Radar has been a cornerstone of military hardware since WWI. The concept is simple: microwave radiation is transmitted and an antenna measures the reflected energy. Unlike optical remote sensing, microwaves ignore bad weather and could care less about whether it's day or night; however, these systems hide as much as they reveal. Unlike the camera in the sky, the radar operator knows something is there, but can't tell what it is.
Just as an optical microscopist has difficulty resolving an object that is smaller than the wavelength of illuminating light, radar is inhibited by the size of the emitting antenna. Add to that the fact that microwave radiation is somewhere around 100,000 times longer than optical wavelengths, and it quickly becomes apparent that radar can resolve the proverbial "side of a barn," but that's about it.
Since the 1950s, scientists have worked on the problem of how to give radar the ability to resolve images with optical precision while remaining immune to environmental conditions. The answer was not so straightforward. Reducing the transmitted wavelength increased the system's resolution, but reduced its ability to penetrate clouds, fog, and even foliage. The answer lay along a different path called Synthetic Aperture Radar (SAR).
Creating a larger antenna
SAR is similar to using an antenna array without the array. A single antenna, generally on a space- or flying platform, sends out a wide beam of polarized microwave radiation at a series of points along a path. Because the radiation is coherent and always in phase upon transmission, the backscatter at each point is collected and then combined with the other data-sets to create high resolution images.
According to Dave Munson of the University of Illinois at Champaign-Urbana, SAR shares some characteristics with optical remote sensing in that the resulting image is a 2D Fourier transform. In fact, early process techniques used optical Fourier transforms to translate the return signals from the SAR into an image. Today, however, improvements in digital technology allow the data to be automatically converted to images that make sense to the human eye.
Early techniques developed by researchers at Goodyear Aerospace kept the radar antenna at one attitude in relationship to the ground and "strip mapped" the land below, usually for military surveillance or battle planning. Newer techniques, such as "spotlight" mode SAR, direct the antenna to a single spot over and over again as the antenna passes high above. By carefully measuring the changes in reflected and polarized light -- and by increasing the frequency from a few gigahertz to several hundred or even a thousand gigahertz (x band) -- intelligence agencies can get SAR images with 1-ft. resolutions regardless of whether it is cloudy, rainy, or dark, explained Munson. Interestingly, spotlight mode shares common methodologies and mathematics with optical tomography. "When a person learns about SAR they learn about a lot of [optical] things whether they know it or not," said Munson.
The military continues to develop new applications for SAR. In addition to using SAR to increase the optical resolution of astronomical images -- such as the moon's surface -- Munson's laboratory is exploring new areas as well: inverse SAR and identifying moving targets.
Inverse SAR is a completely new field where an antenna passively listens for reflected radiometric energy. Munson and his students at the University of Illinois are exploring ways to use television and radio signals that are always present to image objects on the ground -- specifically military targets.
Moving targets create different problems for typical SAR sensing. Because SAR data is collected over time, if the object moves during the collection period then combining the data into a picture can prove nearly impossible. According to Charles "Jack" Jackowatz of Sandia National Laboratory (Albuquerque, NM), interferometric SAR (IFSAR) techniques can alleviate this blurring by placing several antennas along a single platform, each looking down at a slightly different angle. Because the signals are in phase with one another, SAR can better detect the position of moving objects by looking at the phase differences in the return signals.
Another type of interferometric technique involves vertical IFSAR instead of horizontal. Used for high-precision terrain mapping, this method simply places one antenna farther away from the object in vertical space. According to Jackowatz, "You form an interference map as you would with laser interferometry in a nondestructive testing lab to look at the surface defects on a part."
Vertical IFSAR has produced terrain maps of open ground and tree canopies to within a few inches, with an order of magnitude better than the best stereoscopic optical techniques. This information is extremely useful for mission rehearsal, battle planning, and even for developing high-precision terrain models for guided munitions. Jackowatz's lab has even demonstrated multipass interferometric techniques where a single antenna makes several passes to develop the data for later interferometric interpretation. While this works fine for open ground, he said, tree canopies or any kind of moving areas do not produce good images because of temporary decorrelation.
Other groups are actively working on applications that specifically deal with trees. Munson said that, by going to a lower frequency (500 MHz), boosting the bandwidth, and increasing the number of angles from which an antenna images the object, SAR can actually see through tree canopies to detect targets hiding underneath. "This is a very exciting and active area of military research," Munson said. However, he added that the technique is limited by the large number of angles needed to resolve a particular object.
Trees are also at the heart of a growing number of applications that have nothing to do with munitions or battle plans. In the 1980s, Europe and Canada began to build the first SAR satellites for commercial applications, expanding the way the world looks for natural resources.
From munitions to natural resources
European and Canadian governments have been exploiting the powers of SAR for commercial benefits for nearly 10 years. In 1991, The European Space Agency (ESA) launched the European Remote Sensing satellite. ERS-1 contains several sensing systems, including radar. ERS-2 was launched approximately four years later. Riding at a slightly higher orbit and tracking exactly 24 hours behind ERS-1, radar systems on the two satellites can generate interferometric SAR data with 24-hour turnaround for geophysical and geodetic models.
At about the same time ESA began exploring the ERS projects in the 1970s, Canadian officials began looking for ways to exploit arctic resources. When oil prices dropped in the 1980s, the RADARSAT project seemed destined for a bookshelf, but officials determined that SAR could be used for more than strictly oil exploration. The resulting launch in 1995 gave the world a highly flexible SAR instrument, capable of a wide variety of angles from 11 to 60 degrees and resolutions between 8 and 100 m.
Similar to the ERS satellites, RADARSAT is capable of both sending C-band horizontally polarized microwaves, and receiving the same. According to Marc D'Iorio of the Applications Development Section of the Canada Centre for Remote Sensing (CCRS), RADARSAT has been used for a wide variety of applications: for example, providing raw geological data to mining companies in tropical areas where weather is often a factor for remote sending, and monitoring floods and other natural disasters in the U.S.
SAR, which is very sensitive to water, is also used to map ice floes in the St. Lawrence waterway during the winter and in the arctic during the summer. The Canadian government's Ice Services in Ottawa uses SAR data to determine whether these waterways are navigable and what types of ice are present. Updates are issued to vessels within hours of receiving the satellite data.
Both the ESA and the CCRS plan to launch next-generation SAR satellites by 2001. The ESA's satellite for environmental monitoring, ENVISAT, will hold a bipolar SAR unit capable of sending either horizontally or vertically polarized microwaves and receiving both polarization (HH and HV data or VH and VV), increasing its ability to resolve and differentiate objects. The Canadian government intends to launch RADARSAT II later in 2001. This second satellite will be quadpolar, capable of sending and receiving both horizontally and vertically polarized energy. "We'll be able to get up to 3-m resolution," D'Iorio said. The greater number of bands will also mean increased ability to differentiate between species of trees and crops and an increased confidence for search and rescue of floundering ships, ice detection, or other target identification.
The utility of multiband, multipole transmissions has been proven in the past. In 1994, the most recent SAR experiment, Spaceborne Imaging Radar C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR), flew on the space shuttle, collecting 37 terabits of image data. By using multiple transmission wavelengths (C and X bands) with different polarizations, scientists were able to get extremely precise environmental measurements of the equatorial rain forest. In that situation, multiband transmissions and a low orbit resulted in very high resolutions of individual trees -- so high that horizontally polarized light was used to explore the branches while vertically polarized light revealed information about the tree trunks.
Scientists around the world, such as those at the Jet Propulsion Laboratory (Pasadena, CA), the Univ. of British Columbia and others, continue to pore over new data coming from ERS-1 and ERS-2. New generations of SAR satellites will provide even more detailed information on Antarctica's melting ice caps, deforestation, mineral deposits, and even archeaology. As these and even newer projects come online, radar imagery will continue to reveal the world -- whether the lights are on or not.
R. Winn Hardin
R. Winn Hardin is a science and technology writer based in Fairbury, NE.