Larger than Life

Sensing with Synthetic Aperture Radar
Contributed editorial appearing in
Scientific Computing & Instrumentation 20:11, October 2003, pg. 9.

I recently glanced at the standard clipart collection included with Microsoft Office 2000 and discovered the Science & Technology category contains seven images. Two of them depict computers. The others are of a group meeting, a centrifuge containing test tubes, a microscope, a telescope and a financial graph. Not the cornucopia required for a publishing house but, after consideration, a good attempt at generalizing the subject. The two "scopes" have long been a hallmark of scientific instrumentation and getting a good look at things has lead to many revolutionary discoveries.
Making small things appear larger and distant things appear closer permits their study without leaving the laboratory. However, we also require a scope that makes large things, such as the earth, appear smaller. Having a look at the big picture enhances our study of glaciers, coastline erosion, ocean waves, forest fires, volcanic eruptions, land management and urban planning.

Since we can't fit the earth on the stage of our microscope, we are forced to place our imaging equipment at extreme altitude in order to bring it into our field of view using high-flying aircraft or orbiting satellites. Our next choice is which wavelengths of light to use. We could use the visible spectrum, but not without difficulty. Unless we have a powerful flashbulb, we would only be able to take pictures during daylight hours. Secondly, even moderate cloud cover would obscure our view. Alternatively, the water droplets in fluffy clouds and dense thunderstorms are relatively transparent to long-wavelength radio waves that can be projected and reflected in narrow, low-power beams, permitting 24-hour data collection. On the other hand, the typical 25-micron by 25-micron charge-coupled device (CCD) pixel of a visible camera must be replaced by a 10-meter by 1-meter radio antenna. At a resolution of 1024 × 768, the radar antenna would need to be several kilometers on a side.

Determined to overcome this requirement with technology, radar engineers used a few of the group meetings, some of the financial graph, and a large collection of computer clipart to develop an ingenious solution to their problem. Since the aircraft and satellite-mounted radar antennas were moving relative to the earth, a series of radar pulses could be collected in rapid succession and reconstructed in software to create a multi-pixel image ostensibly collected by a large-aperture synthetic radar antenna. The resulting "synthetic aperture radar" or "SAR" image exhibits the image resolution of a much larger instrument.

For instance, the 10 × 1-meter antenna of the ERS-1 satellite collects around 1000 images while traveling a distance of 4 kilometers, resulting in a virtual 4-kilometer-wide radar array capable of resolving a 30-meter ground-based object. The data analysis is performed back on earth using powerful digital signal processing (DSP) hardware. As is often the case, the data processing is simplified by manipulation in frequency rather than temporal space. Ground-based objects in front of the moving SAR antenna reflect a different Doppler frequency shift than objects already passed over and this frequency information is utilized to increase the radar resolution. The SAR system is calibrated for size by placing radar reflectors of known dimensions and geometry on the ground, typically tetrahedron corner cubes a few meters in width, and can be calibrated for flat-field intensity by imaging the Amazon rain forest.

SAR is finding increasing utility in the global war on terrorism. The Sandia National Laboratories Web site contains examples of military SAR images having 1-foot resolution for reconnaissance, surveillance, and targeting. Very low-frequency SAR can be used to image objects hidden under foliage or camouflage nets and buried under sand and soil. It is finding utility in the discovery of underground power and communication lines, arms caches, bunkers, and even land mines. It is being used commercially for aircraft navigation, permitting pilots to have a crisp, detailed view of the approaching runway even at 1 a.m. in a heavy fog. Given the recent advent of 3-D SAR interferometry and its emerging application as a medical imaging diagnostic the SAR clipart image can't be very far behind.