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[177] THE VIKING ORBITER cameras evolved from the cameras flown on the earlier Mariner spacecraft. Each generation of spacecraft sent to Mars has featured cameras with vastly improved capabilities, especially higher resolution and increased light sensitivity. Basically the cameras are high-performance vidicons, similar to those used in television cameras on Earth, and have a telephoto lens assembly in front of them. Each orbiter has two identical cameras. The cameras, along with instruments to measure the surface temperature and amount of water vapor in the atmosphere, are mounted on the science platform, a device that can be moved about two axes to achieve a scanning motion when viewing the Martian surface from orbit. The drawing shows the arrangement of instruments on the science platform.


Each camera, along with its 475-mm focal length telephoto lens, has a field of view of 1.54° X 1.69°. From an orbital altitude of 1500 km, each frame covers a minimum area on the surface of 40 X 44 km. Six selectable filters allow color images to be acquired. Acquisition of a frame and subsequent readout to the tape recorder requires 8.96 seconds, so a frame is taken on alternate cameras every 4.48 seconds. This alternating pattern, coupled with motion along the orbit, combines to produce a swath of pictures, as shown in the illustration of orbiter imagery coverage.


The image on the vidicon is scanned, or read out, as 1056 horizontal lines. Each line, in turn, is divided into 1182 pixels (picture elements), and the brightness of each pixel can range from 0 to 127 arbitrary units. Thus, to record a single frame requires the storage of almost 10 million bits (binary digits) on the orbiter tape recorder. The pictures are stored on the tape recorder in digital form until there is an opportunity to play back the data over the orbiter communications system to a receiving station on Earth.


As the data are received on Earth they are subject to computer-based image processing. All images receive first-order processing that consists of the following: noise removal, contrast enhancement, and shading correction. First-order images are the most widely used for scientific analysis and are the version most commonly seen in this book. Some images also undergo a more complex second-order processing that includes all of the first-order [178] processing plus sophisticated procedures to remove geometric and radiometric distortions and merge black and white images taken through different wavelength filters into a composite color image. Second- order processing can also include techniques such as generating stereoscopic pairs and picture differencing in which an image taken at one time is digitally substracted from another image of the same scene at a different time to highlight any changes that may have occurred during the interval between the images.