for a better understanding of the subject invention , reference is made to the following description and appended claims in conjunction with the above - described drawings . referring now to the drawings , fig1 depicts an aircraft 10 in flight at apredetermined altitude over a body 12 of water such as an ocean having a transmitter 13 for directing a laser beam 14 into the water and a receiver15 for receiving a return signal 16 from the water . transmitter 13 generates a pulsed output beam 14 having a wavelength selected to penetrate the water 12 , and return signal 16 contains brillouin and rayleigh backscatter components . in accordance with this invention , receiver 15 processes the brillouin and rayleigh signals to produce data indicative of the temperature at various depths to provide temperature - depth information . a more detailed block diagram of transmitter 13 and receiver 15 along with associated optics is shown in fig2 . transmitter 13 comprises a laser 18 controlled by a timer 19 to produce output laser pulses 20 . the output of laser 18 preferably is in the blue - green portion of the spectrum ( 4500 to 5500 å in wavelength ) and has a typical pulse repetition frequency of 100 hz . a laser useful for this purpose is a frequency doubled nd : yag at awavelength of 5300 å . laser output 20 is directed by mirrors 21 , 22 and23 to the target area , in this application , sea water 12 below aircraft 10 . return signal 16 containing brillouin and rayleigh backscatter components resulting from the interaction of the laser pulses with the sea water are directed by mirrors 23 , 24 and 25 , to the receiver 15 comprising photodetector 26 having a cathode 26a , discriminator 27 , analog to digitalconverter 28 , analyzer unit 29 and a recorder 30 . photodetector 26 preferably is a photomultiplier tube having a photocathode on which returnsignal 16 is incident and which converts the optical energy ( photons ) to electrons ( current ) while simultaneously amplifying the energy . the photocathode also performs the additional important function of mixing thebrillouin and rayleigh components of signal 16 to provide a difference frequency , called the optical heterodyne frequency . the brillouin backscatter component is frequency - shifted by the acoustic properties of the water whereas the rayleigh backscatter component has a constant frequency and is analogous to the &# 34 ; local oscillator &# 34 ; in a superheterodyne receiver . the acoustic velocity of the water is a function of water temperature . referring now to fig3 the brillouin and rayleigh components incident on cathode 26a of photodetector 26 are represented by curves 37 and 33 at frequencies f 1 and f 3 , respectively , and by curve 34 at frequency f 2 . the signals at frequencies f 1 and f 3 are the frequency - shifted &# 39 ; brillouin components and the signal at f 2 is the frequency - unshifted rayleigh component , the latter functioning as a &# 34 ; localoscillator &# 34 ; in an analogy to a superheterodyne receiver . the two differencefrequencies f b between f 3 and f 2 and between f 2 and f 1 are the same and are additive in the baseband so that the process results in increased sensitivity . moreover , the phase relationship of the brillouin and rayleigh components at photodetector 26 is correct for efficient heterodyning since both components are received from the same angle of view however wide and are produced by the same laser pulse stream . the output ( current ) of photodetector 26 passes to discriminator 27 , such as a delay line discriminator , which produces a voltage proportional to the baseband frequency f b . converter 28 changes the output of discriminator 27 into digital form for analysis in analyzer 29 . analyzer 29 receives the digital frequency f b and , using the equations below , translates it into a signal or number representative of the water temperatures . by way of example , analyzer 29 may comprise a computer whichcompares the digital frequency input with a reference table to derive the equivalent temperature . the output of analyzer 29 is stored in recorder 30 . in typical ocean water , the vertical temperature profile , see fig5 exhibits a reasonably well - defined depth where a marked gradient exists . the surface temperature changes to that of deep water at this point , indicated at 40 and is called the thermocline . as the density of water is dominated by its temperature , thermocline 40 represents the boundary layerbetween waters of different density . as in all density boundaries , wave action can be supported , see &# 34 ; the enclyclopedia of oceanography &# 34 ;, supra . the existence of internal waves in the main thermocline 40 may be 20 - 50 feet high and are exhibited by temperature profiles such as the pattern of &# 34 ; isotherms ,&# 34 ; i . e ., isothermal contours , shown in fig6 . another such profile is a map of temperatures at constant depths . internal wave patterns are thus mapped wherever and whenever they exist and provide a direct indication of the wake of a submarine . because the density gradient at thermocline 40 is relatively weak ( comparedto the gradient at the air - water boundary ) and because of the water viscosity , the internal waves have long periods ( many minutes ) and persistfor long times ( large fractions of hours ). the motion of submerged submarines near the thermocline depth generate unique patterns of internalwaves . because of the persistence of these waves , the pattern may be detected long after the passage of the submarine . this enhances the detectability of the submarine because only the long - persistent wake in the internal waves need be detected rather than the submarine itself . tracking the wake will quickly locate the vessel itself . another underwater disturbance created by a submarine and detectable in accordance with this invention is the &# 34 ; thermal scar &# 34 ; left by the power plant coolant discharge . a submarine cools its prime power plant by using surrounding sea water . the excess heat in the coolant water discharge leaves a trail of relatively warmer water as the submarine moves . the thermal scar persists until the heat is dissipated and provides a long persistent indication of the presence of the submarine . the accuracy of temperature measurement by analysis of brillouin backscatter may be shown mathematically . the relation of signal - to - noise ratio ( snr ) to the standard deviation of frequency measurement accuracy for a radar is given by this standard deviation of the frequency also depends on a parameter t o which is usually taken to be the laser pulse width . in this case , however , the self broadening of the brillouin shifted lines produces a brillouin bandwidth of 480 mhz ( see hirschberg , j . g ., et al , &# 34 ; speed of sound and temperature in the ocean by brillouin scattering ,&# 34 ; applied optics , 23 , 2624 ( 1984 )). this requires that an effective t o of 2 . 1 ns be used in equation ( 1 ) rather than the laser pulse duration . the brillouin backscatter frequency f b is a function of the acoustic velocity v s and the laser optical frequency , f o , as follows where n is the index of refraction and c is the velocity of light in free space . the sound velocity v s is given as a function of temperature by the following expression which when differentiated with respect to temperature and evaluated at 10 ° c . gives dv s / dt = 4 . 82 m / s -° c . substituting this value into equation ( 3 ) and using 6 . 54 × 10 14 hz as the optical frequency for an assumed transmitter wavelength of 459 nm , the result obtained is the temperature accuracy as a function of snr can be obtained by dividing equation ( 1 ) by df b / dt . the resultant temperature accuracy expressedas a function of depth for a representative system is shown in fig4 .