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Radar stereo has been mentioned before in this Section. Again, the crux of its workability is, like aerial photography, based on getting two images that are offset. This can happen through multiple passes, the second time moving over nearly the same ground surface as the previous pass. It is also possible to make a usable stereo pair by acquiring images at two incidence angles. A stereo pairing is achievable during a single pass by having two separate but coordinated radar transmitter/receiver systems operating simultaneous from their moving platform. The radars can be of the same band or different bands. The returned signals will show phase differences that can be analyzed by a mathematical technique known as interferometry, which works well when the signals use coherent radiation. Examples are shown.


Radar Stereo; Interferometry


When flown on aircraft, a radar can point in different look directions and angles during successive passes to simulate the necessary parallax conditions for relief displacements across track. Side-looking Airborne Radar (SLAR) observations, looking out from the same side in successive passes, are best. An example showing the Appalachian Fold Belt in central Pennsylvania, using a SAR X-band system, demonstrates the quality of radar stereo.

SAR radar stereo pair image of the Appalachian Fold Belt in central Pennsylvania.

Stereo radar images from space can produce acceptable stereo models using multiple look angle (off-nadir) viewing. Vertical and horizontal resolutions are hard to achieve by optical systems from comparable orbits. We can significantly reduce the vertical exaggeration because of low B/H (base to height ratio) values. The stereo pair below, showing volcanic terrain in Chile, acquired with the SIR-B L-band SAR from two incidence angles (54° and 45°), has moderate exaggeration of a high relief scene.

Stereo pair representing two incidence angles under which the L-band radar on SIR-B operated while passing over Chile.

Multiple incidence angle imagery serves well in preparing perspective views, as demonstrated in this SIR-C oblique rendition of a group of volcanic calderas on two of the Galapagos Islands.

Colorized SIR-C oblique perspective view of a group of volcanic calderas on two of the Galapagos Islands.

Synthetic Aperture Radar images, generated simultaneously from two antennas on the same platform, have such a small base that they do not, in themselves, function as a stereo pair. But being composed of coherent radiation, signals from each, matched to equivalent pixels in the other, show varying degrees of phase differences, indicative of partial interference. Variations in the signal travel times between two adjacent points induce the interference, and thus, show slightly different heights. When we remove Earth curvature effects and make other adjustments, residual differences become a measure of terrain elevations. SAR Interferometry is a developing technique that you can learn more about by perusing another University of Texas review dealing with this subject. A mathematically-based exposition of interferometric principles is embedded in this JPL site.

You can get some idea of these principles from this illustration showing how SIR-C interferometry produced topographic information for this Long Valley, CA test area.

A set of four images of the Long Valley area in the eastern Sierra Nevadas of California; read text for description of information in each panel.

The upper left panel is a Horizontal-Horizontal polarized L-band image, in which many bright-toned patches correspond to hilly rock and dark patches to smooth valley floors. Lake Crowley (dark) is at the lower left. The ridge is part of the Long Valley volcanic caldera (inactive). The upper right panel shows an interferogram derived by combining two L-band images taken in April and October, 1992, respectively. The colors relate to differences in signal phase, caused by elevation differences. In the lower left is a topographic map derived from the interferogram. Its total relief is 1,300 m (4264 ft). The lower right panel presents a 3-D perspective of this map, made by "draping" a C-band image over the L-band topographic data.

Stereo Pairs from Space

Astronauts took the first Visible-IR stereo-pair photos from space during several missions. Landsat was not designed to acquire in stereo, because its downtrack images result from continuous scanning. However, its orbital track results in 10% to 40% (near the poles) sidelap coverage along adjacent orbits (previous or next day), which provides limited area stereo viewing, if cloud conditions are favorable. The top stereo pair on the linked stereo page ([STEREO ]; print out this page in steps from your printer and cut them apart for appropriate overlaying) consists of two complete Landsat MSS Band 7 images taken on October 10 and 11, 1972, thus just one day apart during a time of cloudfree conditions during the continued presence of a pressure High following an earlier storm. After printing and separating, determine where the scenes are geographically, the approximate percentage of sidelap that produces stereo, and the amount of stereo relief (governed by the B/H ratio).

This works also with Landsat images taken along contiguous orbits at various times of year. Check the stereo strips covering part of the now familiar Blue Mountain and other ridges near Harrisburg, PA: the top image (MSS Band 5) was taken in February (defoliate; low sun angle emphasizes relief), whereas that on the bottom was obtained the previous October (foliage still on trees). If you succeed in getting a stereo image using a printout, note the very low vertical exaggeration (about 0.3) because of the high H value (918 km). (STEREO)

Landsat stereo image (A) of the Blue Mountain area just north of Harrisburg, Pennsylvania, taken in February

Landsat stereo image (B) of the Blue Mountain area just north of Harrisburg, Pennsylvania, taken in October.

11-19 Is a low VE (vertical exaggeration necessarily a good thing? ANSWER

Now, try for 3-D in this Landsat false color composite stereo pairing of images of Katmandu and the surrounding Mountains in central Nepal, taken on March 20/21, 1977. The Environmental Institute of Michigan (ERIM) prepared the mount. (STEREO)

False color Landsat stereo image pair of the Katmandu area of central Nepal, March 20/21 1977.

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Primary Author: Nicholas M. Short, Sr. email: [email protected]