> Because all transmissions are secured by checksums and automatic query, no transmission errors are possible.<p>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.<p>[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.
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.<p>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.
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).<p>[0] <a href="https://www.bbc.co.uk/news/uk-wales-59219380" rel="nofollow">https://www.bbc.co.uk/news/uk-wales-59219380</a>
This site is a bit scarce on details. There's more details and some photos here:
<a href="https://de.wikipedia.org/wiki/Cave-Link" rel="nofollow">https://de.wikipedia.org/wiki/Cave-Link</a>
<a href="http://www.hoehlenverein-blaubeuren.de/index.php/cavelink-telemetriesystem" rel="nofollow">http://www.hoehlenverein-blaubeuren.de/index.php/cavelink-te...</a>
(Use Google Translate)
See also the HeyPhone and Nicola systems built by underground boffins in the UK and France:<p><a href="https://www.shropshirecmc.org.uk/radio.html" rel="nofollow">https://www.shropshirecmc.org.uk/radio.html</a><p><a href="http://www.scavalon.be/avalonuk/technical/radio1.htm" rel="nofollow">http://www.scavalon.be/avalonuk/technical/radio1.htm</a><p><a href="https://www.electronicsweekly.com/blogs/engineer-in-wonderland/british-electronics-thai-cave-rescue-2018-07/" rel="nofollow">https://www.electronicsweekly.com/blogs/engineer-in-wonderla...</a><p><a href="https://www.caverescue.org.uk/nicolaradio/" rel="nofollow">https://www.caverescue.org.uk/nicolaradio/</a><p><a href="http://site2.caves.org.uk/radio/comms_in_caves.html" rel="nofollow">http://site2.caves.org.uk/radio/comms_in_caves.html</a>
Similar tech was used during the rescue of the boys out of a cave in Thailand back in 2018<p><a href="https://bcra.org.uk/creg/heyphone/" rel="nofollow">https://bcra.org.uk/creg/heyphone/</a>
<a href="https://hackaday.com/tag/heyphone/" rel="nofollow">https://hackaday.com/tag/heyphone/</a>
See also <a href="https://en.wikipedia.org/wiki/Communication_with_submarines" rel="nofollow">https://en.wikipedia.org/wiki/Communication_with_submarines</a>
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?
Reminds me of an interesting Great War technology. During WW1 the British used a system called the "power buzzer" which used two spaced electrodes and sent an alternating current through the earth. Apparently this could be picked up and amplified by another ground electrode at some distance, and was used to send morse code communications. I'm struggling to find a good modern explanation but here is a scan of the original military manual:<p><a href="https://cgsc.contentdm.oclc.org/digital/collection/p4013coll9/id/255" rel="nofollow">https://cgsc.contentdm.oclc.org/digital/collection/p4013coll...</a>
Also in caving news this week <a href="https://www.bbc.co.uk/news/uk-wales-59203695" rel="nofollow">https://www.bbc.co.uk/news/uk-wales-59203695</a>
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.
If you liked this you should check out the British Cave Research Association's Cave Radio and Electronics Group at <a href="https://bcra.org.uk/creg/" rel="nofollow">https://bcra.org.uk/creg/</a>
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: <a href="https://www.cavelink.com/cl3x_neu/index.php/en/component-overview/base-unit" rel="nofollow">https://www.cavelink.com/cl3x_neu/index.php/en/component-ove...</a><p>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.