REMOTE SENSING

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Remote Sensing with Radar: Seeing Earth in a New Way

Peering at the Earth below with a new kind of 20/20 vision, scientists are learning more about the planet from afar than they dreamed possible.

The realization that Earth's resources are finite and that environmental changes are occurring at an unprecedented rate has made us view our home planet in a new light. Advances in remote sensing technology allow us to literally "see" outside the visible wavelengths where our eyes are most sensitive. One such technology, Synthetic Aperture Radar, (SAR for short, with radar being a lowercase acronym for radio detection and ranging), allows us to see through darkness and cloud-cover and to peer beneath forest canopies and dry sand so that we can explore inaccessible regions on Earth. SAR also allows us to "sense" moisture content of soils, vegetation, and snow that help us diagnose the health of the planet.

To accomplish all this, SAR transmits pulses of microwave energy toward Earth and measures the strength and time delay of the energy that is scattered back to the antenna. The antenna size defines important characteristics such as ground resolution and image quality. The concept of "synthetic aperture" comes from processing techniques that can simulate a much larger antenna size than is actually flown. SAR was originally developed in the 1950s as a technique for improving the resolution for military reconnaissance radars, but is now widely used in many countries for civilian applications.

The most advanced spaceborne SAR ever built for civilian applications flew aboard two space shuttle flights in April and October, 1994. The unique perspective of space and the advanced capabilities of this Spaceborne Imaging Radar-C and X-Band Synthetic Aperture Radar (SIR-C/X-SAR) Mission have allowed scientists to explore our planet in a way that has not yet been possible. Data from SIR-C/X-SAR and other international SAR systems such as the European Space Agency's Remote Sensing Satellites (ERS-1/2), Japan's Earth Resources Satellite (JERS-1), and Canada's Radarsat are currently being used for studies of ecology, hydrology, oceanography, geology, and ice sheets and glaciers. These sensors have not only provided scientists with a wealth of information about Earth's ever-changing environment, but they have also opened up new areas of potential applications such as archeology and comparative planetology.

Ecological applications of SAR include land cover classification, above-ground woody plant biomass measurement, and wetland inundation delineation. These measurements are essential to our understanding of carbon dioxide increase and global warming because we need to compare how much carbon is being consumed by forests to how much is being released to the atmosphere through clear cutting and burning of fossil fuels. SAR data have also been used for malaria risk assessment in tropical regions by providing spatial and temporal information on the distribution and flooding status of anopheline breeding sites.

Hydrologic applications include measuring snow and soil moisture in order to improve our understanding of where moisture is stored and how it is redistributed. This is critical for adequately modeling the global climate and for managing water resources. In addition, model studies from the 1993 midwestern floods have shown that high levels of soil moisture are essentially recycled back to the atmosphere and can result in severe rain storms. Thus, improving short-term severe weather forecasting may be possible by using actual soil moisture information rather than estimated values.

Oceanographers are using SAR data to study surface and internal waves, wave/current interactions, and sea-ice motion to better understand how the ocean moderates the Earth's climate.

Natural Hazards

SAR data are also being used by geologists in studies of past climates and volcanic and earthquake hazards. The longer L-band radar wavelengths are particularly useful for looking beneath surfaces. SIR-C/X-SAR obtained penetration data of the Sahara Desert that show braided

Figure 1: a previously unknown branch of ancient river buried under thousands of years worth of wind-blown sand in North Africa's Sahara Desert near Kufra Oasis in southeast Libya. This image was discovered by recent flights of the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR).

channels and paleodrainages. The existence of hidden river channels indicates that portions of the Sahara have undergone significant climate change and have evolved from an area of flowing streams to what is now an arid desert. Figure (1) shows an example of a river channel buried under thousands of years worth of wind-blown sand in a region of North Africa's Sahara Desert near Kufra Oasis in southeast Libya.

From the vantage point of space, SAR can be used to study volcanoes that may be too hazardous or inaccessible to study using other methods. The all-weather, day/night capability of radar allows scientists to study volcanoes in tropical regions or other areas that are usually covered in clouds. Radar also penetrates volcanic ash clouds, thereby allowing scientists to study surface changes during eruptions.

The United Nations sponsors a "Decade Volcanoes" program, which is designed to focus scientific and public attention on 15 of the world's potentially dangerous volcanoes based on their activity during historic times and their proximity to large population centers. Thirteen of these Decade Volcanoes were imaged by SIR-C/X-SAR. One these volcanoes, Taal, which is frequently active in the Philippines with several million people living within a 20-kilometer radius of its caldera rim.

Volcano, earthquake, and other natural hazard investigations benefit greatly from a unique capability of SAR called interferometry in which two measurements obtained at slightly different angles are combined to produce a measurement of the surface height. Combining one interferometric data pair with a third measurement, obtained at a different time, allows small-scale shifts in the Earth's surface to be measured over time. This technique, known as differential interferometry, has enormous potential for mapping changes in topography on the order of a centimeter.

Ice and More Ice

High-resoultion maps of topography and topographic change generated from SAR interferometry are also extremely valuable for studies of ice sheets and glaciers. Over 75% of the world's fresh water is presently locked in these frozen reservoirs. Even on short timescales of a few years, changes in ice volume modulate sea level. This has never been more important than it is today, given the increase in economic development of coastal areas. Sustained development of coastal areas worldwide has made the global economy extremely vulnerable to changes in sea level.

While the general retreat of mountain glaciers globally is believed responsible for approximately ¼-½ of the current 2 millimeter-per-year increase in sea level, most of the remainder is still unidentified but is likely the result of yet undiscovered imbalances in the large polar ice sheets. Using interferometric SAR data, it is possible to monitor glacier and ice-sheet velocities and topography, which are crictical to understanding mass balance.

Sizing Up the Planets

SAR data have also proven useful in comparative planetology studies. Radar data of Venus obtained by the Magellan spacecraft has provided an extremely detailed look at a planet that has been shaped by many of the same processes that affect the Earth. Comparisons between tectonic and volcanic features on Venus and Earth allow scientists to better understand the basic physics that govern processes such as mountain building, volcanic-flow emplacement, and impact cratering. Several images of impact craters were acquired during the SIR-C/X-SAR mission, including one of the buried Chicxulub craters in the Yucatan Peninsula, Mexico. This crater was formed by an asteroid or comet striking the Earth more than 65 million years ago and is thought to be linked to the extinction of dinosaurs. The 180- to 300-kilometer-diameter crater is buried by 300 to 1,000 meters of limestone. Although the fracture patterns and wetland hydrology in this region are controlled by the structure of the buried crater, it may be possible to use SIR-C data to help determine the size of the crater, which is a topic of considerable debate.

Another emerging area for SAR study is archeology. For example, a SIR-C image of the region around the site of the lost city of Ubar in southern Oman, on the Arabian Peninsula, was acquired during the April flight. The ancient city was discovered in 1992 with the aid of remote sensing data. Archeologists believe Ubar existed from about 2800 B.C. to about 300 A.D. and that it was a remote desert outpost where caravans were assembled for transporting frankincense across the desert. The actual site of the fortress of the lost city of Ubar, currently under excavation, is too small to be detected in the image; however, tracks leading to the site, and surrounding tracks, appear as prominent, but diffuse, streaks. Since tracks such as these were key to recognizing the site as Ubar in 1992, they are of intense interest and are currently being field checked as to their archeological importance.