How do submarines communicate with radio waves?

In summary, the conversation discusses the use of low frequency signals for communication with submarines, which face significant attenuation in deep waters. The effectiveness of using high power and low frequency signals is mentioned, as well as the option of resurfacing to receive communications. The use of lasers for submarine communication is also mentioned, with the example of a successful DARPA project in the 1980s. However, any current implementation of this technology is likely to be classified.
  • #1
ergospherical
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Treating water as having a conductivity of ##\sigma \sim 5 \ \mathrm{Sm^{-1}} \gg \omega \epsilon_0 \epsilon_{\mathrm{r}}## then Maxwell ##\mathrm{III}## implies\begin{align*}
\nabla \times \mathbf{H} = \sigma \mathbf{E} + \epsilon_0 \epsilon_{\mathrm{r}} \dot{\mathbf{E}} = -i \omega \epsilon_0 \left(\dfrac{i\sigma}{ \omega \epsilon_0} + \epsilon_{\mathrm{r}} \right) \mathbf{E} \equiv \epsilon_0 \epsilon' \dot{\mathbf{E}}
\end{align*}The water has an effective dielectric constant ##\epsilon' \sim \dfrac{i\sigma}{\omega \epsilon_0}## and a refractive index ##n = \sqrt{\mu_{\mathrm{r}} \epsilon'} \sim (1+i)\sqrt{\dfrac{\mu_{\mathrm{r}} \sigma}{2\omega \epsilon_0}}##. A plane electromagnetic wave inside the water has wave-number ##k = \omega n/c \equiv (1+i)/\delta## where the skin depth is given by ##\delta = \sqrt{\dfrac{2}{\mu_{\mathrm{r}} \mu_0 \sigma \omega}}##, and the power decays ##\propto |\mathbf{E}|^2 \propto \mathrm{exp}(-2x/\delta)##. An order of magnitude of power is lost every ##x_*## metres, satisfying\begin{align*}
2x_* / \delta = \log{10} \implies x_* = \dfrac{1}{2} \delta \log{10} \sim \delta
\end{align*}Google tells me that submarines use radio frequencies typically of the order of a few ##\mathrm{kHz}## or tens of ##\mathrm{kHz}##, which results in the power dropping off by an order of magnitude every ##10 \ \mathrm{m}## or so. However, modern submarines are capable of operating at depths of, say, ##\sim 400 \ \mathrm{m}##, at which point the power has decayed by a factor of... ##\sim 10^{40}##. Even at the very lowest limits of radio waves, ##\sim## a couple of ##\mathrm{Hz}##, the power still drops off by an order of magnitude every few hundred metres. Assuming the analysis is vaguely correct, how do submarine engineers get around this issue? Is it merely a case of using very high power radio waves to begin with, so that the signals are still detectable even with this very significant attenuation?
 
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  • #2
They use high power low frequency signals, but also they come near the surface. Yes, they can operate at 400 m, but they tend to go closer to the surface to receive communications.
 
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  • #3
The issue I see with low frequency signals is that the bit rate depends on the carrier frequency; less efficient data transmission is the compromise paid for lower attenuation.

I suppose submarines could also resurface before receiving communications - but this begs the question “how would they know when they’re going to receive a message?” (unless there are scheduled times each day).
 
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  • #4
See https://en.wikipedia.org/wiki/Project_Sanguine

The US Navy system (above) reportedly uses three-letter code groups and requires 15 minutes to transmit one group. So current systems are not used to transmit detailed orders, but serve a "bell ringer" function, to order a specific vessel to surface and receive further orders by ordinary radio or satellite communication.
 
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  • #5
ergospherical said:
The issue I see with low frequency signals is that the bit rate depends on the carrier frequency; less efficient data transmission is the compromise paid for lower attenuation.
Absolutely, that is the main issue.
 
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  • #7
Let's hope that there is not a three letter group code for LAUNCH. :woot:
Or the policy to LAUNCH if you can't hear BBC.

Modern subs carry 75+ nuclear warheads each.

There's a great movie plot, Crimson Tide, around that scenario. Receipt of an ambiguous LAUNCH order.
 
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  • #8
PeroK said:
Apparently, one of the signs that the UK nuclear submarines use to confirm that the country has not been subject to nuclear attack is the BBC Radio 4 "Today" Programme. If it goes off air unexpectedly, then there's a risk of nuclear retaliation!
Why is it this specific programme and not just receiving BBC in general? Is it so British, that it's impossible to fake?
 
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  • #9
A.T. said:
Why is it this specific programme and not just receiving BBC in general? Is it so British, that it's impossible to fake?
Yes, I think that's the idea! You've probably never heard the shipping forecast, for example:

 
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  • #10
How does one fake the absence of a transmission?
 
  • #12
OK, slightly off-topic. Lasers...

in the mid 1980's I worked on a DARPA/NOSC project called SLCAIR which was a feasibility study where we communicated (one-way, in this case) from an airplane to submarines with a green laser (532nm). This was then the basis for SLCSAT which did the same with satellites instead of airplanes (note that SLC = Submarine Laser Communications). The existence of those programs wasn't classified, but some of the results were. Suffice to say, it worked. Blue and green light penetrates seawater deeper than radio waves and has much higher bandwidth. Our demo system transmitted 128 bits of PPM at 10Hz, but bandwidth wasn't the point of those tests.

That program ran its course and I never heard more. I heard of other later experiments in a vague way, but, honestly, I didn't care enough to pay attention. I saw this resurface from 2010, but any real implementation is likely to be classified as much as possible. I suspect it's one of those eternal R&D things, like fusion energy.

There are some security advantages to laser vs. ELF/VLF radio, because you can't receive it in Moscow or Beijing. Plus greater bandwidth. Also, lasers penetrate deeper than VLF antenna depths. OTOH, you're lighting up the sea with a big green light over the submarine. I was told the solution was to shine it in lots of places, only one of which had the submarine, but that never really made complete sense to me. I'm 100% sure they wouldn't have told me the real story, plus it was only R&D about the comm link back then.

BTW, you don't need a lot of bandwidth to say "launch your missiles", you need reliability and security.

SLCAIR84.jpg
 
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  • #13
PeroK said:
Apparently, one of the signs that the UK nuclear submarines use to confirm that the country has not been subject to nuclear attack is the BBC Radio 4 "Today" Programme. If it goes off air unexpectedly, then there's a risk of nuclear retaliation!

https://www.businessinsider.com/bbc-radio-show-may-be-preventing-nuclear-apocalypse-2018-8?r=US&IR=T
There's an Americanized version of that, "the Waffle House Criteria." After a natural disaster such as a hurricane, FEMA calls all the Waffle House restaurants in the area. Locals tend to hang out in Waffle House to wait out the storm. If there is no answer, FEMA concludes that it must be really bad.
 
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  • #14
anorlunda said:
Let's hope that there is not a three letter group code for LAUNCH. :woot:
Or the policy to LAUNCH if you can't hear BBC.

Modern subs carry 75+ nuclear warheads each.

There's a great movie plot, Crimson Tide, around that scenario. Receipt of an ambiguous LAUNCH order.
Don't believe the launch authentication protocol in that movie. There are steps missing.
https://web.archive.org/web/2016010...gov/pubs/foi/Reading_Room/Joint_Staff/320.pdf
:smile:
 
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  • #16
berkeman said:
Wow, talk about redacted. What steps did they miss in Crimson Tide?

That would be telling but:

If you somehow receive/decode just the message header off the 'radio' you know what type of EAM it is just from the header launch, confirm, terminate, test, etc... The boiler plate in the message text has the details.

In the movie the second message fragment has "Nuclear Missile Launch..". Don't you think the word "Terminate" would be the very first thing in a termination EAM message?

https://getyarn.io/yarn-clip/46b4b8e6-fad3-4c47-aa07-9ae1e8b2e972

Even if it was unauthenticated and incomplete, a received and cryptographic decoded message fragment with a Terminate header would be reasonable doubt to discontinue launch until that doubt is eliminated with a complete and properly formatted message. In reality the middle part of the movie would never happen because the CO and XO would wait.
 
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  • #17
nsaspook said:
Even if it was unauthenticated and incomplete, a received and cryptographic decoded message fragment with a Terminate header would be reasonable doubt to discontinue launch until that doubt is eliminated with a complete and properly formatted message. In reality the middle part of the movie would never happen because the CO and XO would wait.
I'm not a sub commander, but my experience in the military and with nuclear weapons tells me that launching a nuclear weapon when virtually any ambiguity exists is beyond idiotic. Unfortunately this exact thing nearly happened before:
https://www.theguardian.com/commentisfree/2012/oct/27/vasili-arkhipov-stopped-nuclear-war
 
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  • #18
https://web.archive.org/web/2009100...time/magazine/article/0,9171,101361-1,00.html

1638587795927.png

A pre-internet system called AUTODIN routed messages to several types of feeds (radio, satcom, avian carriers, smoke-signal...) using a precedence up to what's called Flash-Override for EAMS. There was actually a higher precedence level called CRITIC/ECP Emergency Command Precedence. Never saw it used even in testing. Most of us assumed it was reserved for Alien Attacks. :))
ipv4-header-type-of-service-bits.jpg

The "Type of Service (ToS)" field in the Internet Protocol (IPv4) header
 
  • #19
ergospherical said:
Treating water as having a conductivity of ##\sigma \sim 5 \ \mathrm{Sm^{-1}} \gg \omega \epsilon_0 \epsilon_{\mathrm{r}}## then Maxwell ##\mathrm{III}## implies\begin{align*}
\nabla \times \mathbf{H} = \sigma \mathbf{E} + \epsilon_0 \epsilon_{\mathrm{r}} \dot{\mathbf{E}} = -i \omega \epsilon_0 \left(\dfrac{i\sigma}{ \omega \epsilon_0} + \epsilon_{\mathrm{r}} \right) \mathbf{E} \equiv \epsilon_0 \epsilon' \dot{\mathbf{E}}
\end{align*}The water has an effective dielectric constant ##\epsilon' \sim \dfrac{i\sigma}{\omega \epsilon_0}## and a refractive index ##n = \sqrt{\mu_{\mathrm{r}} \epsilon'} \sim (1+i)\sqrt{\dfrac{\mu_{\mathrm{r}} \sigma}{2\omega \epsilon_0}}##. A plane electromagnetic wave inside the water has wave-number ##k = \omega n/c \equiv (1+i)/\delta## where the skin depth is given by ##\delta = \sqrt{\dfrac{2}{\mu_{\mathrm{r}} \mu_0 \sigma \omega}}##, and the power decays ##\propto |\mathbf{E}|^2 \propto \mathrm{exp}(-2x/\delta)##. An order of magnitude of power is lost every ##x_*## metres, satisfying\begin{align*}
2x_* / \delta = \log{10} \implies x_* = \dfrac{1}{2} \delta \log{10} \sim \delta
\end{align*}Google tells me that submarines use radio frequencies typically of the order of a few ##\mathrm{kHz}## or tens of ##\mathrm{kHz}##, which results in the power dropping off by an order of magnitude every ##10 \ \mathrm{m}## or so. However, modern submarines are capable of operating at depths of, say, ##\sim 400 \ \mathrm{m}##, at which point the power has decayed by a factor of... ##\sim 10^{40}##. Even at the very lowest limits of radio waves, ##\sim## a couple of ##\mathrm{Hz}##, the power still drops off by an order of magnitude every few hundred metres. Assuming the analysis is vaguely correct, how do submarine engineers get around this issue? Is it merely a case of using very high power radio waves to begin with, so that the signals are still detectable even with this very significant attenuation?
Correct.

MO: Periodically come up to 10m or so depth submersion and receive/transmit signals in the LF/VLF bands.

If more data is required, they surface and use satellite telemetry, UHF/SHF.

If at depth, a very slow keyed signal at ELF~ULF can penetrate deep. The signal would be little more than code for 'surface for message'.

Submarines do not have ELF~ULF transmit capability, they shallow submerge for LF/VLF transmissions.
 
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  • #20
One thing I've always suspected of these low frequency radio wave is their very limited data rate , now I wonder if the enemy for example finds out the message style and encryption it could in theory send a similar message from its radio antennas and then find out which subs have surfaced making them easy targets.
There probably is a system in place to exclude this from happening although if a sub is deep underwater they cannot communicate to the outside world so their only line of information is that VLF radio wave.
 
  • #21
artis said:
I wonder if the enemy for example finds out the message style and encryption it could in theory send a similar message from its radio antennas and then find out which subs have surfaced making them easy targets.
They don't have to surface to receive normal frequencies. Periscope depth should be enough to stick out an antenna.
 
  • #22
I doubt a sub would surface on a positive ULF signal alone. They'd shallow submerge and await a coded LF transmission first if they felt there was any risk at all of detection.

Periodic shallow submerge is quite routine anyway, to vent/cycle the air with a snorkel, though not really necessary now with the latest subs, AFAIU.

Once surfaced, a beamed microwave link provides a secure transmit, avoiding radio location.
 
  • #23
artis said:
now I wonder if the enemy for example finds out the message style and encryption it could in theory send a similar message from its radio antennas and then find out which subs have surfaced making them easy targets.
The chances of that are probably very low given modern encryption methods.
 
  • #24
Drakkith said:
The chances of that are probably very low given modern encryption methods.
I believe double agents are still alive today just as they were back in the Cold war era.
 
  • #25
artis said:
I believe double agents are still alive today just as they were back in the Cold war era.
I included those in my estimate of 'very low'.
 
  • #26
Drakkith said:
The chances of that are probably very low given modern encryption methods.
The question is valid, I think the point is that to send a AES 256 encrypted signal on ELF would take a couple of days! The data rate is far too slow. A 3 letter transmission in morse will take a half our or so, I believe the letters are repeated too, as the signals get 'stretched out' and distorted depending on the nature of the local hydrostratigraphy.

I don't think anyone expects to keep ELF signals a secret, so the security is built into the MO of how to respond to it.
 
  • #27
ergospherical said:
Even at the very lowest limits of radio waves, ∼ a couple of Hz, the power still drops off by an order of magnitude every few hundred metres. Assuming the analysis is vaguely correct, how do submarine engineers get around this issue? Is it merely a case of using very high power radio waves to begin with, so that the signals are still detectable even with this very significant attenuation?
I once lived taught at U Maine and way downeast is Cutler, Maine, where the navy converted an entire peninsula into an antenna. to couple to the surface waves at low frequencies (VLF array).
https://en.wikipedia.org/wiki/VLF_Transmitter_Cutler

I was told the power was sufficient that getting in and out of trucks needed to be done quickly and carefully.,
 
  • #28
cmb said:
The question is valid
Of course. But it reminds me of a conversation I had with someone in the Air Force regarding the doors to our nuclear storage structures. They are large, heavy metal doors that you have to pull a chain to run a pulley to slowly slide them open. They have multiple locks, are alarmed, and are blocked by multi-ton concrete blocks stacked a few feet in front of the doors, presumably to stop vehicles from getting close enough to easily load equipment or munitions.

Colleague: "I heard they had a seal team run a test to see how fast they could get into one of these structures."
Me: "Oh?"
"Yeah, the seals just choppered in, attached some C4 to the door, and blew a big hole in it. They were done less than five minutes."
"Okay?"
"What if someone does that?"
"...what?"

The issue here isn't that the doors are vulnerable to C4. It's all in the details of how someone would get the knowledge, manpower, helicopter, equipment, and explosives AND plan and execute this operation without someone finding out beforehand or being noticed flying to and over an active military base.

And then what? They blow open the structure and do what? Steal something? You aren't going to steal an entire nuclear cruise missile with a helicopter. You need specialized handling equipment to move it, the knowledge and expertise to use them, and adapter equipment to load it onto an aircraft. None of the latter even exist to load one of our cruise missiles onto a helicopter by the way. And you need time. You aren't moving and loading one of these things in five minutes. Especially not with large concrete blocks impeding any movement into or out of the storage structure by vehicles and equipment.

So steal the warhead out of the missile? Sure. Do they have the exact tools required to take the panels off and remove it? If so, how did they know which ones to use? Oh they have a technical manual? How did they acquire it? How did they know how to identify which missiles were loaded with warheads vs not loaded? Or which storage structure had nuclear weapons vs one that didn't? A spy you say? How did they coerce this person? How did they even know who to look for to recruit? How did this person smuggle the manual out?

Even if they do manage all of that, how do they escape? A helicopter is relatively easy to track and a nuclear warhead isn't something you can just stick under your jacket. Nuclear weapons are also safe enough that they aren't going to detonate from an external explosion, so yes, you could just shoot the helicopter down.

Please don't answer any of these questions. I only bring them up to show that while each individual issue is possible to overcome, the combination of all of them is very, very difficult. Yes, if someone had the knowledge, manpower, equipment, time, and opportunity to steal a warhead from a nuclear cruise missile they could do it, just like someone could crack the encryption for submarine messaging to target them while surfaced if they had everything they needed.

The devil is in the details.
 
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  • #29
Drakkith said:
Yes, if someone had the knowledge, manpower, equipment, time, and opportunity to steal a warhead from a nuclear cruise missile they could do it,
I agree stealing warheads is not cost effective nor efficient in terms of numbers. That is why anyone with a budget and know how simply builds them. And speaking of stealing just don't forget that one doesn't need to steal the actual thing if one can get the blueprints. Remember how the Soviet's under Stalin pretty much copied every step of the Manhattan project? They cracked that thing like a soda can. In fact so much so that Lavrentiy Beria (the then KGB top man) told the working science teams to put aside their own developed plans for the bomb and just copy/paste the crucial elements from the US version.

C4 and door busting is for laymen like terrorists the pro's simply get your information and use it against you.
hutchphd said:
I was told the power was sufficient that getting in and out of trucks needed to be done quickly and carefully.,
Was that told while being directly in the very territory of the antennas?
 
  • #30
cmb said:
The question is valid, I think the point is that to send a AES 256 encrypted signal on ELF would take a couple of days! The data rate is far too slow. A 3 letter transmission in morse will take a half our or so, I believe the letters are repeated too, as the signals get 'stretched out' and distorted depending on the nature of the local hydrostratigraphy.

I don't think anyone expects to keep ELF signals a secret, so the security is built into the MO of how to respond to it.
It is expected that ELF signals are secret. A proper OTP system makes it unbreakable if the key chain is secure.
https://en.wikipedia.org/wiki/One-time_pad
Advances in technology have made key distribution problems easier to solve.

But:
You do whatever you can and think you have an infallible system, but somehow someone always seems to find a way to screw it up.

http://www.vlf.it/zevs/zevs.htm
The Zevs 82 Hz transmission is technically speaking, making use of a minimum shifted carrier, MSK. This type of transmission mode, is in use by nearly all modern submarine-communication VLF band, 3 – 30 kHz, transmitter facilities. What makes the transmission mode of the ZEVS and similar ELF transmitters so unique, is the very narrow frequencyshift of the transmitter carrier, during the message phase of the transmission.
The widest frequency shift observed, is in the narrow range 81 Hz to 83.3 Hz, at the start of a transmitted message ! The carrier shift of only 2.3 Hz. makes up the difference in a mark and a space of the MSK signal.
That obviously calls for some rather sophisticated decoding software used by the onboard communication computer-system of the submarine. The ELF carrier frequency is shifted from the normal carrier frequency of 82 Hz, down to 81.6 Hz and up to 82.7 Hz, prior to this message. This is most likely the “message waiting” call function of the Russian ZEVS transmitter.
image009.gif

ELF transmissions make use of a very slow data rate, this combined with high natural noise-levels at the frequencies used, put special demand on the coding used for the message.
Messages making use of direct orders like: “ fire ballistic missile 2,4,6,8 with preprogrammed address and ballistic missiles 1,3,5,7 redirected to 59°8’12’’N 11°23’55’’E “( my QTH ! ) is not very likely, as a this certainly will cause an irreversible effect (!) if this NOT was the intended message, but something incorrectly decoded, by the communication computer onboard the submarine.
With a transmission format using only a single three letter code and repeating this single code for the 15 minute message segment of an ELF transmission, it would still be possible to signal some 35937 different codes (33x33x33=35937) when the 33 letter Russian alphabet is used.
The coded message sent is a repeated, error correcting code. The bit rate is a few bits per minute repeated until enough data is accumulated to let the receiver decide if a letter has been received successfully. At the end of the message segment it is expected that three correct characters in the codeword have completed the “journey” from the ZEVS transmitter to the submarine.
 
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  • #31
nsaspook said:
It is expected that ELF signals are secret. A proper OTP system makes it unbreakable if the key chain is secure.
https://en.wikipedia.org/wiki/One-time_pad
Advances in technology have made key distribution problems easier to solve.
That wasn't what I really meant.

I mean, if I was setting up the communication chain, I would make it so that it just wouldn't really matter what the three letter transmission is.

It could just mean 'Moscow calling' and they could publish it on the internet.

But if the MO on the captain's mission delegation sheet says 'shallow surface if you get a 'Moscow Calling' signal at 3PM, Moscow time, because there might be a message waiting for you', then that is all that is needed.

If I was OOW/OC and an ELF signal was detected (from either 'side'!), I might want to just pop up for an update anyway, if I felt it was safe.

I'm unclear what the fuss would be about trying to get messages encoded on ELF. If you make encryption part of the protocol it's just one more thing that can go wrong and create uncertainty. The crew will always be the ones that have to assess whether it is safe to come up, anyway, whatever signal they have received.

But, anyway, I have said that now, no need I repeat, so go ahead and pick up some ELF for yourself! It's not difficult actually, just string a big loop around the garden and wire it up to your computer's sound card and run a FFT app.

You get to see various anomalies and blips in all the LF bands, Schumann resonances and lightning whistlers and 'things'. :)
 
  • #32
cmb said:
I'm unclear what the fuss would be about trying to get messages encoded on ELF. If you make encryption part of the protocol it's just one more thing that can go wrong and create uncertainty. The crew will always be the ones that have to assess whether it is safe to come up, anyway, whatever signal they have received.
It adds redundancy and flexibility to your submarine operations.
 
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  • #33
@nsaspook one can see the 50hz powerline frequency being the most powerful one in that graph.
But then again powerlines with their overall length and location I guess make up the worlds largest low frequency antenna array ever built. Also the most powerful one.

I wonder why they don't use powerline wires to transmit the VLF isn't there a way to combine them or would the two frequencies cause some destructive interference?
 
  • #34
artis said:
I wonder why they don't use powerline wires to transmit the VLF
Because the power companies and their customers would have a fit. I, for one, prefer my AC power in the plain version, not modulated with VLF.
 
  • #35
artis said:
@nsaspook one can see the 50hz powerline frequency being the most powerful one
...I wonder why they don't use powerline wires to transmit the VLF isn't there a way to combine them or would the two frequencies cause some destructive interference?
They could not modulate 50Hz (or 60Hz) because so much industrial kit runs off a reliance on that frequency.

Imagine if all the induction motors around the world had to speed up and slow down, just to sent a sub signal? hmmmm ... not going to happen.

The mains frequencies are tightly controlled, under legal compulsions, for precisely that reason.
 
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