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lloyds mirror effect

Started by rahul1 May 12, 2006
hi all 
i am interested in the lloyds mirror effect and its applications in
sonars. can someone kindly refer me to somewhere i could read up on it.
thanks 
rahul

rahul1 wrote:

> hi all > i am interested in the lloyds mirror effect and its applications in > sonars. can someone kindly refer me to somewhere i could read up on it. > thanks > rahul >
Google has 133,000 hits
rahul1 wrote:
> hi all > i am interested in the lloyds mirror effect and its applications in > sonars. can someone kindly refer me to somewhere i could read up on it. > thanks > rahul >
The basic effect is due to 2 signal paths of different lengths. When a broadband (e.g. white noise) type signal is transmitted it produces a sinusoidal pattern in the spectrogram or more usually for sonar Lofargram. When the source is moving this causes the sinusoidal pattern to change over time. By measuring the period of the sinusoidal pattern at the Closest Point of Approach (CPA) , and by measuring the rate of change of the sinusoidal pattern you can perform some target localization. The basic effect is described in Richard Neilson's book "Sonar Signal Processing" Cheers, David
rahul1 wrote:
> hi all > i am interested in the lloyds mirror effect and its applications in > sonars.
The Lloyd mirror is not an application. It is one of the major causes of trouble when using sonars. I once used the theory of the Lloyd mirror to explain why a radio transmission system broke down at a couple of km range over sea, while it worked on the order of 10 km over land.
> can someone kindly refer me to somewhere i could read up on it.
Try Jensen & al: "Computational Ocean Acoustics" Rune
David Kirkland wrote:
> rahul1 wrote: > > hi all > > i am interested in the lloyds mirror effect and its applications in > > sonars. can someone kindly refer me to somewhere i could read up on it. > > thanks > > rahul > > > > The basic effect is due to 2 signal paths of different lengths. When a broadband > (e.g. white noise) type signal is transmitted it produces a sinusoidal pattern > in the spectrogram or more usually for sonar Lofargram. When the source is > moving this causes the sinusoidal pattern to change over time.
Ant the exact structure of this pattern depends on...?
> By measuring the period of the sinusoidal pattern at the Closest Point of > Approach (CPA) , and by measuring the rate of change of the sinusoidal pattern > you can perform some target localization.
Can you really? There is a long step from "find the CPA" to "localize a target."
> The basic effect is described in Richard Neilson's book "Sonar Signal Processing"
Rune
rahul1 wrote:

> hi all > i am interested in the lloyds mirror effect and its applications in > sonars. can someone kindly refer me to somewhere i could read up on it. > thanks
When a wave is reflected from a boundary between media, either the electric of magnetic field (if electromagnetic) or the pressure or velocity field (if acoustic) is reversed depending on relative magnitudes of the refractive indices. When a wave at grazing incidence is reflected, both the incident and reflected rays travel in substantially the same directing, and the reversal causes destructive interference. At slightly higher angles of incidence, the interference is constructive, and so a series of fringes (of decreasing spacing) becomes evident. The relevance to forward-looking sonar or surface radar is the rather broad low-angle destructive interference that eliminates useful signal. The classical optical Lloyd's mirror is a piece of black plate glass. The black pigment ensures that only reflected, not transmitted, light comes off the glass. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Rune Allnor wrote:
> David Kirkland wrote: >> rahul1 wrote: >>> hi all >>> i am interested in the lloyds mirror effect and its applications in >>> sonars. can someone kindly refer me to somewhere i could read up on it. >>> thanks >>> rahul >>> >> The basic effect is due to 2 signal paths of different lengths. When a broadband >> (e.g. white noise) type signal is transmitted it produces a sinusoidal pattern >> in the spectrogram or more usually for sonar Lofargram. When the source is >> moving this causes the sinusoidal pattern to change over time. > > Ant the exact structure of this pattern depends on...? > >> By measuring the period of the sinusoidal pattern at the Closest Point of >> Approach (CPA) , and by measuring the rate of change of the sinusoidal pattern >> you can perform some target localization. > > Can you really? There is a long step from "find the CPA" to "localize a > target." > >> The basic effect is described in Richard Neilson's book "Sonar Signal Processing" > > Rune >
Hi Rune, As I said, "some target localization". It can at least give you an idea of where to look for your target. How close does the ordinance have to be? From what I've seen, the Lloyd's mirror tools are used mostly with sonobuoys and relatively short ranges at which the dominant propagation effect is due to sound velocity changes (for deep water operation - not littoral). If you get the target localized on multiple buoys then it is more likely than not your target is close to your localization. If it was completely ineffective, then I wouldn't keep receiving operational requirements for it. Cheers, David
David Kirkland wrote:
> Rune Allnor wrote: > > David Kirkland wrote: > >> rahul1 wrote: > >>> hi all > >>> i am interested in the lloyds mirror effect and its applications in > >>> sonars. can someone kindly refer me to somewhere i could read up on it. > >>> thanks > >>> rahul > >>> > >> The basic effect is due to 2 signal paths of different lengths. When a broadband > >> (e.g. white noise) type signal is transmitted it produces a sinusoidal pattern > >> in the spectrogram or more usually for sonar Lofargram. When the source is > >> moving this causes the sinusoidal pattern to change over time. > > > > Ant the exact structure of this pattern depends on...? > > > >> By measuring the period of the sinusoidal pattern at the Closest Point of > >> Approach (CPA) , and by measuring the rate of change of the sinusoidal pattern > >> you can perform some target localization. > > > > Can you really? There is a long step from "find the CPA" to "localize a > > target." > > > >> The basic effect is described in Richard Neilson's book "Sonar Signal Processing" > > > > Rune > > > > Hi Rune, > > As I said, "some target localization". It can at least give you an idea of where > to look for your target. How close does the ordinance have to be? From what I've > seen, the Lloyd's mirror tools are used mostly with sonobuoys and relatively > short ranges at which the dominant propagation effect is due to sound velocity > changes (for deep water operation - not littoral). > > If you get the target localized on multiple buoys then it is more likely than > not your target is close to your localization. If it was completely ineffective, > then I wouldn't keep receiving operational requirements for it.
OK, it's an elaborate set-up, but it makes sense. You know me, I have pretty strong opinions about what can and what can not be done what passive localization is concerned. Rune