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The Cloud Absorption Radiometer measures scattered light in 14 spectral bands. + Larger image (opens in a new window) |
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The CAR instrument is housed in the nose of the CV-580 aircraft. In this photo, CAR's stainless steel sliding door is in the closed position, protecting the scan mirror and optics assembly from dust and other foreign objects during takeoff and landing. + Larger image (opens in a new window) |
The CAR scan mirror scans 360° in a plane perpendicular to the direction of flight and the data are collected through a 190° field of view. In the normal mode of operation onboard the CV-580 aircraft, the CAR views 190° of earth-atmosphere scene around the starboard horizon. This configuration permits observations of both local zenith and nadir with as much as a 5° aircraft roll. In addition to the starboard viewing mode, the CAR instrument can now be rotated in-flight into three other viewing positions: downward-looking imaging mode, upward-looking imaging mode and a dedicated BRDF viewing mode.
The instrument incorporates several novel features. Since it is sometime flown through clouds, there is the possibility that moisture may be deposited on optical surfaces, especially the scan mirror, producing large errors. The instrument is mounted outside the airplane and cannot be observed in flight. In order to check for water on the mirror, a unique detection system was devised. A thin beam of light is shone on the edge of the mirror, and the reflected beam is monitored by a photodiode. If any condensation appears on the mirror the reflected light scatters, reducing the photodiode's output and flagging data likely to be in error. Another novel feature maintains low offset (ensuring that zero volts at the output always correspond to a zero-radiance input) by using the scan mirror as a type of radiation chopper. It works by forcing the electrical output to zero during each backscan while the detectors are all completely darkened by means of a scanner-synchronized moveable shutter. Long time constant coupling in the amplifier then ensures that data measured during the active part of the scan remains accurately related to this zero reference level.
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Table 1: Summaries of the characteristics of the CAR sensor, platform and scanning system. |
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Sample digital output of a complete scan cycle from one of the CAR spectral channels. |
The CAR has undergone several modifications and improvements over the years, including:
- Operator-control of the gain of all channels, enabling signal levels to be optimized for various latitudes and lighting conditions
- Replacement of the original liquid-nitrogen-cooled longwave InSb detector with a self-contained electrically cooled (Stirling cycle) dectector
- Extension of the spectral range into the ultraviolet
Improvement in speed and accuracy of the calibration process by developing an automatic channel/gain stepper and boxcar integrator
Further information can be found in King, M. D., M. G. Strange, P. Leone and L. R. Blaine, 1986: Multiwavelength scanning radiometer for airborne measurements of scattered radiation within clouds. Journal of Atmospheric Oceanic Technology, 3, 513-522.
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