Reply by Fred Marshall March 30, 20052005-03-30
"Tom Cantwell" <NoWay@nope.com> wrote in message 
news:Xns96295C54A2341NoWaynope@129.250.170.86...
> "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in
> --- > Yeah, spent many hours at NOSC/NRAD/what_ever_it_is_now using the real- > time "hybrid simulator" with an ADCAP torpedo panel in the loop. We > shared the facility with the people working on the Mark 46/50 upgrades. >
Tom, I was there managing the advanced lightweight project which funded the simulator upgrade. (The advanced lightweight became the MK50). I left to go to DC in '78. Both ADCAP and MK50 continued well after that of course. Maybe we crossed paths... ? I actually started my engineering career working on the hybrid simulator when it was a UNIVAC 1108 plus a couple of the largest analog computers in existence at the time - in Pasadena, CA which was a Navy / Caltech lab. We moved / merged the lab in San Diego in '73. Fred
Reply by Tom Cantwell March 30, 20052005-03-30
"Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in
news:BsmdnVyx0LZHkqbfRVn-tA@centurytel.net: 

> > "Tom Cantwell" <NoWay@nope.com> wrote in message
<snip>>
> Tom, > > That's interesting. I was in San Diego and worked with Dave > Princehouse, (who was at APL along with Sienkiewicz, Haugen et al) > in writing the paper I mention above. We implemented the first > "production" version or REVGEN in San Diego using Floating Point > Systems AP-120B(?) array processors. Ten of them as I recall. So, > I was interested in efficient computing because that was a lot of > hardware! > > One of my thoughts was to change the range-Doppler map > implementation as follows: > > As you know, the range-Doppler map was split up into cells or > elements that represented range (time) and look angle. The > contents of the R-D map had to be computed completely on the fly > due to platform dynamics (changing velocity vector direction and > beam patterns). It was a lot of computing because there needs to > be a new map computed (a 2-D matrix) for each time sample of each > of the receiver data streams. > > I have an idea that would reduce the processing load per R-D cell > per time sample. Probably by greater than a factor of 2. I never > got around to working on this idea. I wonder if it still might > make sense? I'd be interested in working on it if there's still an > application - and it sounds like there is.... > > Fred >
--- Yeah, spent many hours at NOSC/NRAD/what_ever_it_is_now using the real- time "hybrid simulator" with an ADCAP torpedo panel in the loop. We shared the facility with the people working on the Mark 46/50 upgrades. Dr. Robert Goddard of APL would probably be interested in communicating with you re: increasing the speed of the SST. I'll send you his email addy when I get to work later today. Tom
Reply by Fred Marshall March 18, 20052005-03-18
"Tom Cantwell" <NoWay@nope.com> wrote in message 
news:Xns961D53AD7C708NoWaynope@129.250.170.83...
> "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in > news:X9KdnZmFsoOKX6TfRVn-sg@centurytel.net: > >> > (lotsa' good stuff snipped)> > > >> Since you said you did some reverberation modeling. I published an >> analysis of the UW/APL REVGEN model at IEEE Signals and Signal >> Processing Conference in Hartford, CT around 1975?? looking toward >> efficient algorithmic implementations. If you know this model, it >> provides for a Doppler density map that treats the medium volume as >> a set of discrete cells - each with it's own strength and velocity. >> So, generating realistic volume reverberation was easy to >> manipulate if you have this thing implemented. >> > ---- > We used REVGEN extensively in the late 70's/early 80's (?) while > designing and developing the Mk48 ADCAP torpedo. Did you know it's > still "available" in an extremely enhanced, updated and capable > version, called the Sonar Simulation Toolset (SST)? The > maintainer/evolver is Dr. Robert Goddard of APL. No charge, but I think > you have to request it through a US Navy contact, with a demonstrated > need. > > The model now runs on a standard UNIX/LINUX machine instead of the > dedicated high-speed signal processors of yesteryear. We're > currently using it to evaluate the performance of various waveforms > (PCW, LFM, HFM, SFM, etc) for a relatively high frequency sonar in > shallow water. > > Tom Cantwell > Independent Contractor
Tom, That's interesting. I was in San Diego and worked with Dave Princehouse, (who was at APL along with Sienkiewicz, Haugen et al) in writing the paper I mention above. We implemented the first "production" version or REVGEN in San Diego using Floating Point Systems AP-120B(?) array processors. Ten of them as I recall. So, I was interested in efficient computing because that was a lot of hardware! One of my thoughts was to change the range-Doppler map implementation as follows: As you know, the range-Doppler map was split up into cells or elements that represented range (time) and look angle. The contents of the R-D map had to be computed completely on the fly due to platform dynamics (changing velocity vector direction and beam patterns). It was a lot of computing because there needs to be a new map computed (a 2-D matrix) for each time sample of each of the receiver data streams. I have an idea that would reduce the processing load per R-D cell per time sample. Probably by greater than a factor of 2. I never got around to working on this idea. I wonder if it still might make sense? I'd be interested in working on it if there's still an application - and it sounds like there is.... Fred
Reply by Tom Cantwell March 18, 20052005-03-18
"Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in
news:X9KdnZmFsoOKX6TfRVn-sg@centurytel.net: 

>
(lotsa' good stuff snipped)>
> Since you said you did some reverberation modeling. I published an > analysis of the UW/APL REVGEN model at IEEE Signals and Signal > Processing Conference in Hartford, CT around 1975?? looking toward > efficient algorithmic implementations. If you know this model, it > provides for a Doppler density map that treats the medium volume as > a set of discrete cells - each with it's own strength and velocity. > So, generating realistic volume reverberation was easy to > manipulate if you have this thing implemented. >
---- We used REVGEN extensively in the late 70's/early 80's (?) while designing and developing the Mk48 ADCAP torpedo. Did you know it's still "available" in an extremely enhanced, updated and capable version, called the Sonar Simulation Toolset (SST)? The maintainer/evolver is Dr. Robert Goddard of APL. No charge, but I think you have to request it through a US Navy contact, with a demonstrated need. The model now runs on a standard UNIX/LINUX machine instead of the dedicated high-speed signal processors of yesteryear. We're currently using it to evaluate the performance of various waveforms (PCW, LFM, HFM, SFM, etc) for a relatively high frequency sonar in shallow water. Tom Cantwell Independent Contractor
Reply by David Kirkland March 18, 20052005-03-18
Rune Allnor wrote:

 > Fred Marshall wrote:
 >
 >> "Stan Pawlukiewicz" <spam@spam.mitre.org> wrote in message
 >> news:d14k87$1g0$1@newslocal.mitre.org...
 >>
 >>> Fred Marshall wrote:
 >>>
 >>>> "Stan Pawlukiewicz" <spam@spam.mitre.org> wrote in message
 >>>>
 >>>>> I think that Cook and Bernfield is out of print.  I think that
 >
 >
 > Riachek's
 >
 >>>>> book was reprinted by Peninsula Publishers a while back.  I've
 >
 >
 > heard many
 >
 >>>>> people recomend Skolnick.
 >>>>
 >>>>
 >>>>
 >>>> It's Cook and BernFELD.  It appears to be in print by Artech
 >
 >
 > House.  The
 >
 >>>> original was from Academic Press.  Amazon for $146.00
 >>>> Still not what I'd recommend for directly applying to sonar except
 >
 >
 > as
 >
 >>>> background and interest.
 >>>>
 >>>>
 >>>> Fred
 >>>
 >>>
 >>> Well Fred, what would you recommend?
 >>
 >>
 >> Stan,
 >>
 >> Good question.  Considering it's Rune who's asking, I don't think I
 >
 >
 > should
 >
 >> recommend Urick's Principles of Underwater Sound - and it won't deal
 >
 >
 > with
 >
 >> the question anyway.  I don't know Waite's book and there's not
 >
 >
 > enough
 >
 >> detail on Amazon to get a sense for it.  Bottom line: there aren't
 >
 >
 > that many
 >
 >> books on the subject and most are introductory. After that, a lot of
 >
 >
 > the
 >
 >> stuff was classified.
 >
 >
 >
 > I have the following books on sonar or sonar-related material,
 > in my shelf:
 >
 > Waite: Sonar for Practising Engineers.
 > R.O. Jensen: Sonar Signal Processing
 > Urick: Principles of Underwater Sound
 > Urick: Sound Propagation in the Sea
 > F.B. Jensen & al: Computational Ocean Acoustics
 > Burdic: Underwater Acoustic System Analysis
 > van Trees: Detection, Estimation and Modulation Theory vols I-IV.
 >
 > I have nothing (except for van Trees) on radar.
 >
 > With the probable exception of van Trees, these at best mention the
 > matched filter in the context of pulse compression/increasing range
 > resolution. Most of them mention the ROC, but none that I am aware of
 > study how the matched filter increases the ability to detect weak
 > targets, which is what I want to do. But as you say, that's the kind
 > of stuff that would be classified. I think there may be something in
 > van Trees' volume I, though, but it takes time to read it and figure
 > out exactly what is there and what is not.
 >
 > [... snip...]
 >
 >
 >> Then there is another speed of sound vs. speed of light difference.
 >
 >
 > Radar
 >
 >> has so much more capability to gather data on a target over multiple
 >
 >
 > pulses.
 >
 >> Sonar generally doesn't.
 >
 >
 >
 > True. A few years ago I did a literature survey on synthetic aperture
 > sonar. It turned out all the theory was available in the literature
 > as early as 1976. I don't remember his name, but the author of this
 > paper had apparently been involved with the development of synthetic
 > aperture radar in the 1960ies, and had taken all the theory and
 > rewritten it once and for all in terms of the sonar problem, in one
 > huge article.
 >
 > In the mid 1990ies, the ambition in the underwater acoustics community
 > was to design a synthetic aperture sonar that imaged the sea floor at
 > 1 cm resolution at ranges ~1 km. For a 100 kHz carrier frequency, the
 > wavelength is on the order of 1.5 cm. The synthetic aperture imaging
 > principle requires the position of the platform to be controlled
 > to within a fraction of a wavelength, in this case on the order of
 > millimeters. The slow propagation speed of sound in water means that
 > at 750 m range, the two-way travel time is 1 s. Which means that
 > the sonar platform needs to hang virtually dead in the water. Which
 > isn't very easy to achieve in the real-world sea.
 >
 > So while all the theory was available and served on a silver plate
 > in 1976, the technology needed to actually demonstrate the SAS
 > principle in the field, was not available until some 25 years later,
 > in 2000-2001. I don't know if the SAS ever evolved past the field
 > demonstrator stage, though.
 >
 >
 >> That is, until you get into ultrasound imaging
 >> sorts of applications where the frequencies are in the 100's of kHz
 >
 >
 > and the
 >
 >> ranges are in the centimeters.  In fact, many of the Doppler
 >
 >
 > measuring
 >
 >> methods in radar depend on multiple pulse time differences rather
 >
 >
 > than on
 >
 >> frequency shift - really the frequency shift of a pulse train.
 >>
 >> Rune, what is the center frequency and range you're considering?  Is
 >
 >
 > Doppler
 >
 >> involved? These would shed some light on this question.
 >
 >
 >
 > I don't want to get too much into details about the application,
 > but center frequencies remain to be decided, depending on what field
 > equipment is available or possible to design. We used ~100 kHz in
 > the lab, but expect to end up a bit lower in a field system. At the
 > higher frequencies, we used low-Q transducers and were just able to
 > detect the target we wanted but with reasonable range resolution.
 > At the lower frequencies there were no problems detecting the target,
 > but due to high-Q transducers we couldn't achieve a range resolution
 > anywhere near what would be required in a field system. So unless we
 > can get low-Q low-frequency tranducers (which I suspect could be
 > difficult), we have to stick to the higher frequency ranges in order
 > to both detect the target and achieve the range resolution we need.
 > Hence the need to study alternative source signals. As you suggested
 > in your first post, we *could* try with CWs for longer durations than
 > we did this time.
 >
 > Doppler is not involved. We can use multiple-pulse imaging.
 > Noise is expected to become a severe problem in the field.
 > Rune
 >

Rune,

I'm missing your original post, so I'm not sure if I'm addressing or 
understanding your question properly.

Most texts assume Gaussian noise statistics. In this case the ROC is 
just dependent on the signal energy - it will have nothing to due with 
the pulse characteristics itself.

What the pulse characteristics do determine are things that you are 
trying to estimate - typically this is either the range, or 
Doppler/velocity. You typically trade off one for the other. Also the 
pulse choice affects reverberation performance.

SAS is still a big research area. You may want to check out Peter 
Gough's (I believe that's the correct name) work in Australia - some of 
the papers are available on the web. Some of the newer SAR algorithms 
(chirp scaling and extended chirp scaling) make explicit use of the 
pulse waveform.

The 2 biggest problems I know of in SAS are: motion compensation, and 
failure of the point and shoot assumption. Due to the slower sound speed 
in water these are more serious problems than in the typical SAR 
processing schemes.

I've looked at several of the classic signal detection books, and each 
tend to have their own terminology. You pretty have to fight you're way 
through one to understand it and then can pull the necessary information 
from the others.

If you're interested the texts I'd suggest are:
Fundamentals of Statistical Signal Processing - Steven Kay
(This is a good introduction - and covers quadratic detectors, bayesian 
and MLE approaches)

Detection of Signals in Noise - McDonough and Whalen
(This covers complex signals well, and multiple pulses. It uses mostly 
linear detectors (Amp rather than Amp2) for the worked examples but 
quadratic is mentioned).

Unfortunately Van Trees is a bit voluminous and unwieldly.

Hope that helps.

Cheers,
David
Reply by Rune Allnor March 17, 20052005-03-17
Fred Marshall wrote:

> I wasn't able to tap into the IEEE deep enough to find the article
you
> mentioned. I dunno, why is it that IEEE makes things so hard for us?
It
> seems like a more open interface would be a good thing. I found
OCEANS
> 2001, found abstracts for the papers but no Ward on any of the four
volumes.
> But then, IEEE Explore wouldn't let me see the list of authors as a
plain
> .pdf list .... go figure. But then I don't exclude cockpit error. I
would
> like to see what he's talking about. If you have any suggestions I'd
be
> appreciative.
I found the paper by the search ( ward<in>au ) <and>oceans<and>detection on IEEExplore. Or rather, I went to "basic search" and typed each key word in its own box. The name was tagged as "author". The article is hit no. 7 or 8 down the list. I don't have access rights to the article, so I got stuck too... Rune
Reply by Fred Marshall March 17, 20052005-03-17
"Stan Pawlukiewicz" <spam@spam.mitre.org> wrote in message 
news:d19s0a$n9j$1@newslocal.mitre.org...
> Fred Marshall wrote: >> "Stan Pawlukiewicz" <spam@spam.mitre.org> wrote in message >> news:d17n5h$jv8$1@newslocal.mitre.org... >> >>>Fred Marshall wrote: >>> >>>>"Stan Pawlukiewicz" <spam@spam.mitre.org> wrote in message >>>>news:d16vkb$895$1@newslocal.mitre.org... >> >> . >> >>>>Obviously, Doppler can be a discriminator. So can angle of arrival if >>>>it's being measured and if there's at least a reaonable SRR (signal to >>>>reverberation ratio). If there is no Doppler and if the situation is >>>>reverberation-limited then detection is a tough problem. I know of no >>>>waveform tricks that help out in that case. Do you or Rune? >>>> >>>>Fred >>> >>>Backscatter is a frequency dependent phenomena. In some instances, it >>>can be significant. I can see some advantages of using a range of >>>frequencies to probe a partially characterized environment. There are are >>>other reasons why multiple frequencies make sense that you are probably >>>aware of. >> >> >> "Backscatter is a frequency dependent phenomena." >> >> Ahah! Stan, you gotta be a radar guy. Am I right? > > No a SONAR guy, mostly on the passive side, but I've don some > reverberation modeling. > >> I do understand about surface backscatter which us sonar guys call either >> surface reverberation or bottom reverberation (as long as one isn't >> around icebergs like Rune may be). >> Surface reverberation can be frequency dependent at very long wavelengths >> where the periodicity of gravity waves can be like a resonant crystalline >> structure. The same thing could happen with bottom reverberation if the >> bottom is like wavy sand dunes I suppose. > > Bubbles, surface waves, Rayleigh scattering, internal (salinity) waves, > particle suspensions. Its not like there are no articles in JASA that do > not consider the properties of the bottom in reverberation studies. > > Absorption, while not backscatter, has an effect on backscatter amplitude. > Rune knows more about that kinda >> stuff. So that has an analogy in radar and there's been a lot of work >> done regarding sea surface backscatter characteristics which, at times, >> can be analogous to sea surface backscatter as seen by sonars. >> >> In water there is "volume reverberation" which isn't very frequency >> dependent at all. About the only thing about volume reverberation - which >> often also applies to boundary reverberation - is that it has zero radial >> velocity, thus zero Doppler (unless it's off-axis / velocity vector as >> seen from a moving sonar). > > Water moves ... tides, currents, wind direction, turbulence. Even Morse > and Ingard shy away from looking at a moving media in detail. > It's akin to radar backscatter from rain or fog >> which I do understand is frequency dependent - remember the radars in >> WWII that were designed at the water vapor absorption frequency? Well, >> OK that's absorption and not backscatter... but the frequency dependence >> aspect is similar isn't it? Otherwise radar most often doesn't have such >> a compelling phenonmenon - clean air.... Sonar volume reverberation is >> always there at short range and is substantial. I think that's what >> we're talking about.
Stan, OK, sorry if I was preaching to the choir. There is no doubt that there are reasons for spectral spreading and shifting in volume reverberation and you've mentioned some of those. Platform motion and variations in that motion and beamwidths (for moving platforms) are included in the list. But these things are not about frequency dependence of volume reverberation strength. In my experience, at mid-frequencies, there is not much frequency dependence in volume reverberation. That is not to say that some researcher could not have investigated some second or third-order phenomenon in a particular situation where it is evident, interesting, etc. Volume reverberation is usually *the* system-limiting component of total reverberation unless the object of interest is immediately adjacent to a boundary of the medium. There are lots of techniques to reject boundary reverberation that include beamforming, range gating, etc. So, my emphasis has been on volume reverberation here. Since you said you did some reverberation modeling. I published an analysis of the UW/APL REVGEN model at IEEE Signals and Signal Processing Conference in Hartford, CT around 1975?? looking toward efficient algorithmic implementations. If you know this model, it provides for a Doppler density map that treats the medium volume as a set of discrete cells - each with it's own strength and velocity. So, generating realistic volume reverberation was easy to manipulate if you have this thing implemented. I wasn't able to tap into the IEEE deep enough to find the article you mentioned. I dunno, why is it that IEEE makes things so hard for us? It seems like a more open interface would be a good thing. I found OCEANS 2001, found abstracts for the papers but no Ward on any of the four volumes. But then, IEEE Explore wouldn't let me see the list of authors as a plain .pdf list .... go figure. But then I don't exclude cockpit error. I would like to see what he's talking about. If you have any suggestions I'd be appreciative. Fred
Reply by Rune Allnor March 17, 20052005-03-17
Fred Marshall wrote:
> "Stan Pawlukiewicz" <spam@spam.mitre.org> wrote in message > news:d17n5h$jv8$1@newslocal.mitre.org... > > Fred Marshall wrote: > >> "Stan Pawlukiewicz" <spam@spam.mitre.org> wrote in message > >> news:d16vkb$895$1@newslocal.mitre.org... > . > >> > >> Obviously, Doppler can be a discriminator. So can angle of
arrival if
> >> it's being measured and if there's at least a reaonable SRR
(signal to
> >> reverberation ratio). If there is no Doppler and if the situation
is
> >> reverberation-limited then detection is a tough problem. I know
of no
> >> waveform tricks that help out in that case. Do you or Rune? > >> > >> Fred > > Backscatter is a frequency dependent phenomena. In some instances,
it can
> > be significant. I can see some advantages of using a range of
frequencies
> > to probe a partially characterized environment. There are are other
> > reasons why multiple frequencies make sense that you are probably
aware
> > of. > > "Backscatter is a frequency dependent phenomena." > > Ahah! Stan, you gotta be a radar guy. Am I right? > I do understand about surface backscatter which us sonar guys call
either
> surface reverberation or bottom reverberation (as long as one isn't
around
> icebergs like Rune may be). > Surface reverberation can be frequency dependent at very long
wavelengths
> where the periodicity of gravity waves can be like a resonant
crystalline
> structure. The same thing could happen with bottom reverberation if
the
> bottom is like wavy sand dunes I suppose. Rune knows more about that
kinda
> stuff.
I'm not sure I do. You are right, though, in that surface reverbration is frequency dependent. It has something to do with the wavelength of the sound wave matching the characteristic length of a corrugated surface. Rune
Reply by Stan Pawlukiewicz March 16, 20052005-03-16
Stan Pawlukiewicz wrote:
> Fred Marshall wrote: > >> "Stan Pawlukiewicz" <spam@spam.mitre.org> wrote in message >> news:d17n5h$jv8$1@newslocal.mitre.org... >> >>> Fred Marshall wrote: >>> >>>> "Stan Pawlukiewicz" <spam@spam.mitre.org> wrote in message >>>> news:d16vkb$895$1@newslocal.mitre.org... >> >> >> . >> >>>> Obviously, Doppler can be a discriminator. So can angle of arrival >>>> if it's being measured and if there's at least a reaonable SRR >>>> (signal to reverberation ratio). If there is no Doppler and if the >>>> situation is reverberation-limited then detection is a tough >>>> problem. I know of no waveform tricks that help out in that case. >>>> Do you or Rune? >>>> >>>> Fred >>> >>> >>> Backscatter is a frequency dependent phenomena. In some instances, >>> it can be significant. I can see some advantages of using a range of >>> frequencies to probe a partially characterized environment. There are >>> are other reasons why multiple frequencies make sense that you are >>> probably aware of. >> >> >> >> "Backscatter is a frequency dependent phenomena." >> >> Ahah! Stan, you gotta be a radar guy. Am I right? > > > No a SONAR guy, mostly on the passive side, but I've don some > reverberation modeling. > >> I do understand about surface backscatter which us sonar guys call >> either surface reverberation or bottom reverberation (as long as one >> isn't around icebergs like Rune may be). >> Surface reverberation can be frequency dependent at very long >> wavelengths where the periodicity of gravity waves can be like a >> resonant crystalline structure. The same thing could happen with >> bottom reverberation if the bottom is like wavy sand dunes I suppose. > > > Bubbles, surface waves, Rayleigh scattering, internal (salinity) waves, > particle suspensions. Its not like there are no articles in JASA that > do not consider the properties of the bottom in reverberation studies. > > Absorption, while not backscatter, has an effect on backscatter amplitude. > Rune knows more about that kinda > >> stuff. So that has an analogy in radar and there's been a lot of work >> done regarding sea surface backscatter characteristics which, at >> times, can be analogous to sea surface backscatter as seen by sonars. >> >> In water there is "volume reverberation" which isn't very frequency >> dependent at all. About the only thing about volume reverberation - >> which often also applies to boundary reverberation - is that it has >> zero radial velocity, thus zero Doppler (unless it's off-axis / >> velocity vector as seen from a moving sonar). > > > Water moves ... tides, currents, wind direction, turbulence. Even > Morse and Ingard shy away from looking at a moving media in detail. > It's akin to radar backscatter from rain or fog > >> which I do understand is frequency dependent - remember the radars in >> WWII that were designed at the water vapor absorption frequency? >> Well, OK that's absorption and not backscatter... but the frequency >> dependence aspect is similar isn't it? Otherwise radar most often >> doesn't have such a compelling phenonmenon - clean air.... Sonar >> volume reverberation is always there at short range and is >> substantial. I think that's what we're talking about. > > >> > >> Fred >> >> >>
I hate answering my own posts, but take a look at Steve Ward, The Use of Sinusoidal Frequency Modulated Pulses for Low-Doppler Detection, OCEANS, 2001. MTS/IEEE Conference and Exhibition It is on the IEEE Explore web site.
Reply by Stan Pawlukiewicz March 16, 20052005-03-16
Fred Marshall wrote:
> "Stan Pawlukiewicz" <spam@spam.mitre.org> wrote in message > news:d17n5h$jv8$1@newslocal.mitre.org... > >>Fred Marshall wrote: >> >>>"Stan Pawlukiewicz" <spam@spam.mitre.org> wrote in message >>>news:d16vkb$895$1@newslocal.mitre.org... > > . > >>>Obviously, Doppler can be a discriminator. So can angle of arrival if >>>it's being measured and if there's at least a reaonable SRR (signal to >>>reverberation ratio). If there is no Doppler and if the situation is >>>reverberation-limited then detection is a tough problem. I know of no >>>waveform tricks that help out in that case. Do you or Rune? >>> >>>Fred >> >>Backscatter is a frequency dependent phenomena. In some instances, it can >>be significant. I can see some advantages of using a range of frequencies >>to probe a partially characterized environment. There are are other >>reasons why multiple frequencies make sense that you are probably aware >>of. > > > "Backscatter is a frequency dependent phenomena." > > Ahah! Stan, you gotta be a radar guy. Am I right?
No a SONAR guy, mostly on the passive side, but I've don some reverberation modeling.
> I do understand about surface backscatter which us sonar guys call either > surface reverberation or bottom reverberation (as long as one isn't around > icebergs like Rune may be). > Surface reverberation can be frequency dependent at very long wavelengths > where the periodicity of gravity waves can be like a resonant crystalline > structure. The same thing could happen with bottom reverberation if the > bottom is like wavy sand dunes I suppose.
Bubbles, surface waves, Rayleigh scattering, internal (salinity) waves, particle suspensions. Its not like there are no articles in JASA that do not consider the properties of the bottom in reverberation studies. Absorption, while not backscatter, has an effect on backscatter amplitude. Rune knows more about that kinda
> stuff. So that has an analogy in radar and there's been a lot of work done > regarding sea surface backscatter characteristics which, at times, can be > analogous to sea surface backscatter as seen by sonars. > > In water there is "volume reverberation" which isn't very frequency > dependent at all. About the only thing about volume reverberation - which > often also applies to boundary reverberation - is that it has zero radial > velocity, thus zero Doppler (unless it's off-axis / velocity vector as seen > from a moving sonar).
Water moves ... tides, currents, wind direction, turbulence. Even Morse and Ingard shy away from looking at a moving media in detail. It's akin to radar backscatter from rain or fog
> which I do understand is frequency dependent - remember the radars in WWII > that were designed at the water vapor absorption frequency? Well, OK that's > absorption and not backscatter... but the frequency dependence aspect is > similar isn't it? Otherwise radar most often doesn't have such a compelling > phenonmenon - clean air.... Sonar volume reverberation is always there at > short range and is substantial. I think that's what we're talking about.
>
> Fred > > >