positioning system for navigation, you know the value of accurate The term radar Radar maps. But, have you ever wondered stands for Radio Antenna how accurate the height of Mount Detection and Everest is on a map or how its Ranging. You have height was determined? One of the foundations of many science seen radar images disciplines and their applications of weather patterns to societal issues is accurate on television. Transmitted Energy (pulse) Reected knowledge of the Earth’s surface, Typical radar works Echo including its topography. Accurate like a flash camera, elevation maps have numerous common and easily understood so it can operate civil and military applications, like day or night. But, locating sites for communications instead of a lens towers and ground collision avoidance and a film, radar systems for aircraft. They are also uses an antenna to helpful in planning for floods, volcanic eruptions, and other send out energy natural disasters, and even predicting (“illumination”) and the viewscape for a planned scenic How does radar work? A radar transmits a pulse, then measures computer tapes to the reflected echo. highway or trail. record the reflected It is hard to imagine that the global “echoes” of pulses of “light” that comprise its image. Radar wavelengths are much topographic data sets through the longer than those of visible light so it can “see” through clouds, dust, haze, etc. Radar end of the 20th century were quite antenna alternately transmits and receives pulses at a particular microwave wavelength limited. Many countries created and maintained national mapping (range of 1 cm [0.4 in.] to several meters [feet]). Typical imaging radar systems transmit databases, but these databases varied around 1,500 high-power pulses per second toward the area or surface to be imaged. in quality, resolution, and accuracy. Most did not even use a common What Is Synthetic Aperture Radar? elevation reference so they could not be easily combined into a more global When a radar is moving along a track, it is possible to combine the echoes received map. Space Shuttle radar missions at various positions to create a sort of “radar hologram” that can be further processed significantly advanced the science into an image. The improved resolution that results would normally require a much of Earth mapping. larger antenna, or aperture, thus a “synthetic aperture” is created. Major Scientific Discoveries 361 Space Shuttle Missions: Advancing Earth Observations and Mapping STS-2 STS-41G STS-59 STS-68 STS-99 Columbia Challenger Endeavour Endeavour Endeavour Shuttle Imaging Shuttle Imaging Space Radar Space Radar Shuttle Radar Radar-A Radar-B Laboratory-1 Laboratory-2 Topography Mission November October April September February 1981 1984 1994 1994 2000 Shuttle and Imaging The Working of Shuttle Imaging Radars Radars—A Quiet Tracking and Data Relay Revolution in Satellite System Earth Mapping nk Li ata l D ita The First Mission Dig The Shuttle Imaging Radar-A flew on Space Shuttle Columbia (Space Radar in Orbiter Transportation System [STS]-2) in Payload Bay c /se November 1981 . This radar was km 7 .35 c 15° comprised of a single-frequency, 4 /se les to 60° single-polarization (L-band mi s e l i Data Receiving wavelength, approximately 24-cm m k m Station 5 6 . [9-in.]) system with an antenna capable 1 3 2 3 of acquiring imagery at a fixed angle 1 and a data recorder that used optical film. Shuttle Imaging Radar-A worked perfectly, and the radar acquired images covering approximately 10 million km 2 (4 million miles 2) from regions with surface covers ranging from tropical Space Shuttle’s track at the altitude of 215 km (134 miles) with changing radar antenna look angle allowed the mapping of swaths up to 100 km (62 miles) wide. 362 Major Scientific Discoveries forests in the Amazon and Indonesia Left: Optical view to the completely arid deserts of North of the Sahara region Africa and Saudi Arabia. Analysts (Africa) showing the vast, featureless found the data to be particularly expanse of sand. useful in geologic structure mapping, The white lines depict revealing features like lineaments, the radar flight path. faults, fractures, domes, layered rocks, Right: Radar imagery and outcrops. There were even land-use over the same region, applications since radar is sensitive to taken during STS-2 changes in small-scale roughness, (1981), reveals the surface vegetation, and human-made network of channels and dried-up rivers structures. Urban regions backscatter (radar rivers) beneath strongly, either because the walls of the sand sheets, buildings form corner reflectors with thereby illustrating the surface or because of the abundance the power of radar of metallic structures—or both. for archeological mapping. The Shuttle Imaging Radar-A’s most important discovery, however, resulted from a malfunction. STS-2 For comparison, California’s Death across the entire Northern part of the was planned as a 5-day excursion and Valley—the driest place in the United continent. The existence of wadis the payload operators generated an States—rates no more than a 7 on the (dry valleys) carved in Egypt’s nearby imaging schedule to optimize use of geological aridity index. Gilf Kebir Plateau, as well as other geologic evidence, supports this idea. the radar’s 8-hour supply of film. But when scientists got their first But early on, one of the three Orbiter look at the Shuttle Imaging Radar-A Subsequent field expeditions and fuel cells failed, which by mission images, they said “Hey, where’s the excavations verified the existence of rules dictated a minimum-duration sand sheet?” Instead of the expected what came to be called the “radar flight—in this case, a bit over 2 days. dark, featureless plain, they saw rivers” and even found evidence of So, the operators quickly retooled the what looked like a network of rivers human habitation in the somewhat plan to use the film in that time frame and channels that covered virtually wetter Neolithic period, about 10,000 and ended up running the system all the imaged area and might extend years ago. This discovery of an whenever the Orbiter was over land. for thousands of kilometers (miles). evolving environment was a harbinger The result was a number of additional To everyone’s surprise, the radar of current concerns about global unplanned image passes over Northern waves had penetrated 5 or more meters climate change, evoking historian Will Africa, including the hyper-arid regions (1 6 or more feet) of loose, porous Durant’s statement, “Civilization exists of the Eastern Sahara. sand to reveal the denser rock, gravel, by geological consent, subject to and alluvium marking riverbeds change without notice.” “Radar Rivers” Uncovered that had dried up and been covered over tens of thousands of years ago. This Sahara region, particularly the The Second Mission Selima Sand Sheet straddling the Scientists knew the Sahara had not Egypt/Sudan border, is one of the driest always been dry because some 50 The Shuttle Imaging Radar-B mission places on our planet. Photographs from million years ago, large mammals launched October 5, 1984, aboard the orbit show nothing but vast, featureless roamed its lush savannahs, swamps, Space Shuttle Challenger (STS-41G) expanses of sand, and with good reason. and grasslands. Since then, the region for an 8-day mission. This radar, The area gets rain no more than two has fluctuated between wet and dry, again L-band, was a significant or three times per century, and rates a with periods during which rivers improvement, allowing multi-angle 200 on the geological aridity index. carved a complex drainage pattern imaging—a capability achieved by Major Scientific Discoveries 363 using an antenna that could be mechanically tilted. It was also designed as a digital system, recording echo data to a tape recorder on the Tom Jones, PhD flight deck for subsequent downlink Astronaut on to the ground but with Shuttle Imaging STS-59 (1994), Radar-A’s optical recorder included STS-68 (1994), as a backup. The results, deemed STS-80 (1996), and successful, included the cartography STS-98 (2001). and stereo mapping effort that produced early digital-elevation data. “Space Radar Laboratory-1 Next Generation of Space and -2 orbited Radar Laboratory Missions a state-of-the-art multifrequency radar observatory to examine the The Shuttle Imaging Radar instrument changing state of Earth’s surface.