I have been trying to use the passive sonar example from the mathwork website to simulate how the signal strength decays away from distance. Problem i have is every time the simulation goes to 150000m it drops as shown. Is there is limit or a fix?

    clf
clearvars
fc='What is your operating frequency';
fc=input(fc);
channelDepth =2000
Numberofsourcepaths =1
BottomLoss=0.5
LossFrequencies=1:100
VoltageSensitivity=-200
% xcorr='Please enter the x cordinate of the receiver';
xcorr=0
% ycorr='Please enter the y cordinate of the receiver';
ycorr=0
% zcorr='Please enter the z cordinate of the receiver';
zcorr=0
Speed=1500
forstep=5
maxdistance=15500
for xcorr=14500:forstep:maxdistance

    isopath= phased.IsoSpeedUnderwaterPaths(...    %Creates a  channel for the propagation 
          'ChannelDepth',channelDepth,...
          'NumPathsSource',...                  
          'Property',...                        
          'NumPaths',Numberofsourcepaths,...    
          'PropagationSpeed',Speed,...
          'BottomLoss',BottomLoss,...           
          'TwoWayPropagation',false,...         
          'LossFrequencies',LossFrequencies);   
    channel = phased.MultipathChannel(...
          'OperatingFrequency',fc);                                                
    prf = 1;                 
    pulseWidth = 10e-3;
    pulseBandwidth = 1/pulseWidth;
    fs = 2*pulseBandwidth;
    wav = phased.RectangularWaveform('PRF',prf,'PulseWidth',pulseWidth,...
    'SampleRate',fs);
    channel.SampleRate = fs;

    projector = phased.IsotropicProjector(...                                    %set up the sound projector with frequency range of 0 to 30e3
        'FrequencyRange',[1 100],'VoltageResponse',250,'BackBaffled',false);

    [ElementPosition,ElementNormal] = helperSphericalProjector(8,fc,Speed);
    projArray = phased.ConformalArray(...
        'ElementPosition',[ElementPosition],...
        'ElementNormal',[ElementNormal],'Element',projector);

    projRadiator = phased.Radiator('Sensor',projector,...                   %radiates the sound projector signal outwards to the far field
    'PropagationSpeed',Speed,'OperatingFrequency',fc);

    beaconPlat = phased.Platform('InitialPosition',[0; 0; 0],...   % set a platform for the sound projector
     'Velocity',[0; 0; 0]);

    hydrophone = phased.IsotropicHydrophone('FrequencyRange',[1 1000],...  
     'VoltageSensitivity',VoltageSensitivity);

    array = phased.ULA('Element',hydrophone,...                             %   This object models a ULA formed with identical sensor elements.
    'NumElements',2,'ElementSpacing',Speed/fc/2,...
    'ArrayAxis','x');

    arrayCollector = phased.Collector('Sensor',array,...                    %collects incident narrowband signals from given directions 
    'PropagationSpeed',Speed,'OperatingFrequency',fc);

    arrayPlat= phased.Platform('InitialPosition',[xcorr; ycorr; -zcorr],...
    'Velocity',[0; 0; 0]);

    x = wav(); 
    %Transmit 10 pings, pings appear as a peak in the received signals
    numTransmits = 10;
    rxsig = zeros(size(x,1),2,numTransmits);
    for i = 1:numTransmits

    % Update array and acoustic beacon positions
    [pos_tx,vel_tx] = beaconPlat(1/prf);
    [pos_rx,vel_rx] = arrayPlat(1/prf);

    % Compute paths between the acoustic beacon and array
    [paths,dop,aloss,rcvang,srcang] = ...
         isopath(pos_tx,pos_rx,vel_tx,vel_rx,1/prf);

% Propagate the acoustic beacon waveform
  tsig = projRadiator(x,srcang);
   rsig = channel(tsig,paths,dop,aloss);

% Collect the propagated signal
  rsig = arrayCollector(rsig,rcvang);

% Store the received pulses
  rxsig(:,:,i) = abs((rsig));

    end
    t = (0:length(x)-1)'/fs;
    clf
    figure(4)
    plot(t,rxsig(:,end))
    xlabel('Time (s)');
    ylabel('Signal Amplitude (V)')

    Vpp=peak2peak(rxsig(:,end));
    vdb=20*log10(Vpp);
    Recievelevel=vdb-(VoltageSensitivity);
    table(xcorr,forstep)=Recievelevel;
    tablecor(xcorr,forstep)=xcorr;
    a1=table(table~=0);
    b1=tablecor(tablecor~=0);
end
figure(5)
plot(b1,a1)

xlabel('Distance in x axis(m)') ylabel('Reciever level(dB)') 'Tool finish'

end

Answer 1

[Matthew Farrokhrooz]

From your LossFrequencies assumption, my understanding is that your operating frequency is in the range of 1 to 100 Hz. Comparing with the example, you have reduced the operating frequency which increases the range of sound propgation in the water but didn't change the other parameters of the transmitted signal. In the case of passive sonar, prf is calculated as follows:

prf = c/Rmax;

In your case: prf = 1500/15500;

By calculating prf in this way we let each pulse travels at the Rmax range and gets to the receiver with no ambiguity. for monostatic radars and active sonars, we should consider prf = c/2*Rmax becasue signal travels 2*Rmax to get back to receiver.

The pulse bandwidth needs to be designer accordingly:

bw = c/(2*Rres);

For more information about designing the transmitted signal please refer to:

https://www.mathworks.com/help/phased/examples/waveform-analysis-using-the-ambiguity-function.html

Note that the above mentioned document is about designing signal for monostatic radar and you’ll need to modify the parameters based on one-way travel of the signal in passive sonar.