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A pilot model of a small size, mass, power, and cost, GaAs laser radar altimeter has been developed and tested for use over the ocean. Two versions have been developed, one for triggering a device at a predetermined altitude, and one for providing an altitude 'read out' as an analog dc voltage. Field tests from a bridge over an inland reservoir indicate performance with negligible false-alarm rate up to an altitude of 400 meters over the ocean, under all sea-state conditions, from glassy calm to the 'worst-case' rough sea state condition, and with the sun vertically overhead. A new remote sensing technique was developed to evaluate the roughness state of the water during field tests and to predict the mean laser return signal. This utilizes video imaging of the glitter pattern on the water from a bright point source light at night, or from the sun in the daytime. Tests of the altimeter over the ocean surface at sea are still needed, because extrapolation from ripples on the inland reservoir to full scale ocean waves is not fully understood. (rh).
Satellite remote sensing, in particular by radar altimetry, is a crucial technique for observations of the ocean surface and of many aspects of land surfaces, and of paramount importance for climate and environmental studies. This book provides a state-of-the-art overview of the satellite altimetry techniques and related missions, and reviews the most-up-to date applications to ocean dynamics and sea level. It also discusses related space-based observations of the ocean surface and of the marine geoid, as well as applications of satellite altimetry to the cryosphere and land surface waters; operational oceanography and its applications to navigation, fishing and defense.
Low altitude (81 m.) narrow-beam laser reflectance measurements were made from the nearly ocean-like water surface under the Golden Gate Bridge. For short wavelength waterways superimposed on swell, the signal amplitude probability distribution showed periods of zero return signal, even for vertical incidence, apparently due to tipping of the average water surface. The nonzero signals show an antilog-normal probability distribution, skewed toward higher signal than that provide by a normal (Gaussian) distribution. With incidence angle displaced from the vertical, the distribution shape is retained but with more frequent zero reflections. The decrease with angle of the average signal, including the zeros, 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 signal amplitude probability distribution is log-normal with no zero signal periods, for this case, the distribution shifts toward exponential at large angles from the vertical. For surface states intermediate between the above two extremes the distribution is often normal. The larger return signal resulting form 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 at angles from the vertical remains high. The presence of occasional zero return signal does degrade the accuracy of altitude somewhat for a descending altimeter, but the signal available assures performance at larger altitudes than previously expected.