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Sir-A, -B, and -C on the Space Shuttle

The next step in NASA/JPL's radar entry into space came from the SIR (Shuttle Imaging Radar) series flown on three Space Shuttle missions. SIR-A used an L-band SAR HH-polarized system, which was capable of 40 m resolution in images whose swath widths were 50 km (31 mi). The near to far range depression angles for this fixed-look radar were 43° and 37°, respectively. These relatively small angles diminished foreshortening and layover effects. This setting has led to some spectacular imagery, such as the Hamersley Range in the Western Australian Shield, which consists of strongly folded Precambrian metasedimentary rocks and older granitic cores.

B/W SIR-A radar image of the Hamersley Range in the Western Australian Shield.

Next, we display a somewhat simpler, but still intriguing, geology, as the basaltic volcanic calderas, Volcan Alcedo (top) and Sierra Negra (left center) on Isabela Island, the largest of the Galapagos.

B/W SIR-A radar image of volcanic calderas on Isabela Island, the Galapagos.

8-17: What is unusual about this image (clue: think of the volcanoes themselves)? ANSWER

Below, the remarkable display of dendritic drainage in the SIR-A image of east-central Columbia results from uplands covered with grass that (because the blades are small) strongly reflects away the radar beam (thus dark), whereas the streams stand out as bright because their tree-lined channels produce double bounce reflections between the smooth water and tree trunks.

B/W SIR-A radar image of east-central Columbia.

One property of radar pulses gave rise to an extraordinary image acquired from SIR-A in November, 1981. The color scene below is a Landsat subimage of the Selma Sand Sheet in the Sahara Desert within northwestern Sudan. Because dry sand has a low dielectric constant, radar waves penetrate these small particles several meters (about 10 ft). The inset radar strip trending northeast actually images bedrock at that general depth below the loose alluvial sand and gravel which acts as though almost invisible. It reveals a channeled subsurface topography, with valleys that correlate to specularly reflecting surfaces and uplands shown as brighter.

Color Landsat subimage of the Selma Sand Sheet in the Sahara Desert of northwestern Sudan with a SIR-A inset, November 1981.

Both Seasat and SIR-A were L-Band radars. They differed mainly in altitude of operation and depression angle: Seasat at 790 km, angle = 67-73°; SIR-A at 250 km, angle = 37-43°; their spatial resolutions were similar. It is interesting to compare the same scene as witnessed by each system (Seasat had a near polar orbit, and SIR-A confined to latitudes less than 38°: look at this view of the California coast and mountain ranges near Santa Barbara.)

Comparison of two B/W images, one produced by Seasat and the other by SIR-A, of the California coast and mountain ranges near Santa Barbara.

8-18: Compare the two radar system images, commenting especially on differences (and why)? ANSWER

SIR-B operated in 1984 over eight days aboard another Shuttle mission. It differed from SIR-A in having a variable look angle that ranged between 15° and 55° . In April and October of 1994, a more versatile system flew twice on Shuttle missions. This system was JPL's SIR-C, which had L- and C-band radars, each capable of HH, VV, HV, and VH polarizations, and an X-band (X-SAR) instrument, supplied by German and Italian organizations, that was in the VV mode. All of these radars had variable look angles that imaged sidewards between 20° and 65° , producing resolutions between 10 and 25 m. One advantage of this multiband system was the ability to combine different bands and polarizations into color composites. JPL's SIR-C Web Site describes how to create composites. You can reach it by clicking here. It contains a wealth of information and imagery, including our next set of images showing the Kliuchevskoi Volcano in Kamchatka (Russian Siberia) as captured by SIR-C in three polarization modes.

False color composite SIR-C image of the Kliechevskoi Volcano in Kamchatka.

Next, we show a multiband (multifrequency) image of San Francisco, CA, made from L bands HH (red) and HV (green) and C band HV (blue). It’s one of the most pleasing images to the eye and it shows the city layout, which is a prime example of why so many people want to live in the Bay Area after visiting it.

Color multiband SIR-C image of the San Francisco Bay area.

8-19: Name the bridges you can find in this image.

We can process SIR-C radar imagery taken on two dates (or with two antennas) using interferometric techniques that use signal phase differences to determine differences in distance to point targets to yield information on topographic variations. When combined with Digital Elevation Model (DEM) data (see Section 11), single band or color composite radar images can show perspective views (also, Section 11), as analglyphs (requiring stereo color glasses), or even in simulated flyby videos. A perspective view of Death Valley and adjacent mountains made from SIR-C imagery is a good example.

Perspective view of Death Valley and adjacent mountains produced from SIR-C imagery.

In conjunction with the SIR-C program, JPL flies an airborne system called AIRSAR/TOPSAR. From this system, we present a multiband perspective view of the mountains just north of JPL's home in Pasadena, CA.

Multiband perspective view of Pasedana, California, produced from SIR-C imagery.

Topex/Poseidon; ERS; JERS; Radarsat

NASA/JPL, in conjunction with the French Center for National Space Studies (CNES), has placed a radar altimeter in space on the Topex/Poseidon mission launched on August 10, 1992. Pointing straight down (at nadir), this dual frequency (13.6 and 5.3 GHz) instrument transmits a narrow beam whose variations in round-trip transit time represent changes in altitude or (for oceans) wave heights along the 3-4 km swath line (successive lines are spaced about 345 km [214 miles] apart at equatorial crossings). The Topex altimeter can discriminate elevation differences of 13 cm. They operate Topex primarily for oceanographic studies, measuring the effects of wind on waves, and the influence of currents and tides on marine surfaces, and relating these to global climate change mechanisms. A second French altimeter and a microwave radiometer (for atmospheric water measurements) are among the six instruments onboard.

The Europeans and Japanese have now flown radars on unmanned space platforms. Some information about the European mission is available in JPL's Radar Home Page under the topic Earth Resources Satellites (ERS). The European Space Agency (ESA) launched ERS-1, with a complement of sensors, in July, 1991, at a nominal altitude of 800 km (500 mi). Along with a radar scatterometer, it carried a C band, VV SAR with a fixed look angle extending from 20° to 26°.

Artifically colored ERS-1 SAR image of the Cote d'Azur on the French Riviera.

A nearly identical SAR was on ERS-2, which launched in April, 1995. We can construct innovative color composites from the single band, single polarization radar by using images from different dates. We show here an example of this process: a multi-date image of Sevilla, Spain, in which an ERS-1 image on Nov. 3, 1993, is assigned to red; an ERS-1 image on June 9, 1995, is green, and an ERS-2 image on June 10, 1995, is blue. The city of Seville appears in a cyan tone in the upper right, as does the Sierra de Aracena in the upper left, and agricultural fields (bright, but barren) show as red in the rest of the image.

Color composite ERS SAR image of Sevilla, Spain, taken across several dates from both the ERS-1 and ERS-2 sensors.

JERS-1, launched to a 570 km (354 mi) orbit on February 11, 1992, by the Japanese National Space Agency, contained a seven-band optical sensor and a SAR. The latter was L band with HH polarization. It had a fixed look angle view between 32° and 38°, yielding a swath width of 75 km and a mean resolution of 18 m. One of its first images covered Mt. Fujiyama (volcano) west of Tokyo, as shown here.

B/W JERS-1 SAR image of Mt. Fujiyama west of Tokyo.

8-20: Here is a puzzler. I have determined that the very bright patches are the small city of Fujiyoshida. How did I do that (remember to rely on your World Atlas).ANSWER

As part of its ongoing program, the Canadian Space Agency on November 4, 1995, launched its first Radarsat into a near-polar orbit at a height of 798 km (about 500 mi). This is a C-band SAR, whose look angle can range between 20° and 50° to provide swath widths between 35 and 500 km (22 to 311 mi), providing variable resolution centering around 25 m. The first image collected covered Cape Breton in northern Nova Scotia. As with most of the other systems previously described, users can convert images from Radarsat using separate topographic data into perspective diagrams.

B/W Radarsat SAR image of Cape Breton, Nova Scotia.

The advent of radar systems into space, following their effective demilitarization worldwide, provides the remote sensing communities with a powerful source of environmental and mapping data that are obtainable over any part of the Earth. With altimeter or interferometric processing, radar presents a new capability to generate topographic maps for parts of the global land surface, viewable from near-polar orbits. Information on several aspects of ocean surface states is also a valuable payoff. The prospects of using multi-frequency, multi-polarization beams to obtain distinctive radar signatures offers another means to identify materials that are separable on the basis of dielectric constants, surficial roughness, and other properties.

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Primary Author: Nicholas M. Short, Sr. email:

Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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