Forward-looking sonar for ships and boats

Method and apparatus for detecting and locating underwater obstacles in the path of a ship or boat. The apparatus includes a pulsed wide-angle sonar projector (100) controlled by a digital signal processor (400) that emits sound pulses at frequencies of 30 kHz and less that can penetrate sediment-laden water hundreds of meters or more ahead of the ship or boat. The projector generates echoes from submerged objects. A vector sound-intensity probe (200) receives the echoes and transmits them to the digital signal processor. The digital signal processor determines the location of submerged obstacles ahead of the ship or boat from the echoes received by the probe. This information is displayed on an output device (500). The sonar projector and vector sound-intensity probe are contained separately in streamlined housings aimed in the forward direction under the bow of the ship or boat. The processor, output device and other electronics are located on board the ship or boat.

TECHNICAL FIELD

This invention relates to a sonar system at the bow of a ship or boat for detecting underwater obstacles.

BACKGROUND OF THE INVENTION

1. U.S. Pat. No. 7,054,226 May 30, 2006 issued to R. Hickling entitled “Method and Apparatus for Echolocation”,
discloses the use of the vector sound-intensity probe and ultrasound in air for applications such as robot guidance, aids to the blind, and simulating bat sonar. Use of the vector sound-intensity probe is an important part of the present invention. Sound intensity is sound-power flow per unit area. There are two types of vector sound-intensity probe: a full-space probe that measures vector sound intensity in three dimensions, described in2. R. Hickling, “Acoustic Measurement method and apparatus” U.S. Pat. No. 7,058,184 Jun. 6, 2006
which is hereby incorporated herein by reference; and a half-space vector probe that measures sound intensity in the half space above the ground or next to a wall, described in3. R. Hickling, “Vector sound-intensity probes operating in a half space”, patent application Ser. No. 11/607,376, Dec. 2, 2006.
which is hereby incorporated herein by reference. The half-space probe is used in the present invention to measure vector sound intensity ahead of the boat or ship.

Vector probes in water have been shown to measure direction accurately, as demonstrated in4. R. Hickling and W. Wei, “Use of Pitch Azimuth Plots in Determining the Direction of a Sound Source in Water with a Vector Sound-Intensity Probe”, Journ. Acoust. Soc. Amer., 97, 856-865. 1995.
What is needed is a device that uses low frequency sound in water to enable ships and boats to avoid underwater obstacles. Low-frequency sound at about 30 kHz and below is required to penetrate sediment-laden water ahead of the ship or boat. This is particularly important in riverine and littoral environments. Fish finders, acoustic cameras and narrow-beam search devices operate at higher frequencies and are incapable of penetrating very far in sediment-laden water.

SUMMARY OF THE INVENTION

The forward-looking sonar system for ships and boats has two components: (a) a pulsed wide-angle sonar projector and (b) a half-space, vector sound-intensity probe. These are connected to a digital signal processor. The projector generates echoes from submerged obstacles ahead of the ship or boat, which are received by the half-space vector probe. The digital signal processor controls the pulses from the projector and computes vector sound-intensity from the echoes received by the probe. The computer information is displayed on a suitable output device. The frequency of the pulses from the sonar projector has to be such that the pulses can penetrate sediment-laden water ahead of the ship or boat, for distances of hundreds of meters or more.

As described in patent application Ser. No. 11/607,373 incorporated herein by reference, the half-space vector probe consists of four omnidirectional hydrophones positioned at the vertices of an imaginary regular tetrahedron. The lines joining the mid points of the edges of a regular tetrahedron form a Cartesian set of XYZ axes. The origin of these axes is the measurement point of the sound-intensity vector computed by the signal processor. The Z-axis is the central axis of the probe and points in the probe's forward direction.

A half-space vector probe detects sound from the forward half of the surrounding space. Behind the probe is a solid concave axisymmetric structure lined with sound-absorbing material that is open in the probe's forward direction. This prevents noise intrusion from behind the probe. The axis of the structure coincides with the central axis of the probe. The structure can be attached to a back-plate that supports the four hydrophones of the probe.

The pulsed wide-angle projector and the vector sound-intensity probe are each contained in a streamlined, torpedo-shaped housing. The pulsed wide-angle projector and the vector sound-intensity probe both have the same central axis as their respective housings. The housings and their contents point in the forward direction of the motion of the ship or boat and are located beneath the bow. The processor and other electronics are located on board the ship or boat.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1is a block diagram showing the different components of the forward-looking sonar for ships and boats. A signal processor400drives a pulsed, wide-angle sonar projector100. The projector100operates at frequencies of roughly 30 kHz and below, in order to penetrate sediment-laden water for distances of hundreds of meters or more. The projector100generates echo pulses from submerged obstacles250that are received by a half-space, vector sound-intensity probe200. The probe200is linked, by means of a digital-acquisition system300, to a digital signal processor400. The signal processor400uses the echoes received by the probe200to detect and determine the location of submerged obstacles250ahead of the ship or boat. This information is displayed on an output device500. The distance and direction of an object determine its location. Distance is calculated using time-of-flight and the sound-intensity vector determines direction. The sonar projector100and vector sound-intensity probe200are located under the bow of the ship or boat while the electronic system1000is operated on board.

FIG. 2is a geometric drawing showing the locations of the four hydrophone1,2,3and4of the vector sound-intensity probe200, at the vertices of a regular tetrahedron225. The dashed lines are the four edges of the tetrahedron of length d. The lines joining the mid points of the edges form a Cartesian XYZ coordinate system whose origin is the measurement point M. The Z-axis is the central axis of the vector probe, pointing in the forward direction. The theory underlying the use of vector sound-intensity probes is given in References 2 and 3.FIG. 3is a schematic view of the half-space vector sound-intensity probe200, showing four omnidirectional spherical hydrophones240at the locations1,2,3and4. The figure shows the hydrophones240attached to a back-plate220by narrow tubes that contain electrical wiring. Between the hydrophones1,2,3and4and the back-plate is a sound-absorbing layer260. The wide-angle sonar projector100is shown separately above the vector probe200.

FIG. 4is a side-view of the half-space, vector sound-intensity probe200, with a concave solid shell230lined with absorbing material260behind the probe. The shell230is circularly symmetric and is attached to the front of the back plate220. The shell's axis coincides with central axis of the probe. The measurement point is at225.

Measurement calculations performed by the processor400are based on finite-difference approximations that are satisfied when the quantity 2πd is less than the wavelength of the sound from the sonar projector100. If this condition is not satisfied, heterodyning, as described in Reference 1, can increase or diminish the wavelength to satisfy the condition.

FIG. 5shows a side-view of the half-space vector sound-intensity probe200inside a torpedo-shaped, streamlined housing600. The housing has deturbulator ridges650that extend laminar flow over the housing, reducing turbulence and flow noise. The pulsed, wide-angle sonar projector100is contained in a similar streamlined housing. Streamlined housings in water generally consist of plastic shells filled with oil that have a sound transmission substantially the same as water.

FIG. 6shows the torpedo-shaped housing700of the pulsed wide-angle sonar projector100and the torpedo-shaped housing600of the half-space vector sound-intensity probe200located beneath the bow of a boat1500. The digital processor and other electronics are located on board the boat. These locations are the same for a ship.