Creating 3-D images of the Earth's surface will require the first on-orbit use of single-pass interferometry, which means these topographic snapshots will take just one pass by the Shuttle, using the dual antennas. The Shuttle Radar Topography Mission will attempt to make close to 1 trillion measurements during the 11-day mission.

A reflection of the C-band and X-band Swaths

The power required to operate the Shuttle Radar Topography Mission and its associated equipment will push the edge of the Shuttle's generating capability. The payload will need 900 kilowatt-hours, enough to power a typical home for 2-3 months.

All data will be recorded onboard the Shuttle using payload high-rate tape recorders. Data will be recorded at a rate of 180 megabits per second for the C-band radar and 90 megabits per second for the X-band radar. The total data expected to be recorded are nearly 10 terabytes, enough to fill 15,000 compact disks. The data will be recorded on 300 high-density tapes.

Verifying that the data are properly recorded will be a challenge in itself, as data will be generated at a rate of four times the speed at which it can be downlinked from the Shuttle to the ground. Some data will be played back from the tape recorders at 1/4 speed and downlinked to the ground using the Shuttle Ku-band link to NASA's Tracking and Data Relay System satellites.

History/Background

What Is Imaging Radar?

Since radars provide their own illumination, they can image regions of the world at any time of the day or night. Also, because the radar wavelengths are much longer than those of visible or infrared light, synthetic aperture radar imaging also can "see" through cloudy and dusty conditions that would blind visible and infrared instruments.

An imaging radar works very much like a flash camera. A flash camera sends out a pulse of light--the "flash"--and records on film the light that is reflected back at it through the camera lens. Instead of a camera lens and film, a radar uses an antenna and digital computer tapes to record the reflected pulses of radar "light" that comprise its images. In a radar image, one can see only the light that was reflected back toward the radar antenna.

A typical radar (an acronym for radio detection and ranging) measures the strength and round-trip time of the microwave signals that are emitted by a radar antenna and reflected off a distant surface or object. The radar antenna alternately transmits and receives pulses at particular microwave wavelengths (in the range of 1 centimeter to 1 meter, which corresponds to a frequency range of about 300 MHz to 30 GHz). For an imaging radar system, about 1500 high-power pulses per second are transmitted toward the target or imaging area, with each pulse having a pulse duration, called a pulse width, of typically 10-50 microseconds.

At the Earth's surface, the energy in the radar pulse is scattered in all directions, with some reflected back toward the antenna. This so-called "backscatter" returns to the radar as a weaker radar echo and is received by the antenna. These echoes are converted to digital data and passed to a data recorder for later processing and display as an image.

Radar transmits a pulse and measures reflected echo or backscatter. In the case of imaging radar, the radar moves along a flight path, and the area illuminated by the radar is moved along the surface in a swath, building the image as it moves along building up a radar image using the motion of the platform.

The length of the radar antenna determines the resolution in the azimuth direction of the image, or along the track of the swath that is being taken. The longer the antenna, the finer the resolution will be in this dimension.

Synthetic aperture radar refers to a technique used to synthesize a very long antenna by combining echoes received by the radar as it moves along its flight track. "Aperture" refers to the radar antenna. A "synthetic" aperture is constructed by moving a real aperture or antenna through a series of positions along the flight track.

How Are Radar Images Produced?

Radar images are composed of many dots, or picture elements. Each pixel, or picture element, in the radar image represents the radar backscatter, or the radar pulses that are reflected back from a surface. Darker areas in the image represent low backscatter, brighter areas represent high backscatter. Bright features mean that a large fraction of the radar energy was reflected back to the radar, while dark features imply that very little energy was reflected. Backscatter for an area at a particular wavelength will vary for a variety of conditions, such as the size of the objects being imaged in the desired mapping area, the moisture content of the area, the polarization of the pulses, and the observation angles. Backscatter will also differ when different wavelengths are used.

The rule of thumb in radar imaging is that the brighter the backscatter on the image, the rougher the surface that is being imaged. Flat surfaces that reflect little or no microwave energy back toward the radar always will appear dark in radar images. Vegetation is usually moderately rough on the scale of most radar wavelengths and appears as gray or light gray in a radar image. Surfaces inclined toward the radar will have a stronger backscatter than surfaces which slope away from the radar.

Some areas not illuminated by the radar, like the back slope of mountains, are in shadow and will appear dark. When city streets or buildings are lined up in such a way that the incoming radar pulses are able to bounce off the streets and then bounce again off the buildings--called a double-bounce--and directly back toward the radar, they will appear very bright, or white, in the radar images. Roads and freeways are flat surfaces, so they appear dark. On the other hand, buildings, which do not line up so that the radar pulses are reflected straight back, will appear light gray, like very rough surfaces.

Imaging Different Types of Surfaces With Radar

Backscatter is also sensitive to a mapping area's electrical properties, including water content. Wetter objects will appear bright, and drier objects will appear dark. The exception is a smooth body of water, which will act as a flat surface and reflect incoming pulses away from a mapping area. These bodies will appear dark.

Different observation angles will affect backscatter. The angle of the track will affect backscatter from very linear features, such as urban areas, fences, rows of crops, and fault lines. The angle of the radar wave hitting Earth's surface, called the incidence angle, also will cause a variation in the backscatter. Small incidence angles, which are nearly perpendicular to the surface, will result in high backscatter, whereas the backscatter will decrease with increasing incidence angles.

Radar backscatter is a function of the incidence angle.

What Is Radar Interferometry?

Radar interferometry is the study of interference patterns created by combining two sets of radar signals. There are several ways to explain how interferometry works. The following are two ways to explain radar interferometry.

Ripples in Still Water

If one imagines a person standing with both arms extended to his or her side, holding a pebble in each hand, then dropping the pebbles into a puddle of water, two rippling, concentric circles would emanate from the splash of the pebbles in the water. As the two waves travel outward, they will eventually hit each other and cause interference patterns. These interference patterns, where the two sets of waves meet, are the pulses that are measured by an interferometer. The dual measurements will allow scientists to build a single, three-dimensional image.

SIR-C Multiple-Pass Radar Interferometry

When two radar data sets are combined, the first product to be created is called an interferogram, which is also called a "fringe map." A fringe map looks similar to the ripples in a puddle of water. The main antenna located in the Space Shuttle payload bay will illuminate a portion of the surface of the Earth as it passes over, transmitting a radar wave, much like the ripples in a puddle of water that has been disturbed. When the radar wave hits the surface of the Earth, it will be scattered in various directions. These scattered waves will be collected by the two Shuttle Radar Topography Mission antennas. Using the information about the distance between the two antennas and the differences in the reflected radar wave signals, scientists will be able to obtain very accurate measurements of the heights of land surfaces.

Holograms

SRTM is basically producing a hologram of the surface of the Earth. Holograms are usually produced by shining a laser beam onto an object and recording the patterns resulting from the reflected light interfering with a reference laser beam. The result is a 3-D representation of the object on film. It turns out that radars are just like lasers, except they operate in the long wavelength microwave part of the spectrum.

Benefits

Science and Technology Applications

Geomorphology is the study of Earth's landscapes. These geologic formations are all around us, standing as huge mountain ranges, carved deep into the Earth to form valleys, and stretching flat across thousands of miles of land to create plains.

Plate boundaries cut through continents and oceans and are concealed by them. However, titanic geological events along these boundaries offer clues to their locations. Where plates converge, mountains and volcanoes are often found. Where they pull apart, oceans are born. Wherever they grind against each other, they are jostled by frequent earthquakes.

Digital topographic data of mountain ranges, which will be available for the first time with the retrieval of Shuttle Radar Topography Mission data, will allow geologists to test new models of how mountains form and determine the relative strength of the forces that uplift and crumple mountains and the erosive forces which polish and reshape them.

Lower resolution digital topographic data, available only in the last few years, have yielded some surprising results. It seems that landslides in mountainous areas are responsible for far more of the erosion than previously thought, causing revision of many basic ideas of mountain development. Even more surprising, models based on new digital data have shown that erosion of deep valleys into mountain ranges actually causes the adjacent peaks to rise in elevation due to the buoyant force of the underlying mantle.

As with most disciplines, archaeology has become more interdisciplinary, using cutting-edge technological tools in parallel with detailed field work. Increasingly, archaeologists are studying sites and human activity within their regional context to determine how the sites relate to each other and how they relate to the changing landscape.

This more regional view helps answer questions such as why cities and towns were built in particular locations and how the patterns of settlement relate to natural resources in the area. To do this, it is important to look at why events occurred when they did and how those events might have changed over time. Scientists are intensely interested in examining the interactions of people with the land they inhabited and exploited over time in an effort to explain the changes that occurred in both human societies and in the natural environment.

The Shuttle Radar Topography Mission will provide archaeologists with a topographic view of both ancient sites and the current landscape, which they can use to help determine the boundaries of original sites. They also will be able to learn how and where these sites fit into the regional landscape, as well as probable migration routes through topographic barriers such as mountain ranges.

Shuttle radar data also will enable them to compare large-scale ancient settlement patterns and their distribution around the world. Since many archaeologists working in remote parts of the world rely on outdated maps or no maps at all to conduct these studies, the Shuttle Radar Topography Mission's highly precise 3-D data will provide many with their first comprehensive tools.

Ecology concentrates on the interrelationship of living things and their environment. As civilization and technology advance, people have learned to modify the environment. Human activity has had enormous repercussions, changing ecosystems and depleting natural resources. People use vast amounts of energy and produce massive amounts of waste and exhaust. It is critical that scientists understand the impact humanity is having on planet Earth and that better tools be developed to accurately measure changes in world climate, temperatures, habitats,and species.

Global climate change is another large-scale event occurring in the atmosphere, brought about by the increase of so-called "greenhouse gases" such as carbon dioxide. Like glass in a greenhouse, these gases admit the sun's light but tend to reflect the heat that is radiated from the ground below back down to the ground, trapping heat in Earth's atmosphere.

Scientists continue to work on computer models of climate change to determine how much of an increase in greenhouse gases is occurring in Earth's atmosphere. Shuttle Radar Topography Mission data will allow them to develop more accurate models of the global circulation of the atmosphere.

Mapping of the world's rainforests is an essential ingredient in global protection of Earth in the next century. Another avenue of investigation during the Shuttle Radar Topography Mission will focus on radar-imaging of fragile habitats, such as Earth's tropical forests, to assess vegetation types and determine terrain characteristics. Terrain data that will be collected during the Shuttle Radar Topography Mission will provide near-global-scale coverage of these ecosystems at a much higher resolution and allow scientists to study tropical rainforests in more detail. Combined with data from other remote sensing satellites, three-dimensional data of landforms, waterways, and other types of vegetation will contribute to their understanding of a region's overall health.

Communities nestled near the bases of active volcanoes or on earthquake faults will be of interest to volcanologists and seismologists as well. Scientists can use three-dimensional topographic maps to study the potential of natural hazards. In addition to volcanic eruptions and earthquakes, regions prone to severe flooding by major rivers will be of interest.

Radar imaging will be used as a tool for city planners, land management, and resource conservation, efforts which require highly detailed topographic maps for monitoring land use patterns. Spaceborne radar imaging systems can clearly detect the variety of landscapes in an area, as well as the density of urban development. Examples of previous land management surveys included imaging of major world cities, such as Los Angeles, New York, and Washington, D.C.

Commercial Applications

Some of the commercial products that will be possible using Shuttle Radar Topography Mission data will benefit the transportation industry, as well as the communications and information technologies markets. In telecommunications, wireless service providers and operators will be particularly interested in this digital elevation data. Topographic data can be used for building better transceiver stations and identifying the best geographic locations for cellular telephone towers.

Companies conducting geological and mineral exploration, as well as hydrological and meteorological services, including risk assessment, also will be interested in the data. Providers of satellite data, tourist and leisure maps, and virtual reality software also will reap the benefits of these data, which can be integrated into an absolute geographic grid system that will make all data products uniform and consistent. In fact, just about any industry that requires accurate digital elevation data stands to benefit from this mission.

Terrain height data also may be a valuable addition to current aircraft navigational tools to assist pilots in takeoffs, landings, and pinpointing their locations during flight. Ground collision avoidance systems will become far more accurate with new measurements and topographic maps of Earth's terrain derived from the Shuttle Radar Topography Mission. Flight simulators for crew training will have realistic backgrounds and, by adding information from inflight global positioning system receivers, will become state-of-the-art reference systems, giving pilots a set of "virtual eyes" for use in bad weather or at night.

Automobile navigation displays and digital road maps also will benefit from terrain information provided by the Shuttle Radar Topography Mission. Here terrain height is required, combined with accurate data about the horizon. The newly acquired data will be made available to commercial users and tailored to their specific needs.

Defense Applications

The National Imagery and Mapping Agency (NIMA) plans to use the digital global terrain elevation maps for planning, rehearsal, modeling, and simulation for military and civilian uses. Successful completion of the SRTM data set will provide NIMA with coverage of most of Earth's populated land areas, with three times better resolution than previously available.

Additional information about Defense Department applications is available from NIMA, a partner in the Shuttle Radar Topography Mission.

Source: NASA