Ocean depth sounding from the air by laser beam

Optics: measuring and testing – Range or remote distance finding – With photodetection

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Details

356 4, G01C 308

Patent

active

045182540

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a method of, and means for, ocean depth sounding from the air.
This invention relates particularly to the laser beam geometry, the use of a dual wavelength for ocean bottom and ocean surface reflection, a scanner for beam control, receiver for the reflected subsurface signal and torque motor for platform stabilization.
Features of the invention will be appreciated from the following description but it should be clear that variations within the spirit of the invention are possible and the following description is not to be considered limiting.
The basis of the invention is the use of an airborne station which propagates two laser beams, one of which has a wavelength in the infrared spectrum and is held normal to the ocean surface to give principally a surface reflection, and the other is a transversing beam operating at right angles to the direction of travel of the station with a wavelength in the green to give principally a bottom reflection. Features of the invention are the method of compensation for the additional airpath traversed by the green beam because of the scanning action, and the use of a scanning mirror to scan both a laser beam and the field of view of the receiver to yield an orthogonal pattern of soundings.
A feature of the invention is the receiver means which detect laser light reflected from the surface, water column and the bottom through a telescope and is arranged to discriminate bottom reflections against surface reflection and sunlight and other unwanted relfections, and has a sufficient dynamic range to allow for both varying reflection coefficients of the sea bottom and varying attenuation of the green beam with depth.
Generally the method comprises the steps of directing two laser beams of different wavelengths downwardly from an aircraft, the one a stable vertical beam in the infrared wavelength which reflects back from the ocean surface to allow the distance from the aircraft of the ocean surface to be calculated, the other a beam in the green wavelength which penetrates the ocean surface and is reflected back from the ocean bottom to allow the distance of the ocean bottom from the aircraft to be calculated, the said green beam being traversed transversely of the direction of travel of the aircraft the two beams being preferably but not necessarily produced from one laser by passing the infrared beam through a frequency doubler to generate the green beam and using the residual signal as the infrared beam.
To enable the invention to be fully appreciated, reference will be made to the accompanying drawings in which:
FIG. 1 is a schematic perspective view showing the system in general using a single laser and coupler to produce the infrared and the green beams, showing in block diagram form the basic system and showing in the dotted rectangle the scanning geometry of the green beam.
FIG. 2 is a flow chart in block diagram form to show the sequential processing of a measuring pulse,
FIG. 3 shows the format of the green return signal, S indicating the ocean surface pulse and B the ocean bottom pulse,
FIGS. 4 and 5 show schematically a mirror assembly for directing the scanning beam and receiving the back-reflected signal,
FIG. 6 shows the photomultiplier gain control,
FIG. 7 shows the effect of the time variable gain control of the photomultiplier on the anode signal current, A showing the dynode voltage, B showing the anode signal current,
FIG. 8 shows how g.sub.2 has a critical value, A.sup.1 showing conditions when too high, A.sup.2 showing it too low, and A.sup.3 showing the optimum condition. B.sup.1, B.sup.2 and B.sup.3 show the signals resulting under those conditions,
FIG. 9 shows examples giving A, return signal, B sample period and C sampled backscatter amplitude, the three consecutive conditions again showing conditions when "too high", "too low" and "optimum",
FIG. 10 shows the effect on return signals due to scan angle errors,
FIG. 11 shows at A the laser beam geometry of a vertical beam and at B for an inclined beam, showing below the typical

REFERENCES:
patent: 3523730 (1970-08-01), Hayek et al.
patent: 3533697 (1970-10-01), Hughes
patent: 3604803 (1971-09-01), Kahn
patent: 4050819 (1977-09-01), Lichtman
patent: 4277167 (1981-07-01), Eppel
C. A. Levis et al., Conference: Ocean 1973, IEEE Int'l. Conf. on Engineering in the Ocean Environment, Seattle, Wash., 25-28, Sep. 1973, 356/4, p. 76.
Japan, A, 55-31962 (Nippon Denki K.K.) Mar. 6, 1980, (06.03.80), (JAPATIC English language abstract).

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