The follow-on to Seasat by NASA-JPL was the SIR (Shuttle Imaging Radar) series: SIR-A; SIR-B; and SIR-C (flown twice on the Shuttle. They differed in bands used, in depression angles, and polarization modes. These different operating conditions produced images that could be made into color composites and, in some instances, into stereo pairs from which perspective views could be constructed, using altimeter data. The next system developed by JPL was TOPEX-POSEIDON (on a satellite), with a strong emphasis on gathering data sets from which topographic information is the output. Other nations have also launched satellites with radar systems onboard. Examples from all of these are presented.
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 this color version of folds in the eastern edge of the Atlas Mountains in Morocco.
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. 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. 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. 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 was confined (by Shuttle orbital configuration) to latitudes less than 38°. Look at this pair of views of the California coast and mountain ranges near Santa Barbara, with Seasat on top and SIR-A on bottom. 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°. Here is a SIR-B image L-band image, taken at a 28° incidence angle of a forested area in northern Florida.
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 (L-band HH = blue; L-band HV = green; C-band HV = red); on the left is a photo of the volcano taken at the same time by one of the Shuttle astronauts.
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. 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 page 11-5), single band or color composite radar images can show perspective views (page 11-8), as analglyphs (requiring stereo color glasses) (page 11-10), or even in simulated flyby videos. A perspective view of Death Valley and adjacent mountains made from SIR-C imagery is a good example. The X-SAR instrument on SIR-C was supplied by Deutsches Zentrum fur Luft und Raumfahrt (DLR). This X-band (3 cm) radar operates in the VV mode. Here is an image of Hong Kong and adjacent mountains. Note the many ships in the waters near the city (Kowloon).
The next image was made from all three SIR-C bands: X-band = blue; C-band = green; L-band = red. The scene shows the city of Samarkind in Russia along the Volga River. 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.
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 of pulses 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 data gathered by TOPEX are not normally displayed as images but are used to produce maps of regions or even global hemispheres, as illustrated in the example below (see page 14-12 for other examples). International Radar Systems: ERS; JERS; Radarsat 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°. This next scene shows a color composite, made from multidate images, of the farmlands (including vineyards) along the Rhine near the city of Darmstadt. 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. 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. 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 By way of comparison, here is a false color composite of Fujiyama made by the Japanese MOS-1 (Marine Observation Satellite; see page 14-12). 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. Another radar system, the SRTM (Shuttle Radar Topography Mission) flown in the year 2000, will be discussed in a full page (11-10) in Section 11. 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.
Primary Author: Nicholas M. Short, Sr. email: nmshort@epix.net