CA1186395A - Ocean depth sounding from the air by laser beam - Google Patents

Abstract

ABSTRACT

A system of ocean depth sounding from the air which consists in projecting an infrared beam down normal to the ocean surface along a forward travel path to back-scatter a signal to a receiver to measure distance to the ocean surface, and simultaneously projecting down a green beam to sweep across the direction of travel to backscatter a series of signals from the ocean bottom over an extended width to a second receiver, adjusting the receiver gain from the green signal to achieve optimum ocean bottom measurement and calculating ocean depth from the infrared and green signal differential.

Description

s

2.

This invention relates to a method of, and means Eor, 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 va-riations 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 reElection. 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 oE a scanning mirror to scan both a laser beam and the field of view of the receiver to yield an orthogonal pattern of soundings.

~ feature of the invention is the recei.ver 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 urlwanted reflections, and has a suE:Eicient dynamic range to allow for both varying reflection coefficients of the sea bottom and varying attenuation of the green beam with depth.

L0 Generally the method comprises the steps of clirecting two laser beams of dif.Eerent wavelengths downwardly from an aircraft, the one a stable vertical beam :i.n the infrared wave].ength which reflects back Erom the ocean surface to allow the distance from the ai.rcraft of the ocean surface to be calculated, the other a beam in the green wavelength which pene-trates the ocean surface and is reflected back from the ocean bottom to allow the distance of the ocean bottom Erom the a:i.rcraft to be calc~llated, the said green beam be.ing traversed transversely of the direction of travel. of the aircra:Et the two beams be:ing pre.Eerably but- not necessari:Ly 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 accompanyi.ng clrawings in which:

FIG. l 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 4.

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, indicating also below a typical word format, IIG. 4 and 5 show schematically a mlrror assembLy ~or directing the scanning beam and receiving the back-reELected signal.

FIG. 6 shows the photomultiplier gain control.

.l5 I~IG. 7 shows the e~fect of the time variable gaill control of the photomultiplier on the anode signal current, A showing the clynocle voltage, B
showing the anodc signal current.

FIG. 8 shows how g2 has a critical value, showLng conditions when too high, A2 show;ng it too low, and A showing the optimum condition.
Bl, B2 and B3 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 con-ditions when "too high", "too low" and "optimum".

3~5 5.

FIG. 10 shows the ef-fect 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 typ:ical return signal.

FIG. l2 shows a selection of pulses for deter-rnination of scan angle error, showing A an :invalid condition because g2 is too high~ B a valid condition, ancl C an invalld condition because g2 is too low.

FIG. 13 is a schematic diagram of the photo-muLtipl.ier gain controll.er, and FIG. 14 is a schematic diagram of the Biomation waveEorm recorder and associated timing signals.

Note that throughout this documents the words green and infrarecl are somet:imes usecl as adjectives to clescri.becl e(lui.pment: ;.n this sense they describe the wavelength oE the I.aser ].ight with which that equipment r~e~i.ves, detects or i.n any way is associated.

In ~lG. :l the green traversing beam is indicated by l and the stable infrared beam is indicated by 2. The infrared beam 2 :is reflected from the ocean surfaee 3 while the green beam is reflected from the ocean bottom 4. ~I represents the aircraft height above the ocean surface, while ~B :is the abso].ute inclineation of the green laser beam. D is the depth of the oeean.

In the bloek diagram A" is the aircraEt height ~8~5 6.

counter, B" the waveform recorder, C" the waveform converter, D" the photomultiplier gain control]er PMGC, E" a signaL enhancer and F" the depth counter.

The laser 5 clirects the infrared beam to the coupler 6 which directs the infrared beam to the transmitter 7, the green bearn being transmitted via 8 to the ocean through the scanner mirror 9. 10 represents the infrared telescope and 11 the green telescope.
12 represents the pulse start photo detector. SWG
represents shallow water gain, ancl TC the time constant, both connected to the photomultiplier gain controller D". ST = Start~ SP = Stop.

In the EIow chart designated ~IG. 2 the block clia~ram generally shows the processing of the signals ~5 from just before the Laser fires, the block diagralrl showing the various stages in the horizontaL numbering and the signal processirlg in a timewise manner vert-ically downwarcl.

The various integers of the block diagram represent ~o the ~olLow-ing:
READING HORIZONTALLY

l' Green channel 2' Laser 3' Infrared channel

4' lleight counter (time interval meter)

5' Waveform recorcler

6' SignaL Enhancer and Depth Counter

7' Photornultiplier

8' Waveform Converter 7.

READING DOWN

A. Green Receiver - measures background radiation and sets deep water gain.

B. Green Receiver - gain reduced to shallow water Level determined by PMGC.
C. Green pulse transmitted vertically down to sea surface.
D. Green Pulse reflected from sea surface, backscattered from water column and reflected from sea bottom.
E. Photomultiplier gain increases as surface reflection enters.
F. Photomultiplier gain increases with time con~tarlt ~r determined by PMGC.
G. Photomultiplier gain reaches maximum cleep water gain and after a delay recluces to a very low 'off' va ] ue .
tl. Lase-r fires and produces pulse of mixed ~-reen ancl inErared Iight.
2~ I. Start monitor procluces 'stflrt' signal -s lase~ pulse produced.
J. Coupling un;t separates infrarecl and green pulses K. Infrared pulse transmitted vertically down to sea surface.
L. Scattered infrared pulse from surface collected in infrared telescope.
M. Infrared photo detector produces stop pulse.
N. Time interval meter starts counting up 300 Mtlz pulses and counting down slant height.
O. Time interval meter stops countlng 300 Mtlz pulses. Count represents height H in 0.5 m.

3~

P. SLant height counter reduces to zero.
Presurface pulse produced.
Q. Waveform recorder armed.
R. Waveform recorder triggered and starts recording g-reen signal.
S. Waveform recorder replays under control of waveform converter.
T. Signal enhancer filters replay green signal U. Depth counter measures depth.
V. PMGC computes shallow water gain (g2) and time constant r.
W. Waveform converter reads previous H and real ~B-X. Wave~orm converter computes path delay for current transmittecl pulse and slant height for next transmitted pulse.

From the above chart the various sections and ~heir interreLatiollship can be seen, time being inclicatecl stepw-ise in a downward direction.

The secluence wilL now be clescribecl in more detall.

The Eirst action is the photomultiplier in the green receiver 11 increasing its gain gradually until it detects an output (due to background SUIl and sky illumination) which is just below a pre-cletermined threshold level. The maximum gain isthen set at that level, and the gain then reduced to a level determined from the previous pulse basecl on backscattcr reflection from the water column just below the sea surface.

The laser 5 produces an extremely short (Eive nano-

9.

second) pulse of infrared radiation which is passed through a frequency doubling crystal to convert some of the infrared radiation into green radiation of wavelength 532 nm. The output of the laser is thus a pulse of mixed infrared and green radiation, which is passed into a coupling 6 in which the two clifferent wavelengths are spectrally separated so that they can be directed through separate beam expanding telescopes 7 and 8.

This action of the laser is repeated at a rate of 16~ pulses per second, however the following description is written generally for a single laser pulse, except when discussing the action of the scanning mirror 9 and the production of a pattern oE green laser pulses on the sea surface - each pulse resulting from a separate action of the laser.

A small portion of the outgoing laser pu1se is sampled with a photo detector 12 to produce an electrical pulse which indicates the precise time at which the Laser pulse is transmitted. This eLectrical pulse is called the START pulse and is fed to the aircraft height counter or time interval meter A".

The incrared beam pulse 2 is transmittecl vertically to the sea surface beneath the aircraft, where a 2~ portion of it is reflected. The telescope 10 in the aircraft is arranged to view the surface of the sea illuminated by the infrared pulse and a fraction of the infrared reflection is presented on a photo-sensitive diode surface which produces an electrica]
pulse ca]led the STOP pulse to provide a timing datum for both aircraft height measurement ancl sea i3~

10 .

depth measurement. The time interval meter A" is used to measure with precision the time interval between the START and STOP signals, and thus the height H of the aircraft above the sea surface.

5 ` The green pulse 1 is transmitted via fixed mirror 14 (inclined at 45 to the vertical) onto the mirror 9 which is oscillating in such a manner as to transverse the green beam laterally across the flight path of the aircraft. This mirror is oscillated to a lesser extent along the flight path of the aircraft and the combined motion produces an orthogonal pattern of laser spots on the surface of the sea as shown by the clots in ~IG. 1.

A telescope 11 is arranged to look via a fixed :L5 45 mlrror 15 on to the same oscillating mirror which scans the green laser spots and hence onto the same portion oE sea surface illuminated by the green l.aser light. The output from this telescope passes tt-rough a va-riable Eield stop, which I.irnits ;.ts Eiel<l oE view Erom between 4 mr to 40 mr. In the ccntre oE the Eiel.d oE v:iew Ls a block, which can be set at O rnr, :L m-r, 1.5 mr, 2 mr, 4 mr, 6 mr, and 10 mr, and ls used to obscure as much of the incident surface reflection as is possible, and hence reduce the dynamic range of the received green signal. The reflected green light then passes througl a polarizing Eilter which is adjusted so as to furtller reduce the intensity of the incident surface reflection.
This is possi.ble since the laser light is linearly polari.zed as a consequence of the laser clesign.
I~inally a narrow bancl temperature controlled inter-ference filter is used to pass the received green ~8~
1:1 .

laser light and to block as much as possible of the reflected sun and sky radiation this general assembly not being illustrated as such components forrn part of the general art.

A photomultiplier is used to convert the optical output of the green receiving telescope and filters into an electrical signal which is the sum of four essential cornponents viz, ~a) reflection from the sea surface which can be very intense and is extremely variable, (b) backscatter component from the bulk of the water this component decreases essentially ex-ponentiaL]y with depth (and hence time), (c) reflection from the bottom and (d) background noise due to reElected sun and sky radiation.

The photomultiplier gain is varied to enhance the cletectiorl of a bottom signal. A control dynode close to the photocathode is used to C~lt the tube of completely during the time between the laser puLses: thls allows the tube to be operatecl at high gain at the time of the expected laser return without clamaging the tube. ~ group of dynodes near the centre of the dynode chain is used to increase the gain of the tube during the time that a bottom pulse is expected. The gain is increased smoothly from the level which produces a detectable output (below a predetermined threshold) from the baclcscatter of the water j~st below the surface to a maximum gain level which produces an output due to background noise (sun and slcy) which is also -just below the th-reshold. The timing of this gain increase is such that i~ starts at the time of the expected surface reflections, increases to the maximum level 3~
12.

established before the laser fired at a rate designed to maintain the output due to the exponentially decreasing backscatter from the water column at a constant level which is just below threshold. After a fixed period foll.owing the laser firing, both gain controls return to their low preset "cut-off" levels.

The electrical output of the photomult;plier (GREEN SIGNAL) is then presented to a digitizi.ng waveform recorder B" which digitizes the GREEN SIGNAL
into 6 bit words every two nanoseconds and stores 1.024 such words in a memory. The recording process is continuous from the time the recorder is armed (with an ARM signal). unti.l 1000 two nanosecond increments after the receipt of the trigger (called the STOP signal, an output o.E the infrared recei.ver 10).

Once ~he recording process is comp:lete, the data storecl in memory is replayed at a much slower rate under control of a microprocessor based system called a waveform converter C". Essentially this computes the deLay as a result of the extra path trave:llecl by the scanned green pulse with respect to the verl:ical :i.nEra-red pulse. To do this it Lakes the aircraft he:ight ~l, as measured by the ti.me interval meter, and the scan angl.e reLat:ive to the vertica:L, ~B
and computes A `- 3.3 I-l(Sec~B ~ N, where N
represents relative delays in the electronics between the two receivers and A i.S the number of words in memory which must be OUtpLIt such that the replayed waveform commences at a time representing a position effectively 5.0 m before the expected position of the surface reflection. It then commands the waveform recorder to OUtpLIt this number at high speed (1.4M~Iz), and the data 13.

contained within is dumped. Then 267 words are commancled to be output at 50 KHz word rate which enables the data to be recorded. The 267 words correspond with a depth A of approximately 60 m.
The remainder of the 1024 words are commanded to be output at the high rate and the contents dumped.
The waveform recorder is then rearmed ready for the next laser pulse.

The waveform converter also calculates the slant height of the scanned beam, based on H slant - }I sec~ - dh, (where dh represents a small distance above the sea surface at whlch the photo-multiplier gain is to be commanded to increase) and presents the result to the time interval meter.

~5 At the instant oE the STA~T pulse the time interval meter counts the slant height down to zero at which time Lt produces a PRESURFACE pu]se. This PRESURFACE pulse is used to initlate the increase in photomultip:Lier gain from the shallow water level to the deep water level.

The output of the waveform recorder is available t~ ar~cl read by the photomultiplier Ga:in Controller (P~GC), D". This microprocessor based system cletermines (frolll ttlC' amp:Li.tucle oE the backscatter component at a shallow depth in the water column and the measured gain of the photomultiplier) the gain required for shallow water and the rate at which it should increase with depth as described before.

A digital to analogue converter in the waveform recorder produces an analogue output signal which is a reproduction of the output of the photomultiplier 3~5 4 .

but is 10,000 times slower and can be more easily displayed and processed, and recorded. A high pass filter is used in the Signal Enhancer E" to remove the relatively slow backscatter component from the signal.

This filtered version of the signal is then presented to a Depth Counter F" which measures the time interval between the reference timing datum (efEectively 5.0 m above the sea surface) and any bottom signal whlch occurs after a time determined by a Range Gate ancl which exceeds a predetermined Threshold.
This tirne interval is thus a measure of the depth of the sea (adjusted for the 5.0 m extra depth).

The entire optical components of the system are mounted on a Drift platform which rotates so that the acrosstrack scan motion is normal to direction o~ fLight; on a Roll platform which is gyro stabilized to maintain verticality of the infrared transmissions i.n the rol] sense, and on a Pitch patEorm which is manually adjusted to maintain verticality similarly 2~ in the pitch sense.

The stabilization of the Roll platEorm is main-taincd usirlg a vertical gyroscope, the roLL output o~ which is used to drive a torque motor which directly drl.vcs the RolL platform. This permits the natural inertia o~ the Roll platEorm to assist maintain a stabilized mount, and only deviations from the desirecl verticality of the infrared beam are used to drive the torque motor.

The true angle (~B) oE the scanned green beam relative to the vertical is the sum of the angle of the scanned beam relative to the Roll platform (produced by 15.

the major scan transducer) and the angle of the Roll platform relative to the horizontal (detected by the gyroscope). It is this composite angle which is digitized by the Scan A/D (at the time of the laser firing) and used in the computation in the ~aveform Converter of the airpath delay due to beam angle.

T~IE SYSTEM AND ~EAM GEOMETRY
.. . . _ From the above it will be realised that in order to measure sea depth from an aircraft using a laser beam, a time interval must be recorded, i.e. the elapsed time between reElection of the beam from the sea surface and reflection for the bottom. The method relates to measuring the specified time interval in a mode of operation where the laser beam ;s scanned transversely, across the track of the aircraft to generate an area of depth soundings, using as said a green traversing beam l and infrared vertical surface beam 2 as shown par-ticularly in Figures 1 and 2.

As reEerred to earlier herein the infrared vertical]y stabilised laser beam 2 is refLected frorn the surface 3 of lhe sea to generate a tirnlng clatum for clepth measuremerlts. The technique permits, with suitable instrurnentation, the integration of returned signals to establish a mean sea level datum. Integration can be applied for a short period (i.e. 1 or 2 seconds) when aircraft height can be considered constant, or for a longer period over which vertical motion of the aircraft, measured by separate instrumentation~ is taken into account.

3~5 16.

The transversely scanned component of the laser beam propagates through the water column, is diffusely reflected from the bottom, 4, propagates upward through the water column, is then subject to refraction at the surface, and a small component of the reflected laser energy is received at the aircraft. A position fixillg signal is used, indicating the aircraft and the Elight path.

At beam inclinations away from the vertical the need arises for compensation of the additional airpath traversed by the scanned component of the laser beam. This correction is H(sec~B - 1) where H is the aircraft height above the sea and ~B is the absoLute inclination of the laser beam.

[his Eunction is carried out by a microprocessor system the output of which contro]s the presentation of data in the Eormat shown in FIG. 3 where B represents the bottom signal. In this A indicates the depth measuring range oE system, l3 indicating the bottom 2~ ~lignal, S the surEace signal. The word Eormat can be as ShOWll at the Lower part oE FIG. 3. Note that the reEcrence timing edge C occurs 5.0 m before expected surface returl~.

The parameter H is obtained from a 300 M~lz counter which measures aircraft height above the sea with a single shot resolution of O.S metres.
Beam inclination ~B is also available as a measured parameter.

rhe wavelength of laser energy propagated through a water column must be in the blue green band.

It fo]lows therefore that the frequency doubled component of a Nd:YAG laser at a wavelength of 532nm should be used for this task. The frequency doubling process, i.e. the conversion of infrarecl energy at 1064 nm to energy at 532 nm (green) is inefficient and à residua] infrarecl component remains after the process. It is this infrared component ~hich is used to sense the sea surface.

The ].aser rod used is preferabLy 3 mm diameter and is positioned ;.n a cavity having a mirror of tota].
refl.ection at one end and a mirror o:E low reflectivity at the other end to pass about 90% of the signal. Tlle l.ash is designed to minimi.se hea~ing problems in the l.aser rod and flash lamp and to allow high energy o~lt~ut.

T~IE SCANNER
.

The scanni.ng m:;rror, in this instance used ~o scan a l.aser beam and the f:ield o.E vi.ew of a receLver, :is used to p:lace on the ocean an orthogona].
20 trace o~ soulld:ings. Provided laser p~.llse rate and scann:i.ng frecluency are aclj~lstecl in accorclance with grouncl speecl, a squclre matrix of depth sound.ings is g~nerated.

The scanner, shown in ~IG. ~ in conjurlction w:ith FIG. 5 which shows the signal. generator and amplifiers, comprises a major actuator 16, a minor actuator 17, a scanning mirror 18, a major actuator transducer 19, a minor actuator transducer 20, a signal generator 21 and a dual scanner ampl.ifier i3~3S

18.

22.

The operation of the scanner, using a single ba]l joi.nt mount 23, approximates a gimbal system.
For a given ]aser beam deflection ~ the required mirror incLination is ~. Thus, for smal] angle variation of the laser beam the mirror angles are small and the coupling from one orthogonal axis to another is correspondingly small.

The assembly is mounted in a stable configuration free from mechanical constraints (6 degrees of freedom fixed) by using the major actuator 16 with a connecting rod 24 with two ball jointed ends and by pivoting the minor actuator 17 on its base by a trunion mount 2~ w:ith one ball jointed rod end 26 at its point o application. This is not illustrated as stabi.]izing .15 clevices are known in the art.

The scallner can be operated in a fi.xed con-~ ulAr:lti.on to provi.de a nominal:Ly square pattern o~ depth soundings uncler zero wind cond:itions, :Ln the event cl:istortion of the pattern due to wind 2~ car) be determi.rlecl ancl accepted, THE, REC~IVER

The purpose of the receiver is as fol].ows:

(a) to detect laser light reflected from the sea bottom resulting from the illumination of the bottom by a laser beam.

(b) To cliscriminate bottom reflection signals 3~5 19.

against surface reflections caused by the laser beam, backscatter of laser light in bulk water and reflected sun and sky light.

(c) To provide an adequate dynamic range of detection to cater for bottom reflections which vary significantly due to varying coefficients of the sea water and bottom and more importantly, varying depth.

The principle fo]lowed, for convenient and efficient detection of bottom reflections, is to modify the gain of the detector, (a photomultiplier) over the time gate when bottom reflections can be anticipated.

The relevant invellti.ve features are :

(i) The method of achieving "programmed gain cont-rol" of the photomultiplier detector to provide ex~remely fast control of gain and change of gain ove-r the time gate, when bottom signals can be expected.

(:ii.) Ttle method and princi.pl.e usecl in controlL:ing 2~ th~ ~ain Eor the deep water case.

(iii) The method and principle used for controlling the gain :in the shallow water case.

The invention preferably comprises a telescope and a selected EocaL length, say 1100 mm, and an effective aperture o:E say 250 mm, and i.ncorporating a number o.E presel.ectable field-stops which attenuate 3~
20.

the direct surface reflections, and an interference type filter centred on 532 nm to ensure a good signal-to-noise figure. The fie]d can be adjusted to between 4 and 40 mrad with centre field stops ranging from 0 to 10 mrad.

The detector head, employing a 14-stage photo-multiplier, incorporates a control grid, enabling the tube to be switched on on:Ly during the interval frorn time of transm;ssion until the return signals have been rece;ved. With this mark/space ratio of approximately 1:1000, it is possible to operate the tube at greater peak anode currents than woulcl otherwise be the case.

To achieve the wide dynamic range of gain necessary (in order o,E 104:1), a number of dynodes in the later stages of the photomu:Ltiplier are controlled, in such a manner as to effectively change it from an optimised l~-stage conf,iguration to a configuration where the tube is vi.rtually cut-off.

2~ l~y mealls of the configuration shown in I~IG.
6, se'Lcctecl acljacent pa:irs of clynodes are controlled ~rom a t:ime var.iable voltage generator in a syrn-metrical.:ly opposed manner. The amp:Litude of these signclls are such that the photomultiplier is progress-ively turned on as the potential increases between dynodes D6-7 ancl D9-10.

When this potential. reaches that which exists between D4-5 and D9-1~, a linear arrangement exi.sts between all dynodes and the photomultiplier exhibits max:imum gain.

21.

The time variable voltage generator emp]oyes a number of power MOS FETS in order to achieve the fast switching times required. The maximum amplitude of the voltage swing is related to the E.H.T. on the photomultiplier, which in turn controls the dynamic range of the detector system.

Changing the E.H.T. aLone to control the required gain would result in a transit time varying inversely proportional to halE power of the applied E.H.T.
With the method employed the transit time remains constant at approximately 50 nS. The spread in anocle pulse width is kept to a minimum by maintaining a constant voltage between the cathode and first clynode.

In deep water in daylight, reflected sky and sunlight constitutes a limiting noise source. In this case the backgrourld energy entering the receiver is assessecl a Eew microseconds before the Laser is operatecl. 'rhe~ gain of the receiver is adjusted ~0 by means of servo action to amplify the background surllight to a speciEic level. Ttlis predetermined gain is then appLlecl to the photomultiplier at that part of the receiver time gate when deep bottom signals are expected.

The predominating noise in shallow water is backscatter induced by the laser beam. The receiver which measures this backscatter is described later herein. This part of the receiver which is micro-processor controlled achieves the following:

(a) Backscatte-r is measured for each laser 3~5 transmission.

(b) Optimised averaging processes are used to predict optimum gain settings for the photomultiplier in the shallow depth section of the receiver time gate.

(c) Water turbidity can be estimated from the backscatter measurements. This permits the prediction of maximum depth sounding capability of the system.

(d) By analysing the relative timing of surface reflections, the verticality of the scanning system can be estimated.

The part oE the receiver which optimises the photomultiplier gain for signals from shallow water, ~5 is described with reference to FIGS. 7 to 1~.

The principle of operation is to control the gain oE the photomultiplier used to detect the green return slgnal; that it reduces the clynamic range o~ the signal to optimise the detection of the bot~om.

The ti.me variable gain of the photomultiplier is iLLustrat~d schematically in FIG. 7. About 15 ~s before each laser pulse is expected the ~ain of the photomult;plier is increased until the anode noise current reaches a predetermined value. This is achieved by controlling the voltage between two adjacent dynodes in the photomultiplier. When the noise current reaches the set value the dynode voltage stops rising and thus the gain (gl) is held constant until about 3 ~s beEore the return signal is expected.

3~5;

The gain is then reduced to (gz) in order to reduce the amplitude of the reflections from the surface and from the bottom in shallow water. After the surface reflection has been received the gain is again increased to gl with a time constant I

For optimum detection of the reflection from the bottom the noise level shoulcl be constant over the whole time that bottom signals are expected.
The thresholcl Eor detecting the bottom reflection can then be set at a fixed level above the noise.

FIG. ~3 shows the influence of g2 on the noise profile. If g2 is too high as shown at Al - Bl the backscatter from the water produces a larger signaL that prevents the use of a constant threshoLd.
LE g2 is too Low as shown at A2 _ B2, noise is sup-pressed and a small bcttom signal might also be supressed arld remain undetected. The optimum value oE g2 prod~lces a constant noise level. This is shc)wrl at A3 -- 133; B~, I32 and B3 show respectively 2~ "aJse depth", "bottom uncletected" and "true bottom".

Tlle v~lue of the shallow water gain (g~) will be controllecl so that the amplitude of the backscatter noise signaJ Erom shal]ow water is equal to the background noise signal in deep water. This will be achievecl by sampling the amplitucle of the backscatter signal from the water within a few metres of the surface and controLling g2 and ~ to keep the back-scatter amplitude constant. This is illustratecl in FIG. 9, where A shows the return signal, 13 the sample period, ancl C the sampled backscatter amplitude reading f-rom Left to right "too large" "too small"

24 .

and "optimum".

The sample period will be variable but initially it will correspond to water depths between one and two metres. Sampling cannot occur too close to the water surface otherwise surface reflections and small variations in the timing could introduce errors. Similarly sampling cannot continue too deep otherwise bottom reflections in shallow water would distort the measurement.

The laser beam is scanned from side to side by a scanning mirror mounted on a roll stabilized platform. The time at which the surface reflection is received depends on the scan angle and timing errors result from any roll errors in the stabilized platEorm. Because the sampling interval fo-r cleterming backsccltter must be accurately related to the water surface, the dependence of the time of the surface reflection on scan angle must first be established experimental'ly. For a given roll angle erro-r, the 2~ tlme error is proportional to scan angle as shown ;in F,IG. lO which shows the scan angles in degrees agaiinst time. Tlile surface expected is designatecl .

The transverse scanning of the laser beam provides an important aid to the determination of backscatter amplitudes. When the laser beam is vertical as at A in FIG. ll there is a large probability that a strong reflection from the surface will be detected.
On the other hand, when the beam is inclined at an angle as at B in FIG. 11 a reflection from the surface is ]ess likely to be detected as il,lustrated in the lower part of F'IG. 11. Thus, accurate sampling 3~5 25.

of the backscatter amplitude can be enhanced by ignoring all pulses exhibiting a strong surface reflection ancl averaging the remainder to determine the mean backscatter ampl:itude and transverse gradient for each transverse scan.

The parameter averaged will be a measure of water turbidity rather than system gain because it is water turbi.dity that determines the amplitude of the backscatter envelope. It is assumed that the water turbidity changes sufficiently slowly over the 270 metre scan width that it can be adequately described by a mean value and a transverse gradient.

~ second averaging process will calculate a mearl turbicli.ty and turbidity gradient aLong the 1~ El.ight path oE the aircraft. These parameters wil]
then be used to predict the mean turbidity and trans-verse gradient along the next transverse scan.

~ rom those values wi'll be caLculated the required val.ues of the shall.ow water gain g2 and the time c~orlStant r.

'I'he ~heoretical considerations are as follows:
The main assumption, on which this invention is based, is that water turbidity changes relatively slowl.y with position. Specifical'ly, it is assumed that the turbidity along a scan about 270 m wicle can be predicted from measured values of turbidity over an adjacent 270 m square. Measu-rements of the beam attenuation coefficient in South Australian and Queensland coastal waters show changes o F less than 20% over distances of this order. It is assumed, 3~35 26.

therefore, that the attenuation coefficient can be adequately represented by a linear function of position.

The laser power P entering the receiver at a time t after the light reflected from the surface is given by P(t) ~ U b exp (-2 k ct/n) /h2 sec2 ~, where U is the energy in the transmitted pulse, b wil.l be called the backscatter coefficient and is a measure of the mangitude of backscatter in the water, h is the aircraft height, and (p is the nadir angle of the laser beam. The exponential term, wh:ich describes the attenuation of the backscatter signal with depth, clepends on the diffuse attenuation coefficient k, ~5 the speecl of ].ight :in vacuum c and the refractive index of sea water n. The.factor 2 accounts for the atten~lclt:i.on oE the light on both downward and upward paths. Over the small scan angles used (+ 15) the approximation cos~ (P = 1 _ (p2 where (p is in raclians, is accurate enough and the total correction for angle 1~ al.ways Less than 7%. The a~ten-lation coe:EEicient k is relat~cl to the backscatter coefficient b. An empiri.caL re:Lati.onsh-ip of the .Eorm l<(b) - ko -1- k:L b ~5 wil]. be :invcstigated when the necessary data becomes ava;il.able.

The anocle current of the photomultiplier is gi.ven by I(t)~ g(t) P (t), where g(t) i.s the time variable gain of the tube.
The vari.able gai.n is achieved by changing the photo-muLtiplier vo:Ltage Vp and the voltage Vd applied to two pairs o:E dynodes. The empirical relationship ~3bi3~5 27.

g = gO Vpl2 exp -26000 (Vd/Vp)2~

will be assumed, where gO is a constant. In this expression the Vp term describes the overall gain of the photomultiplier with a linear dynode voltage divider. The exponential term gives the gain reduction due to the voLtage applied to the controlled dynocles, where Vd is the voltage deviation from the linear chain value. The dynode voltage decays from V2 to Vl with a time constant 1, so that Vd(t) = Vl - (Vl - V2) exp (- t/l) Using the above equations the backscatter coeficient can be dervied from the photomultiplier current generated by backscatter from water just ~elow the surface (i.e. t-~0).
lS ~rt,~ expressiorl is b = AI(o) h2/ (1 - ~) U Vpl2 exp ~-26000(V2/Vp) where A is A scaliTlg constant.

When ~he average amplitucle and transverse gradient o~ the backscatter is cletermined Eor each transverse scan, clLl pulses exhibiting a strong surface reflection wiLl be excluded from the averaging. A selection process is now described.

~ irstly, the backscatter amplitude will be sampled as close to the surface as possible after the surface reflection. Timing is therefore critical.
Timing errors occur if the roll stabilisation of the laser transmitter is in error by a small amount, as ilLustrated iTl ~IG. 10. To correct for this 3~5 28 .

error a thresholcl (Ll) is set below the amplitude of the backscatter that would result when the system is operating correctly. The crossing of this threshold (at time ti for transverse position xi) indicates the presence of the water surface. Invalid pulses, due to g2 being too low (see FIG. 6), are excluded ~rom the analysis. Invalid pulses, due to g2 being too high, are prevented by limiting the maximum value of g2-The average surface position S and the transverse gradient S' are then calculated by standard linearregression analysis, which yields S = (~xi2 ~t; - ~xi ~xi t ~ /D
S' = ~n ~xi ti - ~Xi ~ti ~ / s i5 where Ds = ns ~xi2 - (~xi)2 ~llld Wllere nS iS the number of vaLid pulses. If nS i.s too smnJ:L for an acceptable analysis, S should be set to the value expectecl if the system operates per~ectly clncl S' should be set at zero.

When the average surface position has been determined in this way, it is possible to define a sampling interval for determining the backscat~er that is less subject to timing errors. Small errors will remain, due to variation in wave height and to errors in the scan angle of the scanning mirror, but these should be small. A typical dependence of the sarnpling interval on scan angle is shown in ~IG. 10.

jj3~5 29.

A selection process is needed to exclude those pulses having a strong surface reflection. This is achieved by establishing a second threshold (L2) and sampling interval corresponding to depths between O and 1 metre. Any pulses which exceed the L2 threshold in this sampling interval are excluded.

The analysis will include all other pulses, everl where the gain is too low to exceed threshold Ll. If the gain is too low, a low backscatter amplitude wil.l be recorded and the servo system will increase the gain. On the other hand, if the gain is too high, so that even the backscatter amplitude exceeds L2 and a]l the pulses are excluded, a "latch-up"
could result. This would be prevented by setting the backscatter amplitucle equal to L2, in this case, so that the servo system would force the gain down a~airl.

It wiLl be assumed that the backscatter coefEicient is a linear function of position along a scan, thus ~ b(x) -- b ~ b'x, where x is the position along the scan (measured ~rom a point clirectly below the laser transmitter ill the starboard clirection) ancl b and b' are the mean and gradiellt of the coe~fient along the transverse scan. Stanclarcl linear regression analysis yields Xi bi ~Xi Xi bi 3 /D

b~ = (n ~xi bi ~ ~Xi ~bi) /D

where D = n~xi2 - (~xi)2 3~
30.

and n is the number of valid measurements along the scan that are included in each summation.

The mean backscatter coefficient and its transverse gradient along the next scan, which will be designated bo and bo', can be predicted from the values of these parameters for the previous m scans. It can be shown that bo = ~aj bj and bo' = ~aj bj' where aj = nj (~j n~ j n~ nj ~j nj - (~j nj)2¦

clnd bj,bj', and nj are the va]ues of b, b' and n for the jth scan prior to the one being predicted.

The ~ain seltings for the next scan can then L>e calcul~ated from an inversion of equcLtlon. This yielcls ~he ith value of V2 al.ong the next scan, n~lmely V2i = V~ {~ 'Pl) Ui (bo + bol xi) Vp /A In h ~/26000 where In is the desired noise level, and ~i~ Vi' and xi are the scan angle, transmitted pulse energy, and scan number of the ith pulse in the next scan.

The time constant ~ may be fixed by requiring the anode current to be the same at time ~ as it is .

31.

initially. This requires g( T ) P ( T ) = 1 ~
which reduces to i ~ V 2 ~V2i2 ~~ Vl - (Vl - V2i)e~ l ) A schematic diagram of microprocessor based photomultiplier gain controller is shown in FIG.
13.

In this 1 represents BUS, 2 to 6 represent respectively CPU, RAM, ROM, INTERFACE, and ARITHMETIC.
7 to 10 represent respectively the BIOMATION WAVEFORM
REC,ORDFR, the SEQUENCER, the DATA CONTROILER, and the PHOTOMUlJrIPLIER.

The Biomation waveform recorder supplies the return signals to be processed by the gain controller .L5 in the ;Eorm o~ 6 bit paralLel output. A schematic d;iagram of the sys~em and the associated timing signaLs a~e shown in F[G. 14. In this 11, 12 and 13 represent the SEQUENCER, the WAVEFORM CONVERTER, and the SIGNAL ENI-IANCER, the BIOMATION WAVEFORM
RECORDER being 7 as in FIG. 13. AO is the analogue ouput and OT the output. When the recorder receives the arm pu]se (RMA) from the sequencer it starts recording the photomultiplier signal. It stops recording 2 lls after receiving a trigger T from the infrared surface pulse. The waveform converter then supplies an output signal OPT which allows the recorded waveform to be clocked out at a rate ~8~3~5 32.

contro]led by the word command signal WDC. The words initialLy clocked out at the 1.4 MHz rate correct timing errors associated with slant heights and should be ignored. The data to be analysed is available on the 6 bit parallel output when the flag F is set and the pedestal P is present.

The sequencer provides timing pulses for the system, inclucling the 168 Hz laser pulse rate and the 50 kHz clock rate.

Data required Eor calculations, such as transmitted laser pulse energy and aircraft height~ will be supplied by the data controller on an 8 bit parallel interface with handshake control. Values of the mean and transverse gradient of the backscatter coefficierlt (b and b') for each scan will need to be sent to the data controller on the parallel interface for recording on magnetic tape.

The two controls that need to be supplied to the photomultiplier are grid voltage V2 and the time constant T. The grid voltage is required as an anaLogue voLtage. The time constant is controlled by switching two capacitors with relays. It is envisaged that the number of capacitors will be increased to four, in a binary sequence of values, to provide 16 combinations. The interface should provide the necessary levels for 4 parallel lines.

Claims (3)

33.

THE EMBODIMENTS OF THE INVENTION ON IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The method of ocean depth sounding from the air by laser beam which comprises the following steps:
(a) directing two laser beams of different wavelengths downwardly from an aircraft, one a beam in the green spectrum traversed laterally across the flight path of the aircraft to read ocean depth, the other a vertical stable beam in the infrared spectrum to read ocean surface position, (b) receiving the reflected infrared surface signal and the reflected green ocean bottom signal in separate-receivers, (c) processing the data from the red beam reflec-tions to determine ocean surface distance and to generate a timing datum for depth measurement, (d) adjusting the dynamic range of the green receiver, for optimum reception of the relatively weak bottom return signal, and (e) processing the data from the green beam to measure the ocean depth by determining the differ-ential between distances indicated by the two reflected signals.

2. The method of ocean depth measurement according to claim 1 wherein the depth measurement is effected by the steps of first determining the level of back-ground energy entering the said receiver, adjusting 34.

the gain of the said receiver to compensate for background noise, triggering the said laser after the said gain has been adjusted, further adjusting the gain of the said amplifier when the green reflected signal is received to reduce the dynamic range of the signal to optimise the detection of the bottom, and sampling the signal at a time interval for a selected duration to detect the bottom.
3. The method of claim 2 which comprises storing the received data in a memory and using a microprocessor to change the word rate to record the data.

4. The method of ocean depth measurement according to claim 2 characterised by the step of controlling the gain of the said amplifier so that the amplitude of the backreflected noise signal from shallow water is equal to backreflected signal from deep water.

5. The method of ocean depth measurement according to claim 1. or 2 characterised by the step of discrim-inating the variation of backreflection of the green beam, due largely to water turbidity, to ignore pulses exhibiting a strong surface reflection and averaging the remainder to determine mean back-reflection and applying the resultant signal to effect amplifier gain control.
6. The method of ocean depth measurement according to claim 1 or 2 characterised by the step of adjusting the timing of the green beam to compensate for additional airpath traversed by the scanned component 35.

of the green laser beam according to the formula H(sec.PHI.B - 1) where H is the height of the aircraft above the ocean surface and .PHI.B is the absolute inclination of the laser beam.
7. The method of ocean depth measurement according to claim 1 or 2 wherein the said laser (preferably a Nd:YAG laser) is selected to provide a beam in the infrared wavelength and by the step of frequency doubling to produce a beam in the green wavelength, and using the said green beam for the ocean bottom depth determination and the residual infrared component for ocean surface determination.

8. The method of ocean depth measurement according to claim 1. using a telescope of selected focal length for receiving the reflected signals to amplifiers and attenuating the direct surface reflections in the green-receiving telescope by use of preselectable field stops, and passing the received signal through an interference type filter centered on the green signal wavelength.

9. The method of ocean depth sounding according to claim 2 wherein the gain is adjusted by a photo-multiplier which incorporates a control grid, and regulating the control grid to actuate the tube only from the time of transmission until the return signals have been received.

10. The method of ocean depth sounding according to claim 1 using a pair of telescopes to pass the signal to receivers, one for each beam, measuring the background radiation received by the green receiver 36.

and setting the output by a variable field stop, limiting ocean surface reflection, setting the green receiver to deep water gain, using a photomultiplier gain control to reduce the green receiver gain to simulate shallow water firing the laser to produce infrared and green beams and starting a time interval meter to count up a selected pulse frequency and count down slant height, separating the infrared and green pulses, causing the green receiver gain to increase as surface reflection enters, triggering a waveform recorder and increasing green gain to reach required deep water gain, feeding scattered infrared pulse from ocean surface to said waveform recorder, causing the waveform recorder replay under control of a waveform converter, enhancing replayed green signal and using a counter to measure ocean depth.
11. The method of ocean depth sounding according to claim 10 wherein the sequence defined is inter-mittently repeated as the green beam traverses the ocean floor both forwardly and laterally to give a pulsed wide-path ocean floor scan.

12. Apparatus for ocean depth sounding from the air by laser beam carried by an aircraft along a defined path, comprising:

(a) means to produce a laser beam in the infrared wavelength and a beam in the green wavelength, (b) means to direct the infrared beam normal to the surface of the ocean in stable condition to reflect from the ocean surface, (c) means to direct the green beam to traverse laterally across the said path to reflect in part from the ocean surface and the ocean bottom, (d) means to separately receive the reflected signal from the two beams to identify ocean bottom reflections and surface reflections, (e) means to determine the background energy from the ocean surface and to adjust the gain of amplifier means to which the return laser signals are fed, (f) means to process the data from the infrared beam to generate a timing datum for ocean depth measurement by the green laser beam and to adjust the gain of the said amplifier to reduce the dynamic range of the signal to optimise detection of the bottom, and (g) means to determine the elapsed time between the reflection from the ocean surface and the reflection from the ocean bottom and to calculate ocean depth.

13. Apparatus for ocean depth sounding according to claim 11 or 12 wherein said means to produce a laser beam in the infrared wavelength and a beam in the green wavelength comprises a laser for producing a primary beam in the infrared wavelength, and means for splitting said primary beam and doubling the frequency of a portion thereof to provide said green beam, said infrared beam being derived from a residual portion of the primary infrared beam.

14. Apparatus for ocean depth sounding according 38.

to claim 12 or 13 wherein the said laser (optimally a Nd:YAG laser) is selected to provide a beam in the infrared wavelength, and by means to double the frequency to give a green beam and a secondary infrared beam derived from the residual of the infrared beam during doubling.

15. Apparatus for ocean depth sounding according to claim 14 wherein the said laser rod is of about 5 mm diameter and is positioned in a cavity defined between a total mirror and an output mirror of about 90% transmission characteristic.
16. Apparatus for ocean depth sounding according to claim 12 wherein the infrared beam is projected normal to the ocean surface and wherein the green beam is directed generally on a plane parallel to the said infrared beam but is caused to swing with a transverse movement by being projected from a mirror mounted to move about two axes normal the one to the other by generators activated from a two-signal initiating generator through amplifiers.

17. Apparatus for ocean depth sounding according to claim 16 characterised by at least a transducer coupled to link the transverse movement of the said mirror to a compensating circuit arranged to correct for the additional airpath traversed by the scanned component of the green laser beam according to the formula H(sec.PHI.B - I) where H is the height of the aircraft above the ocean surface and .PHI.B is the absolute inclination of the laser beam.

18. Apparatus for ocean depth sounding according 39 .

to claim 12 or 13 wherein a photomultiplier is arranged to detect the green reflected signal and is arranged to reduce the dynamic range of the said signal to optimise detection of the bottom, the said photo-multiplier having gain control means arranged so that the gain is increased until the internal noise current reaches a predetermined level and is held level until about 3 us before a return signal is expected whereupon the gain is reduced to reduce the amplitude of reflections from the surface and bottom in shallow water but when these are received the gain is increased for a period of time sufficient only to record the depth signal return.

19. Apparatus according to claim 12 wherein the said laser operates in the infrared wavelength and is frequency multiplied to produce two beams, one in the green wavelength and the other in the infrared wavelength, a coupler arranged to divide the said beams, means to transmit the said infrared beam to the ocean surface normal to the said surface a telescope and amplifier means to receive radiation from the said infrared beam backreflected from the ocean surface, means to transmit the said green beam to the ocean fLoor, a telescope and amplifier means to receive radiation from the said green beam back-reflected from the ocean floor, and a mirror in the path of the said green beam mounted to be oscill-atable about the axis of flight of the aircraft.

20. Apparatus according to claim 19 wherein the said telescope for the green beam has a field of view substantially larger than the infrared beam at the ocean floor to amplify the backreflected energy received by the receiver.

21. Apparatus according to claim 19 wherein the received signal from the infrared backreflected beam is amplified and fed to an aircraft height counter and to a waveform recorder, and the received signal from the backreflected green beam is also fed tothe waveform recorder, a signal enhancer and depth counter being connected to receive the signals from the said waveform recorder, and wherein a photomultiplier gain controller is connected to also receive the signal from the said waveform recorder and is arranged to control the output amplitude of the said green receiver.

22. Apparatus according to claim 21 characterised by means to store the signal from the said waveform recorder in a memory, and by a microprocessor to change the word rate to record the data as the difference between ocean surface and ocean bottom measurement.

3. An apparatus according to claim 13 wherein said laser is a Nd:YAG laser.