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Space Topics: Cassini-HuygensCassini RADARRadio Remote Sensing Instrument for the Cassini OrbiterScientific Objectives - How It Works - More about SAR - Saturn Exploration Context Scientific ObjectivesCassini RADAR is performing radio observations of Titan and the icy satellites to determine their topography and surface properties. Specific goals include:
How It WorksCassini's RADAR instrument is actually the same radio dish that is used for the spacecraft's communications with Earth. To perform measurements on Titan, it is operated in three modes: Radiometry: A passive mode, in which Cassini points at a target and "listens" for radio energy emanating from Titan. Radiometry roughly correlates with temperature, and radiometry results are often described as "brightness temperatures" of surfaces. In this mode, Cassini can create maps of Titan with 7 to 310 kilometer (4 to 200 mile) pixel resolution. Altimetry: An active mode, in which Cassini pings a radio signal at Titan's surface and waits for the echo to return, then pings again, and so on. After the effects of the spacecraft's forward motion and Titan's overall spherical shape are removed from the data, scientists can produce an altimetric profile along Cassini's track over Titan, showing the shape of the landscape. In this mode, Cassini acquires profiles with 24 to 27 kilometer (15 to 18 mile) horizontal resolution and 90 to 150 meter (300 to 500 foot) vertical resolution. Synthetic Aperture Radar (SAR): An active mode, in which Cassini can build up images of the surface. SAR images may be the sharpest that Cassini can achieve on the surface of Titan, with 350 to 1700 meter (1100 to 5600 foot) horizontal resolution. More About SARSynthetic Aperture Radar, or SAR, was employed with great success on the Magellan mission to Venus from 1990 to 1992. Although they look like photographs, SAR images are very different. To collect SAR data, Cassini looks off to one side of the ground track as it flies by, instead of straight down as it does for altimetric measurements. The radio antenna chirps a signal at the planet, then listens for the echo and measures the "delay time," the time between the chirp and the echo. But the echo is not exactly like the original chirped signal. The echo will be stretched out over time, because the signal returns first from the ground closest to Cassini, and later from the ground farther from Cassini. Also, the echo is Doppler shifted to slightly different wavelengths depending upon whether the echo comes from the ground ahead of the spacecraft along its track -- which appears to be moving toward Cassini, shifting the wavelength shorter -- or from the ground behind the spacecraft, which appears to be retreating and thus has a longer-wavelength shift. By processing the data into a map of echo strength in "delay-Doppler space" the RADAR team can see how the strength of the signal varies relative to the spacecraft's position. This processing must be performed for each chirping signal produced by the spacecraft. SAR processing depends on incredibly accurate knowledge of the speed and position of the spacecraft relative to the surface that it is mapping. Instead of the inherent albedo, or brightness of the surface that is seen in optical images, SAR images show the strength of a reflected radio signal. Variation in the strength of a reflected radio signal can result from three main factors: slope, roughness, and "dielectric constant." Slope. If the ground surface is tilting toward the spacecraft, then the radio signal will bounce off the surface and back to the spacecraft, resulting in a strong signal. If the surface slopes away from the spacecraft, then it will reflect the signal off in a different direction, and consequently the surface will appear dark. So if they are oriented the right way, ridges, cliffs, fractures, or volcanic flow fronts can appear radar-bright, while horizontal, flat surfaces will usually appear darker. Roughness. If the surface is rough at the scale of the radio wavelength (that is, if it is blocky or pebbly with block sizes of one to a few centimeters), then the facets of the blocks that are oriented perpendicular to the radio beam will reflect the signal like myriad little mirrors, making the surface appear bright. Dielectric constant. The "dielectric constant" of a material describes how well it reflects radio waves. Water and water ice have a very high dielectric constant, while rocks and hydrocarbons have a low dielectric constant. Understanding what the bright areas on radar images can be a challenge because of these three entangled effects. What's worse, it's probable that the radio waves actually penetrate some distance, 1 to 2 meters (3 to 7 feet) into the icy ground on Titan. SAR data comes in the form of long "swaths," parallel to the spacecraft's ground track, located either to the right or to the left of that ground track. Where the brightness variations are due to topography, the swath appears to be illuminated from the direction of the spacecraft. How Does the Cassini RADAR Fit in the Context of Saturn Exploration?No RADAR mapping instrument has ever been sent to the outer solar system. These instruments are primarily used to map solid surfaces that are obscured by hazy atmospheres: Venus and Earth. On Venus, the Magellan mission mapped the entire planet at a resolution of about 0.5 kilometer (1/3 mile) using Synthetic Aperture Radar imaging. On Earth, the Space Shuttle has carried the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) instrument aloft to map the Earth. However, both Magellan and SIR-C/X-SAR operated at longer radio wavelengths than the Cassini RADAR does. Magellan's radar was operated in the S-band (13 cm wavelength), while SIR-C/X-SAR operated in L-band (23 cm wavelength), C-band (6 cm wavelength), and X-band (3 cm wavelength). Cassini's radar is Ku-band, with a 2.2 cm wavelength. Therefore, there is no extant data that was captured under the same conditions as Cassini's RADAR data, which means that scientists are venturing into uncharted territory as they attempt to interpret it. |
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