Abstract:
A sonar system comprises a transmitter adapted to provide pulse sequences, wherein each of the pulse sequences includes pulses that reflect off an object, a receiver adapted to receive the reflected pulses, and a processor. The processor is configured to transmit a first pulse sequence via the transmitter to obtain a first distance to the object in a first distance range and to transmit a second pulse sequence via the transmitter to obtain a second distance to the object in a second distance range. The processor is configured to transmit the second pulse sequence in response to being unable to obtain the first distance to the object in the first distance range.

Description:
BACKGROUND  
       [0001]     Sonar systems are used to detect, navigate, track, classify, and locate objects in water using sound waves. Sonar systems can be used in defense and civilian applications. Military applications include using underwater sound for depth detection, navigation, ship and submarine detection, ranging, tracking, underwater communications, mine detection, and guidance and control. Civilian applications include using underwater sound for depth detection, navigation, object location such as fish finding, bottom topographic mapping, underwater beacons, wave-height measurement, underwater imaging, telemetry and control, underwater communications, ship handling and docking, anti-stranding alerts for ships, and vessel velocity measurement.  
         [0002]     A typical active sonar system includes a transmitter to generate sound waves and a receiver to sense reflected sound waves. The transmitter includes a transducer that generates sound waves and the receiver includes a transducer commonly referred to as a hydrophone that receives reflected sound waves. A short burst of energy, referred to as a sonar pulse, is generated by the transmitter. The sonar pulse travels to a target object and is reflected by the target object. The reflected sonar pulse is received by the hydrophone and the sonar system measures the time between the transmitted sonar pulse and the received reflected sonar pulse to determine the distance to the object. Often, each sonar pulse is transmitted, reflected, and received before transmitting another sonar pulse.  
         [0003]     Typically, a substantial number of transmissions are needed to enable integration of the reflected sonar pulses and to accurately determine the distance to an object. Transmitting and receiving a substantial number of sonar pulses can be a time consuming process. Also, transmitting a substantial number of sonar pulses may give away the transmitters position, which is unacceptable in some military applications. Other problems encountered include high false alarm rates, inaccuracies, and false measurements. These problems may be due to transmission reverberations being received by the hydrophone and interpreted as reflected sonar pulses or sonar pulses bouncing from the bottom to the surface and back to the bottom, in double and triple bounces, before being received by the hydrophone.  
         [0004]     For these and other reasons there is a need for the present invention.  
       SUMMARY  
       [0005]     One aspect of the present invention provides a sonar system comprising a transmitter adapted to provide pulse sequences, wherein each of the pulse sequences includes pulses that reflect off an object, a receiver adapted to receive the reflected pulses, and a processor. The processor is configured to transmit a first pulse sequence via the transmitter to obtain a first distance to the object in a first distance range and to transmit a second pulse sequence via the transmitter to obtain a second distance to the object in a second distance range. The processor is configured to transmit the second pulse sequence in response to being unable to obtain the first distance to the object in the first distance range. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a diagram illustrating one embodiment of a sonar system.  
         [0007]      FIG. 2A  is a diagram illustrating a sonar pulse sequence and a receive window for finding a depth candidate in a moderate depth range.  
         [0008]      FIG. 2B  is a diagram illustrating a sonar pulse sequence and a receive window for finding a depth candidate in a deep depth range.  
         [0009]      FIG. 2C  is a diagram illustrating a sonar pulse sequence and a receive window for finding a depth candidate in a shallow depth range.  
         [0010]      FIG. 3  is a flow diagram illustrating normal processing in one embodiment of a sonar system.  
         [0011]      FIG. 4A  is a flow diagram illustrating one part of the system process flow in one embodiment of a sonar system.  
         [0012]      FIG. 4B  is a flow diagram illustrating another part of the system process flow in one embodiment of a sonar system.  
         [0013]      FIG. 5A  is a flow diagram illustrating one part of special processing in one embodiment of a sonar system.  
         [0014]      FIG. 5B  is a flow diagram illustrating a second part of special processing in one embodiment of a sonar system.  
         [0015]      FIG. 5C  is a flow diagram illustrating a third part of special processing in one embodiment of a sonar system. 
     
    
     DETAILED DESCRIPTION  
       [0016]     In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.  
         [0017]      FIG. 1  is a diagram illustrating one embodiment of a sonar system  20 . Sonar system  20  includes a data processing unit  22 , a transmitter  24 , and a receiver  26 . Data processing unit  22  is electrically coupled to transmitter  24  via conductive path  28  and to receiver  26  via conductive path  30 . Sonar system  20  can be part of any suitable vessel, such as a surface ship, a submarine, or any other water craft, or part of any suitable equipment, such as a depth finder, or any autonomous on the water device.  
         [0018]     Data processing unit  22  includes a processor  32  and a memory  34 . Processor  32  is electrically coupled to memory  34  via conductive path  36 . Also, processor  32  is electrically coupled to transmitter  24  via conductive path  28  and to receiver  26  via conductive path  30 . Processor  32  can be any suitable computing unit, such as a micro-processor, micro-controller, digital signal processor or main frame computing system. Memory  34  can be any suitable memory or combination of memories including random access memory (RAM), read only memory (ROM), programmable read only memory (PROM), and electrically erasable programmable read only memory (EEPROM).  
         [0019]     Transmitter  24  includes transmitter circuitry  38  and a transmitter transducer  40 . Transmitter circuitry  38  is electrically coupled to processor  32  via conductive path  28  and to transmitter transducer  40  via conductive path  42 . Transmitter circuitry  38  receives electrical signals from processor  32  via conductive path  28  and supplies corresponding electrical signals to transmitter transducer  40  via conductive path  42 . Transmitter transducer  40  converts received electrical signals to sound waves, i.e. sonar pulses.  
         [0020]     Receiver  26  includes receiver circuitry  44  and a hydrophone or receiver transducer  46 . Receiver circuitry  44  is electrically coupled to processor  32  via conductive path  30  and to receiver transducer  46  via conductive path  48 . Receiver transducer  46  receives reflected sound waves, i.e., reflected sonar pulses, and converts received reflected sonar pulses to electrical signals. The electrical signals are received by receiver circuitry  44 , which supplies corresponding electrical signals to processor  32 .  
         [0021]     Sonar system  20  is a depth detection system that rapidly, accurately, and covertly determines the distance or depth from sonar system  20  to the bottom of a body of water, such as the ocean. Sonar system  20  includes a software program that is stored in memory  34  and executed by processor  32 . Processor  32  executes the software program to transmit sonar pulses via transmitter  24 , receive electrical signals from receiver  26 , and process the received electrical signals to obtain a depth result.  
         [0022]     In operation, sonar system  20  transmits one or more sonar pulse sequences to the bottom of the body of water to obtain a depth result. Each sonar pulse sequence includes multiple sonar pulses. Processor  32  transmits a sonar pulse sequence in electrical signals to transmitter  24 . Transmitter circuitry  38  receives the sonar pulse sequence and transmits corresponding electrical signals to transmitter transducer  40 , which converts the received corresponding electrical signals into sonar pulses. In one embodiment, transmitter  24  transmits the sonar pulses directly toward the bottom of the body of water to enhance covert operation of sonar system  20 .  
         [0023]     After the sonar pulses in a sonar pulse sequence are transmitted, processor  32  opens a receive window to receive electrical signals from receiver  26 . Reflected sonar pulses are received by receiver transducer  46 , which converts the received reflected sonar pulses to electrical signals. Receiver circuitry  44  receives the electrical signals from receiver transducer  46  and provides corresponding electrical signals to processor  32 . Processor  32  processes the received corresponding electrical signals to obtain a depth candidate.  
         [0024]     Processor  32  executes the software program to set sonar pulse characteristics, sonar pulse sequence characteristics, and receive window characteristics to search for a depth candidate in a selected depth range. If a depth candidate is not found in the selected depth range, processor  32  provides another set of sonar pulses in another sonar pulse sequence and opens another receive window to search for a depth candidate in another depth range. Once a depth candidate is obtained, the depth candidate is processed to reduce false alarms and refine the distance measurement prior to declaring a depth result.  
         [0025]      FIGS. 2A-2C  are diagrams illustrating sonar pulse sequences, indicated at  100 ,  200 , and  300 , and receive windows  102 ,  202 , and  302  in one embodiment of sonar system  20 . Sonar system  20  includes three sonar pulse sequences  100 ,  200 , and  300  and three receive windows  102 ,  202 , and  302 . Sonar pulse sequence  100  and receive window  102  are used to search for a depth candidate in a moderate depth range. Sonar pulse sequence  200  and receive window  202  are used to search for a depth candidate in a deep depth range, and sonar pulse sequence  300  and receive window  302  are used to search for a depth candidate in a shallow depth range. In other embodiments, the sonar system can include any suitable number of sonar pulse sequences and receive windows, such as more than three, to search for a depth candidate and obtain a depth result.  
         [0026]      FIG. 2A  is a diagram illustrating sonar pulse sequence  100  and receive window  102 . Sonar pulse sequence  100  includes sonar pulses  100   a - 100   e . Sonar pulses  100   a - 100   e  reflect off items, such as the water, particles in the water, and schools of fish to create reverberations  104 . Processor  32  opens receive window  102  after the last sonar pulse  100   e  in sonar pulse sequence  100  to receive electrical signals corresponding to reflected sonar pulses, indicated at  102   a - 102   e . The reflected sonar pulses  102   a - 102   e  correspond to sonar pulses  100   a - 100   e , where reflected sonar pulse  102   a  corresponds to sonar pulse  100   a , reflected sonar pulse  102   b  corresponds to sonar pulse  100   b , reflected sonar pulse  102   c  corresponds to sonar pulse  100   c , reflected sonar pulse  102   d  corresponds to sonar pulse  100   d , and reflected sonar pulse  102   e  corresponds to sonar pulse  100   e . Reflected sonar pulses  102   a - 102   e  are received in the presence of ambient noise (not shown) that is created in part by weather above the surface of the water and wave motion.  
         [0027]      FIG. 2B  is a diagram illustrating sonar pulse sequence  200  and receive window  202 . Sonar pulse sequence  200  includes sonar pulses  200   a - 200   e . Sonar pulses  200   a - 200   e  reflect off items, such as the water, particles in the water, and schools of fish to create reverberations  204 . Processor  32  opens receive window  202  after the last sonar pulse  200   e  in sonar pulse sequence  200  to receive electrical signals corresponding to reflected sonar pulses. Receive window  202  is held open for a longer time than receive window  102  to receive stretched reflected sonar pulses in the deep depth range. The reflected sonar pulses are stretched due to the sonar pulse beam pattern propagating to a deeper depth. As the beam pattern propagates to a deeper depth, the beam pattern spreads out and is reflected off the bottom to provide stretched reflected sonar pulses.  
         [0028]      FIG. 2C  is a diagram illustrating sonar pulse sequence  300  and receive window  302 . Sonar pulse sequence  300  includes sonar pulses  300   a - 300   e . Each of the sonar pulses  300   a - 300   e  is at a low energy level, which minimizes reverberations. Processor  32  opens receive window  302  after the last sonar pulse  300   e  in sonar pulse sequence  300 .  
         [0029]     Each of the sonar pulse sequences  100 ,  200 , and  300  includes five sonar pulses, indicated at  100   a - 100   e ,  200   a - 200   e , and  300   a - 300   e , respectively. In other embodiments, each of the sonar pulse sequences  100 ,  200 , and  300  includes any suitable number of sonar pulses, such as from 3 to 10 sonar pulses. Also, in other embodiments, each of the sonar pulse sequences  100 ,  200 , and  300  includes a different number of sonar pulses as compared to another one of the sonar pulse sequences  100 ,  200 , and  300 .  
         [0030]     Processor  32  executes the software program to select a search depth range and set values for sonar pulse characteristics, sonar pulse sequence characteristics, and receive window characteristics. Sonar pulse characteristics include the sonar pulse energy, which is related to the sonar pulse width and the magnitude of the sonar pulse pressure, referred to as the sonar pulse power. Sonar pulse sequence characteristics include inter-pulse spacing of sonar pulses in a sonar pulse sequence. Receive window characteristics include the receive window width and the time between the last sonar pulse in the sonar pulse sequence and opening the receive window.  
         [0031]     Sonar pulse characteristic values are different for each set of sonar pulses  100   a - 100   e ,  200   a - 200   e , and  300   a - 300   e . Each of the sonar pulses  100   a - 100   e  is at a moderate energy level and includes a moderate width (Wmod) and a moderate power level (Pmod). Each of the sonar pulses  200   a - 200   e  is at a high energy level and includes a wide width (Wwide) and a high power level (Phigh). In contrast, each of the sonar pulses  300   a - 300   e  is at a low energy level and includes a narrow width (Wnar) and a low power level (Plow). In one embodiment, each of the sonar pulses  100   a - 100   e  includes a wide width similar to wide width (Wwide) of sonar pulses  200   a - 200   e.    
         [0032]     Sonar pulse sequence characteristic values are different for each sonar pulse sequence  100 ,  200 , and  300 . Sonar pulse sequences  100  and  200  include two inter-pulse spacing values to reduce ambiguity in received reflected sonar pulses. Sonar pulse sequence  100  includes inter-pulse spacing S 1  and S 2  and sonar pulse sequence  200  includes inter-pulse spacing S 3  and S 4 . In sonar pulse sequence  100 , inter-pulse spacing S 1  is greater than S 2 . In sonar pulse sequence  200 , inter-pulse spacing S 3  is greater than S 4 . As between sonar pulse sequences  100  and  200 , inter-pulse spacing S 3  is greater than inter-pulse spacing S 1  and inter-pulse spacing S 4  may be greater than inter-pulse spacing S 2 . The greater inter-pulse spacing in sonar pulse sequence  200  is used to search for a depth candidate in a deep depth range and reduce problems associated with the sonar pulse stretching. The moderate inter-pulse spacing in sonar pulse sequence  100  is used to search for a depth candidate in a moderate depth range. In contrast, sonar pulse sequence  300  includes one inter-pulse spacing S 5 , which is small to search for a depth candidate in a shallow depth range. In one embodiment, inter-pulse spacing S 5  is less than any other inter-pulse spacing S 1 , S 2 , S 3 , and S 4 . In one embodiment, moderate depth range inter-pulse spacing value S 1  is 1.5 or more times wider than shallow inter-pulse spacing value S 5 , and deep depth range inter-pulse spacing value S 3  is 2 or more times wider than shallow inter-pulse spacing value S 5 .  
         [0033]     Receive window characteristic values are different for each receive window  102 ,  202 , and  302 . Receive window  102  is opened a time TI after the last sonar pulse  100   e  in sonar pulse sequence  100  and receive window  102  remains open for a time T 2 . Receive window  202  is opened a time T 3  after the last sonar pulse  200   e  in sonar pulse sequence  200  and receive window  202  remains open for a time T 4 . Receive window  302  is opened a time T 5  after the last sonar pulse  300   e  in sonar pulse sequence  300  and receive window  302  remains open for a time T 6 . As between receive windows  102  and  202 , time T 3  is greater than time T 1  and time T 4  is greater than time T 2 . The greater times T 3  and T 4  are used to open receive window  202  later and search for a depth candidate in a deep depth range, including receiving stretched reflected sonar pulses. The moderate times T 1  and T 2  are used to open receive window  102  in a moderate time frame and search for a depth candidate in a moderate depth range. In contrast, receive window  302  is opened in a short time T 5 , which is less than time T 1  and time T 3 , to search for a depth candidate in a shallow depth range. Also, receive window  302  is closed in a short time T 6 , which is less than time T 2  and time T 4 , as sonar pulse sequence  300  is shorter than sonar pulse sequence  100  and sonar pulse sequence  200 .  
         [0034]     Sonar pulses  100   a - 100   e  are moderate energy sonar pulses in a moderately spaced sonar pulse sequence  100 . Receive window  102  is opened a moderate length of time after the last sonar pulse  100   e  in sonar pulse sequence  100  and left open a moderate length of time to search for reflected sonar pulses in a moderate depth range. Sonar pulses  200   a - 200   e  are high energy sonar pulses in a widely spaced sonar pulse sequence  200 . Receive window  202  is opened a greater length of time after the last sonar pulse  200   e  in sonar pulse sequence  200  and left open a greater length of time to search for reflected sonar pulses in a deep depth range. Sonar pulses  300   a - 300   e  are low energy sonar pulses in a narrowly spaced sonar pulse sequence  300 . Receive window  302  is opened a short length of time after the last sonar pulse  300   e  in sonar pulse sequence  300  and left open a short length of time to search for reflected sonar pulses in a shallow depth range.  
         [0035]     In operation, processor  32  transmits one of the sonar pulse sequences  100 ,  200 , and  300  and opens one of the receive windows  102 ,  202 , and  302  to search for a depth candidate in a selected depth range. To search for a depth candidate in a moderate depth range, processor  32  transmits sonar pulse sequence  100  and opens receive window  102 . To search for a depth candidate in a deep depth range, processor  32  transmits sonar pulse sequence  200  and opens receive window  202 . Processor  32  transmits sonar pulse sequence  300  and opens receive window  302  to search for a depth candidate in a shallow depth range. In one embodiment, the moderate depth range is from 350 fathoms to 2300 fathoms, the deep depth range is from 2000 fathoms to the maximum depth of the ocean, and the shallow depth range is from 100 fathoms to 400 fathoms. In other embodiments, the moderate, deep, and shallow depth ranges can be any suitable distance ranges, such as a moderate depth range of 35 fathoms to 230 fathoms, a deep depth range of 200 fathoms to 400 fathoms, and a shallow depth range of 10 to 40 fathoms.  
         [0036]     Processor  32  receives electrical signals corresponding to reflected sonar pulses during receive windows  102 ,  202 , and  302 . Processor  32  receives electrical signals corresponding to reflected sonar pulses  102   a - 102   e  during receive window  102  if the bottom of the body of water is detected in the moderate depth range. Processor  32  receives electrical signals corresponding to reflected sonar pulses during receive window  202  if the bottom of the body of water is detected in the deep depth range, and processor  32  receives electrical signals corresponding to reflected sonar pulses during receive window  302  if the bottom of the body of water is detected in the shallow depth range. Processor  32  processes the received electrical signals to obtain a depth candidate and a depth result.  
         [0037]     To obtain a depth candidate, processor  32  samples the received electrical signals and digitizes each sample. Processor  32  calculates amplitude correlations for groups of reflected sonar pulses. In one embodiment, amplitude correlations are calculated for groups of three reflected sonar pulses as shown in Equations I and II.  
               RO   ⁢           ⁢     (   j   )       =       ∑   i     ⁢           ⁢     A   ⁢           ⁢     (     j   +   i     )     *   A   ⁢           ⁢     (     j   +   KL   +   i     )     *   A   ⁢           ⁢     (     j   +   KL   +   KM   +   i     )                 Equation   ⁢           ⁢   I                 RE   ⁢           ⁢     (   j   )       =       ∑   i     ⁢           ⁢     A   ⁢           ⁢     (     j   +   i     )     *   A   ⁢           ⁢     (     j   +   KM   +   i     )     *   A   ⁢           ⁢     (     j   +   KL   +   KM   +   i     )                 Equation   ⁢           ⁢   II             
 
         [0038]     In Equation I, RO is the amplitude correlation for a group of three reflected sonar pulses beginning with an odd numbered pulse. RO is equal to a summation of amplitude products for sample j over summation variable i. Sample amplitudes from three consecutive reflected sonar pulses are multiplied to obtain one amplitude product. In one embodiment, i is varied from minus four to plus four to obtain nine amplitude products. That is, four amplitude products prior to sample j, the sample j amplitude product, and four amplitude products after sample j. The nine amplitude products are summed to obtain RO for sample j.  
         [0039]     Sample amplitudes are taken from three consecutive reflected sonar pulses, where A is the amplitude of a sample, K is the sampling rate used to sample the received electrical signals in samples per second, L is a first pulse spacing from the beginning of an odd numbered pulse to the beginning of an even numbered pulse in seconds, and M is a second pulse spacing from the beginning of an even numbered pulse to the beginning of an odd numbered pulse in seconds. KL is the number of samples in first pulse spacing L and KM is the number of samples in second pulse spacing M.  
         [0040]     In sonar pulse sequence  100 , first pulse spacing L equals Wmod+S 1  and second pulse spacing M equals Wmod+S 2 . In sonar pulse sequence  200 , first pulse spacing L equals Wwide+S 3  and second pulse spacing M equals Wwide+S 4 . In sonar pulse sequence  300 , first pulse spacing L equals Wnar+S 5  and second pulse spacing M equals Wnar+S 5 .  
         [0041]     In Equation II, RE is the amplitude correlation for a group of reflected sonar pulses beginning with an even numbered pulse. RE is equal to a summation of amplitude products for sample j over summation variable i. Sample amplitudes are taken from three consecutive reflected sonar pulses, where A is the amplitude of a sample, KL is the number of samples in first pulse spacing L, and KM is the number of samples in second pulse spacing M. Sample amplitudes from three consecutive reflected sonar pulses are used to obtain one amplitude product. In one embodiment, i is varied from minus four to plus four to obtain nine amplitude products. That is, four amplitude products prior to sample j, the sample j amplitude product, and four amplitude products after sample j. The nine amplitude products are summed to obtain RE for data sample j.  
         [0042]     Processor  32  obtains amplitude correlations and calculates amplitude correlation product values RP. An amplitude correlation product value RP is equal to amplitude correlations beginning with odd numbered pulses RO multiplied times amplitude correlations beginning with even numbered pulses RE. The number of product terms is based on the number of sonar pulses transmitted in a sonar pulse sequence and the number of reflected sonar pulses used to calculate the amplitude correlations.  
         [0043]     In one embodiment, five sonar pulses are transmitted in each sonar pulse sequence. The five reflected sonar pulses are taken three at a time to obtain three amplitude correlations, namely, amplitude correlations for the first, second, and third sonar pulses, the second, third, and fourth sonar pulses, and the third, fourth, and fifth sonar pulses. These three amplitude correlations are multiplied together to obtain the numerator of the amplitude correlation product value RP for a given sample h, as shown in Equation III.  
               RP   ⁢           ⁢     (   h   )       =       RO   ⁢           ⁢     (   h   )     *   RE   ⁢           ⁢     (     h   +   KL     )     *   RO   ⁢           ⁢     (     h   +   KL   +   KM     )           [     R   ⁢           ⁢       (   h   )     max       ]     n               Equation   ⁢           ⁢   III             
 
         [0044]     In Equation III, the denominator is a normalizing factor, where n is the number of product terms in the numerator and R is the maximum amplitude correlation value. In other embodiments, any suitable number of sonar pulses can be transmitted and any suitable number of amplitude correlations obtained to calculate the amplitude correlation product value RP.  
         [0045]     Processor  32  uses amplitude correlation product values RP to find the leading edges of received reflected sonar pulses, such as leading edges  106   a - 106   e  of reflected sonar pulses  102   a - 102   e . The time between transmitted sonar pulses, such as  100   a - 100   e , and the leading edges of reflected sonar pulses, such as leading edges  106   a - 106   e  of reflected sonar pulses  102   a - 102   e , is related to the distance or depth from the sonar system  20  to the bottom of the body of water. Processor  32  compares the maximum amplitude correlation product value RP from the received reflected sonar pulses to an amplitude correlation product threshold value. If the maximum correlation product value RP is greater than the amplitude correlation product threshold value, processor  32  determines the distance from sonar system  20  to the bottom of the body of water using the time between transmitted sonar pulses and the leading edges of reflected sonar pulses. Processor  32  stores the distance as a depth candidate. If the maximum correlation product value RP is less than the correlation product threshold value, processor  32  continues processing.  
         [0046]      FIG. 3  is a flow diagram illustrating normal processing in one embodiment of sonar system  20 . At  400 , sonar system  20  executes a moderate depth range search to find a depth candidate. Sonar system  20  transmits sonar pulse sequence  100  and processor  32  opens receive window  102 . Processor  32  samples and processes electrical signals received during the open receive window  102 . At  402 , if a depth candidate is found in the moderate depth range search, sonar system  20  continues processing at  404 . If a depth candidate is not found in the moderate depth range search, processing continues at  406  with a deep depth range search.  
         [0047]     In the deep depth range search at  406 , sonar system  20  transmits sonar pulse sequence  200  and processor  32  opens receive window  202 . Processor  32  samples and processes electrical signals received during the open receive window  202 . At  408 , if a depth candidate is found in the deep depth range search, sonar system  20  continues processing at  404 . If a depth candidate is not found in the deep depth range search, processing continues at  410  with a shallow depth range search.  
         [0048]     In the shallow depth range search at  410 , sonar system  20  transmits sonar pulse sequence  300  and processor  32  opens receive window  302 . Processor  32  samples and processes electrical signals received during the open receive window  302 . At  412 , if a depth candidate is found in the shallow depth range search, sonar system  20  continues processing at  404 . If a depth candidate is not found in the shallow depth range search, sonar system  20  repeats the series of searches beginning with the moderate depth range search at  400  until a depth candidate is found. In one embodiment, a depth candidate may be found by lowering the amplitude correlation product threshold to find a depth candidate and confirming the depth candidate with multiple distance measurements, such as when the bottom is sloping.  
         [0049]     Sonar system  20  continues processing depth candidates at  404 . In one embodiment, sonar system  20  confirms a depth candidate is not a false alarm by comparing the magnitude of one or more electrical signals that resulted in the depth candidate to a signal strength threshold. If the magnitude of the one or more electrical signals is greater than the signal strength threshold, the depth candidate is confirmed as a valid depth candidate and not a false alarm. If the magnitude of the one or more electrical signals is less than the signal strength threshold, the depth candidate is discarded and processing continues to find another depth candidate. In one embodiment, sonar system  20  refines the depth candidate distance by executing look back processing on the electrical signals processed to find the depth.  
         [0050]      FIGS. 4A and 4B  are flow diagrams illustrating the system process flow in one embodiment of sonar system  20 . At  500 , sonar system  20  executes an initial search to find a depth candidate in a moderate depth range. Sonar system  20  transmits sonar pulse sequence  100  and processor  32  opens receive window  102 . Processor  32  samples and digitizes electrical signals received during the open receive window  102 . Processor  32  calculates amplitude correlations and amplitude correlation product values from the digitized samples to find a depth candidate. At  502 , if a depth candidate is found in the initial moderate depth range search at  500 , sonar system  20  continues processing at  504  (shown in  FIG. 4B ). If a depth candidate is not found in the initial moderate depth range search at  500 , processing continues at  506  with a second search. In one embodiment, in the initial search at  500 , processor  32  opens receive window  102  earlier and leaves receive window  102  open longer than in other moderate depth range searches to find depth candidates in a shallower depth range as well as the moderate depth range.  
         [0051]     In the second search at  506 , sonar system  20  executes a deep depth range search. Sonar system  20  transmits sonar pulse sequence  200  and processor  32  opens receive window  202 . Processor  32  samples and digitizes electrical signals received during the open receive window  202 . Processor  32  calculates amplitude correlations and amplitude correlation product values from the digitized samples to find a depth candidate. At  508 , if a depth candidate is found in the deep depth range search at  506 , sonar system  20  continues processing at  504 . If a depth candidate is not found in the deep depth range search at  506 , processing continues at  510  with special processing.  
         [0052]     In special processing at  510 , sonar system  20  including processor  32  executes moderate, deep, and shallow depth range searches in a search order determined by search results, such as finding a depth candidate, the maximum amplitude correlation product value, and depth candidate distances. Also, in special processing sonar system  20  guards against false detections, such as false detections due to reverberations and false detections due to sonar pulses bouncing from the bottom to the surface and back to the bottom in double and triple bounces or more. Special processing at  508  is further described later in this specification. At  512 , if a depth candidate is found during special processing at  510 , sonar system  20  continues processing at  504 . If a depth candidate is not found in special processing at  510 , processing continues at  514  with normal processing.  
         [0053]     In normal processing at  514 , sonar system  20  executes moderate, deep, and shallow depth range searches to find a depth candidate. Normal processing at  514  is similar to the normal processing of  FIG. 3 . At  516 , if a depth candidate is found during normal processing at  514 , sonar system  20  continues processing at  504 . If a depth candidate is not found in normal processing at  514 , sonar system  20  repeats the search process until a depth candidate is found. In one embodiment, sonar system  20  repeats the search process beginning with the initial moderate depth range search at  500 . In one embodiment, sonar system  20  repeats the search process beginning with special processing at  510  until a depth candidate is found.  
         [0054]     Low correlation processing at  520  can be reached if sonar system  20  obtains low amplitude correlations and low amplitude correlation product values RP during previous searches. Low amplitude correlations and low amplitude correlation product values RP may be due to searching for a depth candidate in a highly sloped bottom region or in rough terrain.  
         [0055]     In low correlation processing at  520 , sonar system  20  stores the maximum amplitude correlation product value RP and the corresponding depth if the maximum amplitude correlation product value RP is greater than a low amplitude correlation product value threshold. The maximum amplitude correlation product value and corresponding depth are stored in a table. In one embodiment, sonar system  20  builds the table from previous searches. In one embodiment, sonar system  20  executes additional searches in low correlation processing at  520  to build the table.  
         [0056]     Sonar system  20  including processor  32  locates a preliminary depth candidate DEPp in the table. Sonar system  20  also locates depth entries in the table that fall within a depth window of the preliminary depth candidate DEPp. The depth window is centered at the preliminary depth candidate DEPp and the depth window size is a function of the preliminary depth candidate DEPp as shown in Equation IV. 
 
DEPTH WINDOW SIZE=MAXIMUM (MIN,  R*DEPp )   Equation IV 
 
         [0057]     In Equation IV, the depth window size is equal to either a MIN value or R*DEPp, whichever is larger, where MIN is a predetermined minimum window size, R is a constant multiplier, and DEPp is the preliminary depth candidate.  
         [0058]     Processor  32  sums the maximum amplitude correlation product values RP that correspond to depth entries located within the depth window. If the sum is greater than a low correlation sum threshold value, the preliminary depth candidate DEPp is declared a depth candidate and, at  522 , processing continues at  504 . If the sum is less than the low correlation sum threshold value, the preliminary depth candidate DEPp is discarded and, at  522 , sonar system  20  repeats the search process. In one embodiment, sonar system  20  repeats the search process beginning with the initial moderate depth range search at  500 . In one embodiment, sonar system  20  repeats the search process beginning with special processing at  510 .  
         [0059]     At  504 , sonar system  20  executes a low signal amplitude test to guard against false detections and water noise. In the low signal amplitude test, processor  32  calculates one or more mean signal levels of electrical signals corresponding to the received reflected sonar pulses used to obtain the current depth candidate. The one or more mean signal levels are compared to a signal strength threshold level. If the one or more mean signal levels are greater than the signal strength threshold level, the depth candidate is confirmed. The depth candidate is discarded if the one or more mean signal levels are less than the signal strength threshold level.  
         [0060]     In one embodiment, if the depth candidate was the result of a moderate depth range search, the mean signal level of electrical signals corresponding to the first received reflected sonar pulse is calculated and compared to a fixed signal strength threshold level. If the mean signal level corresponding to the first received reflected sonar pulse is greater than the signal strength threshold level, the depth candidate is confirmed. The depth candidate is discarded if the mean signal level is less than the fixed signal strength threshold level.  
         [0061]     In one embodiment, if the depth candidate was the result of a deep depth range search, the mean signal levels of electrical signals corresponding to the second and third received reflected sonar pulses are calculated and compared to a noise adaptive signal strength level NAL, shown in Equation V. 
 
 NAL= MINIMUM (FIXED,  K *AMBIENT)   Equation V 
 
         [0062]     In Equation V, NAL is equal to either FIXED or K*AMBIENT, whichever is less, where FIXED is a fixed signal strength threshold level, AMBIENT is the minimum noise amplitude from three noise measurements taken during the receive window corresponding to the current depth and K is a constant multiplier. If each of the mean signal levels corresponding to the second and third received reflected sonar pulses is greater than NAL, the depth candidate is confirmed. The depth candidate is discarded if each of the mean signal levels is less than NAL.  
         [0063]     At  524 , if the depth candidate is not confirmed, sonar system  20  repeats the search process. In one embodiment, sonar system  20  repeats the search process beginning with the initial moderate depth range search at  500 . In one embodiment, sonar system  20  repeats the search process beginning with special processing at  510 . If the depth candidate is confirmed, sonar system  20  continues processing at  526  with look back processing.  
         [0064]     In look back processing at  526 , sonar system  20  refines the distance measurement of the confirmed depth candidate to provide a more precise depth result at  528 . Look back processing at  526  refines the location of the leading edge of the first received reflected sonar pulse to provide a more precise depth measurement. Look back processing at  526  is especially helpful in refining depth measurements taken over highly sloped ocean bottoms in the deep depth range, where the reflected sonar pulses are stretched and do not have well defined or sharp leading edges.  
         [0065]     Processor  32  repeats amplitude correlation and amplitude correlation product value calculations starting a depth dependent time prior to the depth candidate time. Processor  32  compares the amplitude correlation product values to a lower amplitude correlation product threshold, where the first crossing of the lower amplitude correlation product threshold provides the final depth result at  528 .  
         [0066]      FIGS. 5A-5C  are flow diagrams illustrating special processing in one embodiment of sonar system  20 . The special processing of  FIGS. 5A-5C  is one embodiment of special processing at  510  (shown in  FIG. 4A ). At  600  (shown in  FIG. 5A ), sonar system  20  executes a moderate depth range search to find a depth candidate. Sonar system  20  transmits sonar pulse sequence  100  and processor  32  opens receive window  102 . Processor  32  samples and digitizes electrical signals received during the open receive window  102 . Processor  32  calculates amplitude correlations and amplitude correlation products from the digitized samples to find a depth candidate. In one embodiment, in the moderate depth range search at  600 , processor  32  opens receive window  102  earlier and leaves receive window  102  open longer than in other moderate depth range searches to find depth candidates in a shallower depth range as well as the moderate depth range.  
         [0067]     At  602 , if a depth candidate is found in the moderate depth range search at  600  and the depth candidate distance value is less than a reverberation threshold value, sonar system  20  continues processing at  604 . The reverberation threshold value is set to distinguish pressure readings received by receiver  26 , which may be due to reverberations from the transmitted sonar pulse sequence  100 . If the depth candidate value is less than the reverberation threshold value, the depth candidate may be due to reverberations and processing continues at  604 . If a depth candidate is not found in the moderate depth range search at  600  or the depth candidate value is greater than the reverberation threshold value, sonar system  20  continues processing at  606 .  
         [0068]     At  606 , sonar system  20  determines if the depth candidate may be due to multiple bounces, such as sonar pulses in sonar pulse sequence  100  bouncing from the bottom to the surface and back to the bottom. The depth candidate may be due to multiple bounces if the depth candidate value is greater than a predetermined distance threshold. In one embodiment, sonar system  20  checks to see if the maximum amplitude product value RP is greater than an amplitude product threshold value and if the depth candidate distance value is greater than a predetermined distance threshold. If the maximum amplitude product value RP is greater than the amplitude threshold value and the depth candidate distance value is greater than the predetermined distance threshold, sonar system  20  continues processing at  608  (shown in  FIG. 5C ). If a depth candidate was not found in the moderate depth range search at  600  or the maximum amplitude product value RP is not greater than the amplitude threshold value or the depth candidate value is less than the predetermined distance threshold, sonar system  20  continues at  610  with system processing, such as at  512  (shown in  FIG. 4A ). In one embodiment, sonar system  20  continues at  610  with system processing, such as in normal processing and a deep depth range search at  514  (shown in  FIG. 4A ).  
         [0069]     At  604 , the depth candidate obtained in the moderate depth range search at  600  may be due to reverberations. Sonar system  20  discards this depth candidate and searches for another depth candidate in the same data gathered during receive window  102  of the moderate depth range search at  600 , but starting later in the receive window  102 . Processor  32  calculates amplitude correlations and amplitude correlation products from the digitized samples to find another depth candidate. At  612 , if another depth candidate is found in the data from the moderate depth range search at  600 , sonar system  20  continues processing at  614 . If another depth candidate is not found, processing continues at  616  with a deep depth range search.  
         [0070]     At  614 , sonar system  20  stores the depth candidate found at  604  as a moderate depth range candidate and executes a shallow depth range search. The shallow depth range search is executed to sort out whether or not there is a shallow depth range candidate that is not due to reverberations. Sonar system  20  transmits sonar pulse sequence  300  and processor  32  opens receive window  302 . Processor  32  samples and digitizes electrical signals received during the open receive window  302 . Processor  32  calculates amplitude correlations and amplitude correlation products from the digitized samples to find a shallow depth range candidate. At  618 , if a shallow depth range candidate is found in the shallow depth range search at  614 , sonar system  20  continues processing at  620 . If a shallow depth range candidate is not found in the shallow depth range search at  614 , processing continues at  622 .  
         [0071]     At  620 , the special processing depth candidate is set equal to the shallow depth range candidate and processing continues at  610 . At  622 , the special processing depth candidate is set equal to the moderate depth range candidate and processing continues at  610 . Sonar system  20  continues at  610  with system processing, such as at  512  (shown in  FIG. 4A ).  
         [0072]     At  616 , sonar system  20  executes a deep depth range search. Sonar system  20  transmits sonar pulse sequence  200  and processor  32  opens receive window  202 . Processor  32  samples and digitizes electrical signals received during the open receive window  202 . Processor  32  calculates amplitude correlations and amplitude correlation products from the digitized samples to find a deep depth range candidate.  
         [0073]     At  624  (shown in  FIG. 5B ), if a deep depth range candidate is found in the deep depth range search at  616 , sonar system  20  continues processing at  626 . If a deep depth range candidate is not found in the deep depth range search at  616 , sonar system  20  continues at  628  with system processing, such as at  512  (shown in  FIG. 4A ). In one embodiment, sonar system  20  continues at  628  with system processing, such as in normal processing and a shallow depth range search at  514  (shown in  FIG. 4A ).  
         [0074]     At  626 , sonar system  20  stores the deep depth range candidate found at  616  and executes a shallow depth range search. The shallow depth range search is executed to sort out whether or not there is a shallow depth range candidate that is not due to reverberations. Sonar system  20  transmits sonar pulse sequence  300  and processor  32  opens receive window  302 . Processor  32  samples and digitizes electrical signals received during the open receive window  302 . Processor  32  calculates amplitude correlations and amplitude correlation products from the digitized samples to find a shallow depth range candidate. At  630 , if a shallow depth range candidate is found in the shallow depth range search at  626 , sonar system  20  continues processing at  632 . If a shallow depth range candidate is not found in the shallow depth range search at  626 , processing continues at  634 .  
         [0075]     At  632 , the special processing depth candidate is set equal to the shallow depth range candidate and processing continues at  628 . At  634 , the special processing depth candidate is set equal to the deep depth range candidate and processing continues at  628 . Sonar system  20  continues at  628  with system processing, such as at  512  (shown in  FIG. 4A ).  
         [0076]     At  608  (shown in  FIG. 5C ), the depth candidate from the moderate depth range search at  600  is greater than the threshold value for possible multiple bounce issues. In resolving possible multiple bounce issues, sonar system  20  sets a flag and stores the moderate depth range candidate found at  600  as a first moderate depth range candidate. Sonar system  20  also executes a deep depth range search at  608 . In the deep depth range search, sonar system  20  transmits sonar pulse sequence  200  and processor  32  opens receive window  202 . Processor  32  samples and digitizes electrical signals received during the open receive window  202 . Processor  32  calculates amplitude correlations and amplitude correlation products from the digitized samples to find a deep depth range candidate.  
         [0077]     At  636 , if a deep depth range candidate is found in the deep depth range search at  608 , sonar system  20  continues processing at  638 . If a deep depth range candidate is not found in the deep depth range search at  608 , sonar system  20  clears the flag and discards the first moderate depth range candidate. At  640 , sonar system  20  continues with system processing, such as at  512  (shown in  FIG. 4A ). In one embodiment, sonar system  20  continues at  640  with system processing, such as in normal processing and a shallow depth range search at  514  (shown in  FIG. 4A ).  
         [0078]     At  638 , sonar system  20  including processor  32  checks to see if the flag is set and if the deep depth range candidate is twice the first moderate depth range candidate. The deep depth range candidate is in a double bounce range if the deep depth range candidate is twice the first moderate depth range candidate, and processing continues at  642  to resolve the multiple bounce issue. If the deep depth range candidate is not in the double bounce range, sonar system  20  sets the special processing depth candidate equal to the deep depth range candidate at  644  and processing continues at  640  with system processing, such as at  512  (shown in  FIG. 4A ).  
         [0079]     At  642 , sonar system  20  stores the deep depth range candidate found at  608  and executes a second moderate depth range search. In the second moderate depth range search at  642 , sonar system  20  transmits sonar pulse sequence  100  and processor  32  opens receive window  102 . Processor  32  samples and digitizes electrical signals received during the open receive window  102 . Processor  32  calculates amplitude correlations and amplitude correlation products from the digitized samples to find a moderate depth range candidate.  
         [0080]     At  646 , if a moderate depth range candidate is found in the second moderate depth range search at  642 , sonar system  20  stores the moderate depth range candidate as a second moderate depth range candidate and continues processing at  648 . If a moderate depth range candidate is not found in the second moderate depth range search at  642 , sonar system  20  continues processing at  650  by clearing the flag and setting the special processing depth candidate equal to the deep depth range candidate found at  608 . Processing continues at  640  with system processing, such as at  512  (shown in  FIG. 4A ).  
         [0081]     At  648 , if the second moderate depth range candidate is near the first moderate depth range candidate, sonar system  20  clears the flag and at  652  sets the special processing depth candidate equal to the second moderate depth range candidate found at  642 . Processing continues at  640  with system processing, such as at  512  (shown in  FIG. 4A ). If the second moderate depth range candidate is not near the first moderate depth range candidate, sonar system  20  clears the flag and at  650  sets the special processing depth candidate equal to the deep depth range candidate found at  608 . Processing continues at  640  with system processing, such as at  512  (shown in  FIG. 4A ).  
         [0082]     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.