Another minor update. I’ve been messing around with some transducer designs for the SAS transmitter. Given that the wavelengths of ultrasonics underwater are very short (due to the high propagation velocity), it’s difficult to achieve 1/2λ spacing for array elements, which is needed to suppress grating sidelobes in an array.
Regardless, I think I’ve found a potentially useful arrangement for transducers. Initially, with a uniformly driven array, the sidelobes are rather quite atrocious (~7dB down from the peak for the first sidelobes). Using some very simple, non-scientific shading (read: hasty trial and error), I can suppress the sidelobes to better than 15 dB down from the carrier, which will greatly reduce unwanted effects in the SAS image. The shading also has an effect of widening the beam, which is actually desirable to increase the insonified swath on the ocean floor.
Unfortunately, shading can’t be used to significantly mitigate the grating sidelobe at 45° off boresight, but since it is >15 dB down hopefully it shouldn’t cause much of a problem. The lobe facing down to the ground will have its most significant impact via ground-bounce, will be mitigated by not opening the receive window until after the ground bounce has arrived. The lobes facing towards the surface will manifest themselves in multi-path reflections off the surface and back to the receive, or down to the ground and back to the receiver — These are important to keep to a minimum for the best imaging performance.
Next steps are to continue developing some software to do some NESZ calculations before finalizing the transducer design and forking over a large amount of money on piezoceramic elements…