Monday, February 22, 2021

Networks and Royal Navy Signal Flags

In several posts, we’ve examined historical naval battles and analyzed them from today’s perspective to learn lessons applicable to our current strategies, doctrine, and tactics.  The timelessness of the lessons has been striking.


Today, we’re going to look not at a battle but at a technology and see what lessons can be learned.  Specifically, we’re going to look at the English Royal Navy system of signal flags, implemented in the age of sail.  As we do, we’ll keep in mind the parallel between the English flag system and today’s networks and command and control (C2) systems.


Before the advent of radio, communication between ships was problematic, to say the least.  Ships that were out of sight of each other were, essentially, out of communication.  Even within sight of each other, communications were difficult.  Simplistic signals involving sails or flags or lanterns or some such were used but their effectiveness was subject to degradation by darkness, weather, distance, and the confusion of combat.


Eventually, the Royal Navy attempted to standardize the meaning of a limited set of signal flags, as described below.


Between 1337 and 1351 the British Navy lists two signals in their old “Black Book of the Admiralty.” The first was to hoist a flag of council high in the middle of the mast, to notify all captains to come aboard the admiral's flagship for a meeting. Hoisting another flag aloft reported the sighting of the enemy. (3)


By the late seventeenth century things still had not progressed much. A code book issued for the British Navy in 1673 defined 15 different flags, each with a single predefined meaning, which was probably not too much different from what had been used since antiquity. (3)


Following this, the English came up with a major step forward when they established an extensive system of standardized signal flags.  From Wikipedia (4),


Several wars with the Dutch in the 17th century prompted the English to issue instructions for the conduct of particular fleets, such as (in 1673) the Duke of York's "Instructions for the better Ordering of His Majesties Fleet in Sayling". Signals were primitive and rather ad hoc ("As soon as the Admiral shall loose his fore-top and fire a gun..."), and generally a one-way communication system, as only flagships carried a complete set of flags. In 1790 Admiral Lord Howe issued a new signal book for a numerary system using numeral flags to signal a number; the number, not the mast from which the flags flew, indicated the message. Other admirals tried various systems; it was not until 1799 that the Admiralty issued a standardized signal code system for the entire Royal Navy. This was limited to only the signals listed in the Signal-Book. In 1800 Captain Sir Home Popham devised a means of extending this: signals made with a special "Telegraph" flag referred to a separate dictionary of numbered words and phrases. (4)


The system was straightforward and simple.


The signaling system in use by the Royal Navy at the time of the Battle of Trafalgar had been introduced in 1799 by Admiral Lord Howe and was further developed by Captain Sir Home Popham. Howe's original system was based on a set of flags numbered 0 to 9 that when hoisted in various combinations could transmit words, numbers, or messages listed in the Signal Book. Each letter of the alphabet was allotted a number, enabling words to be spelled out, The numeral flags could also be used to send numbers as such. But most signals were two-or three-flag combinations which referenced messages in the Signal Book, e.g. to discontinue the engagement, to pursue the enemy, to anchor, etc. The Signal Book was so arranged that the sender and receiver could quickly compose and interpret flag hoists. (2)


Howe's system was impractical for sending long messages due to the large number of flags required to spell out individual words. In 1800, therefore, Captain Popham developed a "vocabulary" system by which three- or four-flag hoists referenced words or phrases in the Signal Book. This was the system in use at the time of Trafalgar and Nelson's famous signal is a fine example of how it worked. This type of hoist was preceded by the so-called telegraph flag, indicating that a vocabulary message was to be sent. The signal was terminated by a finishing flag, functioning like the period at the end of a sentence. Receiving ships acknowledged the signal by hoisting the Affirmative flag, indicating that it had been seen and understood. If the signal was seen but not understood, receiving ships hoisted the Affirmative flag over numeral flag no. 8. For repeated numbers a substitute flag was provided. In Nelson's signal this flag, yellow with black stripes along the top and bottom, was employed in the hoist for the word do (No. 221), substituting for a second no. 2 flag.


The second signal that Nelson ordered to be hoisted was No. 16 in the Signal Book: "Engage the enemy more closely." This remained aloft until it was shot away in the heat of battle. (2)



Standardized signal flag systems, while a vast improvement over ad hoc signals, were still subject to distance limitations.  The sender and receiver had to be within visual range.  This was partially remedied by using signal chains whereby the sender could disseminate a signal to ships out of visual range by using a repeater chain of ships, each ship relaying the message to the next ship in line.  Of course, this required time for the message to slowly repeat its way along the chain and required a string of ships.  Fortunately, naval battles in the age of sail were generally fairly slow affairs, often requiring many hours to lead up to the actual battle.  The system also suffered from the confusion of battle where signals might be missed, misread, shot away, or otherwise fail since both the sender and receiver had, understandably, higher priorities.


Here is an example of the signal chain from the Battle of Trafalgar:


Accordingly the French admiral Villeneuve hoisted the signal to weigh anchor, and at six in the morning of 19 October the British frigate Sirius, waiting outside Cadiz, signalled to the fleet below the horizon ``Enemy have topsails hoisted.'' An hour later it hoisted signal no. 370, ``Enemy ships are coming out of port.'' The hoists were made to the next frigate in the signalling chain, Euryalus, which in turn signalled no. 370 to Phoebe with the accompanying admonition--superfluous in a service schooled to such discipline--``Repeat signals to lookout ships west.'' And so no. 370 travelled down the chain, from Phoebe to Naiad, Naiad to Defence (a line-of-battleship), Defence to Colossus and Colossus to Mars, standing in Nelson's line of battle itself, 48 miles from the mouth of Cadiz harbour. The news reached Nelson at 9:30. He immediately ordered ``General chase southeast'' and steered to place the fleet between Cadiz and the Straits of Gibraltar. The opening move of the battle of Trafalgar had begun. (3)



Regarding Nelson and Trafalgar, it is interesting that Nelson opted not to depend on signals for his C2, choosing, instead, to emphasize doctrine over command and control.  In fact, he conducted the entire battle while sending only two signals, neither of which was of a tactical nature.  Nelson understood that even a simple system like signal flags could, and would, be misapplied and misunderstood in the heat of battle and that the preferred method of exercising C2 was to not attempt to exercise C2.  He chose, instead, to depend on clearly defined doctrine which allowed local commanders (the individual ship Captains) to act without higher command direction and yet still unerringly act with total adherence to his intent.





As with the historical battles we’ve examined, nothing has changed from age of sail to today in terms of the lessons about communications and networks.  The same problems that plagued naval communications then still plague us today and will continue to do so on the future naval battlefield.  Weather will still degrade communications.  Confusion will still rear its head.  Signals are still susceptible to detection and interception.  And so on …


The technology has changed but the lessons remain constant.


Inevitable Confusion – Anyone who has ever played the child’s party game of passing a whispered message around a circle of people and seeing how distorted it comes out at the end can testify to the inevitable confusion and garbling of any communication.  A message that is crystal clear to the sender may be confusing gibberish to the receiver – and that’s without considering the effects of weather, distance, light, jamming, false signals, combat, or whatever other form of disruption occurs.  The sources of confusion may have changed (jamming, false signal induction, cyber attack, etc.) but the effects and the lessons remain the same.  Communications will be misunderstood and the advent of radio and electronic signals has done nothing to mitigate the confusion.  A famous example of this is the Admiral Nimitz’s radio message to Admiral Halsey during the battle of Leyte, in WWII, where the meaningless padding phrase, “the world wonders”, was accidentally left in the message and interpreted by Halsey as a sharp rebuke of his actions and wound up changing his plans.


Distance – Distance has been the nemesis of communications since time immemorial.  Even today, electronic signals degrade with distance as interfering phenomena are encountered.  Line of sight communications are impacted by distance and impose constraints and altered behavior by the sending platform. 


Interception – The mere sending of a signal, and frequency of signaling, even if the meaning is not understood, can provide the enemy with valuable intelligence (see, “The Battle of Heligoland Bight”). 


Here is an anecdotal story from a reader,


I spent a while working with a blokey from EW element of the Royal Signals, they would spend an entire exercise doing SIGINT traffic analysis of our own side during exercises in Germany.

It is really scary what can be discerned by traffic analysis alone (all the more prevalent when crypto is so effective - in the context of the battlefield timescale). (5)



The only truly secure signal, the only signal that cannot be detected, intercepted, and analyzed is the one that is not sent.



Doctrine and Commander’s Intent – The only sure form of communication is no communication and the best communication is no communication.  This is where doctrine and Commander’s Intent comes in.


During the Cold War, the US Navy carrier groups would routinely launch entire strike forces without a single radio transmission and aircraft could conduct air-to-air refueling with no transmissions.  Today, our forces have acquired a severe case of verbal diarrhea and are utterly lost without constant communications. 


We need to re-establish doctrine and relearn comms-out operations.  The corollary to this is that we need to drop our obsession with ever more intrusive C2 and return to allowing local commanders to command.  If we allow local commanders to command then we don’t need to communicate with them and that is, after all, the best form of communication.





Despite all the historical and current evidence to the contrary, we still cling to the fantasy of region-wide networks of sensors and shooters all tied together in a mammoth, flawless, seamless, real time network of instantaneously shared data and intel.  Just as the Royal Navy learned that the best form of communication was no communication, we, too, must learn to operate without communication and that means emphasizing doctrine and commander’s intent instead of the ceaseless – and useless – drone of higher authority clogging the airwaves.






(1)Richard Howe, Signal and Instruction Book, ca. 1776






(4)Wikipedia, “Naval Flag Signaling”, retrieved 25-Jan-2021


(5)Navy Matters, “The Battle of Heligoland Bight”, Dave Wolfy, January 6, 2020 at 12:10 PM


  1. In the Pacific War, mistaking a cruiser for a tender or friendly for enemy was a very common occurrence.

    Decades later, with total superiority, USN cannot definitively tell whether a warship has been attacked or not.

    How's that going to work when fighting the Chinese who everyone admits to be very, very good at hacking into every network they can find?

  2. Also note apparently expensive subs still seem to having a difficult time detecting what I assume are not running silent cargo ships when they surface...

    1. That could ruin your whole day.

      I guess the one sort of positive comment might be that at least we aren't the only ones. But exactly how in the heck does that happen?

  3. Good article. I believe Nelson also said something to the effect of "No Captain can do wrong by laying his ship alongside the enemy". Clear enunciation of "Commander's intent" if there ever was one. We used to do "Silent Unreps" in the Med, using only flags (or flashing light at night). Worked well.

    1. "Silent Unreps"

      Great reminder!

      We've developed communications diarrhea and feel the need to constantly vomit forth (sorry for all the imagery) inane commentary. We need to relearn how to conduct professional, silent evolutions and operations.

      You know what I'd like to see? I'd like to see a naval task force given an exercise assignment and told to execute it in total EMCON, from leaving dock to return docking … not a word, not an emission, nothing - total silence. It's how we'll fight (not at first but we'll quickly learn the hard way not to emit) so let's start practicing and relearning silence.

      Of course, we'll also find out that much of our equipment no longer has an EMCON capability (EMALS, for example) and that our ships are continuous electromagnetic beacons. We need to reintroduce EMCON as a spec in equipment design and procurement.

    2. As staff ops officer, I found that the less we talked, the more we understood. We used signal flags a lot, and we did silent UNREPs (RASs for our Brit friends) and a number of other silent operations.

      Then on an amphib for the 1973 Yom Kippur War we went several days under total EMCON (trying to hide from the Russkis) and that worked okay. Then we turned things back on and got more chatter than a party line, and soon there was mass confusion.

      I absolutely believe we should do an exercise (or many exercises) with total EMCON, no SATNAV, just good old basic seamanship. We'd be awful at first, but as we gained proficiency we would enhance our operational capability.

      As far as EMALS, I keep hearing conflicting reports. Of course, the ones telling me it's no problem in terms of electromagnetic radiation are the same ones telling me it works great, so I think I know whom to believe.

    3. "As far as EMALS, I keep hearing conflicting reports."

      Well, CNO Greenert stated that EMALS was not EMCON capable during a discussion of emissions control. That would seem believable.

      I documented it in a post but I don't remember which one off the top of my head.

    4. I assume it's still an EMCON failure, otherwise the Navy would be trumpeting the result everywhere.

    5. "I documented it in a post but I don't remember which one off the top of my head."

      I know, and I believe you were/are correct. At very best we are introducing something else into the electromagnetic spectrum that potentially could be identified and tracked. Or worse, used by large anti-ship missiles (and I wonder who has those) to home in on our carriers.

    6. "No captain can do wrong"

      A Nelson quote very appropriate for the discussion at hand. Part of Nelson's ability and success was recognising the need to not overburden his subordinates. Other quotes:

      "I shall never distract the attention of my fleet on the day of action by a superabundance of signals" (Naval Chronicle, Volume 14, p411)

      "You will see by this loose letter than it is almost impossible at this distance to give precise orders for such various Services but I rely on your abilities and Zeal to do what is right."
      (Nelson the New Letters, p367)

      And the fuller quote, which is also pertinent:
      "But, in case Signals can neither be seen or perfectly understood, no Captain can do very wrong if he places his Ship alongside that of an Enemy."
      (Nelson's Trafalgar Memorandum)

    7. And for discussion, while signals in the current day have a much greater limit than back in 1804, they must still have a limit. Their capacity and human frailty in their interpretation is finite, just as they were finite in 1805. This rather successful Commander understood that there was a limit, wherever it might be, and took prudent steps to mitigate it's effect.

  4. Relearning to fight under EMCON is a good starting point but probably not enough. In the Battle of Heligoland the Germans used destroyers as bait. The same concept of bait can use radio traffic. If you hear increased traffic at a location, it will peak your curiosity. These type of head games can help keep an enemy confused. And if they chose to ignore the extra traffic it can be used to allow you to communicate more freely. The combination of minimal controlled traffic and deception should be even more effective than just EMCON.

    Of course if you can't reliably fight under EMCON, all this isn't possible.

  5. E-2, especially latest E-2D is able to let F-35C turn off their radar. They become missile launcher. E-2D provides 300 miles search range (at least as manufacturer boast). F-35C can then rely on information from E-2D without sending any signal out. Even though enemies know there are E-2D behind F-35C but except one, none of them can attack E-2D which are so far away from them.

    1. " F-35C can then rely on information from E-2D"

      Not in any peer threat combat scenario. Every modern fighter aircraft is semi- to fully stealthy. The E-2 radar is not going to see semi- to full stealth aircraft at 300 miles. Depending on the degree of stealth, detection ranges would be on the order of 20-50 miles.

      "none of them can attack E-2D which are so far away from them."

      In a previous post, we covered the Chinese Very Long Range Air to Air Missile (VLRAAM) and the impact it would have on E-2 operations. It is reported to have a range of 300 miles and speed of Mach 6. See, Goodbye Hawkeye

      You need to come up to speed on the archives!

    2. Currently in service stealthy fighter jets - F-22, F-35, J-20, SU-57.

      If E-2D does work as manufacturer says, see through 300 miles and direct every F-35C (or F/A-18) do whatever, basically, they turn every navy fighter jet to a "manned" drone. E-2D gives all commands and none of any navy pilot allows to say anything in mission. Pilots only do whatever, E-2D tells them.

      Yes, you question E-2D's true ability but let field data talk. Nevertheless, E-2D can achieve no communications among navy pilots.

    3. Not that I am answering for ComNavOps or anything but where is this field data that you mention? One that's not designed to look good? One that's faced a probable real chaos and repeated attacks of enemy EW and still works perfectly as you suggests (E2D can achieve no communications among Navy pilots)?

      You stated in the beginning that were to believe E2D manufacture specifications, your assessment would work. Unfortunately for us, it's almost a given that manufacturers data is highly unrealistic and only achieve in the most unimaginable conditions. I suspect that it's highly and extremely likely that your assessment is optimistic to the extreme.

    4. "let field data talk"

      There is no field data. Neither the military nor industry would ever release any specifics about detection ranges and certainly not about stealth detection ranges.

  6. I would like to know how far you believe that this line of thinking should be pushed. If you go to extremes you end up with :

    - No long range strike with cruise missiles because GPS will be disrupted (astro-inertial is not working yet below clouds as far as I know).
    - No AMRAAM or equivalent radar guided stuff.
    - LGBs working only in good weather.

    Basically you are left with IR and EO guidance and good old guns, rockets and dumb bombs. True or not ?


    1. You have correctly identified three troubling issues, yes.

    2. "I would like to know how far you believe that this line of thinking should be pushed."

      I'm not sure exactly what you're asking or what you mean by 'pushed'?

      You've obviously grasped the basic concept that all signals will be disrupted on a peer combat scenario so you've kind of answered your own question, I suspect. Any form of communication will be degraded, disrupted, and interrupted to varying degrees. Thus, networks, data sharing, Link X, C2, etc. will all be affected and only sporadically (and unreliably) available.

      The only assured forms of signals will be organic, meaning those that originate from the sending platform (the seeker head on a missile, for example, or the radar on a ship) and even then the returns from those signals will be subject to jamming, scatter, decoy, etc.

      None of this is just theory. We've already seen that UAVs are subject to signal disruption, loss, or hijacking. We've seen the history of electronic warfare disrupting guided missiles. The Russians have amply demonstrated communications and GPS disruption. And so on.

      So, yes, passive signals are the only solidly reliable signals and even they are susceptible to decoys, obscurants, weather, etc.

      Now, all of this is not to say that no signal will ever be successfully transmitted and received on the battlefield. That kind of total disruption by the enemy is not possible. What this is telling us is that the reverse - total success - which the US military seems to be assuming as their birthright, is equally wrong and we need to be preparing for it. We need to learn to function with no comms, few comms, less guidance, disruption, and confusion. We need to train to that level of signal disruption and become comfortable with it. We need to reverse the trend of micro-managing and go back to local command using doctrine and commander's intent.

      I don't know if that answered your question or not?

    3. I grapsed the concept, I just wanted to know if the implications I mentionned were right. It seems that they were. So yes you answered.


    4. Alternatives to GPS
      Army opened research lab to develop open systems architecture with PNT, Positioning, Navigation and Timing System, several technologies as alternatives to GPS. This years Army exercise with Project Convergence 21 they will test how its new technologies function without GPS.

      One option under research, "Quantum" navigation, accelerometers used in INS have existed for a long time, however they cannot maintain their accuracy over time without an external reference so researching quantum accelerometers which can reduce navigation drift from approx 1 km a day to 1 m, but quantum accelerometer can’t distinguish between tiny gravitational effects, a mountain, and accelerations caused by a vehicle's movement, will require detailed gravitation maps.

      Would note USAF/USN/Lockheed in full development of the long range AIM-260, same size as AIM-120 so presuming using advanced propellants motors, gels?, would allow throttling or even stopping and restarting motors as required.

    5. "I just wanted to know if the implications I mentionned were right."

      Yes, they're right! This is telling us that our precision guided, micro-managed way of warfare, developed over the last two decades of fighting technologically deficient terrorists, is wrong for the peer war battlefield. Instead of precision and micro-managing we need to place more emphasis on unguided, area munitions that can't be jammed or decoyed (a naval or artillery shell is going to land where you aimed it and is invulnerable to jamming or decoys). When precision fails, we need massive amounts of unguided firepower to fall back on and we don't have that, currently. When our local commanders are cut off from communication with higher command, they need to be able to function on their own. And so on.

      In short, we need to develop a new way (largely an old way, actually!) of fighting. We're currently locked into the "preparing to fight the last war" mentality and we need to break out of that thought prison.

    6. Above just emphasizes the need to undertake as many varied exercises as possible in as many conditions as possible.
      Eg. An enemy sub has just sunk one of your ships (could be part of a convoy or a carrier group or DDG flotilla, doesn’t really matter).
      Nearby destroyers (because for this scenario DDG’s are the only anti sub vessels the navy currently has, but that is a different discussion) have to search and destroy it.
      First time round you have all the toys, Sonar, P8 help, helicopter, flat sea etc. (perfect conditions)
      Then do it at night
      Then do it without a P8 and /or helicopters.
      Then you try with sonar issues
      Then you do it with No coms, Disrupted coms, No radar/emissions
      Then you do it with a damaged carrier, do you chase the sub (or subs), if how much of the carrier group do you leave with the carrier/s.
      Practice, practice, practice.
      Going silent should be just as familiar to all the crew’s as working with all the gizmo’s working (and any option in between).
      In football (sorry Soccer to you lads, not Rugby with padding, sorry again couldn’t resist, no offence meant, lol), the team practices different scenarios , eg how to take a corner or free kick or a penalty, but also how to change tactics if one player is sent off/ injured. All the players don’t run round with ear pieces getting instructions from the manager; they know what to do because they have practiced it.
      After this we assess and develop a plan. We may find we need more signal lamp practice, or new 21st century laser signal lamps that and send/read in milliseconds or even some ASW frigates/Corvettes. Nobody really knows until you try out the current system of work / plans.
      Sorry rambled on a bit, feel free to delete if I’ve gone to far off topic.

    7. "Sorry rambled on a bit, feel free to delete if I’ve gone to far off topic."

      Quite the contrary. You're spot on!

    8. And do it over and over and over.

      The inter-bellum Fleet Problems were a good model, as I think you have suggested, ComNavOps. Figure out a war plan for China (and Russia) and then practice it, practice it, practice it. And do so against some realistic opposition forces. I don't know exactly how many exercises I did on active duty, but I don't ever remember seeing any live opposition.

    9. "I don't ever remember seeing any live opposition."

      All the Perrys we retired and the Ticonderogas we're going to start retiring and the Cyclones we're retiring and the LCS we're retiring would make a good start on a credible, live OPFOR.

    10. In my idea of a proposed fleet, I incorporate a lot of CAPT Wayne Hughes's NNFM littoral combat fleet, and one role that I would have for them is forming an OPFOR for Fleet Problems.

    11. "No long range strike with cruise missiles because GPS will be disrupted"

      There are other technologies can be used to address this, for instance, back to digital maps (require long loading time before fire). Certainly, you cannot move mountains and large building away in hours.

      There are also more advanced technologies, just use relativity among key land features than entire digital map, such as major mountains. This is what China's DF-26 use. While a missile flies close to target, its guiding system check relativity among reference points and target.

  7. 253-269-863-261-471-958-220-370-4-21-19-24

  8. What's better is that Collingwood, Nelson's second in command, is reported by his Flag Captain of deriding Nelson's 'Duty' signal with the comment ''Why does he keep signalling?We know what we must do''


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