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SONAR : 

SONAR  Is an acronym for ‘Sound Navigation and Ranging.’  Uses acoustic signals propagated through water to detect, classify, and localize underwater objects.  A sonar system generally consists of:-  A transmitter that transmits a signal through water.  A receiver that receives the transmitted signal which has been degraded due to:  Underwater propagation effects.  Ambient noise.  Interference from other signal sources such as surface ships.  A processing unit that processes the received signal.  Various displays that aid human operators to detect, classify and localize signals.

SONAR : 

SONAR  Sound (i.e. acoustic energy), propagates better underwater than do other types of energy.  For example, both light and radio waves are attenuated to a far greater degree underwater than are sound waves.  Because of the above, sound waves are generally used to extract information about underwater objects.  There are two types of sonar which are: -  Passive sonar.  Active sonar.

Passive sonar : 

Passive sonar  Passive sonar, uses sound radiated from the underwater object itself.  Transmission through the ocean, from the source to a receiving sensor, is “one way.”  Passive sonar signals are primarily modeled as random signals.  In the ocean environment, an arbitrarily narrow frequency width is never observed and signals have some finite bandwidth.  The full spectrum of most underwater signals is quite colourful.  In fact, the signals of interest are not unlike speech signals, except that the signal-to-noise ratio (SNR) is much higher for speech than for sonar.

Active sonar : 

Active sonar  Active sonar involves echo ranging where an acoustical signal is transmitted from a source and reflected echoes are received back from the object.  Transmissions from a transmitter to an object and back to a receiving sensor are two way.  There are three types of active sonar systems. These are: - (i) Monostatic In this, the source and receiver are located on the same platform (e.g. a surface ship). (ii) Bistatic The transmitter and receiver are on different platforms. (iii) Multistatic One or more transmitters and multiple receivers are located on different receiving platforms or ships.

Active sonar : 

Active sonar  Received active sonar signals can be viewed as consisting of the results of: -  The transmit signal convolved with the medium and reflector transfer functions.  A random (noise) component.  The Doppler imparted frequency shift to the reflected signal makes the total system effect non-linear, thereby complicating analysis and processing of these signals.  In addition, in active systems, the noise is typically correlated with the signal, making detection of signals more difficult.

SONAR : 

SONAR A typical underwater acoustical signal processing scenario is as shown in the figure below.

Underwater Propagation : 

Underwater Propagation  Sound speed c in the ocean, in general lies between 1450-1540 m/s and varies as a function of several physical parameters, such as:  Temperature.  Salinity.  Pressure (depth).  Variations in sound speed can significantly affect the propagation (range or quality) of sound in the ocean.

Sound Velocity Profiles : 

Sound Velocity Profiles  Sound rays that are normal to the signal acoustic wave front can be traced from the source to the receiver by a process called ray tracing.  Ray tracing models are however only used for high frequency signals and in deep water.  Generally, if the depth-to-wavelength ratio is 100 or more, ray tracing models are accurate. Below this ratio, corrections must be made to these models.  The acoustic ray paths are not straight, but bend in a manner similar to optical rays focused by a lens.  The ray paths are determined by the sound velocity profile (SVP) or sound speed profile (SSP) – that is, the speed of sound in water as a function of water depth.  The sound speed not only varies with depth but also varies in different regions of the ocean and with time as well.

Sound Velocity Profiles : 

Sound Velocity Profiles A typical deepwater sound velocity profile as a function of depth is shown in figure 9.2 below. In deep water, the SVP fluctuates the most in the upper ocean due to variations of temperature and weather. Just below the sea surface is the surface layer where the sound speed is greatly affected by temperature and wind action.

Sound Velocity Profiles : 

Sound Velocity Profiles Below the surface layer lies the seasonal thermo cline where the temperature and speed decrease with depth and the variations are seasonal. In the next layer, the main thermo cline, the temperature and speed decrease with depth and surface conditions or seasons have little effect. Finally, there is the deep isothermal layer where the temperature is nearly constant at 39°F and the sound velocity increases almost linearly with depth. If the sound speed is a minimum at a certain depth below the surface, then this depth is called the axis of the underwater sound channel - also called the sound fixing and ranging (SOFAR) channel. The sound velocity increases both above and below this axis.

Sound Velocity Profiles : 

Sound Velocity Profiles When the sound wave travels through a medium with a sound speed gradient, the direction of travel of the sound wave is bent toward the area of lower sound speed. In shallow water (water depth of less than 100 m ), the SVP is irregular and difficult to predict because of:  Large surface temperature and salinity variations.  Wind effects.  Multiple reflections of sound from the ocean bottom.

Propagation Modes : 

Propagation Modes There are three propagation modes that depend on the range between the sound source and the receiver and these are: - (i) Direct Path (DP) Usually at short ranges. Sound energy travels in a nominal straight line path between the source and the receiver. (ii) Bottom Bounce Path (BB) At intermediate ranges. Sound energy is reflected from the ocean bottom (iii) Convergence Zone (CZ) In the convergence zone, sound energy converges at longer ranges where multiple acoustic ray paths add or recombine coherently to reinforce the presence of signal energy from the radiating/reflecting source.

Propagation Modes : 

Propagation Modes The figure below, shows the various propagation paths in fathoms (or depth).

Multipaths : 

Multipaths The ocean splits signal energy into multiple acoustic paths. When the receiving system can resolve these multiple paths, they should be coherently recombined by optimal signal processing in order to fully exploit the available signal energy for detection. In the case of first-order bottom bounce transmission (i.e. only one bottom interaction), there are four paths (from source to receiver). (i) A bottom bounce ray path (B). (ii) A surface interaction followed by a bottom interaction (SB). (iii) A bottom bounce followed by a surface interaction (BS). (iv) A path that first hits the surface, then the bottom and finally the surface again (SBS).

Multipaths : 

Multipaths Typical first-order bottom bounce propagation paths are as shown in the figure below.

Sonar System Performance : 

Sonar System Performance  The performance of sonar systems can be assessed by the passive and active sonar equations.  The major parameters in the sonar equation, measured in decibel, are as follows: - LS = Source level. LN = Noise level. NDI = Directivity index. NTS = Echo level or target strength. NRD = Recognition differential.  LS is the target-radiated signal strength (for passive) or transmitted signal strength (for active), and LN is the total background noise level.  NDI, or DI, is the directivity index, which is a measure of the capability of a receiving array to electronically discriminate against unwanted noise.  NTS is the received echo level or target strength.

Sonar System Performance : 

Sonar System Performance  Underwater objects with large values of NTS are more easily detectable with active sonar than are those with small values of NTS.  In general, NTS varies as a function of:  Object size.  Aspect angle i.e., the direction at which impinging acoustic signal energy reaches the underwater object.  Reflection angle - the direction at which the impinging acoustic signal energy is reflected off the underwater object.  NRD is the recognition differential of the processing system.

Sonar System Performance : 

Sonar System Performance  The figure of merit (FOM), a basic performance measure involving parameters of the sonar system, ocean and target, is computed for active and passive sonar systems (in decibel) as follows: (a) For passive sonar FOMP = LS - (LN - NDI) - NRD (b) For active sonar FOMA = (LS + NTS) - (LN - NDI) – NRD

Sonar System Performance : 

Sonar System Performance  Sonar systems are designed so that the FOM exceeds the signal propagation loss for a given set of parameters of the sonar equations.  The amount above the FOM is called the signal excess.  When two sonar systems are compared, the one with the largest signal excess is said to hold the acoustic advantage.  It should be noted however, that the set of parameters in the FOM equations given here is simplified.  Furthermore, due to multi-paths, differences in sonar system equipment and operation and the constantly changing nature of the ocean medium, the FOM parameters fluctuate with time.  Thus, the FOM is not an absolute measure of performance but rather an average measure of performance over time.

Sonar System Performance Limitations : 

Sonar System Performance Limitations  Noise and interference can degrade the performance of the sonar system and limit its ability to detect signals.  The effects of these degradations must be considered when any sonar system is designed.  The noise or interference could be:  From a school of fish.  Shipping (surface or sub-surface).  Active transmission operations (e.g. jammers)  Use of multiple or sonar systems simultaneously.  Unusual vertical or horizontal directivity in ambient noise.  Environmental - in some environments (e.g. Arctic), noise due to ice motion produces unusual interference.  Unwanted backscatter (this, can cause a signal- induced noise that degrades processing gain.

Sonar System Performance Limitations : 

Sonar System Performance Limitations  Some other performance-limiting factors are: -  The loss of signal level and acoustic coherence due to boundary interaction as a function of grazing angle.  The radiated pattern (signal level) of the object and its spatial coherence.  The presence of surface, bottom, and volume reverberation (in active sonar).  Signal spreading (in time, frequency, or bearing) owing to the modulating effect of surface motion.  Biologic noise as a function of time (both time of day and time of year).  Statistics of the noise in the medium (e.g., does the noise arrive in the same ray path angles as the signal?).

Imaging And Tomography Systems : 

Imaging And Tomography Systems Underwater sound and signal processing can be used for bottom imaging and underwater oceanic tomography. Signals are transmitted in succession and the time delay measurements between signals and measured multi-paths are then used to determine the speed of sound in the ocean. This information along with bathymetry data is used to map depth and temperature variations of the ocean. In addition to mapping ocean bottoms, such information can also aid in quantifying global climate and warming trends.

Underwater Sound Systems: Components and Processes : 

Underwater Sound Systems: Components and Processes  Except for the signal generator (present only in active sonar), there are many similarities in the basic components and functions of the active and passive sonar systems.  In an active sonar system,  An electronic signal generator produces a signal.  The signal is then inverse beam-formed by delaying it in time by various amounts.  A separate projector is used to transmit each of the delayed signals by transforming the electrical signal into an acoustic pressure wave that propagates through water.  Thus an array of projectors is used to transmit the signal and focus it in the desired direction.

Underwater Sound Systems: Components and Processes : 

Underwater Sound Systems: Components and Processes  Depending on the desired range and Doppler resolution, different signal waveforms can be generated and transmitted.  At the receiver (an array of hydrophones), the acoustic waveform is converted back to an electrical signal.  The received signal consists of the source signal embedded in ambient noise and interference from other sources present in water.  The signal then goes through a number of signal processing functions.  In general, each channel of the analog signal is first filtered in a signal conditioner.  It is then amplified or attenuated within a specified dynamic range using an automatic gain control (AGC).  For active sonar we can also use a time varied gain (TVG) to amplify or attenuate the signal.

Underwater Sound Systems: Components and Processes : 

Underwater Sound Systems: Components and Processes  The signal, which is analog until this point, is then sampled and digitized by A/D converters.  The individual digital sensor outputs are next combined by a digital beam former to form a set of beams.  Each beam represents a different search direction of the sonar.  The beam output is further processed (band shifted, filtered, normalized, down sampled etc.) to obtain detection, classification and localization (DCL) estimates, which are displayed to the operator on single or multiple displays.  Based on the display output (acoustic data) and other non- acoustic data (environmental, contact, navigation, radar and satellite), the operators make their final decision.

Signal Waveforms : 

Signal Waveforms  The transmitted signal is an essential part of an active sonar system.  The properties of the transmitted signal will strongly affect the quality of the received signal and the information derived from it.  The main objective in active sonar, is to detect a target and estimate its range and velocity.  The range and velocity information is obtained from the reflected signal.  Commonly used signals in active sonar are continuous wave (CW), linear frequency modulation (LFM), hyperbolic frequency modulation (HFM) and pseudorandom noise (PRN) signals.

Signal Waveforms : 

Signal Waveforms  CW signals have been used in sonar for decades.  Signals such as frequency hop codes (FHC) and Newhall waveforms are recently rediscovered signals that work well in high reverberation shallow water environments.  The simplest signal is a rectangular CW pulse, which is a single-frequency sinusoid.  The CW signal may have high resolution in range or Doppler but not in both simultaneously. LFM signals are waveforms whose instantaneous frequency varies linearly with time. In HFM signals, the instantaneous frequency sweeps monotonically as a hyperbola. Both the above signals are good for detecting low Doppler targets in reverberation-limited conditions.