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I am working on beam forming of high frequency (>400 KHz) linear array for imaging sonar. I have gathered sensor data of 80 channel linear array sub merged halfway in 8 m deep water tank with a transmitting probe 6 m away from it at same depth. I recorded multiple pulses and perform offline analysis on it. The problem i am facing is very high side lobe levels in a beam pattern developed using recorded data. Can any one kindly guide me what could be possible causes of higher side lobe levels?

Thanks & Regards

Nauman

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There are several practical items that may cause reduced beamforming performance and lead the higher sidelobes. You haven't said how high exactly these sidelobes are, nor what levels you expected. Some common areas to investigate further:

  1. Array element spacing. To prevent the presence of grating lobes your element spacing needs to be less than $\frac{\lambda}{2}$. You haven't indicated the bandwidth of your signal, so I'll use the 400 KHz you gave and assume a speed of sound to be 1500 m/s. Then your spacing needs to be less than $\frac{1500}{2\cdot 400\cdot10^3}=1.875$ mm. Of course you should use the highest frequency of signal of interest for this calculation.
  2. I'm assuming that you are using IQ sampling. Phase errors in the IQ demodulation signal or slew in the clocking (synchronization) across the 80 channels may degrade the performance. At this high frequency you will need very small tolerances to limit the effect of these phase errors.
  3. Have you considered the effect of Multipath propagation? The effect of the signals bouncing off the side of the tank or the water surface can lead to signal levels with a higher intensity (constructive interference) than the level of your direct path.
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  • $\begingroup$ Thanks very much for detailed reply. My array coverage area is very limited i.e. <+/- 8 deg and in this area grating lobes can not occur theoretically due to selected frequency (420 +/-10 KHz) and spacing (14.3 mm). I am doing IQ demodulation in software after digitization on high sampling frequency (2 M samples/s/channel) $\endgroup$ – naumankalia Apr 9 at 12:57
  • $\begingroup$ One strange issue i observed during individual channel analysis is that phase of array sensors w.r.t. reference sensor are changing for each ping i.e. if for example phase of channel 10 is 80 degree w.r.t. reference channel #50 for one ping, for next ping it will change dramatically to say 300 deg and in next ping it will again change. $\endgroup$ – naumankalia Apr 9 at 12:58

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