This page is for realistic scientifically plausible slower-than-light communication. For unrealistic science-fictional faster-than-light communication see this page.
This deck contains communication gear, perhaps even with something like a Morse code key for use when radio interference becomes a problem (If this was a Metalunan ship, this is where you'd find the interociter).
Sometimes this deck is called a "radio shack", manned by a communication officer whose nickname is "Sparks." Communication officers are the ship's ears and mouth. They direct incoming messages to the proper departments and send outgoing messages in the proper format to the proper channels. Communication noise must be monitored and auxiliary channels used if required. All messages must be logged.
Distress signals are sent to the watch officer, but never responded to without authorization. Responding binds the ship to render assistance, a decision reserved for the captain. John Reiher points out that given the reality of the spartan limitation on a ship's delta-V, there is probably little they could do to render assistance besides helpful advice over the radio. If they tried to match postion and vector they'd use up all their delta-V, so now there are two ships in distress. The best they can do is notify the Orbit Guard.
The communication officer must also maintain the ship's transponder, which broadcasts the ship's ID.
The communication officer may also be responsible for encrypted communications, using the proper keys to encrypt and decrypt. If this is a military spacecraft, the comm officer is responsible for destroying the code book if the ship is captured by a hostile power. Hit the red "incinerate" button to keep the one-time pad and Captain Midnight secret decoder pin from falling into enemy hands. The safe containing the code book may be on the communications deck, but on some ships it is in the captain's cabin.
Communication will generally be by radio or laser.
Back in the 1900's, Amateur Radio (aka "ham radio") was very popular. Back in those days people didn't travel very much. But if you got your amateur radio license and a radio rig, you could talk with people between 30 and 100 kilometers away. Or even over 400 kilometers away if you link to a radio repeater station. It was sort of like a more powerful and regulated version of the CB radio used by truckers. Science fiction writers like Poul Anderson postulate that rock rat asteroid miners would also be avid ham radio operators. A miner at their claim would probably be located at an inconveniently large distance and large delta V from the closest neighbor. Ham radio is a cheap way to keep in touch, and pass news & gossip. You probably can get more range if you use Morse code instead of speech.
In the communication version of the Kzinti Lesson, note that long range laser communication gear can be used as short ranged laser weapons. In many of Larry Niven's "Known Space" stories comm lasers have been used as weapons, e.g., "There is a Tide" and "The Ethics of Madness."
Radiators are not the only thing spoiling the Polaris' sleek external lines. Roger Manning's radio needs large dish antennas. They might not be as large as the monsters on 2001's Discovery, but they won't be much smaller than the ones on the Apollo service module. It might also be a good idea to have landing radar on an outrigger or boom, so it won't be blinded by the exhaust. Both of these will be retracted during atmospheric re-entry, with the landing radar deployed when the air speed drops low enough so it won't be ripped off.
Range is how far you can transmit a message. Major factors are:
- Power in the signal, including loss due to inverse-square law
- Sensitivity of the receiver
- Noise in the signal, including electromagnetic interference from natural and artificial sources
By "bandwidth", I mean in the computing sense: how many bits per second of digital information can be trasmitted. The other sense (signal bandwidth) is width between the upper and lower set of frequencies used.
As an example, using old 1977 technology the Voyager space probes use S Band (2 to 4 GHz) communications with 3.7 meter high-gain antennas. As the ranged to Earth increased, the bit-per-second rate decreased due to the inverse-square law. Using the 62 m Deep Space Network dishes as receivers, at Jupiter it was 115 kBits/sec. At Saturn it was 57 kBits/sec. It went rapidly downhill from there.
As far as signal bandwidth goes, voice transmission typically takes 2400 Hz, while Morse code takes only 100 to 150 Hz.
A final matter is that there is only so many radio frequencies to go around, so some authority has to be in charge of radio spectrum allocation.
Radio proper typically use the electromagnetic spectrum from 3 kHz to 300 GHz.
Laser communicators typically use visible, infrared, or ultraviolet frequencies. Currently the maximum range that laser communication has managed in the field is 24 million kilometers, but that will no doubt be improved by many orders of magnitude.
Some times masers are used in recievers in order to amplify weak radio signals.
Also read about the Interplanetary Internet.
That party-pooper Einstein codified the ultimate speed limit on communication in his Special Relativity. The limit is the speed of light in a vacuum, which is quite rapid at 299,792,458 m/s. For distances found on the surface of a planet this speed limit is not much of a problem (unless you are doing ultra-high-speed stock market trading or something). But for distances within a solar system the speed limit problem becomes moderate to major, and for interstellar distances they become overwhelmingly outrageous.
If you are trying to have a conversation with somebody, the delay is doubled, because it becomes a round trip. Alice says a sentence which travels by radio to Betty. Betty responds with a sentence which travels to Alice. From the viewpoint of Alice, Betty's answer is delayed by double the transit time.
For a conversation between Terra and Luna the double delay will be from 2.4 to 2.7 seconds (perigee and apogee), which is annoying but manageable. There is a nice scene illustrating this problem in The Expanse Season 2 Episode 5: “Home”, where Chrisjen Avasarala on Terra tries to talk to her husband on Luna. They keep stepping on each other's sentences. They know better but they are preoccupied with the fact Terra might be destroyed in a couple of hours, and this might be the very last time they talk to each other.
For any longer distance conversation is more or less impossible. It will be more like sending emails, or even snail-mail. The most efficient strategy is probably to constantly talk, and as questions arrive just insert the answer into your monologue.
This is why trying to control asteroid mining drones by telepresence from Terra is probably impractical.
For conventional radio and other electromagnetic communication the time delay is obviously proportional to the distance, which is science-speak for "the longer the distance the longer the delay". For handwavium faster-than-light communication it depends upon what mad ideas the author comes up with. Most of them simply have the communication speed set faster, e.g., a subspace radio with a speed of five-hundred times lightspeed. Which would have a timelag 1/500th of a conventional radio. But occasionally you find weird communication devices with strange limits, such as a constant time delay regardless of distance. These have to be handled on a case-by-case basis. Sometimes there will be two or more kinds of FTL communication: a commonly available but slow kind and a highly restricted but fast kind.
James Blish had many stories featuring his "Dirac" communicator, which was instantaneous. Timelag of zero.
With NASA and other space agencies, they assume all us humans are living on Terra, while the unmanned space probe can be a long way out there. It is going to take some time for the messages from the space probe to make their way back to Terra.
For instance, the New Horizons space probe went whizzing by trans-Neptunian object (486958) 2014 MU69 on 1 January at 05:33 UTC. It immediately sent back a radio message "I'm OK, I didn't smack into the asteroid or anything, photos and data to follow". But the radio message didn't make it back to Terra until 1 January at 15:28 UTC, nine hours and 55 minutes later. Such is the timelag. The people at Mission Control knew theoretically that New Horizons had its close encounter at 5:33, but they had to sweat it out until 15:38 until they found out what happened.
This can be confusing when NASA wants to send a command to a space probe ordering it to perform an action at such-and-such a time. So space agencies use Spacecraft Event Time (SCET, also known as Orbiter UTC) and Earth-received time (ERT, also known as Ground UTC).
The table below indicates the communication delay that occurs in a one-way transmission.
Circuit Distance Delay Time HF link (UK-NZ) ~20,000 km 0.07 s (67 ms) Submarine cable(UK-NZ) ~20,000 km 0.07 s (67 ms) Geosat Link (US-Aus) ~80,000 km 0.25 s Earth-Moon 384,000 km 1.3 s Earth-Mars 55 - 378 million km 3 - 21 minutes Earth-Jupiter 590 - 970 million km 33 - 53 minutes Earth-Pluto ~5800 million km 5 hours Earth-Nearest Star ~9.5 million million km 4 years
Only the first two circuits listed have a delay that is barely noticeable in a two-way voice conversation. On telephone calls between continents that are routed via a geosynchronous satellite, the time between when one person stops speaking and then hears the other person reply is half a second (0.25 s × 2). This can cause immense confusion if the other person starts speaking before the first has finished. It can take several sentences before the confusion is finally sorted out. Two-way digital communication between machines at high data rates suffers from this problem even more acutely, and protocols must be established which prevent conflicts arising.
Communication with a future lunar base will be worse, and for voice communications will necessitate an "over to you" simplex radio communications type approach. Two-way interactive communication with any station beyond the moon is basically impossible. Although there are no manned bases currently on other planets, this delay is presently of great concern to people who send remotely controlled spacecraft to Mars. There is no possibility of detecting an incipient vehicular crash in time to do anything about it. The vehicle must thus be given a very large degree of autonomous control (e.g., for navigation).
TIMELAG LIMITS REACTION TIME
From a military or interstellar empire standpoint, the important point is the speed of your communication puts an upper limit to how rapid your reaction time is. If it takes a year for news of a rebellion on the outer marches of the galactic empire to reach the capital (or sector capital) the rebels were already gifted with an entire year to win the rebellion and fortify in preparation for the arrival of the imperial starfleet. The time it takes the imperial starfleet to travel to the rebel world is just icing on the cake.
In the Traveller RPG and the Starfire game the fastest FTL communication is by courier starship. Starfire ships carried lots of FTL courier drone missiles used with the Imperial Command And Communication Network in a desperate attempt to reduce reaction time to a minimum. The Traveller based boardgame The Fifth Frontier War is mostly about the players trying to cope with reaction times measured in weeks.
In the real world of ultra-high-speed stock market trading, a few more milliseconds of reaction time could cost you millions of dollars. A company called Spread Networks entire business model is offering stock traders a super fast connection between the Chicago and New York City with a latency three milliseconds shorter than all the competitors. Their arrow-straight data path required boring a tunnel through the Allegheny Mountains of Pennsylvania.
If you are an asteroid miner on an isolated rock or a shipwrecked castaway on a deserted planet, and circumstances have deprived you of your radio transmitter, you have a problem. You will have to some how jury rig a transmitter out of whatever you can scrounge up so you can call for help. Naturally the simpler the transmitter, the easier will be the scrounging and the construction.
Which brings us to the Spark-gap Transmitter.
This is pretty much the simplest radio transmitter possible. Yes, this is real-science electromagnetic transmitter. If you have created for your science fiction novel some sort of superluminal radio powered by handwavium, you are on your own.
In the pictorial diagram:
- The "Leyden jars" are capacitors
- The "aerial" is an antenna
- The "tuning coil" is an inductor
- The "induction coil" is NOT an inductor, it is a "spark coil"
Sometimes solar storms or enemy jamming might fill the communication lines with static. If it is real bad, one might have to revert to good old Morse code. The dot and dash symbols can punch through interference much easier than the spoken word. Even present day naval vessels can send Morse code by Signal Lamps, when the enemy might overhear a radio message.
By its nature, Morse code is sent over the radio by Amplitude Modulation (AM), not Frequency Modulation (FM) or other kind of modulation. Radio Morse code requires less signal bandwidth (100 to 150 Hz for typical speeds in the 20 to 40 word per minute range) compared to voice (2400 Hz), though it has a lower data rate.
Morse code is usually received as a medium-pitched audio tone (600–1000 Hz), so transmissions are easier to copy than voice through the noise on congested frequencies (too many conversations taking place on the same band), and it can be used in very high noise / low signal environments (loud static and faint voice, never a happy situation). The transmitted power is concentrated into a limited bandwidth so narrow receiver filters can be used to suppress interference from adjacent frequencies (narrowing the opening that static and competing conversations can leak through).
The narrow signal bandwidth also takes advantage of the natural aural selectivity of the human brain, further enhancing weak signal readability (translation: it is easier to hear a dot or dash than it is to decipher somebody slurring their words). The signal can be weak if the sender is distant, has low amounts of power available for the radio, or both. Very well could be good news for castaways in a wrecked spacecraft in the back of the beyond. In fact the required bandwidth can be reduced and the signal range increased by simply drastically slowing down the word per minute rate. Dave Hinerman (WD8CIV) told me about some hams who experimented with computer generated Morse code with the dots lasting ten seconds and the dashes lasting 30. They were trying to see what was the least amount of power needed to transmit a receivable decodable signal.
In addition, a Morse code signal is easier to jury-rig than a full two-way radio (for instance: by tapping two wire together). Which could come in handy for an interplanetary spy trying to transmit information using the equipment at hand before the bad guys burn open the hatch with their laser pistols. Or by survivors of a drastic shipboard accident that destroys the communication equipment. Or if Our Hero is in a holding cell of the evil villain's lair, covert communication would be possible with prisoners in other cells by tapping on the walls (though actually prisoners would want to use a Tap Code because it is hard to tap a "dash" sound). Or sending a message by turning on and off a flashlight or using a mirror to flash sunlight at the intended message recipient (in olden days this was the mechanism used by heliographs).
Skilled telegraphers can comprehend ("copy") Morse code signals at rates in excess of 40 words per minute. Top sending speed by experts might be around 75 to 100 words per minute. Individual telegraphers might use slightly longer or shorter dashes or gaps, perhaps only for particular characters. This is called the telegrapher's fist, and other telegraphers can recognize specific individuals by it alone. A good telegrapher who sends clearly and is easy to copy is said to have a "good fist". A "poor fist" is a characteristic of sloppy or hard to copy Morse code.
There are three main types of Morse code "keys" (devices used by telegraphers to tap out the dot-dash Morse code signals). They are Straight Keys, Semiautomatic Keys and Iambic Paddles. Improvised keys generally take the form of two wires that the telegrapher taps together.
My dear departed maternal grandfather used one of those as a radio HAM. I tried to use it but was never very good at it.
My dear departed maternal grandfather rigged up one of these for a friend of his who had a disability in his arms. My grandfather bolted the paddles under the table and crafted a long lever his friend could control with a knee.
So in times of imperfect reception one can fall back on Morse Code. But even with perfect reception, some spoken items are hard to distinguish. The letters "T" and "D" for instance. Misunderstood words and letters can be catastrophic, especially in a military situation. To avoid this the NATO phonetic alphabet is commonly used. Also useful are military "brevity words."
Another useful source is the International Code of Signals. These are a set of international code signs and words that do not depend upon the two people communicating to share a language in common. They include multicolored flags, semaphore, blinking lights, Morse code, and radio. For instance, AJ means "I have had a serious nuclear accident and you should approach with caution" and EO means "I am unable to locate vessel/aircraft in distress because of poor visibility". Those signals can be understood even if the sender only speaks Mandarin Chinese and the receiver only speaks Czechoslovakian. Refer to the manual found here.
This was invented by Dream Pod 9 for their Jovian Chronicles game. It is a set of quickly drawn symbols used as emergency writing where there is no radio contact. Say, painted on the side of a spacecraft in distress. For details, refer to the Symbols page.
This is only a short list. For a fuller list go here.
- FREE at targets not identified as friendly in accordance with current rules of engagement (ROE).
- TIGHT at targets positively identified as hostile in accordance with current ROE.
- HOLD (USA, USMC) in self-defense or in response to a formal order.
- SAFE (USN) NOTE: USN and NATO use weapons safe to avoid confusion with the phrase hold fire.
- As a request: Communication is difficult, please send each word twice.
- As information: Since communication is difficult, I will send each word twice.
Lots of communication will have to be encrypted for security reasons, especially if a crew member is doing some online banking. For non-military purposes, a useful innovation is Public-Key Cryptography. It allows two valuable functions:
 Encrypting Messages
Ordinary encryption allows specific persons to send you a message that only you can read. Problem is that the encryption key they use is also the decryption key. Which means anybody with the key can read the encrypted message, not just you.
There is also a problem when you try distributing the key to the specific people you want. If an evil spy intercepts the key, they can read any message sent to you. Worse: the evil spy might swipe the key and subsitute their own secret key. Sending the key safely to your friends is really hard.
With public key encryption this is not a problem. The encryption key is NOT also the decryption key. You can broadcast the encryption key far and wide with no attempt at security, it won't hurt anything. This makes distributing the encryption key easy, just post it on the internet. It doesn't matter if evil spy obtains it, they cannot use to decrypt your messages.
 Digital Signature
When it comes to digital signature, the trouble with ordinary encryption is that it cannot do it at all. Public-key encryption can.
Digital signature is an extra bit of encoded message you attach to a cleartext message. It "gives a recipient reason to believe that the message was created by a known sender, that the sender cannot deny having sent the message (authentication and non-repudiation), and that the message was not altered in transit (integrity)." Better than a pen-and-ink signature on a contract, since a digital signature sort of rots away if somebody changes the contract.
How Does It Work?
If you only wanted to know why public-key encryption is so popular, you can stop reading now. What follows is the esoteric inner workings of the scheme.
Encryption is taking a message (the "plaintext") and scrambling it into an unreadable mess (the "cyphertext") so that the Wrong People cannot read it. The Right People can unscramble the message back into plaintext and read the message. It is much like two last-century children sending messages using their Captain Midnight secret decoder badges ("Be sure to drink your Ovaltine").
Old fashion garden-variety encryption uses what is called a Symmetric-key algorithm. The sender takes the plaintext plus an encryption key (in the form of a huge number with many digits) and feeds both into the encryption algorithm (meaning you put both into your encryption sofware or smartphone app). It spits out the cyphertext.
The cyphertext is sent to the recipient. They feed the cyphertext and the encryption key to their encryption algorithm and the cleartext pops out. The Wrong People cannot intercept the message (unless they manage to crack the encryption or swipe the encryption key).
What makes this "symmetric-key" is the fact that both sender and recipient are using the same key.
Public-Key Cryptography is different. There are two keys: A and B. A message encrypted with key A can be unencrypted with key B and a message encrypted with key B can be decrypted with key A. And a message encrypted with key A cannot be decrypted with key A. This is also called an asymmetric-key algorithm.
The two keys are generated by the user using a key-pair generating software, by feeding the software a huge random number.
How do you use it?
The user calls key A the "public key", and broadcasts it far and wide. Key B is the "private key", and the user keeps it a deep dark secret.
For  encrypting messages, anybody can use your public key to encrypt a secret message intended just for you. The message can only be decrypted with your private key, which presumably only you know. Thus the sender is reasonably sure you are the only one who can read the secret message.
In the example at right, Bob sends the secret message "Hello Alice!", encrypting it with Alice's public key. Only Alice can decrypt it by using her private key.
For  digital signature, it is a bit more complicated. To do it simplistically, you would "sign" a document (say, a contract) by encrypting the document with your private key. Anybody else in the universe can use your public key to decrypt the contract. The fact that your public key worked is proof that the document was encrypted with your private key, which presumably only you have. Thus it is proof you "signed" the document.
In the example at right, Alice digitally signs the contract "I will pay $500" by encrypting it with Alice's private key. Bob can verify Alice's signature since the message decrypts with Alice's public key.
In practice, since encrypting a huge document can take an annoyingly large amount of computing time, digital signature generally use a cryptographic hash (message digest) of the document. The hash is encrypted with the private key instead of the entire document. The encrypted hash is transmitted along with the plaintext document. To verify that you digitally signed the document a person has to:
- Decrypt the encrypted hash with your public key
- Generate a hash of the plaintext document
- If the two hashes are identical, the digital signature is valid
Clever readers will have spotted the flaw in the system: How do you know the public key is really associated with the person? Some disaffected young person living in their mother's basement can claim they are Stephen Hawking and distribute a public key. How is anybody going to be sure that any given public key actually comes from who claims it?
What is needed is a Public key infrastructure. Among other things, the infrastructure binds a person with their public key, that is, they ensure that Stephen Hawking is actually associated with Stephen Hawking's public key and not some disaffected young person living in their mother's basement. This is done with a Certificate Authority (which is also a Trusted third party).
A person who has generated a public and private key makes an application to the certificate authority. The authority does some checking to ensure that person is actually who they say they are. Usually this is via some simplistic domain validation but the validation becomes more stringent as the amount of money at stake increases.
Assuming the person passes the check, the certificate authority issues a public key certificate, which is a fancy way of saying the CA broadcasts the public key and assures everybody that it actually belongs to the person. Of course the public key certificate is digitally signed with the CA's private key.
An Automatic Identification System (AIS) is a radio system on a vehicle that continually broadcasts the vehicle's unique ID, position, course, speed, and any other vital navigational information. Currently these are found mainly on naval vessels.
A Transponder is a radio system on a vehicle that responds to a radio interrogation message with a radio reply containing the information. Interrogations such as "identify yourself", "what is your altitude?" etc. If there are no interrogations, the transponder is silent. Currently these are found mainly on aircraft. "Transponder" is short for "transmitter-responder".
AIS and Transponders are the responsibility of the ship's communication officer. Which is why this section is in the "Communication Deck" page.
In the Traveller role playing game, by law civilian starships are equipped with something they call a transponder, but which acts more like an AIS (it is constantly broadcasting). In Traveller it is illegal for a civilian starship to turn off its transponder, unless there are mitigating circumstances. Which usually means a fear of pirate corsairs using your transponder signal to home in on you.
Pirate ships will need illegal "variable transponders" that can be set to broadcast fake identities, in order to lull the suspicions of both victims and police.