On 10 Nov, 04:06, glen herrmannsfeldt <g...@ugcs.caltech.edu> wrote:> Eric Jacobsen <eric.jacob...@ieee.org> wrote: > > (snip, someone wrote) > > >>it would be really rewarding for someone to develop an > >> emergency comm system for miners trapped underground... > > There are standards for such things, and products that meet the > > standards. � I don't know how well any of them work in practice, but > > some seem to be pretty useful. > > I used to know stories about systems for communicating with > submarines at either 15kHz or 7Hz. �The latter, as I remember it, > is the resonant frequency of the earth, such that one can't do > direction finding. � I don't know how deep you can get at > the lower frequencies with enough power. �Presumably the electrical > noise level is low down there (after a cave in with all power > turned off).Direction finding has nothing to do with any of that. The point of those systems is to have a one-way shore- to-ship method to issue mission orders at any time, regardless of how deep the sub might be submerged. The problem is that sea water is a dielectric EM medium, so it reflects EM waves. These LF/VLF/ULF systems depend on the evanescent 'tail' of the wave into the water beind detected by the sub. The longer the wavelength of the transmitted signal, the deeper penetration into sea water by that evanescent tail. The transmitters of such signals are on shore, in fixed locations that presumably would be known to anyone who care about tracking such comms. There is no way any man-made moving object ever will come even remotly close to carrying that kind of transmitter on board: Use an inefficient antenna and the transmitter itself will need to be ridiculously large. Efficient antennas require less power from the transmitters but will themselves be impractically large, wavelengths being on the scale of Earth itself. Of course, one only need a handful of such transmitters to cover the globe. It is hard to track the fleet of subs, as tracking transmitter activity does not give away any clues as to where your fleet might be operating. There was movie some time ago, 'Crimson Tide', http://www.imdb.com/title/tt0112740/ where the plot revolved around a boomer recieving a partial launch order, the transmission having been scrambled by some hickup in the comm system. The problem was how the sub crew should deal with a *partial* order to launch their cargo at whatever target was specified. As I remember, some people on board meant the scrambling might be a result of the transmitter being attacked; others that a *partial* launch order did not authorize a launch in anger, irrespective of what might have caused the comm problem. Rune
Modulation/Signaling scheme for transfering information over an acoustic channel
Started by ●November 9, 2011
Reply by ●November 10, 20112011-11-10
Reply by ●November 10, 20112011-11-10
On 11/10/2011 3:56 AM, Rune Allnor wrote: ...> The problem is that sea water is a dielectric EM medium, > so it reflects EM waves. These LF/VLF/ULF systems depend > on the evanescent 'tail' of the wave into the water beind > detected by the sub. The longer the wavelength of the > transmitted signal, the deeper penetration into sea water > by that evanescent tail.Not merely a dielectric, but a very lossy one. Actually, a fairly good conductor.> The transmitters of such signals are on shore, in fixed > locations that presumably would be known to anyone who > care about tracking such comms. There is no way any man-made > moving object ever will come even remotly close to carrying > that kind of transmitter on board: Use an inefficient antenna > and the transmitter itself will need to be ridiculously large. > Efficient antennas require less power from the transmitters > but will themselves be impractically large, wavelengths being > on the scale of Earth itself. Of course, one only need a > handful of such transmitters to cover the globe. It is hard to > track the fleet of subs, as tracking transmitter activity > does not give away any clues as to where your fleet might > be operating.... The US navy's ELF carrier frequency was 67 Hz. (Not a typo.) The Russians' was higher: 82 Hz. The US antenna (at Clam Lake, Wisconsin?) was only 32 miles long. There was a dedicated power plant to power it, and in the end it radiated only a few watts. The names "Sanguine" and "Seafarer" come to mind. Jerry -- Engineering is the art of making what you want from things you can get.
Reply by ●November 10, 20112011-11-10
On Thu, 10 Nov 2011 00:07:01 +0000, Eric Jacobsen wrote:> On Wed, 9 Nov 2011 18:49:19 -0500, "MarkK" <makolber@yahoo.com> wrote: > > >> >>> The OP could state just how "open air" it needs to be? I suspect you >>> could do a lot better if the user was expected to hold the phone up to >>> the speaker than if it had to work from across the room. >> >> >>speaking of difficult channels >> >>it would be really rewarding for someone to develop an emergency comm >>system for miners trapped underground... >> >>Mark > > There are standards for such things, and products that meet the > standards. I don't know how well any of them work in practice, but > some seem to be pretty useful. > > We did some consulting for these guys several years ago for some early > work on a magnetically-coupled system: > > http://www.kuttaradios.com/ > > The idea is to use magnetic coupling to existing infrastructure, pipes, > rail, cables, whatever. Naturally, if such infrastructure isn't in > place or gets disconnected in an event, it won't be useful. > > There are other approaches as well, some using traditional radio > techniques, but it's a difficult proposition to make something that will > work in all cases. The through-the-ground stuff is probably not > practical, either. > > Over the years and especially lately I've spent a fair amount of time in > an extensive tunnel network in a silver mine. Recently I took some FRS > radios in there with me, keeping my fingers crossed that the tunnels > would provide some degree of waveguide or power concentration effect, > but the range of the radios was about the same range as just shouting at > each other, so it didn't work well at all. I have some ideas of how to > make that better, especially around corners, but haven't had a chance to > try anything yet. > > It is a difficult and interesting problem. I'm hoping to use the > access to the tunnels for some experiments, but won't be back in there > until next year. > > > Eric Jacobsen > Anchor Hill Communications > www.anchorhill.comIf it works well and you commercialize it, you'll need to change your name to "Under Hill Communications" -- www.wescottdesign.com
Reply by ●November 10, 20112011-11-10
Jerry Avins <jya@ieee.org> wrote: (snip in submarine radio)> Not merely a dielectric, but a very lossy one. Actually, > a fairly good conductor.(snip)> ...> The US navy's ELF carrier frequency was 67 Hz. (Not a typo.) The > Russians' was higher: 82 Hz. The US antenna (at Clam Lake, Wisconsin?) > was only 32 miles long. There was a dedicated power plant to power it, > and in the end it radiated only a few watts.> The names "Sanguine" and "Seafarer" come to mind.I wonder how much 60Hz there is down there. I would have thought that 67Hz wouldn't be a good choice in a 60Hz world, but maybe not so bad in a 50Hz world. I remember one homework problem with a submarine and long wire antenna at 15kHz, using 9 for the index of refraction (sqrt(80)). For that, the submarine has to come close to the surface, but not actually above the surface. So, no 7Hz transmitters? That was the story I remembered. -- glen
Reply by ●November 10, 20112011-11-10
On 10 Nov, 01:07, eric.jacob...@ieee.org (Eric Jacobsen) wrote:> On Wed, 9 Nov 2011 18:49:19 -0500, "MarkK" <makol...@yahoo.com> wrote: > > >> The OP could state just how "open air" it needs to be? �I suspect you > >> could do a lot better if the user was expected to hold the phone up to > >> the speaker than if it had to work from across the room. > > >speaking of difficult channels > > >it would be really rewarding for someone to develop an emergency comm system > >for miners trapped underground... > > >Mark > > There are standards for such things, and products that meet the > standards. � I don't know how well any of them work in practice, but > some seem to be pretty useful....> It is a difficult and interesting problem. � I'm hoping to use the > access to the tunnels for some experiments, but won't be back in there > until next year.You might want to take a long and hard look at the prioirities, what functionality is actually *needed* and what is merely *convenient*? The *necessary* properties are simple. The system needs to 1) Communicate that somebody is still alive in the mine. 2) Be used for locating where the transmission originates from. Anything else, even acknowledgemnts of messages recieved, are mere conveniences that kan be skipped with no *essential* loss. If one limits the scope of the system to the two items above, one all of a sudden can make efficient systems at least on the workstation / squad level; maybe even personal systems. The basic principle is simple, although implementation might require some work. The key is to think in the correct terms, focus on the essentials, and otehrwise keep the relevant perspective. Rune
Reply by ●November 10, 20112011-11-10
On Thu, 10 Nov 2011 11:03:56 -0600, Tim Wescott <tim@seemywebsite.com> wrote:>On Thu, 10 Nov 2011 00:07:01 +0000, Eric Jacobsen wrote: > >> On Wed, 9 Nov 2011 18:49:19 -0500, "MarkK" <makolber@yahoo.com> wrote: >> >> >>> >>>> The OP could state just how "open air" it needs to be? I suspect you >>>> could do a lot better if the user was expected to hold the phone up to >>>> the speaker than if it had to work from across the room. >>> >>> >>>speaking of difficult channels >>> >>>it would be really rewarding for someone to develop an emergency comm >>>system for miners trapped underground... >>> >>>Mark >> >> There are standards for such things, and products that meet the >> standards. I don't know how well any of them work in practice, but >> some seem to be pretty useful. >> >> We did some consulting for these guys several years ago for some early >> work on a magnetically-coupled system: >> >> http://www.kuttaradios.com/ >> >> The idea is to use magnetic coupling to existing infrastructure, pipes, >> rail, cables, whatever. Naturally, if such infrastructure isn't in >> place or gets disconnected in an event, it won't be useful. >> >> There are other approaches as well, some using traditional radio >> techniques, but it's a difficult proposition to make something that will >> work in all cases. The through-the-ground stuff is probably not >> practical, either. >> >> Over the years and especially lately I've spent a fair amount of time in >> an extensive tunnel network in a silver mine. Recently I took some FRS >> radios in there with me, keeping my fingers crossed that the tunnels >> would provide some degree of waveguide or power concentration effect, >> but the range of the radios was about the same range as just shouting at >> each other, so it didn't work well at all. I have some ideas of how to >> make that better, especially around corners, but haven't had a chance to >> try anything yet. >> >> It is a difficult and interesting problem. I'm hoping to use the >> access to the tunnels for some experiments, but won't be back in there >> until next year. >> >> >> Eric Jacobsen >> Anchor Hill Communications >> www.anchorhill.com > >If it works well and you commercialize it, you'll need to change your >name to "Under Hill Communications"The name "Anchor Hill" comes from the historic location of an underground gold mine, coincidentally. ;) The other day I found out that a client is entering the underground (mining) comm business. I told them I have a test facility available if they're interested. I'm hoping that works out. ;) Eric Jacobsen Anchor Hill Communications www.anchorhill.com
Reply by ●November 10, 20112011-11-10
On 11/10/2011 2:25 PM, glen herrmannsfeldt wrote: ...> So, no 7Hz transmitters? That was the story I remembered.Maybe that came later Jerry -- Engineering is the art of making what you want from things you can get.
Reply by ●November 10, 20112011-11-10
Rune Allnor <allnor@tele.ntnu.no> wrote: (snip, someone wrote)>> >it would be really rewarding for someone to develop an >> > emergency comm system for miners trapped underground...(snip)> You might want to take a long and hard look at the prioirities, > what functionality is actually *needed* and what is merely > *convenient*?> The *necessary* properties are simple. The system needs to> 1) Communicate that somebody is still alive in the mine. > 2) Be used for locating where the transmission originates from.So, either direction finding (hard at low frequencies) or enough bits of information and a map. Should be low enough cost that it can be used in poor countries.> Anything else, even acknowledgemnts of messages recieved, are > mere conveniences that kan be skipped with no *essential* loss.Hmm. Remembering the Chile mine rescue, they first drilled a small enough hole to be sure someone was alive, get food and water down. It might have been nice to notify the miners that help was coming. That is, so they don't get discouraged. But that only takes one bit. It would seem that more power would be available on the surface, though also one could build more sensitive receivers, and use bigger antennae.> If one limits the scope of the system to the two items above, > one all of a sudden can make efficient systems at least on > the workstation / squad level; maybe even personal systems.-- glen
Reply by ●November 10, 20112011-11-10
On 11/9/2011 4:07 PM, Rune Allnor wrote:> On 10 Nov, 00:00, Fred Marshall<fmarshallxremove_th...@acm.org> > wrote: >> On 11/9/2011 12:28 PM, Rune Allnor wrote: > >>> On 9 Nov, 21:26, Jerry Avins<j...@ieee.org> wrote: > >>>> How do you classify acoustic modems? >> >>> As an acoustic interface to an EM comm channel. >> >>> Rune >> >> Reminds me somewhat of the term: "external locus of control".... :-) >> >> I think the "acoustic interface" is an acoustic communication channel >> pure and simple. What else can it be? > > An minor acoustic component in an otherwise totally EM system? > In the EM case the modem would hardly be considered a major > part of the propagation channel, but rather a component in > the interface between the source of the signal, and > the waveform that propagates as EM waves from the antenna. > > RuneRune, Well, if it were just a "transducer" then I'd likely agree. But it isn't. It includes a modem (modulator-demodulator), a selected coding scheme, a transmit encoder and transducer, a receive transducer and decoder (the transducers each have both roles), etc. etc. The physical length of the channel, the fact that it's relatively "tightly coupled", the physical size of the components on a comparative basis, minor vs. major, etc. etc. aren't the issue. WHAT EM waves? WHAT antenna? Such things are peripheral to the discussion at hand. Fred
Reply by ●November 10, 20112011-11-10
On 11/10/2011 4:23 PM, glen herrmannsfeldt wrote:> Rune Allnor<allnor@tele.ntnu.no> wrote: > > (snip, someone wrote) >>>> it would be really rewarding for someone to develop an >>>> emergency comm system for miners trapped underground... > > (snip) >> You might want to take a long and hard look at the prioirities, >> what functionality is actually *needed* and what is merely >> *convenient*? > >> The *necessary* properties are simple. The system needs to > >> 1) Communicate that somebody is still alive in the mine. >> 2) Be used for locating where the transmission originates from. > > So, either direction finding (hard at low frequencies) or enough > bits of information and a map. > > Should be low enough cost that it can be used in poor countries. > >> Anything else, even acknowledgemnts of messages recieved, are >> mere conveniences that kan be skipped with no *essential* loss. > > Hmm. Remembering the Chile mine rescue, they first drilled a > small enough hole to be sure someone was alive, get food and > water down. It might have been nice to notify the miners > that help was coming. That is, so they don't get discouraged. > > But that only takes one bit.They lowered a note and the miners sent one up. Fortunately, they knew how to read.> It would seem that more power would be available on the surface, > though also one could build more sensitive receivers, and use bigger > antennae. > >> If one limits the scope of the system to the two items above, >> one all of a sudden can make efficient systems at least on >> the workstation / squad level; maybe even personal systems. > > -- glenJerry -- Engineering is the art of making what you want from things you can get.
