[HN Gopher] Cave-Link: Underground text communication system
___________________________________________________________________
Cave-Link: Underground text communication system
Author : L_226
Score : 187 points
Date : 2021-11-09 13:02 UTC (1 days ago)
(HTM) web link (www.cavelink.com)
(TXT) w3m dump (www.cavelink.com)
| dll wrote:
| This was used in the recent rescue in Wales [0]. I seem to recall
| a colleague telling me years ago (in about 2000) that he got
| stuck in a cave once (somewhere in South Wales I think) due to
| unexpected rain causing flooding and there was a phone available
| that could be used to request rescue (or to tell people you were
| going to sit it out).
|
| [0] https://www.bbc.co.uk/news/uk-wales-59219380
| jlturner wrote:
| Extremely / Very Low frequency radio is fascinating and is
| surprisingly not used that often. Radio propagation
| characteristics include being able to pass through water and
| ground, and continually bounce across the ionosphere / circle the
| globe. The navy experimented with this for some time to talk to
| submarines but seems to have ended the experiment.
|
| Additionally I learned that lightning strikes generate a high
| burst of low frequency radio and commercial lightning detectors
| (which I use while hiking) actually is just a radio receiver on a
| harmonic of the lightning signal.
| superkuh wrote:
| At the frequencies and distances that these cave "radio"
| systems are operated they act more like two coils of wire with
| a mutual inductance in the near field (<1/4 wavelength away).
| This isn't propagating electromagnetic waves (radio), at least
| not much or with any fraction of efficiency.
| xvf22 wrote:
| I'm reasonably sure VLF and ELF are very much in use but it
| seems as though ELF was abandoned by the US Navy. The Navy does
| continue to use VLF though.
|
| https://www.thedrive.com/the-war-zone/25728/chinas-new-york-...
|
| http://www.nukeresister.org/static/nr135/135elfcloses.html
| mrfusion wrote:
| I never understood why visible light goes through water but
| almost every other frequency is blocked?
| colechristensen wrote:
| If there were a open band somewhere else, that is what we
| would have evolved to see.
|
| Is just the dynamics of how light interacts with the atom and
| its bonds.
| hexane360 wrote:
| Because people haven't really explained the specific
| mechanisms:
|
| You can think of a material as a bunch of simple harmonic
| oscillators, which are driven by an applied field (electric
| or magnetic). These oscillators have different natural
| frequencies, and so couple to different wavelengths. For
| instance, if you play a loud note next to a piano, you can
| see the corresponding piano string start to vibrate, but the
| others may not.
|
| The interaction between these oscillators and the wave are
| what lead to the dielectric constant (and therefore the
| slowing of waves), as well as absorption (which can be
| thought of as the imaginary component of the dielectric
| constant).
|
| In a real material, these "oscillators" are really any method
| of energy storage that can couple to the motion of charges
| (i.e. current). These include, but are not limited to: -
| rotations (in a gas or liquid) - vibrations (in any state) -
| electronic transitions - electronic movement (in the case of
| a metal) - displacement (in any state)
|
| In a single molecule, many of these mechanisms would have
| discrete natural frequencies. But in a solid or liquid,
| interactions lead to a continuous band structure (especially
| for things like vibrations).
|
| For water specifically, the below visible range is quickly
| absorbed by vibrational and rotational energy modes, while
| the high end of the UV range is absorbed by electronic
| transitions. Other materials have similar sweet spots for
| transmission, but at different frequencies. For instance,
| materials like indium tin oxide (ITO) are designed to be
| conductive, but not at the high frequencies of visible light,
| making them transparent. As another example, metals are
| reflective below their plasma frequency (related to the speed
| the 'electron sea' can move at), and transparent above
| (X-rays operate in this region of transparency).
|
| If you want more information, I can recommend "Optical
| Properties of Solids" by Mark Fox.
| i000 wrote:
| Why? The properties of water were optimized to make life
| possible!
| ohwellhere wrote:
| I think this reasoning may be right, if expressed backward.
| "Visible light" is just that which life evolved to care
| most about, because it was what was available in the
| (shallowish) water.
| 867-5309 wrote:
| in the case of the whales, whom returned to the deep sea
| 40-50mya and have since rarely had a use for visible
| light, it's interesting to wonder what they have evolved
| to care most about if no longer visible light, especially
| in the sense of their song, and when you compare the
| visual acuity of their cousins the dolphins in the same
| timespan, and the incredible hyper-evolution of the human
| eye since ~1mya
| mrfusion wrote:
| I just meant the physics of it.
| graderjs wrote:
| The properties of vision were optimized to make seeing
| through water possible!
| jfrunyon wrote:
| It's the other way around. Our eyes evolved to be sensitive
| to (some of) the wavelengths to which air and water are
| transparent.
|
| There's nothing innately special about visible light. Just
| about every substance is transparent to some wavelength or
| other.
| Archelaos wrote:
| The issue is even more complicated. The absorbtion at the red
| end of the visual spectrum is two magnitudes larger than at
| the blue end. This is why water appears often blue, even
| against a white background.
|
| Wikipedia has some more details: https://en.wikipedia.org/wik
| i/Electromagnetic_absorption_by_...
| mrfusion wrote:
| I just need an ELI5 of that Wikipedia page and I'm all set.
| Good find though.
| bityard wrote:
| This reminds me of the time I went looking for a
| satisfactory answer for why the sky is blue.
|
| It turns out a surprising number of people have a
| surprising number of different opinions and explanations
| on this. You pretty much need a PhD in physics to fully
| grok most of them. And every time you think you've read
| something that at least _sounds_ concrete, someone else
| comes along and says, "yes and no, the _real_ mechanism
| is this," ad infinitum until you wind up on the fringes
| of scientific knowledge.
|
| In the end, hours of research later, as best as I could
| tell, it came down to: the sky is blue because air is
| blue. But it's a very faint blue, so you can't actually
| discern the blue until you look through a _lot_ of air,
| for example in the sky.
| cmurf wrote:
| There's a huge antenna in Colorado, I think managed by the
| Denver Air Route Traffic Control Center which is located in
| Longmont, that's used as a backup for transatlantic flights
| communications.
| tomfanning wrote:
| Trans-oceanic flights go far beyond the VHF radio horizon to
| land so routinely use HF radio. It's not a backup, it's the
| norm.
|
| Example: https://www.iaa.ie/air-traffic-management/north-
| atlantic-com...
|
| "Huge" is probably not that big vs VLF. Maybe on the order of
| ~50m width (~3MHz / ~100m wavelength for ~50m half-wave
| antennas). VLF is more like wires across entire valleys -
| kilometres.
| ale42 wrote:
| How huge? Aviation uses HF (shortwaves) and not VLF as a
| backup for translatlantic flights (see for example
| https://thepointsguy.com/guide/how-pilots-communicate-
| with-a...). HF antennas can already look huge ;-)
| JoeDaDude wrote:
| The US Navy still uses VLF, though ELF comms seem to be retired
| at this point.
|
| https://en.wikipedia.org/wiki/Communication_with_submarines#...
| mkup wrote:
| Russians still use extremely low frequencies for communication
| with submarines (an unidirectional communication system called
| "Zevs"). Transmitter is located on the Kola Peninsula,
| frequency is 82 Hz, bit rate is a few bits per minute, power
| consumption is in the multi-megawatt range. Waveguide is formed
| by the surface of the earth and the ionosphere.
| bollu wrote:
| I'd like some intuition --- is that very low frequency? Is
| that very high power? Can I have some context for how I
| should think about 82 hz and multi megawatt? (For example, I
| know that few bits a minute = well, that's a couple ASCII
| characters a minute)
| jfrunyon wrote:
| If you don't already know just how low 82 Hz is - then the
| lowest frequency band you're probably familiar with is AM
| radio. Which uses around 1,000,000 Hz.
|
| And as others have mentioned, yes, megawatts is a _lot_.
| That 's the same order of magnitude as what most power
| plants produce.
| mcpherrinm wrote:
| Here are some names we give to radio bands. These names are
| somewhat dated, as even "high" frequency is pretty low.
|
| Low Frequency (LF) (30 kHz to 300 kHz)
|
| Medium Frequency (MF) (300 kHz to 3 MHz)
|
| High Frequency (HF) (3 MHz to 30 MHz)
|
| Very High Frequency (VHF) (30 to 300 MHz)
|
| Ultra High Frequency (UHF) (300 MHz to 3 GHz)
|
| HF is used by amateur radio operators to communicate around
| the world, and has sufficient bandwidth to carry voice or
| low speed data (300 bits per second). Commercial airliners
| use HF over the oceans too. Probably some military stuff
| too.
|
| Most other radios on the planet is operating on a higher
| frequency, in VHF/UHF or higher.
|
| As for power, 100 watts is enough to send an HF signal
| around the globe. Commercial AM radio stations might
| operate up to 50,000 watts, so a megawatt is a lot more.
| pja wrote:
| 82 Hz implies a wavelength of ~4000km in vacuum. It's
| extremely low frequency by comparison with the kind of
| radio spectrum we usually use.
|
| The Wikipedia page gives a good overview:
| https://en.wikipedia.org/wiki/Radio_spectrum
| TedDoesntTalk wrote:
| Isn't that audible? And with megawatts of power... think
| of the whales!
| vel0city wrote:
| Its still electromagnetic waves not pressure waves so
| they're not audible.
|
| Still, I'd recommend not being very close to the
| transmitter if its putting out megawatts of power.
| pja wrote:
| Nah, it's electromagnetic & your ears are too small to
| intercept the transmission to any kind of significant
| extent.
|
| A few MW sounds a lot, but the inverse square law applies
| - once you get to the ocean the power density is going to
| be very low. Plus penetration into the ocean itself isn't
| going to be great - there's an impedence mismatch between
| the water and the air.
| _def wrote:
| I'm a noob regarding radio: do I understand it correctly that
| low frequency radio signals can more easily be used with more
| solid carriers while higher frequencies allow more/faster
| bandwidth? (I'm totally oversimplifying here)
| ajsnigrutin wrote:
| You need bandwidth to transfer more data (there is a
| theoretical limit of how much data you can transfer at a
| given bandwidth with a given signal to noise ratio).
|
| The lower you go with frequencies, the larger antennas you
| need (quarter wavelength is usually the minimum... for FM
| radio (around 100MHz) it's 75cm which is very realistic
| length for a car or a house radio... at 14mhz (ham radio
| freq), it's 5.3 meters, which is still doable if you live in
| a house, and at 1.8MHz (another ham band) is ~40 meters,
| which is hard.. going lower is even harder, and special
| "tricks" are needed.
|
| The second problem is the bandwith itself... some wifi
| equipment supports bands up to 100MHz of width (to simplify,
| you use all the frequences from eg. 5.1GHz to 5.2GHz), and if
| you want to stay at low frequencies, it's impossible to
| create such a wide band, because the antenna sizes are so
| different at 1MHz and 101MHz, the propagation is different,
| etc., so usually you're stuck with narrower bandwiths at
| lower frequencies (and of course, many of the low frequencies
| are already in use.. eg for fm radio).
|
| But generally yes, higher the frequency, more directly (in a
| straight line) it goes, and you get more losses in stuff
| between the transmitter and receiver.
| _def wrote:
| Thanks!
| TedDoesntTalk wrote:
| Can the antennas be coiled to achieve a long length while
| not being strung , for example, 40 meters? I recall car
| antennas that used to have coils long ago.
| ajsnigrutin wrote:
| Not really, atleast not the classic designs and if you
| want to transmit. You can bent them a bit, but the total
| lenght has to be around the quarter wavelength
| namibj wrote:
| There are coil antennas, but they work magnetically. The
| 40m straight wire works electrically. The key difference
| is that the electric antenna interacts with more of the
| wave, so it's stronger in the far-field, while the
| magnetic antenna interacts more localized. The latter is
| why/how NFC works at 13 MHz (23 m wavelength) with an
| antenna that fits on a credit card.
| colechristensen wrote:
| Bandwidth, literally the width of the band. Don't think of it
| as transmitting at one frequency but a range between two, a
| band.
|
| Information rate depends on the width of the band. For
| example wifi uses 20, 40, or 80 MHz bands. Easy when your
| signal is around 2.4 or 5 GHz.
|
| But you can't do that at 100 Hz or 100 kHz or 100 MHz
| because... well the numbers aren't big enough. You could have
| a funny radio that transmitted from 1 to 80 MHz, but you
| wouldn't say your signal was "at" 100kHz.
|
| That make sense?
| _def wrote:
| Yes thank you!
| brk wrote:
| Generally speaking, yes. With RF there are a handful of
| encoding methods, but in very simple terms binary 1's and 0's
| correspond to the high/low peaks of the frequency. Higher
| frequencies can therefore transmit data faster because you
| have more "peaks", and opportunities to encode data per
| interval of time.
|
| Higher frequencies have less power in terms of ability to
| travel a great distance or through materials. It is very much
| like audible sounds, you can hear bass notes from someone
| playing music far away, but you'll lose the highs and then
| the mids after a relatively short distance.
|
| One of the many tradeoffs in RF networks is trading range for
| throughput.
| zild3d wrote:
| > but in very simple terms binary 1's and 0's correspond to
| the high/low peaks of the frequency
|
| This is AM, "Amplitude Modulation"
|
| Wiki has some good articles on other modulation techniques
| https://en.wikipedia.org/wiki/Amplitude_modulation
| _def wrote:
| Oh the comparison with audible sounds is brilliant, thank
| you.
| williamtwild wrote:
| Yes
| hoseja wrote:
| Lighting strikes also feed the very cool
| https://en.wikipedia.org/wiki/Schumann_resonances
| sideshowb wrote:
| Also in caving news this week https://www.bbc.co.uk/news/uk-
| wales-59203695
| binbag wrote:
| Yeh, this is the system they used to communicate during the
| rescue.
| rendall wrote:
| How do the Mole People feel about this?
| twic wrote:
| See also the HeyPhone and Nicola systems built by underground
| boffins in the UK and France:
|
| https://www.shropshirecmc.org.uk/radio.html
|
| http://www.scavalon.be/avalonuk/technical/radio1.htm
|
| https://www.electronicsweekly.com/blogs/engineer-in-wonderla...
|
| https://www.caverescue.org.uk/nicolaradio/
|
| http://site2.caves.org.uk/radio/comms_in_caves.html
| Isamu wrote:
| > Because all transmissions are secured by checksums and
| automatic query, no transmission errors are possible.
|
| Just a reminder to people implementing systems with error
| detection: undetected errors are always possible. Checksums may
| be just fine for this application but if you need to achieve some
| target error rate you may have to consider error detecting or
| correcting codes that fit with the interference you find in your
| transmission channel.
|
| [edit] Error detecting checks work not by making errors
| impossible but by making them unlikely. It is part of the work to
| quantify your design to show that probability is low enough for
| your goal.
| jareklupinski wrote:
| "Only a Sith deals in absolutes."
|
| that line (along with enough experience) helped erase words
| such as "always", "never", and "impossible" from my technical
| vocabulary :)
| selfhoster11 wrote:
| I started to reintroduce them into my vocabulary. I found
| myself qualifying nearly everything, and it decreases the
| clarity of the discussion, feels lawyer-like. I'm not
| drafting a contract (most of the time), but trying to
| communicate a complicated principle in a few words, where
| every extra word detracts from the meaning.
| WithinReason wrote:
| If you hashed every 1000 symbols with a 512 bit hash to check
| message integrity and retransmit, a hash collision would be
| practically impossible, so "no transmission errors are
| possible" is perfectly fair to say under some circumstances.
| lazide wrote:
| Also possible they are using a 1 byte CRC heh
| kabdib wrote:
| A piece of communications gadgetry even mentioning
| checksums is like a car salesman proudly claiming "... and
| this baby comes with _working brakes_! " The very
| pronouncement leads to worry, and additional questions
| (like this very thread, QED :-) ).
|
| My inexpert digging came up with: They use APRS packets,
| which use AX.25, whose framing includes a 16-bit Frame
| Check Sequence, which looks like it came from HDLC, and is
| a 16-bit CRC-CCITT. Phew.
| Isamu wrote:
| Is this what they are doing? Seems unlikely that you would
| transmit so much overhead when the data rate is so low,
| unless you needed to overcome a lot of noise.
|
| It's about the numbers, there's trade-offs for each specific
| application. I would encourage people to do the math and see
| if their design makes sense for their goals.
| AlotOfReading wrote:
| There's virtually no benefit to dedicating so much of your
| message to checksums with any remotely efficient algorithm.
| More practically, any CRC >= 32 bits is probably overkill
| and any CRC > 64 bits is definitely overkill until you get
| up into gargantuan message sizes.
| Isamu wrote:
| Agreed, for this application (texting) a simple approach
| is probably best. You have to look at the particulars of
| your domain, it's not one size fits all.
| malik9 wrote:
| If you're concerned about one or two bits being toggled, a
| 512-bit hash will merely increase the chance of bit errors;
| on a noisy medium, using such a big "checksum" will BOTH
| detect and cause a high number of packet faults.
| xyzzy21 wrote:
| Sounds like you don't understand secure communications and
| basic concepts like bit-error-rate, QoS and reliability.
|
| Hash collisions have NOTHING to do with effectiveness or
| reliability of communication. A feature is a benefit!
| WithinReason wrote:
| Sounds like you simply didn't understand my point.
| meibo wrote:
| The German version is more clear, "automatic query" is
| translated more literally as "automatic callback", by which I
| assume is meant that the device requests retransmission in case
| of an error.
| Isamu wrote:
| Yes, if an error is detected there's an automatic retry. I
| just want to remind others that checksums or any scheme has
| undetected errors, in which case a retry doesn't happen. The
| human in the loop will say, that was garbled, could you
| repeat?
|
| Again this design may be totally fine for this application. I
| am bringing this up for other engineers because people tend
| to hand-wave this away.
| aidenn0 wrote:
| True story: an embedded target that was loaded over the network
| (I think TFTP) would boot slowly and then crash when loaded
| from Workstation 1, but not Workstation 2.
|
| Turns out there was a bad port on the Ethernet hub, but 1/65536
| corrupted packets would get through because of the 16-bit
| checksum.
| SamuelAdams wrote:
| Could see this being used in the Mammoth Caves National Park. A
| lot of the cave is undiscovered and having reliable comms might
| make that work a bit safer.
| baq wrote:
| See also
| https://en.wikipedia.org/wiki/Communication_with_submarines
| foobarbecue wrote:
| If you liked this you should check out the British Cave Research
| Association's Cave Radio and Electronics Group at
| https://bcra.org.uk/creg/
| [deleted]
| marcodiego wrote:
| Suppose you're lost inside a cave... In a great feat of luck, you
| find a cave-link base unit. Nice let's ask for help! You then
| discover the system is in German and you can't understand how to
| use it: https://www.cavelink.com/cl3x_neu/index.php/en/component-
| ove...
|
| Ok, I know it supports other languages, but there's no reason for
| the main site not to use English to illustrate it. Or better:
| make the interface intuitive enough that you can use even without
| reading instructions. We did it with smartphones.
| pmyteh wrote:
| These are highly specialised pieces of technical equipment with
| a tiny potential market. The use case isn't fixed unattended
| public installations to be used by untrained randoms, but ad-
| hoc use by trained users like cave rescue teams and
| speleologists. As for the UI, production (like much in the
| caving world) is semi-commercial at best; an iPhone it isn't
| and frankly it doesn't need to be.
| c_o_n_v_e_x wrote:
| Similar tech was used during the rescue of the boys out of a cave
| in Thailand back in 2018
|
| https://bcra.org.uk/creg/heyphone/
| https://hackaday.com/tag/heyphone/
| ChrisMarshallNY wrote:
| That's pretty cool!
|
| [V|E]LF stuff is used for many things. For example, the Navy uses
| it to communicate with subs.
| graderjs wrote:
| A quantum entangled communication system wouldn't have any
| 'transmission' issues (through mediums, etc), correct?
| colechristensen wrote:
| You can't transmit information with quantum entanglement.
| graderjs wrote:
| Really? But can't you influence 1 side of an entangled pair,
| then measure the other side, and you can pass information
| through the correlation? I thought quantum-entangled FTL
| communication devices were a legitimate thing.
| genewitch wrote:
| Information cannot be transceived faster than light. The
| thought experiments and whatabouts and whatifs are fun to
| discuss, especially after drinks - but seeing which spin or
| collapse or whatever isn't information in the actual
| definition of information.
|
| Information involves entropy and all that stuff is bound by
| the speed limit.
| colechristensen wrote:
| Nope. You can't change the outcome, you can only make an
| observation on one and then and only then know the state of
| the other. What is being observed is entirely random.
| Ellipsis753 wrote:
| This site is a bit scarce on details. There's more details and
| some photos here: https://de.wikipedia.org/wiki/Cave-Link
| http://www.hoehlenverein-blaubeuren.de/index.php/cavelink-te...
| (Use Google Translate)
| goodpoint wrote:
| I'm really surprised ELF are not commonly used for this. Even a
| simple single-frequency CW transmitter for emergencies.
|
| I wonder if audio frequencies below 20KHz can be used to make a
| cheaper system.
| ttyprintk wrote:
| At the extremity of that band, fewer nations have constructed
| transmitters than have nuclear weaponry.
|
| You can combine acoustic with radio in TARF (yes, really).
| ale42 wrote:
| see e.g. https://www.mit.edu/~fadel/papers/TARF-paper.pdf
| for those who didn't hear about TARF (translational
| acoustic-RF communication)
| goodpoint wrote:
| I meant acoustic frequencies in ELF, not using sound... but
| TARF looks amazing.
| dll wrote:
| I wonder how the licensing issue referenced in the Wikipedia
| article is handled in the UK. I imagine Ofcom will have had to
| approve and license these installations, or will they have made
| these exempt (I can't find any evidence of this)?
| gnfargbl wrote:
| I can't see these devices specifically mentioned in https://w
| ww.ofcom.org.uk/__data/assets/pdf_file/0028/84970/i...,
| though the frequency range they operate in (20-140kHz) is
| allowed to be used by induction loop type applications, at a
| suitably low power.
| ale42 wrote:
| ... but the cave link uses relatively high power levels
| (20-30 Watts if I remember correctly)
| lazide wrote:
| Inductive stove tops are also In the KW range
| detaro wrote:
| as far as I know similar general licensing exists for such
| purposes. Lots of inductive applications need it.
| meibo wrote:
| Something that is mentioned here and not on the main page: the
| devices form a mesh network and can relay messages between each
| other.
|
| Seems strange, feels like a pretty big selling point.
| seiferteric wrote:
| This uses VLF RF, but there was another interesting thing I have
| read about long ago called "earth mode" or "conduction mode"
| maybe. You put two conductors in the ground some distance apart
| (10s or hundreds of feet) and inject the signal into the ground.
| The signal causes a current in the ground which will cause
| voltage differentials that can be picked up miles away with a
| similar receiver. I believe it is also low bandwidth, but I often
| wonder if you could create a mesh network of these. Also would
| the FCC actually be able to regulate this since it is not
| actually a radio wave?
| TedDoesntTalk wrote:
| I think it is still a radio wave, but propagated through soil
| and moisture rather than air.
| seiferteric wrote:
| Maybe, but I thought it was different since it uses
| conduction. For example I am pretty sure you could detect a
| DC signal, whereas with radio you cannot.
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