Signal systems let a community send critical information without electricity: flags are visible at 1-3 km, horn blasts carry 1-3 km, drums 2-5 km in still air, and a mirror heliograph can reach 50+ km but only between high points, since level ground limits it to far less, while a shuttered lantern is visible at 3-5 km on a dark night but announces your presence to everyone in line of sight. Keep the code book to a maximum of 10-12 distinct messages, make the emergency signal the simplest action, include authentication and an under-duress variation, and change every code immediately if the book is compromised. Relay beyond a single signal’s range with a chain of hilltop stations 5-15 km apart or with runners stationed every 5-10 km.
Signal Systems and Communication
When electronic communication fails — no phones, no internet, no radio — communities must fall back on methods that predate electricity. Flags, mirrors, fire, drums, and horns carried critical information across entire civilizations for thousands of years. These systems are simple, reliable, and require no infrastructure beyond human ingenuity.
The challenge is not the technology. The challenge is establishing codes, training operators, and maintaining discipline so that signals are sent correctly, received accurately, and acted upon immediately.
Visual Signal Methods
Mirror Heliograph
The heliograph — using reflected sunlight to flash coded messages — is the longest-range visual signal method available without electricity. Under clear conditions a mirror signal can be seen at 50+ km, but only between high points, and that is a constraint on where you put the stations rather than a property of the mirror.
Line of sight over a curved earth is about d = 3.57 × √h kilometres for an eye height h in metres, and two elevated points add their separate ranges. So:
- 50 km between two stations needs about 49 m of eye height at each end — a substantial hill or tower at both.
- At the 5-15 km station spacings recommended later in this article, you need under 4.5 m, which is a modest rise or a short tower.
- A watcher standing at 1.7 m sees 4.65 km of horizon unaided.
In other words: for a relay chain, terrain decides your sites, not the equipment. Walk the line of sight before you build anything.
One practical limit on the mirror itself: the reflected beam spreads about 0.0093 of the distance, so it is roughly 46 m wide at 5 km and 465 m at 50 km. That sounds forgiving until you notice it means aiming to within about 0.13° of mirror tilt — finer than an unsupported hand holds. Rest the mirror on something, use a sighting hole or a foresight stick, and expect to re-aim every minute or so, because the sun moves 15° an hour and walks the beam off a 5 km target in a little over a minute.
Equipment:
- A flat mirror, 7-15cm across. Glass mirrors are best but polished metal works
- A sighting device — a small hole in the center of the mirror, or a separate sighting stick
Technique:
- Hold the mirror near your face with one hand
- Extend the other hand toward the target, making a V with two fingers
- Angle the mirror so the reflected sunlight hits your extended fingers
- The target is now illuminated. Tilt the mirror to flash the signal on and off
- To send code: long flash = dash, short flash = dot. Standard Morse code works but requires training
Simplified mirror code (recommended for communities):
| Signal | Meaning |
|---|---|
| 3 flashes, pause, repeat | Attention — look for follow-up |
| Continuous rapid flashing | Emergency — send help |
| 2 slow flashes | Acknowledged/understood |
| 1 flash, long pause, 1 flash | All clear |
Flag Signals
Flags are visible at 1-3 km with the naked eye, farther with binoculars or a telescope.
Simple flag code (no semaphore training needed):
Use 3 flags of different colors. Red = danger/emergency, white = peaceful/all clear, and any dark color (black, blue) = attention/information.
| Display | Meaning |
|---|---|
| Green or marked flag raised | All clear, safe to approach |
| White flag | Truce or parley only — never “all clear” |
A white flag does not mean what your code book says it means
An earlier version of this table used a raised white flag for “all clear, safe to approach”. A white flag means truce, parley or surrender nearly everywhere, and the people whose reading of it matters are precisely the outsiders who have never seen your code book. A local code can define anything it likes internally; it cannot redefine a signal whose audience is strangers.
Both failure directions are bad. Your routine all-clear, flown daily, reads to an approaching group as an offer to surrender — which invites exactly the confident approach you did not want. And when you genuinely need to parley, your own people read the flag as “normal operations”. Use a distinctly coloured or marked flag for all-clear and leave white alone for the one thing the whole world already agrees it means. | Red flag raised | Danger — do not approach, stay alert | | Dark flag raised | Attention — message follows | | Red over dark | Hostile contact — request assistance | | White over dark | Visitors approaching — standby | | Flag waved left-right | Negative / no / denied | | Flag waved up-down | Affirmative / yes / understood |
Placement: Flags must be on tall poles (3-5m minimum) in positions visible from all observation posts and from approaching roads.
Fire and Smoke Signals
Fire signals work at extreme range, especially at night. Smoke signals work during daylight.
Fire signals:
- A single fire visible from a distance is ambiguous — it could be a campfire
- Two fires side by side = attention/look for message
- Three fires in a triangle = universal distress signal (recognized internationally)
- A fire lit and extinguished repeatedly = urgent message, come to the signal point
Smoke signals:
- To generate visible smoke: build a hot fire, then smother it with green leaves, grass, or damp wood
- White smoke (damp green material on fire) = all clear, camp here
- Dark smoke (resinous softwood, pine branches, pitch-soaked wood) = danger, warning. Do not burn rubber, plastic or oil-soaked material to make it, which an earlier version of this line specified. Burning plastic and rubber gives off hydrogen chloride, dioxins and — from nitrogen-bearing plastics — hydrogen cyanide, along with dense particulates, and the person making the signal is crouched over it, generally upwind of their own settlement. You would be poisoning your sentry and your neighbours to say “danger”. Resinous wood gives a dark enough smoke and the corpus’s beacon article already restricts signal fuel to wood
- Pulsed smoke = message being sent; count pulses for meaning. Interrupt the smoke with a hinged lid or a cover on a long handle, standing upwind — not with a wet blanket in your hands, which an earlier version of this line described. Dropping wet cloth onto a fire and lifting it off again puts your face over a burst of steam, and steam at the same temperature as air carries vastly more energy and scalds on contact. A sheet of tin on a pole, or a damp mat on a pivot, does the same job from arm’s length
Pre-positioned signal fires: At observation posts and along escape routes, maintain pre-built fire stacks that can be lit in seconds. A pile of dry kindling, with tinder at the base, covered by a waterproof sheet. Pull the sheet, apply flame, signal within 60 seconds.
Lantern Signals (Night)
A lantern, candle, or flashlight behind a shutter allows coded signals at night.
- Construction: attach a flat board (the shutter) to the front of a lantern on a hinge or with a handle. Open the shutter to flash, close to hide the light
- Signal pattern: same codes as mirror flashing — short exposure = dot, long exposure = dash
- Range: visible at 3-5 km on a dark night with good atmospheric conditions
- Caution: light signals at night announce your presence to everyone in line of sight, not just the intended recipient. Use only when the security benefit outweighs the exposure risk
Audio Signal Methods
Horns and Trumpets
A horn blast carries 1-3 km depending on wind and terrain. Purpose-made instruments carry farther.
Construction:
- Animal horn — a cattle horn, cleaned and hollowed, with the tip cut off to form a mouthpiece. Produces a single note that carries well over distance
- Conch shell — if available, a large conch shell with the tip cut off is one of the most powerful natural horns
- Improvised horn — a section of PVC pipe (2-3cm diameter, 30-60cm long) with a funnel-shaped bell at one end. Buzz your lips into the narrow end
Horn codes:
| Signal | Meaning |
|---|---|
| Continuous repeated blasts, not stopping | Immediate danger — take cover or evacuate |
| Long–long–long | Emergency assembly — all hands to rally point |
| Long–short–short | Stand down — threat passed |
| Long–short–long | Attention — all watch posts alert |
| Short–short–long | Acknowledged |
Every message is exactly three blasts, except the danger signal, which never stops. That is the whole design and it is worth understanding rather than memorising, because the obvious alternative is broken.
The previous code had "Acknowledged" as the opening of "take cover"
An earlier version of this table read: 1 long = Attention, 2 short = Acknowledged, 3 long = Emergency assembly, repeated short = Immediate danger, 1 long + 2 short = Stand down. Now hear that as a listener, one blast at a time, which is the only way anybody ever hears it:
- Two short blasts is the beginning of “repeated short blasts”. So the sentry sounds the alarm for immediate danger, and the first thing the settlement hears is the code for acknowledged — nothing is wrong. That is the worst possible confusion in the set and it is built into the code.
- One long blast could be Attention, or the first of the three longs for Emergency assembly, or the first element of Stand down. Three-way ambiguity on the signal most likely to be sent first.
This is a solved problem: no message may be the opening of another message. Fixed-length codes guarantee it, because a three-blast code cannot be the start of a different three-blast code. Three elements give eight combinations, which is more than this table needs; if you want more messages, go to four elements and sixteen. Keep exactly one signal outside the scheme — the one that does not stop — and make it the emergency, so that it is recognised by its character rather than by counting under stress.
Apply the same test to every code table in this article, and to your own. Write the messages out as sequences, then check whether any one of them is the start of another. If it is, the code will be misread on the night it matters.
Drums
Drums carry farther than horns in forested terrain because low-frequency sound bends around obstacles.
- A large drum (40cm+ diameter) with a taut animal-skin or synthetic head, beaten with a heavy padded stick, can be heard at 2-5 km in still air
- Drum codes work on the same principle as horn codes: different patterns of beats convey different messages
- Advantage: drums can be played continuously without the fatigue that wind instruments cause
Whistles
For short-range communication between patrol members, observation posts within 500m, and immediate alerts.
- Metal or plastic whistles are louder and more consistent than shouting
- Keep codes simple — no more than 4-5 signals total. Complexity causes errors under stress
- Every community member should carry a whistle and know the codes
Code Systems
Designing Your Code
Keep it simple. In an emergency, people forget complex codes. Design for the stressed, tired, scared operator.
Principles:
- Maximum 10-12 distinct messages in your code book. More than that causes confusion
- The most urgent messages should be the simplest — the emergency signal should be one action (continuous blasts, rapid flashing), not a complex pattern
- Include authentication — how does a receiver know the signal is from a friendly source? Use a daily code word or a specific prefix pattern known only to your community
- Include an “under duress” signal — a subtle variation that tells receivers the sender is being forced to signal. Example: adding one extra flash to the “all clear” signal means “I am signaling all clear but I am not free to communicate honestly”
Sample Community Code Book
| Code | Visual (flag) | Audio (horn) | Meaning |
|---|---|---|---|
| ALERT | Dark flag raised | 1 long blast | Attention, stand by for information |
| ALL CLEAR | Green or marked flag up | Long–short–short | No threats, normal operations |
| VISITORS | White over dark | 2 long blasts | Unknown people approaching, non-hostile |
| DANGER | Red flag raised | 3 rapid blasts | Confirmed threat, all posts alert |
| EMERGENCY | Red flag waved | Continuous blasting | Immediate danger, execute emergency plan |
| RALLY | Dark flag waved | 3 long blasts | All personnel assemble at rally point |
| ACKNOWLEDGED | Any flag dipped once | 2 short blasts | Message received and understood |
| NEGATIVE | Flag waved side-to-side | 1 short blast | No / denied / do not proceed |
Inter-Settlement Communication
Relay Networks
For communication beyond the range of a single signal, establish relay stations.
- Signal tower chain — observation posts positioned on hilltops, each within visual range of the next (5-15 km apart). A signal originating at one end can traverse 50+ km in minutes as each station relays the message
- Historical example: the beacon chain in Lord of the Rings is fiction, but the concept is real. The Roman Empire maintained signal tower chains across its borders. The British Admiralty used a shutter-telegraph chain from London to Portsmouth (100 km) that could transmit a message in 15 minutes
- Requirements: each relay station needs a trained operator on duty, a clear line of sight to stations on either side, and a standardized code book
Runner Systems
For complex messages that exceed the capacity of signal codes, human runners carry written or memorized messages between settlements.
- Speed: a fit runner covers 10-15 km per hour on roads, 5-8 km per hour on trails
- Relay running: station runners every 5-10 km. Each runner covers only their segment at maximum speed, handing the message to the next runner. This sustains a pace that no single runner could maintain
- Message security: runners should memorize short messages and destroy any written message if capture is imminent. For written messages, use simple ciphers (letter substitution, coded references to landmarks)
- Runner safety: runners travel alone and fast. They should know the terrain intimately, travel armed minimally (a knife, whistle), and know escape and evasion routes along their path
Training and Drills
- Weekly signal drills — send a practice message from one post to another. Verify it arrives correctly. This catches misunderstandings and equipment failures before they matter
- Night signal drills monthly — night signaling is harder and less practiced. Train for it specifically
- Cross-train operators — every community member should know the basic emergency signals. At least 3-4 people should be proficient in the full code book
- Update code books after any compromise — if your code book falls into outside hands, change every code immediately