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Numerous reports have demonstrated the applicability of thermal infrared imagery surveys to geologic, environmental, and agricultural problems.
Most thermal infrared imaging for geologic purposes is flown at night to avoid the effects of differential solar heating. Existing aeronautical navigation aids are designed for point-to-point flights rather than the precisely spaced and oriented parallel flight lines required to cover a project area. The traditional method of navigating nighttime imagery flights requires personnel with signal lights to occupy a sequence of ground control points and this method has numerous drawbacks.
A newly developed aircraft navigation system utilizes the 9 very low-frequency U.S. Navy radio transmitters located around the globe to fly very accurate courses with no ground aids. Using this system at night to navigate from the base air strip to a known starting point within the project area, parallel survey lines up to 50 mi long have been flown with precise spacing to obtain the necessary imagery sidelap for preparing mosaics. Comparison of actual aircraft flight lines (obtained by plotting nadir line of imagery onto a map) with the programmed flight lines indicates a satisfactory agreement. The navigation system also provides real-time readout of true ground speed which is essential for processing of imagery magnetic tapes.
After solving the navigation problems, the flight program is designed. Each project area has unique combinations of terrain and geology that require special consideration, but some generally valid concepts have been established. Normally the orientation of the structural grain of an area is known before the survey. Flight lines should not cross the structural grain at right angles because subtle linear patterns may be obscured by the closely spaced scan lines on the imagery. Selection of flight altitude above average terrain elevation is a tradeoff between imagery spatial resolution versus imagery scale and ground coverage. Cost is also a factor, for at higher flight altitudes the ground coverage is greater and fewer flight lines are required. Atmospheric attenuation of the infrared s gnal at higher flight altitudes must also be considered. As a practical guide to altitude selection, imagery of the same flight line at altitudes ranging from 1,000 to 8,000 ft above terrain is shown.
Thermal infrared imagery has been improved greatly by recent advances in electro-optical technology and solid-state physics. Doped germanium detectors which required closed cycle cryostats or liquid helium cooling have been replaced with tri-metal detectors which operate with simple liquid nitrogen cooling to obtain imagery in the 8-14 micrometer wavelength region of greatest geologic significance. Quantitative scanners are now available in which gray-scale variations are calibrated to exact temperature values rather than relative temperature variations.
Early airborne scanners which recorded imagery directly on film had many disadvantages. Modern scanners record the imagery on magnetic tape which is later played back onto film in the laboratory. The magnetic tape may be replayed to obtain optimum imagery contrast and to adjust the scale for any variations in aircraft ground speed. Conventional scanner imagery is compressed and distorted at the margins because of the geometry of the system. The magnetic tapes, however, may be processed with a secant-square function to produce rectilinearized imagery with constant scales in the X and Y directions. Various image enhancement techniques also may be applied to the tapes, such as level slicing, digitizing, and conversion to color display for greater resolution of subtle thermal differences. /P>
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