Abstract:
A system for producing a decoy to enable a target to avoid a homing torpedo that uses a sonar ping signal for homing in on the target comprises a transducer that operates in a receive mode in which it receives sonar signals and produces corresponding electrical signals and in a transmit mode in which it emits a sonar return pulse. A digital signal processor connected to the transducer is arranged to analyze the electrical signals corresponding to sonar signals received by the transducer and to determine whether they were emitted by the homing torpedo. The digital signal. processor is further arranged to switch the transducer to the transmit mode in response to receipt of a sonar ping signal from the homing torpedo and to cause the transducer to transmit a return pulse that acts as a decoy signal to the homing torpedo.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of signal generators, and in particular signal generators for producing decoy signals to a homing torpedo. In even more particularity, the subject invention relates to a signal generating method using basic Digital Signal Processing (DSP) techniques to effectively enable Counter-Measures (CM) so that the torpedo can be avoided. 
     BACKGROUND OF THE INVENTION 
     The days have long since passed when a submarine launched three or four torpedoes, each separated by half a degree of bearing, in order to ensure at least one hit on the target. Modern torpedoes incorporate powerful sonar seekers with advanced signal processing and computer technology to search for (and destroy) their targets. These torpedoes are faster and more maneuverable than any submarine or surface ship. While early acoustic torpedoes relied upon a target&#39;s radiated noise, today&#39;s torpedo uses its own active sonar to detect the target or the targets wake. The most advanced of these weapons uses target motion information from the Doppler shift of its sonar ping to eliminate false targets and increase the signal to noise ratio for better detection. 
     Torpedo sonar systems operate at relatively high frequencies (compared to a search sonar) in order to obtain the resolution necessary for target classification. One trade off is that higher frequency means higher absorption of the sonar energy in seawater, which limits the detection range of the weapon. Frequently the target knows that a torpedo has been launched and has time to initiate evasive action. Torpedo countermeasures are then deployed to prevent the torpedo from detecting and homing on the evading ship. While there are many different kinds of torpedo countermeasures employed by the world&#39;s navies, they all try to accomplish two things: attract the torpedo&#39;s attention (present a false target) and reduce the torpedo&#39;s ability to detect the true target. 
     Most of these countermeasure devices are simple noisemakers, programmed to produce noise at a high sound pressure level. Just as it is difficult to carry on a normal conversation when there is loud music playing nearby, these countermeasure devices increase the ambient noise level, which in turn decreases the torpedo&#39;s signal to noise ratio and lowers the probability that the torpedo will detect the target. 
     The major drawback to noisemakers is that their signal typically remains the same during its entire active lifetime. A particular countermeasure may be very effective against one particular type of torpedo but may not be effective against another. However, there is usually no way for a noisemaker to change its output after it has been deployed from the target ship, particularly since acoustic communications are blocked by the noise itself. There are over two dozen kinds of torpedoes available on the open market, manufactured by countries such as the U.S., U.K., Germany, Italy, France, Russia, China, and North Korea. All of these weapons have very different sonar characteristics, which makes it difficult to produce a noisemaker that will be effective against them all. 
     SUMMARY OF THE INVENTION 
     Digital signal processor (DSP) technology as used in the present invention solves many of the problems associated with prior countermeasure devices. The overall goal of the present invention is to provide a torpedo countermeasure design that is superior in both performance and cost effectiveness when compared to previous countermeasures. A primary consideration is use of inexpensive commercial DSP technology in a countermeasure design that is able to recognize and properly respond to torpedo sonar signals. 
     The entire electrical/electronic equipment package is contained within a single waterproof housing of sufficient strength to operate at tactical water depths. An external mechanical flotation system is attached and can be preset prior to launch to maintain the encased equipment package at the appropriate water depth. Further, electrical and electronic connections penetrate the waterproof hardware casing, thus permitting battery charging and complete updates of software and firmware, so that the acoustic and tactical capabilities of the device can be improved over time with minimal expense and minimal hardware modifications. 
     An object of the present invention is to overcome the disadvantages of the prior art by providing a torpedo countermeasure that produces a signal that attracts the technically sophisticated homing torpedoes. 
     An object of the invention is to provide a countermeasure that uses a single transducer to detect a torpedo sonar ping, analyze its characteristics (frequency, pulse length, etc.), and, using the transducer, transmit a return pulse that will be interpreted by the torpedo as the refection off a valid target. 
     Accordingly, a system according to the invention for producing a decoy signal to enable a target to avoid a homing torpedo that uses a sonar ping signal for homing in on the target comprises a transducer arranged to operate in a receive mode in which it receives sonar signals and produces corresponding electrical signals and in a transmit mode in which it emits a sonar return pulse. The invention further comprises a digital signal processor connected to the transducer. The digital signal processor is arranged to analyze the electrical signals corresponding to sonar signals received by the transducer and to determine whether they were emitted by the homing torpedo. The digital signal processor is further arranged to switch the transducer to the transmit mode in response to receipt of a sonar ping signal from the homing torpedo and to cause the transducer to transmit a return pulse that acts as a decoy signal to the homing torpedo. 
     The system according to the invention preferably further comprises an analog to digital converter connected to the transducer to receive analog signals therefrom and to output corresponding digital signals and a filter connected between the analog to digital converter and the digital signal processor. The filter is arranged to determine whether signals input thereto from the analog to digital converter are within a defined frequency range. 
     The system according to the invention preferably further comprises a digital to analog converter arranged to receive digital signals output from the digital signal processor and produce corresponding analog signals and an amplifier connected between the digital to analog converter and the transducer. 
     The invention preferably further comprises a front-end filter connected to the transducer and arranged to pass signals having frequencies less than a predetermined frequency, an analog to digital converter connected to receive signals passed by the front-end filter and to output corresponding digital signals and a filter connected between the analog to digital converter and the digital signal processor, the filter being arranged to determine whether signals input thereto from the analog to digital converter are within a defined frequency range. 
     A method according to the invention for producing a decoy signal to enable a target to avoid a homing torpedo that uses a sonar ping signal for homing in on the target comprises the steps of (a) receiving sonar signals with a transducer that produces electrical signals corresponding to the sonar signals, (b) determining whether the electrical signals have frequencies that are within a defined frequency range (c) processing the electrical signals to determine whether they correspond to sonar ping signals, (d) calculating an estimate of the torpedo&#39;s motion, (e) calculating a transmit time for transmitting a decoy signal, (f) calculating a Doppler frequency for the decoy signal such that the decoy signal will appear to the homing torpedo to be a return pulse from the target and (g) switching the transducer to a transmit mode to transmit the decoy signal having the calculated Doppler frequency at the calculated transmit time. 
    
    
     Other objects and many of the advantages will be readily appreciated as the subject invention becomes better understood by reference to the following detailed description, when considered in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a torpedo countermeasure system; 
     FIG. 2 is a block diagram of a first embodiment of circuitry according to the invention that may be included in an electronics module that may be included in the torpedo countermeasure of FIG. 1; 
     FIG. 3 is a signal flow diagram for the circuitry of FIG. 2; 
     FIG. 4 is a block diagram of a second embodiment of circuitry that may be included in the electronics module of FIG. 1; and 
     FIG. 5 is a signal flow diagram for the circuitry of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This disclosure describes a torpedo countermeasure. Specific circuitry and components that may be included therein are disclosed to provide a thorough description of the invention. However, it will be apparent that the present invention may be practiced without these specific circuits and components. Well-known components of the present invention are shown in block diagram form, rather than in detail, to avoid unnecessarily obscuring the invention. 
     As shown in FIG. 1, a torpedo countermeasure  10  includes an electronics module  12  placed in a watertight enclosure  14 . The torpedo countermeasure  10  also includes a float  16 , a spool and tether assembly  18 , an electrical power system  20 , which may be a conventional battery package, a payload section  22  and a transducer  24 . After the torpedo countermeasure  10  is launched into water, the float  16  is released from the enclosure  14 . The float  16  rises to the surface and supports the enclosure  14  via the spool and tether assembly  18 . This invention is directed in particular to the electronics module  12  and algorithms used thereby. 
     The torpedo countermeasure  10  might be deployed from a submarine (not shown) when the targeted submarine detects an inbound torpedo. The torpedo countermeasure  10  listens for the torpedo&#39;s acoustic incoming sonar ping, analyzes the ping, and after obtaining information such as frequency, amplitude and pulse length, the torpedo countermeasure  10  sends out a sonar pulse similar to an actual return from a target. This false target return fools the torpedo by tricking the weapon into thinking there was a valid target in the direction of the torpedo countermeasure  10 , while the evading submarine slips away. 
     Referring now to FIGS. 1 and 2, the transducer  24  is arranged to receive an input sonar ping when the torpedo countermeasure  10  is operating in a receive mode. The transducer  24  acts as hydrophone when receiving the input solar ping. The countermeasure  10  preferably includes a single transducer  24  that is used both for receiving and sending sonar signals. Transducers suitable for use in the countermeasure  10  are typically made of piezoelectric ceramic materials. Pressure variations in an input acoustic signal cause the piezoelectric crystal to produce a small time-varying voltage characteristic of the input signal. When an external time-varying voltage is applied to the piezoelectric crystal, it vibrates and produces sound waves having the same frequency as that of the applied voltage. 
     After receiving the input sonar ping, the transducer  24  produces an analog electrical signal that is input to an analog to digital converter (ADC)  26  that converts the analog signal into a corresponding a digital signal. The digital signal is then input to a filter circuit  28  that is arranged to determine the frequency of signals input thereto. Signals output from the filter circuit  28  are buffered by a buffer circuit  30  and then input to a digital signal processor (DSP)  32  that analyzes the signal to determine whether it has come from a torpedo (not shown). 
     The countermeasure  10  also operates in a transmit mode in which the DSP  32  provides a signal to a digital to analog signal converter (DAC)  34  that produces corresponding analog signals. These analog signals are input to a linear amplifier  36  that is connected to the transducer  24 . Upon receipt of the signals from the amplifier  36 , the transducer  24  produces an output sonar pulse. 
     FIG. 3 illustrates a logic flow diagram for the DSP  32  circuitry of FIG.  1 . In the receive mode, digital signals from the buffer  30  are input to the DSP  32 . The DSP  32  executes a read buffer, read clock step  38  in which a signal value is read from the buffer  30  and stored as a variable. The DSP  32  also reads and stores a time corresponding to the signal from the buffer  30 . The DSP  32  analyzes the stored signal with a frequency determination function  40  to determine its frequency. After the frequency is determined, the DSP  32  executes a ping determination step  42  to determine whether the stored signal represents a torpedo sonar ping or merely background noise. 
     If the signal is found to represent a torpedo sonar ping, then an update ping data step  44  is performed. The DSP  32  includes a memory (not shown) adequate to store ping parameters such as ping length, ping start and stop time and average frequency. If the stored signal is not found to represent a torpedo ping, then the DSP  32  evaluates whether it is time to transmit. If a torpedo sonar ping is being received, the DSP  32  executes a step  46  to determine whether the ping has ended and whether the ping was valid. At the end of a valid ping the DSP  32  performs a calculate torpedo motion estimate step  48  and also performs a calculate transmit time step  50 . 
     The DSP  32  next performs a step  52  in which to determine whether it is time to transmit. If is time to transmit, the DSP  32  executes a calculate false target Doppler step  54  in which the Doppler shifted frequency for a return pulse to the torpedo is calculated. The DSP  32  then does a transmit step  56  in which it sends a signal to the DAC  34  to initiate transmittal of a pulse that the incoming torpedo may interpret as being an echo from a fleeing target. 
     While the return pulse is being transmitted the ADC  26  is turned off to avoid confusion between the return pulse being transmitted and the incoming sonar ping. If it is not time to transmit, the DSP reads a new signal value from the buffer  30  and repeats the steps described above until it is time to transmit a return pulse. 
     Determining the frequency of the incoming sonar ping typically includes the step of performing a baseband frequency conversion performed on the digital signal output of the ADC  26  to filter out the carrier signal. The frequency of the resulting data can be determined by using methods such as a discrete time Fourier transform, a fast Fourier transform, a zero-crossings algorithm, or hybrid methods that include both a zero-crossings algorithm and a fast Fourier transform. All of these techniques for determining the frequency of a sonar ping signal are well-known in the art and are not described in detail herein. 
     FIG. 4 is a block diagram of a second embodiment of a countermeasure electronics module  58  according to the invention. The electronics module  58  includes a transducer  60  that may be substantially identical to the transducer  24  of FIG.  1 . In the receive mode, the transducer  60  produces an analog electrical signal in response to an incident sonar ping. The analog signal is input to a front-end filter  62 . 
     The front-end filter  62  includes a low-pass filter that preferably is designed to pass and amplify signals less than 100 kHz. The front-end filter  62  matches impedance and measures voltages produced by the transducer  60  when it is receiving the external acoustic signals. Signals of interest include the input sonar ping by an incoming torpedo. Initial amplification raises the low-level voltages to a signal of suitable strength for further digital processing. Initial hardware filtering eliminates unwanted low-frequency components and improves signal-to-noise ratio. 
     The filtered signal is then digitized by an ADC  64 . A suitable device for this application is commercially available from National Semiconductor as ADC model 14061, which is a 14-bit AID converter with a power rating of 390 mW. The conversion rate range is from 312.5 kHz to 2.5 MHz, which means the signal sampling rate will also be in this range. Using a zero-crossings algorithm in which the sampling rate is ten times the maximum frequency of interest gives frequency coverage of up to 250 kHz. 
     Signals output from the ADC  64  are input to a filter  66  that is preferably formed as a finite impulse response (FIR) device. Signals output from the filter  66  pass through a buffer  67  and are then input to a DSP  68  for processing. 
     A general-purpose digital signal processor such as the Motorola DSP56301 processor may be used as the DSP  68 . The Motorola DSP56301 processor may also be used for multimedia and telecommunication applications such as videoconferencing and cellular telephony. As a general purpose device with up to 42 programmable General Purpose I/O (GPIO) pins, this processor provides a great deal of flexibility making it a good fit in the architecture for this application. The DSP56301 runs at 80 MIPS using an internal 80 MHz clock. It uses 3V logic and 24-bit addressing. On-chip memory consists of 4096×24 Program RAM (or the cache option can be selected, giving a 1024×24 bit instruction cache and a 3072×24 bit Program RAM), and two data RAM spaces (X and Y) as 2048×24 bit data RAM. 
     When the countermeasure  10  operates in the transmit mode, the DSP  68  provides a digital signal to a DAC  70  that converts the digital signals into corresponding analog signals. The DAC  70  may be a model number ML2375 DAC chip manufactured by Micro Linear Corporation. Chip capabilities include a 10-bit A/D converter, a 10-bit D/A converter, an 8-bit D/A converter and a four channel multiplexer. Signals output from the DAC  70  are amplified by an amplifier  72  that is preferably a pulse width modulating (PWM) amplifier. The PWM amplifier  72  makes efficient use of the available power by using a digital signal to approximate the analog signal. Pulse width modulation maintains longer “high” pulses when the signal amplitude is high and shortens the pulse when the signal amplitude is low. 
     A suitable PWM amplifier  72  is commercially available as amplifier model number SAO7 from Apex MicroTechnology. This is a 40 volt, 500 kHz PWM amplifier requiring a power source of between 10 and 16 volts. It is designed for a wide variety of applications including high fidelity audio equipment, brush type motor control and vibration canceling amplifiers. This amplifier has a full-bridge output circuit that is capable of providing a continuous 5 amp output current to a step-up transformer  74  that drives the transducer  60 . 
     FIG. 5 illustrates a logic flow diagram for the DSP  68  circuitry of FIG.  4 . FIG. 5 shows a first group of logic steps identified as a listen mode  80  and a wait-then transmit-mode  82 . 
     The listen mode  80  includes a do loop  84  that continues until a valid ping has been received. Signals from the buffer  67  are read in a read step  86  that also reads a time corresponding to the buffer signal value. The signal is then processed by a zero crossings function  87  that includes an inspect and compare step  88  that compares the incoming sample to the preceding sample. The DSP  68  then performs a zero crossing test step  90  to determine if the buffer signal crossed zero between the present sample and the preceding sample. If the signal crossed zero, the frequency of the signal is estimated. If the data point did not cross zero since the last stored point, a value of zero is returned for the frequency. 
     The DSP  68  next executes a ping determination step  92  to determine whether the signal represents a ping. Receiving a signal in the proper frequency range is not sufficient to establish that the signal represents a sonar ping. Several signal samples must be processed to determine if their frequencies are similar and thus part of a sonar ping. The DSP  68  next performs a step  94  to determine whether the ping has ended and whether the ping was valid. After a ping has been verified, it is assumed to have ended when subsequent signals have frequencies that are not in the proper frequency band. After the ping ends, its length is calculated. 
     After the end of a valid ping, the DSP  68  performs a calculate torpedo motion estimate step  96 . The DSP  68  then executes a calculate false target Doppler step  98  in which the frequency of the decoy signal is calculated. The DSP  68  also executes a false target evasion time delay step  100  in which the proper time for sending the return pulse is calculated. At the appropriate time, the DSP  68  executes a transmit step  102  in which it sends a signal to the DAC  70  that results in the return pulse being transmitted by the transducer  60 . 
     The structures and methods disclosed herein illustrate the principles of the present invention. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects as exemplary and illustrative rather than restrictive. Therefore, the appended claims rather than the foregoing description define the scope of the invention. All modifications to the embodiments described herein that come within the meaning and range of equivalence of the claims are embraced within the scope of the invention.