Doppler Velocimetry Using Fabry-Perot Imaging
NAVAL AIR WARFARE CENTER WEAPONS DIV CHINA LAKE CA
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The feasibility of measuring velocities of potentially multiple objects via the Doppler effect using Fabry-Perot imaging has been explored. This report consists of two parts an experimental, active approach where the target scene is illuminated by a multimode Ar-ion laser and a passive approach assessing the possibility of using naturally occuring spectrally narrow lines for illumination. For the experimental approach it was found that it is advantageous to use multimode lasers with narrow longitudinal mode spacing, especially for the lower velocity regime. If the cavity length is chosen to be an integer multiple of the Fabry-Perot plate distance, a dense interference ring pattern results and overlapping of stationary and dynamic ring parts occurs at already low speeds of tens of ms. The light intensity provided by laser illumination is compared to the intensity of the brightest natural line source in the atmosphere, the OH-Meinel bands. The most intense ones of these are emitted from vibrational transitions of the OH molecule between 1.5- and 1 .8-micrometer wavelength. Because they are generated in the coldest atmospheric layer -90 deg. C, about 90 km above ground, the Doppler linewidth is the smallest available, promising also the best resolution.
- Atmospheric Physics