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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: acoustic imager
Date: Fri, 18 Apr 2025 22:18:33 +0100
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On Fri, 18 Apr 2025 19:10:07 -0000 (UTC), piglet
<erichpwagner@hotmail.com> wrote:

>Cursitor Doom <cd@notformail.com> wrote:
>> On Fri, 18 Apr 2025 10:34:25 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>> 
>>> On 2025-04-17 03:45, John R Walliker wrote:
>>>> On 17/04/2025 03:12, john larkin wrote:
>>>>> On Wed, 16 Apr 2025 22:01:28 -0400, Phil Hobbs
>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>> 
>>>>>> On 2025-04-16 10:41, john larkin wrote:
>>>>>>> On Wed, 16 Apr 2025 09:01:00 +0100, Cursitor Doom <cd@notformail.com>
>>>>>>> wrote:
>>>>>>> 
>>>>>>>> On Tue, 15 Apr 2025 15:04:15 -0700, john larkin <jl@glen--canyon.com>
>>>>>>>> wrote:
>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> https://www.google.com/aclk?sa=L&ai=DChsSEwjTjaDVg9uMAxW3Hq0GHVmKOlYYACICCAEQARoCcHY&co=1&cce=2&sig=AOD64_3aGs74magNuXwdRGFo7oP8zK-LMQ&ctype=5&q=&adurl=
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>> 
>>>>>>>> 
>>>>>>>> For 42,000 dollars? There's a product there you could develop, John.
>>>>>>> 
>>>>>>> Seems like it needs maybe a dozen electret mikes, one mux'd ADC, an
>>>>>>> FPGA, and some code.
>>>>>> 
>>>>>> In the last few decades, there's been a lot of work done on imaging with
>>>>>> sparse arrays.
>>>>>> 
>>>>>> A full NxN rectangular antenna array has an enormous amount of
>>>>>> duplicated information from an imaging point of view. To make a good
>>>>>> image, you need spatial frequency information corresponding to all
>>>>>> values of dx and dy, with some regular spacing, i.e. in an NxN array,
>>>>>> 
>>>>>> dx and dy go from -N/2 to +N/2-1 (or equivalently, from 0 to N-1) in
>>>>>> integer steps.
>>>>>> 
>>>>>> In principle you only need one estimate per spacing, but in a dense
>>>>>> array, every pair of adjacent pixels gives an estimate of the dx = +-1
>>>>>> components, i.e. essentially the same information as every other
>>>>>> adjacent pair.  The redundancy is less at wider spacing, of course.
>>>>>> 
>>>>>> If one is willing to trade off SNR and computational expense, you can
>>>>>> get the resolution of a full array with far less than N**2 antennas--I
>>>>>> forget what the the number is, but it's a lot more like N log N than
>>>>>> N**2.  A pal of mine in grad school, Yoram Bresler, did his thesis on
>>>>>> that problem, which is where I first heard of it.
>>>>>> 
>>>>>> So a sparse array of microphones can in principle do quite a bit better
>>>>>> than one might suppose.
>>>>> 
>>>>> And it looks like the Fluke acoustic imaging is primitive, like those
>>>>> hybrid visual+thermal gadgets.
>>>>> 
>>>>>> 
>>>>>> Cheers
>>>>>> 
>>>>>> Phil Hobbs
>>>>> 
>>>>> I'd expect that a bunch of wideband antennas and ADCs listening to the
>>>>> world would have the same effect, see everything. Radar without the
>>>>> transmitter. No doubt that is being done.
>>>> 
>>>> It is.  Look up "passive bistatic radar"
>>>> For example:
>>>> https://sspd.eng.ed.ac.uk/sites/sspd.eng.ed.ac.uk/files/attachments/basicpage/20171219/Session%201.0.pdf
>>>> 
>>>> 
>>>> John
>>>> 
>>> 
>>> For a long time, too.
>>> IIRC the first successful radar experiment used the reflection from a 
>>> BBC transmitter.
>>> 
>>> Cheers
>>> 
>>> Phil Hobbs
>> 
>> Radar and code-breaking really saved Britain's bacon in WW2. Plus a
>> bit of assistance from the old colonies. :->
>> 
>
>Yes but until the cavity magnetron came to fruition British radar was
>technically grossly inferior to German radar. Think of the wurzburg system
>with uhf, parabolic dishes, high prf rates, coax cable etc. 
>
>In the early stages the main factor was operational in that the brits
>thoroughly integrated radar into air defense whilst the Germans only used
>it on an individual basis. The chain home radar was so totally
>different/primitive/worse than the Nazis radars that initially they mistook
>it for something else.

Scene: Churchill's War Room under Whitehall:-

"Bandits incoming! Bandits incoming! Dispatch 810 Squadron to
intercept immediately!"

"Tally-ho chaps! Let's get those Spitfires up! Where do the radar
chaps say gerry's position is? North West Europe?? Ginger, Corky - get
the bloody binoculars out again!"