phased.IsoSpeedUnderwaterPaths

Isospeed multipath sonar channel

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Description

The phased.IsoSpeedUnderwaterPaths System object™ creates an underwater acoustic channel to propagate narrowband sound from point to point. The channel has finite constant depth with air-water and water-bottom interfaces. Both interfaces are planar and horizontal. Sound speed is constant throughout the channel. The object generates multiple propagation paths in the channel using the acoustical method of images (see [3]). Because sound speed is constant, all propagation paths are straight lines between the source, boundaries, and receiver. There is always one direct line-of-sight path. For each propagation path, the object outputs range-dependent time delay, gain, Doppler factor, reflection loss, and spreading loss. You can use the channel data as input to the multipath sound propagator, phased.MultipathChannel.

To model an isospeed channel:

  1. Create the phased.IsoSpeedUnderwaterPaths object and set its properties.

  2. Call the object with arguments, as if it were a function.

To learn more about how System objects work, see What Are System Objects?

Creation

Syntax

channel = phased.IsoSpeedUnderwaterPaths
channel = phased.IsoSpeedUnderwaterPaths(Name=Value)

Description

channel = phased.IsoSpeedUnderwaterPaths creates an isospeed multipath underwater channel System object, channel.

channel = phased.IsoSpeedUnderwaterPaths(Name=Value) creates an isospeed multipath underwater channel System object, channel, with each specified property Name set to the specified Value. You can specify additional name and value arguments in any order as (Name1=Value1,...,NameN=ValueN).

Properties

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Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the release function unlocks them.

If a property is tunable, you can change its value at any time.

For more information on changing property values, see System Design in MATLAB Using System Objects.

Isospeed underwater channel paths, specified as a phased.IsoSpeedUnderwaterPaths System object.

Example: phased.IsoSpeedUnderwaterPaths

Channel depth, specified as a positive scalar. Units are in meters.

Example: 250.0

Data Types: double

Underwater sound propagation speed, specified as a positive scalar. Units are in meter per second. The default value is a commonly-used underwater sound propagation speed.

Example: 1502.0

Data Types: double

The source of the number of propagation paths, specified as "Auto" or "Property". If you set this property to "Auto", the object automatically determines the number of paths based on spreading and reflection losses. If you set this property to "Property", you specify the number of paths using the NumPaths property.

When NumPathsSource is set to "Auto", only paths having a total loss greater than 20 dB below the direct path loss are returned.

Example: "Property"

Data Types: char

The number of propagation paths, specified as a positive integer between 1 and 51, inclusive.

Example: 11

Dependencies

To enable this property, set the NumPathsSource property to "Property".

Data Types: double

Channel coherence time, specified as a nonnegative scalar. Coherence time is a measure of the temporal stability of the channel. The object keeps a record of cumulative step time. When the cumulative step time exceeds the coherence time, propagation paths are recomputed and the cumulative step time is reset to zero. If you set this quantity to zero, the propagation paths are update at each call to step. Units are in seconds.

Example: 5.0

Data Types: double

Bottom reflection loss, specified as a nonnegative scalar. This value applies to each bottom reflection of a path. Units are in dB.

Example: 10

Data Types: double

Frequencies for which to compute absorption loss, specified as a positive real-valued vector. Units are in Hz.

Example: [1000:100:3000]

Data Types: double

Enable two-way propagation, specified as a false or true. Set this property to true to perform round-trip propagation between the signal origin and the destination. Set this property to false to perform only one-way propagation from the origin to the destination.

Example: true

Data Types: logical

Usage

Syntax

pathmat = channel(srcpos,destpos,srcvel,destvel,T)
[pathmat,dop,aloss,destang,srcang] = channel(srcpos,destpos,srcvel,destvel,T)

Description

pathmat = channel(srcpos,destpos,srcvel,destvel,T) returns the propagation paths matrix, pathmat, for a multipath underwater acoustic channel. The matrix describes one or two-way propagation from the signal source position, srcpos, to the signal destination position, destpos. The velocity of the signal source is specified in srcvel and the velocity of the signal destination is specified in destvel. T is the step time interval.

When you use this method for one-way propagation, srcpos refers to the origin of the signal and destpos to the receiver. One-way propagation modeling is useful for passive sonar and underwater communications.

When you use this method for two-way propagation, destpos now refers to the reflecting target, not the sonar receiver. A two-way path consists of a one-way path from source to target and then along an identical one-way path from target to receiver (which is collocated with the source). Two-way propagation modeling is useful for active sonar systems.

example

[pathmat,dop,aloss,destang,srcang] = channel(srcpos,destpos,srcvel,destvel,T) also returns the Doppler factor, dop, the frequency dependent absorption loss, aloss, the receiver arrival angles, destang, and the srcang transmitting angles.

When you use this method for two-way propagation, destang now refers to the reflecting target, not the sonar receiver.

example

Note

The object performs an initialization the first time the object is executed. This initialization locks nontunable properties and input specifications, such as dimensions, complexity, and data type of the input data. If you change a nontunable property or an input specification, the System object issues an error. To change nontunable properties or inputs, you must first call the release method to unlock the object.

Input Arguments

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Source of the sonar signal, specified as a real-valued 3-by-1 column vector. Position units are meters.

Example: [1000;100;500]

Data Types: double

Destination position of the signal, specified as a real-valued 3-by-1 column vector. Position units are in meters.

Example: [0;0;0]

Data Types: double

Velocity of signal source, specified as a real-valued 3-by-1 column vector. Velocity units are in meters per second.

Example: [10;0;5]

Data Types: double

Velocity of signal destination, specified as a real-valued 3-by-1 column vector. Velocity units are in meters per second.

Example: [0;0;0]

Data Types: double

Elapsed time of current step, specified as a positive scalar. Time units are in seconds.

Example: 0.1

Data Types: double

Output Arguments

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Propagation paths matrix, returned as a real-valued 3-by-N matrix. N is the number of paths in the channel. Each column represents a path. When you set NumPathsSource to "Auto", N is 51. In this case, any columns filled with NaN do not correspond to found paths. The matrix rows represent:

RowData
1Propagation delays for each path. Units are in seconds.
2Total reflection coefficient for each path. Units are dimensionless
3Spreading loss for each path. Units are in dB.

Except for the direct path, paths consists of alternating surface and bottom reflections. The losses for multiple reflections are multiplied. Bottom loss per reflection is specified by the BottomLoss property. The loss at the surface is –1 indicating no loss, but only a 180° phase change. This is because the air-water interface surface is a pressure-release surface.

Data Types: double

Doppler factor, returned as a real-valued N-by-1 row vector where N is the number of paths. The Doppler factor multiplies the transmitted frequency to produce the Doppler-shifted received frequency for each path. The Doppler shift is defined as the difference between the transmitted frequency and the received frequency. The Doppler factor also defines the time compression or expansion of a signal. Units are dimensionless.

Data Types: double

Frequency-dependent absorption loss, returned as a real-valued K-by-(N+1) matrix. K is the number of frequencies specified in the LossFrequencies property. N is the number of paths returned. The first column of aloss contains the absorption loss frequencies in Hz. You specify the frequencies using the LossFrequencies property. The remaining columns contain the absorption losses for each frequency. There is one column for each path. Units are in dB.

Data Types: double

Angles of paths at destination, returned as a real-valued 2-by-N matrix. Each column contains the direction of the received path with respect to the destination position as azimuth and elevation, [az;el]. Units are in degrees.

Data Types: double

Angles of paths from source, returned as a real-valued 2-by-N matrix. Each column contains the direction of the transmitted path with respect to the source position as azimuth and elevation, [az;el]. Units are in degrees.

Data Types: double

Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named obj, use this syntax:

release(obj)

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stepRun System object algorithm
releaseRelease resources and allow changes to System object property values and input characteristics
resetReset internal states of System object

Examples

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Open Live Script

Create a 5-path underwater sound channel and display the propagation path matrix. Assume the source is stationary and the receiver is moving along the x-axis towards the source at 20 kph. Assume one-way propagation.

speed = -20*1000/3600;
numpaths = 5;
channelpaths = phased.IsoSpeedUnderwaterPaths(ChannelDepth=200,BottomLoss=10, ...
    NumPathsSource="Property",NumPaths=numpaths,CoherenceTime=5);
tstep = 1;
srcpos = [0;0;-160];
rcvpos = [500;0;-50];
srcvel = [0;0;0];
rcvvel = [speed;0;0];
pathmat = channelpaths(srcpos,rcvpos,srcvel,rcvvel,tstep);
disp(pathmat)
    0.3356    0.3556    0.4687    0.3507    0.3791
    1.0000   -1.0000   -0.3162    0.3162   -0.3162
   54.1847   54.6850   57.0766   54.5652   55.2388

The first row contains the time delay in seconds. The second row contains the bottom reflection loss coefficients, and the third row contains the spreading loss in dB. The reflection loss coefficient for the first path is 1.0 because the direct path has no boundary reflections. The reflection loss coefficient for the second path is -1.0 because the path has only a surface reflection.

Open Live Script

Create a 7-path underwater sound channel and display the propagation path matrix. Assume the source is stationary and the target is moving along the x-axis towards the source at 20 kph. Assume two-way propagation.

speed = -20*1000/3600;
numpaths = 7;
channelpaths = phased.IsoSpeedUnderwaterPaths(ChannelDepth=200,BottomLoss=10, ...
    NumPathsSource="Property",NumPaths=numpaths,CoherenceTime=5, ...
    TwoWayPropagation=true);
tstep = 1;
srcpos = [0;0;-160];
tgtpos = [500;0;-50];
srcvel = [0;0;0];
tgtvel = [speed;0;0];
[pathmat,dop,aloss,tgtangs,srcangs] = channelpaths(srcpos,tgtpos,srcvel,tgtvel,tstep);
disp(pathmat)
    0.6712    0.7112    0.9374    1.0354    0.7014    0.7581    1.0152
    1.0000    1.0000    0.1000    0.1000    0.1000    0.1000    0.0100
  108.3693  109.3699  114.1531  115.8772  109.1304  110.4775  115.5355

The first row contains the time delay in seconds. The second row contains the bottom reflection loss coefficients, and the third row contains the spreading loss in dB. The reflection loss coefficient for the first path is 1.0 because the direct path has no boundary reflections. The reflection loss coefficient for the second path is -1.0 because the path has only a surface reflection.

Open Live Script

Create an underwater sound channel and plot the combined received signal. Automatically find the number of paths. Assume that the source is stationary and that the receiver is moving along the x-axis toward the source at 20 km/h. Assume the default one-way propagation.

speed = -20*1000/3600;
channel = phased.IsoSpeedUnderwaterPaths(ChannelDepth=200,BottomLoss=5, ...
    NumPathsSource="Auto",CoherenceTime=5);
tstep = 1;
srcpos = [0;0;-160];
rcvpos = [500;0;-50];
srcvel = [0;0;0];
rcvvel = [speed;0;0];

Compute the path matrix, Doppler factor, and losses. The propagator outputs 51 paths output but some paths can contain Nan values.

[pathmat,dop,absloss,rcvangs,srcangs] = channel(srcpos,rcvpos,srcvel,rcvvel,tstep);

Create of a 100 Hz signal with 500 samples. Assume that all the paths have the same signal. Use a phased.MultipathChannel System object™ to propagate the signals to the receiver. phased.MultipathChannel accepts as input all paths produced by phased.IsoSpeedUnderwaterPaths but ignores paths that have NaN values.

fs = 1e3;
nsamp = 500;
propagator = phased.MultipathChannel(OperatingFrequency=10e3,SampleRate=fs);
t = [0:(nsamp-1)]'/fs;
sig0 = sin(2*pi*100*t);
numpaths = size(pathmat,2);
sig = repmat(sig0,1,numpaths);
propsig = propagator(sig,pathmat,dop,absloss);

Plot the real part of the coherent sum of the propagated signals.

plot(t*1000,real(sum(propsig,2)))
xlabel("Time (millisec)")

Figure contains an axes object. The axes object with xlabel Time (millisec) contains an object of type line.

Open Live Script

Compare the duration of a propagated signal from a stationary sonar to that of a moving sonar. The moving sonar has a radial velocity of 25 m/s away from the target. In each case, propagate the signal along a single path. Assume one-way propagation.

Define the sonar system parameters: maximum unambiguous range, required range resolution, operating frequency, and propagation speed.

maxrange = 5000.0;
rngres = 10.0;
fc = 20.0e3;
csound = 1520.0;

Use a rectangular waveform for the transmitted signal.

prf = csound/(2*maxrange);
pulseWidth = 8*rngres/csound;
pulseBW = 1/pulseWidth;
fs = 80*pulseBW;
waveform = phased.RectangularWaveform(PulseWidth=pulseWidth,PRF=prf, ...
    SampleRate=fs);

Specify the sonar positions.

sonarplatform1 = phased.Platform(InitialPosition=[0;0;-60],Velocity=[0;0;0]);
sonarplatform2 = phased.Platform(InitialPosition=[0;0;-60],Velocity=[0;-25;0]);

Specify the target position.

targetplatform = phased.Platform(InitialPosition=[0;500;-60],Velocity=[0;0;0]);

Define the underwater path and propagation channel objects.

paths = phased.IsoSpeedUnderwaterPaths(ChannelDepth=100, ...
    CoherenceTime=0,NumPathsSource="Property",NumPaths=1, ...
    PropagationSpeed=csound);
propagator = phased.MultipathChannel(SampleRate=fs,OperatingFrequency=fc);

Create the transmitted waveform.

wav = waveform();
nsamp = size(wav,1);
rxpulses = zeros(nsamp,2);
t = (0:nsamp-1)/fs;

Transmit the signal and then receive the echo at the stationary sonar.

[pathmat,dop,aloss,~,~] = paths(sonarplatform1.InitialPosition, ...
    targetplatform.InitialPosition,sonarplatform1.InitialVelocity, ...
    targetplatform.InitialVelocity,1/prf);
rxpulses(:,1) = propagator(wav,pathmat,dop,aloss);

Transmit and receive at the moving sonar.

[pathmat,dop,aloss,~,~] = paths(sonarplatform2.InitialPosition, ...
    targetplatform.InitialPosition,sonarplatform2.Velocity, ...
    targetplatform.Velocity,1/prf);
rxpulses(:,2) = propagator(wav,pathmat,dop,aloss);

Plot the received pulses.

plot(abs(rxpulses))
xlim([490 650])
ylim([0 1.65e-3])
legend("Stationary sonar","Moving sonar")
xlabel("Received Sample Time (sec)")
ylabel("Integrated Received Pulses")

Figure contains an axes object. The axes object with xlabel Received Sample Time (sec), ylabel Integrated Received Pulses contains 2 objects of type line. These objects represent Stationary sonar, Moving sonar.

The signal received at the moving sonar has increased in duration compared to the stationary sonar.

Open Live Script

Create an underwater sound channel and display the propagation paths which are found automatically. Assume the source is stationary and the receiver is moving along the x-axis towards the source at 20 kph. Assume two-way propagation.

speed = -20*1000/3600;
channelpaths = phased.IsoSpeedUnderwaterPaths(ChannelDepth=200,BottomLoss=5, ...
    NumPathsSource="Auto",CoherenceTime=5,TwoWayPropagation=true);
tstep = 1;
srcpos = [0;0;-160];
tgtpos = [500;0;-50];
srcvel = [0;0;0];
tgtpos = [speed;0;0];
[pathmat,dop,aloss,rcvangs,srcangs] = channelpaths(srcpos,tgtpos,srcvel,tgtpos,tstep);

Display the first 7 columns of pathmat. Some columns are filled with NaNs.

disp(pathmat(:,1:7))
    0.2107    0.2107       NaN       NaN       NaN       NaN       NaN
    1.0000    1.0000       NaN       NaN       NaN       NaN       NaN
   88.1753   88.1753       NaN       NaN       NaN       NaN       NaN

Select the column indices of the valid paths from the entire matrix. 

idx = find(~isnan(pathmat(1,:)))
idx = 1×4

     1     2    27    28

Display the valid paths information.

validpaths = pathmat(:,idx)
validpaths = 3×4

    0.2107    0.2107    0.3159    0.3159
    1.0000    1.0000    0.3162    0.3162
   88.1753   88.1753   95.2131   95.2131

The first row contains the time delays in seconds. The second row contains the bottom reflected loss coefficients, and the third row contains the spreading losses.

References

[1] Urick, R.J. Principles of Underwater Sound, 3rd Edition. New York: Peninsula Publishing, 1996.

[2] Sherman, C.S. and J.Butler Transducers and Arrays for Underwater Sound. New York: Springer, 2007.

[3] Allen, J.B. and D. Berkely, “Image method for efficiently simulating small-room acoustics”, J. Acoust. Soc. Am, Vol 65, No. 4. April 1979.

Extended Capabilities

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Version History

Introduced in R2017a

See Also

Objects

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