Explain dual freq. FM pulse homing

Excuse me for surfacing here, but I am a bit of a submarine buff in need of technical insight.

Can anyone familliar with range-finding, etc., explain what is meant by "Mod 0 is a homing torpedo using active acoustic, dual-frequency FM pulses and preformed beams."

In general terms, how does this operate?

More specifically, what is the design rationale for this system, eg. the use of FM, two beams, and beamforming?

Why acoustic frequencies?

Bob Litton

Reply to
Bob Litton
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"Bob Litton"

** That line is from "British Torpedoes after World War 2 "
** Plenty of info on the net about " active acoustic torpedoes ".
** Much of this info is classified, for obvious reasons. The system used is apparently an advanced SONAR with lots of DSP used to generate the beams & extract the wanted info from return echoes. Looks like FM is used to locate the direction from which an echo is arriving.

Imagine that there is one beam of pulsed, HF sound that is rapidly scanned over a range of angles, both vertically and horizontally. If at the same time, the frequency is varied about so that the bursts of sound are never the same frequency until the scanning pattern repeats - then there is a unique frequency associated with each point in the scan pattern.

If the bean strikes a target sub at some point, then the echo is also at that specific frequency corresponding to its position in the scan pattern. No matter when the echo arrives, its precise frequency gives the direction.

Two beams means the system can track two targets simultaneously - important in determining if the echo is a sub, a whale or something else.

It also appears that when such a torpedo gets close to its target, it switches over to passive acoustic and relies on sound (water screw noise etc) coming from the sub to guide it in. They can also dive to depths of up to 1000 metres and destroy their targets.

Who'd wanna be inside a sub ??

** A silly way of saying sound is used.

.... Phil

Reply to
Phil Allison

--
Why do you want to know?
Reply to
John Fields

As far as I know, it's got two sensors, and they use the different return times to steer the device by comparing return times; the FM part, I believe, gives better distance resolution by comparing the freq. of the return signal to the instantaneous freq. of the xmitted signal. But don't quote me on this, I'm just guessing.

FM, because it gives better distance resolution. Two beams, so it knows which direction to turn.

"Beamforming" means the same as "focusing;" it's just techno-buzzword speak to impress the roobs.

Because RF doesn't travel well in sea water. It's SONAR, after all.

Hope This Helps! Rich

Reply to
Rich Grise

"John Fields"

** Errr - try reading the first line of your post above.

The OP's query is about current electronics technology, albeit of a military nature.

Hard for him to get specifics anywhere.

I have tried too.

..... Phil

Reply to
Phil Allison

So the FM is linked to the sweep angle of the synthetic aperture, and there are two of same to provide directional info.

In the latter function maximum gain occurs when the two beams coincide. Does this imply both FM signals are identical?

Or ... perhaps one is anti-phase so _minimum_ gain occurs when they coincide. This is more logical in terms of being able to differntiate the two return signals.

What say ye experts?

This is interesting. Thanks.

Bob Litton

Reply to
Bob Litton

A more or less directional acoustic signal is sent out and a receiver looks for echos - signal reflected back from a target,

For the use of FM, search on chirped sonar - bats use it for locating insects, and there are other applications.

They propagate further through water. In general, the lower the frequency, the lower the attentuation with distance.

When I worked on phase-array ultrasound systemes for looking at human internal organs, we worked with 2MHz signals, with which we could get reflections from the back of an adult liver, if the patient/test subject wasn't too fat. We also used 4MHz for less extensive organs, and gear for looking at the eye-ball uses anything from 7.5MHz to

10MHz.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

In general terms, those who know don't say and those who say don't know.

The design rationale is to make a big hole in the water where the target used to be.

--
Rich Webb     Norfolk, VA
Reply to
Rich Webb

You might be surprised -- many a SBIR contract goes to companies consisting of "just a couple of guys" who start out knowing nothing more than what Google and a few good libraries can tell them: Much of what's classified in military technology isn't anything that isn't quite well-known in commercial sectors as well, it's just the exact details (e.g., frequencies used, pulse rates, etc.) that one doesn't want to divulge. Heck, while spread spectrum RF systems did initially come out of military research, the commercial implementations rapidly outstripped the performance of the military implementations of the day. Military hardware often lags cutting-edge commercial research, simply because it takes so much longer to develop and "harden" it and because the military often doesn't have as many constraints (e.g., cost, spectrum efficiency) as commercial developers do. (Recall all the folks sending their kids off-the-shelf GPS receivers during the first Gulf War -- since selective availability wasn't degrading the signals, the inexpensive Garmin/Magellan/etc. units were more usable with better features than many of the military GPS receivers available.)

My understanding is that during the cold war, most Russian spies spent the vast majority of their time doing nothing more exciting than reading at publicly-accessible libraries; very few engaged in the "cloak and dagger" stuff.

---Joel

Reply to
Joel Koltner

AFAIUI there are some interesting details in this sort of business.. the ocean surface (and bottom if you're close enough to see it) are rather noisy. Googling on "synthetic aperture sonar" will probably bring up a whole bunch of links and some scholarly articles on the subject that could be had through any University library. Hmm.. I just got an interesting resume from a guy who has worked in this area.

Reply to
Spehro Pefhany

As a point of interest, relative to my most recent post, could you please ask him if the two FM beams in this type of homing system are

1) in phase, 2) anti-phase, or 3) phase modulated?

As someone pointed out, this is not classified info, adaptive beam forming is now widely used in commercial applications.

Bob Litton

Reply to
Bob Litton

e

Nowhere near reality- military hardware lags behind because the development and procurement is HORRIBLY mismanaged. Clearly, if your full time job is to oversee procurement AND there are absolutely no deleterious career consequences to horrendous cost overruns and schedule slips- hell, go for it, stretch that develpment out to eternity even to the point of cancellation- then move on to the next one.

Reply to
Fred Bloggs

Question:

Perhaps the "FM" part allows for the use of a Doppler shift in the return signals and hence a calculation for the speed of the target. The amount of "lead" would be very important for a torpedo that may have only a 50% (or so) speed advantage. No?

Reply to
David Eather

Passive? Perhaps they're taking advantage of a change in propagation time vs. frequency. Solve the set of equations and the distance falls out? Sorta like finding the distance to a thunder storm.

Reply to
krw

"David Eather"

** Doppler shift can only determine the speed of a target that is moving directly away or towards the sonar. One that is moving across the path of the sonar shows no shift. So your "lead" idea fails.

Sonar determines the range of a target by the time delay in the return echoes - depending on whether echoes are arriving sooner, later or remaining the same indicates if the range is closing, receding or is relatively stationery.

A homing torpedo simply needs to know what direction to go to close in on the target - long as the torpedo is faster than the target, return echoes should always be getting shorter and stronger. If a pattern of sound pulses is being sent out, each with a characteristic direction and frequency, then the torpedo simply selects the strongest echo and heads in the direction indicated by the particular frequency.

... Phil

Reply to
Phil Allison

You're nuts. Never did. Never will. Most of the chips that were made back then were by military behest and the companies enjoyed commercial sale gains as well. Not as much now, but definitely back then.

You've obviously been out of the loop for too many years. We have COTS now. We turn out products with very little "hardening" overhead. We have advanced environmental stress evaluation labs. It is quite easy to prove a design, and hone it for mission critical function, if that is what the product is.

Total bullshit. Military electronics ARE the cutting edge. The idiots in Wahington nearly killed recent projects, before someone finally wised them up that we DO need what we have contracts for and MORE. They were going to kill it due to cost over-runs but neglected to note that their requisites morphed over the years REQUIRING development time that was guaranteed to blow the contract timeline, much less the budget.

It is the most advanced set of waveforms that have ever been produced.

Receiving a GPS signal does NOT "degrade the signal". It is a damned read only device. The satellites don't know a damned thing about the receivers on the ground.

You are really truly in the dark. Mil GPS resolves ten times better than consumer grade back then, and is still five times better than the "high res" consumer units now available as both have evolved.

There is not a single in motion unit (mil transport)in service that doesn't have them now, and fixed installations have big 1U and 2U rack mount precision units.

Stop making shit up.

Reply to
The_Giant_Rat_of_Sumatra

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There isn't any change of propagation time vs. frequency. The speed of sound in water is widely supposed to be essentially independent of frequency.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

On a sunny day (Tue, 26 Apr 2011 19:12:49 -0700) it happened The_Giant_Rat_of_Sumatra wrote in :

Fruitcake. If you had a memory you would have remembered the drones sending not encrypted analog? video, while the rest of the world uses smart cards and digital TV. Some of the enemy used free Russian TV software on the PC to see the picture from the drones. There are many more cases.

Reply to
Jan Panteltje

Does "military" and "intelligence" belong to the same sentence ?

Reply to
upsidedown

My recollection is that some military grade computer gear in the 80's was essentially civilian kit with a few key components swapped for more robust parts. It wasn't that hard for a well equipped electronics lab to upgrade those parts either. Several academic groups did just that to increase uptime significantly as and when parts failed.

analog? video,

It is a lot more variable than you might hope. They tend to have gold plated glamorous and expensive sensors with amazing capabilities hooked up to nearly jerry rigged things that lack basic security features on their telemetry like encryption.

And some of their prototyping procurement procedures done by the book were so incredibly long winded and cumbersome that they would give a pair of free sensors to certain astronomy research groups for testing on condition they made *two* prototypes. Terahertz images have largely disappeared from the web as they show too much of the capabilities.

from the drones.

Unfortunately this is true. Military intelligence is an oxymoron.

Regards, Martin Brown

Reply to
Martin Brown

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