Has anyone ever tried to design a radar using phase-coded ultrasonic waveforms for short range foliage penetration or possibly for imaging through buildings or under water? The much lower velocity of acoustic waveforms relative to RF waveforms should allow ultra-fine resolution in both range and cross-range for a given bandwidth and aperture size. Additionally, given that the acoustic calls of certain animals can be detected over great distances in dense jungles and through ocean waters, it would appear that the attenuation of acoustic waves may be very reasonable, especially for short range applications. This may even have applicability to "radar for the blind" where the entire system is built into a pair of eye glass frames in which the ultrasonic acoustic waveform is radiated from the bridge of the nose and received & processed through each of the ear pieces.
we tried making Doppler for sport fishing, and for traffic. It works, with some specific. Yes, in water there is very small loss. And sound travels well above the surface. But, diffraction is a general wave effect, which is worse on longer waves. So, depending on what you want to achieve, it is better or worse. And don't forget that in medicine it is used for many decades.
I don’t understand why diffraction should be more of a problem than at RF since the much lower acoustic velocity results in extremely small wavelengths despite the much lower frequencies.
Let me explain how I see the problem. In the air at distance you have rapidly growing absorption. In medical imaging MHz signal is used by the reasons of short range and 4 times higher velocity; with attenuation lower by 2 orders. In air ranging it should be opposite. Turning to biology, and evolution for optimal solution we find that the band of bat sonar is 100kHz. 100kHz is 3.4 mm, which is equivalent to 88 GHz radar. Modern consumer radars work at frequencies from 20 to 120 GHz. So, purely by my impression, it was very reasonable in the past, but not today. And on RF you get a benefit of lower air absorption, possibility of characterizing the foliage depending on polarization, probably better control over radiation pattern.
why okay in past, but not now? Intended applications would be ranges of essentially zero through up to max of 500 ft. Air attenuation should not be significant at these ranges. Also attenuation due to trees and other vegetation would predominate in FOPEN applications (100 to 500 ft ranges). Also, wouldn’t care about characterizing or precision locating foliage, but primary interest would be to detect enemy combatants or large animals.
Note that “radar for the blind” application would need ultra short minimum range (made possible by ultra short pulses coupled with slow propagation velocity) with max range of only 25 to 50 feet.
I believe, 150 meters in the air is very optimistic for ultrasound, at least you have to radically decrease the frequency. When I was speaking with , why don't they use it in their car radars, they said that beyond interference from environment, and relatively large doppler (which I am not sure is a bad thing anyway), they couldn't get an effective range longer than a few metters. it is not acceptable for safety. So everyone use 60GHz and above EM with equivalent wavelength. In addition, you can play with EM polarization for characterizing targets better (still not very popular in marketed products).
BTW. many animals are sensitive to lower band ultrasound. and it should not be expensive to make a direction combat detector for ultrasound... just a capsule mic, low frequency silicon amplifier and simplest micro-logic. no expensive RF components, painful wideband antenna design...
But making it for blind people is an idea. Like adopting vehicle parktronic.
In Japan stores and restaurants use lower ultrasound for scaring small animals... they scare me as well, because I can not withstand that painful hiss.
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