Sea test development of laser altimeter

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Author
Crittenden, Eugene Casson
Rodeback, George Wayne
Milne, Edmund Alexander
Cooper, Alfred William
Date
1991-01Metadata
Show full item recordAbstract
Low altitude (81 m.) narrow-beam laser reflectance measurements were made from the nearly
ocean-like water surface under the Golden Gate bridge. This site allowed precise measurements
not possible from flying platforms. For short wavelength water waves 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 provided 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 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 signal amplitude probability distribution is lognormal
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 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 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.
NPS Report Number
NPS-PH-91-006Collections
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