This invention relates generally to devices for detecting sound pressure waves in water and, in particular, to large scale passive sonar arrays capable of detecting weak sound waves.
To detect weak coherent sound signals in an ocean environment, a sound detecting apparatus must overcome many obstacles a few of which are ambient noise, attenuation of sound as a function of frequency and array gain.
Ambient noise in the ocean comes from many sources including biological noise such as whales and other underwater animals, miscellaneous ship noises such as gear and propeller noises, wind generated wave noise, and storm generated (rain) noise.
This ambient noise level tends to mask the noise from a particular sound source.
For long range (over 50 miles) sonar detection of a man-made sound signal in an ocean environment, almost 100% of the sound propagating in the vertical direction is ambient noise while 90-100% of the sound propagating in the horizontal direction is man-made sound signal. In other words, the main long range signal path of man-made sound in an ocean environment is in a generally horizontal plane.
Long range sound propagation in the ocean depends upon many factors such as sound frequency, ocean depth, salinity water temperature, surface waves and nature of underwater terrain.
In deep water, in temperate latitudes where there is a temperature gradient from warm surface water to deeper colder water, sound velocity will gradually decrease with depth up to a particular depth below which sound will gradually increase in velocity due to steady increase in water pressure. In the region where sound velocity reaches a minimum, underwater sound signals will be refracted back and forth across this region and can travel great distances. Depending upon the temperature gradient of the water, latitude and season, this minimum sound velocity region, defining generally horizontal plane or "sound channel", can vary in depth from 0 to 3,000 feet.
Where the sound source is located on the surface of the water or below the surface but above the sound channel, this phenomenon can produce what are known as "shadow zones" in the region above the sound channel measuring 20 to 50 miles in width where the sound intensity of the source will be very weak or non-existent. Between these shadow zones are "convergence zones" where the sound signals tend to concentrate and are more readily detected.
To avoid these shadow zones, sonar detectors should be placed at or near the convergence zone or submerged to the sound channel depth. Few prior art sonar devices used apparatus for controlling the depth of a towed detector. Most prior art sonar devices were either placed or towed on the surface or in upper several 100 feet of the ocean or placed on the ocean floor.
In relatively shallow water, that is, water shallower than the depth of the sound channel, the sound pressure wave can propagate over relatively long distances by multiple reflections from the water surface and ocean bottom. In this situation the ocean acts in the manner of a waveguide permitting long wavelength sound waves to be attenuated less than shorter wave length sound waves.
In the process of being reflected, the sound wave is further distorted and modified depending upon the size and period of surface waves and the topography of the ocean bottom.
Propagation of sound in the ocean is also very frequency dependent. For frequencies below approximately 100 Hz, sound energy is attenuated, for the most part, by losses due to reflection from the ocean surface and bottom. This would be represented by an absorption coefficient of about 0.001 to 0.003 dB per kiloyard.
For frequencies ranging from 100 Hz to about 10 KHz, sound energy is attenuated primarily by B(OH).sub.3 in the water. This would be represented by an absorption coefficient of about 0.001 to 1.0 dB per kiloyard. Above 10 KHz sound energy is principally attenuated by MgSO.sub.4. This would be represented by an absorption coefficient of about 0.001 to 1.0 dB per kiloyard at 10 KHz to 200 dB per kiloyard at about 500 KHz.
Thus, the transmission losses for an unknown sound source will be lowest for frequencies of about 1,000 Hz and below. This would be represented by an absorption coefficient of about 0.10 dB per kiloyard down to about 0.001 dB per kiloyard or less.
Because most man-made sounds in the ocean fall below 2,000 Hz, the frequency range of most underwater sound detectors or hydrophones fall between 600 Hz and 2,000 Hz with the most used range being 600 Hz to 1,200 Hz.
Since ambient noise and sound propagation in the ocean are an uncontrollable parameters, the sonar devices of the prior art have all attempted to increase array gain by various techniques including increasing the length or size of a linear array of hydrophones, design circuits that analyze the incoming signal by frequency domain techniques using Fourier analysis, or use electronic circuits to discriminate the direction of the incoming signal by various beam, aperture or receiving lobe forming techniques.
Nearly all of the prior art signal processing circuits used analog circuits and methods to increase the signal-to-noise ratio based on the RMS value of the hydrophone output signal.
Although increasing the number of hydrophones will increase hydrophone array signal-to-noise ratio, there is, however, a practical limit as to how many hydrophones can be used both as to cost and ability of the electronic circuits to control beam direction and signal processing.
Since control of beam or aperture lobe direction for receiving an incoming signal is important for increasing the signal-to-noise ratio, most prior art sonar arrays have used this technique.
The primary technique for discriminating signal direction is to measure the time delay of the pressure wave between hydrophones. Most of these circuits measure such time delay in real time, as opposed to initially storing the time delay data for later processing.
For sound pressure waves arriving perpendicular, 90 degrees, to the array, there will be no time delay detected between hydrophones for a linear array.
For sound pressure waves arriving at an angle between 0 and 90 degrees or between 90 and 180 degrees to the array, there will be a measurable time delay between adjacent hydrophones, the maximum delay being at 0 and 180 degrees.
One prior art apparatus utilized a towed linear array of equally spaced hydrophones several thousand feet long in order to increase array gain according to the theory that array gain is equal to 10 times the log of the number of hydrophones used.
This array was generally towed behind a ship. In some cases the linear array was provided with a means for controlling its depth but not necessarily its horizontal attitude.
The problem with a linear array is its flexibility and inability to always deploy into a straight line, particularly when the towing vessel makes a turning maneuver.
If a time delay method of determining signal direction is used, the ability to discriminate direction is substantially reduced while the array is in the curved condition.
Since the ability to discriminate direction greatly increases the signal-to-noise ratio of the array, such a maneuver substantially reduces the effectiveness of such a linear configuration.
In addition, for such a linear array, there is an ambiguity as to which side of the array the signal is coming from. The pressure wave front arriving from a sound source 45 degrees to the right in front of the array will produce a received signal identical to a sound source 45 degrees to the left in front of the array.
The only way for such a linear array to discriminate as to which side the signal is coming from is to use individual hydrophones sensitive to sound pressure waves only in one hemisphere.
Furthermore, many of the prior art sonar array systems are adapted to simultaneously receive signals propagating along the vertical as well as the horizontal direction. Because of the very high noise level in the vertical direction produced during a storm by the rain striking the surface of the water, sonar systems adapted to detect sound waves without distinguishing between vertical and horizontal direction are virtually useless during even a light rain storm at sea.
Because a vertically disposed linear array of hydrophones cannot be towed horizontally and maintain its linearity, such an array configuration would not be practical as a mobile sonar listening platform.