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1 September 1991 Vertical ocean reflectance at low altitudes for narrow laser beams
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A narrow-beam laser altimeter was used to measure the reflected signal from the ocean surface as represented by the waters beneath the Golden Gate Bridge. This site allowed precise measurements as a function of angle from the vertical not possible from flying platforms. For short-wavelength water waves superimposed on swell, the signal amplitude probability distribution for the reflected signals showed periods of zero reflection, even for vertical incidence, apparently due to tipping of the water surface. The nonzero signals showed a distribution that could be fitted with an antilog-normal distribution. This is skewed toward higher signals than a normal (Gaussian) distribution. With incidence angle displaced from the vertical, the distribution shape was retained but with more frequent zero reflections. The decrease with angle of the average signal, including the zeroes, is well fitted with a Gram- Charlier distribution, as seen by earlier observers using photographic techniques which masked these details of the structure. For the simpler wave pattern due to a long sustained wind direction, the probability distribution is log-normal with no zero signal periods. At large angles from the vertical the log-normal distribution shifts toward exponential. For surface states intermediate between the above two extremes the distribution is often normal. The larger return signals resulting from the skew toward larger amplitudes from lognormal are more favorable for disposable laser altimeters than previously believed. Also, for an altimeter which may be swinging from a parachute or balloon, the return remains high at angles other than vertical. The presence of occasional zero return signal does somewhat degrade the accuracy of altitude measurement for a descending altimeter, but the signal available assures performance at larger altitudes than previously expected.
© (1991) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Eugene C. Crittenden Jr., G. Wayne Rodeback, Edmund A. Milne, and Alfred W. Cooper "Vertical ocean reflectance at low altitudes for narrow laser beams", Proc. SPIE 1492, Earth and Atmospheric Remote Sensing, (1 September 1991);

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