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. 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."

Radio Antenna

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 and Bandwidth

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.

Rick Robinson:

One of the biggest differences between real space and rocketpunk era space is comms. There is physical isolation but rarely communications isolation in the Vastness of Space.

Citizen Joe:

The range we're getting on long range communications is due to directional antennae. If you're broadcasting omnidirectionally, the signal drops fast. So you could get radio transmissions, but there would like be just the one line. That one line would also be blocked by planets and other stuff on occasion. That one line would be in high demand and thus not readily available. You might spend thousands on a couple minute recording that gets compressed and burst fed to a destination. Real messages, sent parcel post, would probably get stored digitally and then transmitted once you arrived at the destination orbit.


To expand on Citizen Joe's point, would you like to receive "Male enhancement" emails with the olympusmon.mars address? How about "Dear Sir, I am the last surviving member of the Europan Resistance front and need your help to transfer $10 billion solars from the Bank of Callisto...."

On a more serious note, the high bandwidth links would probably be reserved for ship traffic, government and military communications and corporate communications (for companies with the financial clout to get in line for email). Certainly the Uranus Space Navy would not want the high bandwidth links clogged during their showdown with the Imperial Jovian Navy, nor would they want to risk malware or botnet attacks coming through those links; which suggests interplanetary comms would be tightly controlled and subscribers carefully vetted.

Citizen Joe: The range we're getting on long range communications is due to directional antennae.


This is really just an engineering issue, and not that hard to solve. Signal multiplexing, multiple frequencies, different modulations, and even using lasers (which can pack in massive amounts of data) could (and would) increase the amount of bandwidth available in space.

There's really nothing in the laws of physics that says you can't beam 1,000+ Tb/sec of data from earth to mars. It'll just take a few hours to get there. That itself could create some interesting cultural dynamics. No sane 'belter would ever play day trader on Venus' stock markets, for example.

That said, I agree with the points made above that you would see a massive amount of data security being used in space. The military forces would be the most paranoid, probably using laser transmissions most of the time to prevent eavesdropping. I like the idea of using a cost formula for data transmission that discourages spamming. I could see something like that being used to govern the use of non-corporate civilian data traffic.

Stevo Darkly:

Since the subject of communication has come up in these comments, at some point I would love to see some discussion of what kind of communication technology an interplanetary civilization is likely to use for long-distance messaging. Radio? (Isn't that what we use now, for communication with interplanetary probes?) Maser? Laser?

Even the amazing nuts-and-boltsy Atomic Rockets site of Winchell Chung doesn't seem to address this question specifically, that I can find.

For example, my intuition tells me that any spacecraft outfitted with a laser capable of communication between, say, Neptune orbit and the Earth would also be able to use that laser as a weapon at shorter distances. If so, even a "peaceful unarmed merchantman" would have a potential short-range antiship/antimissile weapon. Interesting implications.


Radio? Maser? Laser?" My guess would be, all of the above. As for lasers, terrestrial free-space laser communication systems get about 1Gb/sec for high-end systems. That's actually much lower than what is possible with optics, because of the problems inherent in trying to punch that laser through a messy atmosphere. In space, you could expect to get performance in excess of 1Tb/sec, YMMV.

Radio is likely to be used extensively, to the point that the general space around planets and other points of interest could get pretty noisy. I'm not sure whether it's easier to collimate a laser beam or a radio signal over stupendous range. Whichever one wins would be the one you'd see more in interplanetary communications.


Communication with electromagnetic radiation is subject to diffraction just like lasers. The diffraction equation is on Atomic Rockets in the laser cannon section. Basically, the shorter wavelength you use, the less diffraction you get and the smaller angle you can focus the beam. Now, radio waves have much longer wavelength than light, so light from lasers could be used in a more efficient way for communication, as it can be concentrated better.

There are phased arrays for radio waves, and optical phased arrays are possible but cannot be built by existing technology. These would help to focus the transmission. I do not know any equations for phased arrays, though, so someone with more knowledge on these might want to enlighten us.


Since power and surface area aren't problems on the ground (and to a lesser extent, in orbit), communications would likely be predominantly TO ships while ship to shore (home base) comms would be limited to a confirmation of receipt. On Earth, we can afford to put up huge arrays to catch the smallest radio signal. Not so much in space. Likewise we can pump a lot of energy into the antenna to send back a longer message with a lot of bandwidth and strength even at stupendous ranges.

Now there are some tricks, like omnidirectional beacons and antenna that act as targets for the directional antennae. But you can only listen to data in the direction of the directional antenna. That might be limited to a single stream. Comm relays would likely have at least 4: Signal in, Signal out, Previous relay, Next Relay. By using multiple relays (at least 4 would get you around the sun) you wouldn't have black outs (except at ship orbit). And then there is the problem with the fragile gimbals needed for all the antennae.

In the end, yes, you can communicate via radio. No, it isn't broadband. No, it won't service a population comparable to the internet. It will likely be biased communication. It will still be expensive. There will be extreme needs that keep every carrier busy. So, although you COULD send a digital copy of Pluto Nash to Pluto, you would never get enough priority to use the carriers and thus it would be simpler to put it on a torch or fling it out the airlock.

Rick Robinson:

Communications. This whole subject gets discussed far too little, so I am glad to see it come up here.

My intuition, worth what you paid, is that point to point comms bandwidth, probably by laser, will be surprisingly cheap even across interplanetary distances.

Very quick and dirty. If I am using the formula from Atomic Rockets correctly, a visible band laser beaming through a 1 meter telescope will have a beam radius of 90 km at 1 AU.

But an ordinary light bulb is visible at 45 km (in ideal conditions). It is shining in all directions, so the projected surface is the same, and a 100 watt visible band laser should be naked eye visible at 1 AU.

Anyone want to check this? A laser beam bright enough to be visible should be able to sustain a huge data bandwidth, and even a spacesuit helmet could probably carry the receiver. A 100 watt laser and 1 meter telescope aren't a huge order either, as such things go.

If all of this is more or less correct, high bandwidth space comms should be cheap. By my rule of thumb for a mature midfuture space tech, the transmitter rig might be a ton or so all up, and cost $1 million, no extravagant fitting for a spaceship.

Receivers could cost a few kilobux and fit on a spacesuit helmet, though high bandwidth interplanetary comms in your spacesuit is probably taking 'mobile Internet' to needless extremes.

Proviso also that all this applies only to tight beam comms, for which you need to know exactly where to point your telescope. Which makes precise locational data crucial at least to high volume comms.

Citizen Joe:

90km at 1 AU means that your signal has dropped to like one ten billionths of the original signal strength. 90km at 1 AU is also a very small spec to aim at. Additionally, if the receiver isn't pointed at just the right angle, you get nothing. Plus it is hard to distinguish a visible laser from a star. The real trick is to use a wavelength that is not generally present in space. I just don't see the high bandwidth at that range.

Jean Remy:

What about relays at Lagrangian points?

Everyone knows where those are, and if someone needs to connect they can just link into the network. I don't even think lasers would even be needed. The relays would have high gain antennas to receive the data on broadband signals, and when a ship or colony needs to link in they can query the closest available platform with a much lower-gain antenna.

If we can be in contact with various probes (like Voyager probes) at interplanetary distances, on 1970s technology, then I hardly think you would need a giant technological leap to create a system-wide comm network.

Granted you won't have the bandwidth of fiberoptic cables, so no browsing for Earth-porn from Callisto, but I don't see communication as much of an impediment. The only real reason to use lasers (that I see) would be for private (read: military) communications that you don't want intercepted, but with a target area of 90 km in radius, it's not really very private anymore.

Citizen Joe:

Viral propagation is another option. It doesn't guarantee speed or privacy. The idea would be that a message would be sent to any ship that is heading in the right direction that is within range. This could be a very long route. However, since there would likely be relatively few interplanetary vessels (compared to airplanes), Solar Space Traffic Controllers would have the full list of vessels and thus able to chart a route based on predicted paths. While the InterPlanetary Space ships would carry parcels, they would probably also serve as communication hubs for the viral network.


That sounds pretty close to how the modern internet functions, with data tracing geographically indirect paths as it goes from place to place. I like the efficiency of such a scheme: it would use the infrastructure that's already there to create an ad-hoc network backbone. On the other hand, you'd be trading out data security unless you've got some very good encryption (or just plain don't care who else reads your messages).


Ultra high bandwidth communications will be easy enough between "base stations" which can support not only powerful transmitters but also huge receiver stations (large antenna arrays, photodetectors, neutrino tanks or whatever tech we care to use). Finding a planet, asteroid or Venus Equilateral should not be too difficult, and tracking demands will be quite modest.

Moving spacecraft will be a lot more difficult to accurately track with a tight, high bandwidth beam, and they will generally not have such large scale transmitter or receiver arrays. The two exceptions would be a laser battlestation, which would simply use the targeting array and laser (dialed down to a suitable frequency), and torch ships under weigh, which can be tracked by their drive emissions. Of course putting a transmission beam through the drive plume might be difficult in itself.

Ships would probably have more need for local high bandwidth communications between ships in a constellation, or to constellations of sensor drones around the ship itself.


Good point, Thucydides.

Ships en route wouldn't have the means to relay a huge amount of data.

Perhaps station-ship-station relay networks would be more common as the system is just starting out, and later on only out in the boonies. Data routes with a high demand would soon get their own dedicated infrastructure.

One other interesting feature of intrasystem communications: the transmission-time "tides" that everything (minus parent-satellite combinations) is subject to. Mars would cycle between a 3-minute delay and a 20-minute delay for getting a signal to the earth-side servers, with its tidal cycle taking more than a year to complete. It'll be interesting to see how people deal with the variability. Here, if you purchase internet access you can expect the average connection speed to stay constant.

Jean Remy:

Viral dissemination works on Earth because of thousands and thousands of privately owned servers.

However, even in the best-case scenario of a very developed interplanetary infrastructure, I don't see a lot of traffic in space. Say two cyclers between every major colonial epicenters (say 2 for Mars, 2 for the Jovian colonies, 2 for Saturn etc...) and a few "moon hopper" shuttles, but those would be so close to their giant primaries getting a Line of Sight on them would be an issue.

However the Comm relay platforms suggested are basically Voyager probes without the scientific instrumentation and a known stable orbit. Rather that throwing your message out omnidirectionally and hope that eventually it will reach your destination (because viral dissemination is kind of like shooting blind) you simply bounce the signal of a set number of known (and if want, secure) predetermined platforms. If your goal is to reach as many people as possible (the entire point of viral dissemination in the first place) then target the Cyclers. Chances are good the passengers en route back and forth have personal computers linked in to the ship's server, which keeps updated by linking in to the platforms.

Luke Campbell:

Re: communication speed. I'm not an electrical engineer, so the following calculations may have grievous errors, but I'm going to try to throw some numbers out and see what sticks.

Let's assume a spread-spectrum laser communicator, using Rick's assumptions of 100 W and a 1 meter aperture at 1 AU. We choose a frequency spread of 125 THz centered around 1 micron infrared (roughly corresponding to 0.8 micron wavelength to 1.25 micron wavelength). We will use a 1 square meter aperture telescope to receive the signal, focused onto a detector cooled to 4 K.

This laser will deliver 4 nW/m2 at 1 AU, so we pick up 4 nW of signal.

The thermal (Johnson) noise of the detector is roughly the temperature (in energy units) times the frequency spread. 4 K is 5.5 × 10-23 J, so we have about 7 nW of noise.

The Shannon-Hartley theorem says the maximum rate of data transmission is equal to the bandwidth times the log base 2 of (1+(signal)/(noise)).

Using log_2(1+4 nW/(7 nW))=0.65, we find that we can move data at a rate of about 80 THz under optimum conditions using optimum engineering.

Note that this analysis neglects shot noise (noise due to quantization into photons) and background noise (nearby stars, zodiacal light, an operating torch drive, etc.).

An interesting thing I just noticed — in the limit of low signal to noise and assuming only thermal noise, the rate at which you can transmit data is independent of your carrier bandwidth. This is because

log_2(1+S/N) = ln(1+S/N)/ln(2)

~= (S/N)/ln(2) ~= (S/(TB))/ln(2)

where S is the signal power, N is the noise power, T is the temperature (in energy units), and B is the bandwidth. Plugging this in to the Shannon-Hartley theorem

R = B log_2(1+S/N)

for R the rate of data transmission, we can see that

R = S/(T ln(2))

In other words, the rate of data transfer depends only on your signal strength and noise in the detector. Ultimately, this is because the noise you pick up decreases linearly with bandwidth. Using a narrow-band signal also cuts down on background noise in that band. Then all that matters is dumping enough power into the signal to be heard.


Luke, Marcus, et al...all that is correct if you're using Frequency Division Multiplexing (FDM), but with Time Division Multiplexing (TDM), you can get a much higher data rate from the same bandwith...Polarizing Division Multiplexing also has a higher data transfer rate than FDM...Composite Multiplexing (CM),the combination of two or more of those, would give you a much higher data transfer rate than any single multiplexing technique. Modern-day laser communications can carry millions of channels of data in a single frequency beam; I see no reason that a resonable data rate, even at interplanetary ranges, couldn't be fesible between Earth and a theoretical Callisto colony in as little as 50 years or even less.


Luke: you assumed a 4 kelvin receiver. I believe this would require active cooling, especially on the side which is turned towards the Sun. Assuming radiative equilibrium, the temp at Earth orbit (semi-major axis) is approx. 390 K, at Mars orbit it is 320 K. These would produce some 100 times the noise, reducing the bitrate from 80 Tbps to 1 Tbps if cooling was not used.

Still, that's pretty impressive with a 100 watt laser. Presumably the relays would have more powerful transmitters, since they don't need many other systems, being automatic and orbital.

Now, if there was a conflict in our solar system, could communications be jammed? One could blind the relays and cut off communication to half the system. Laser battlestars could perform this role with their multi-gigawatt lasers from great distances away. Could you defend against this kind of jamming? Would anything get through if an opponent was shining the receiver with 1 mW/m2 while you're trying to reach it with 10 nW/m2?

Rick Robinson:

Much good information as usual, and in balance it confirms my sense that interplanetary communications will be (relatively!) cheap and plentiful.

Though it may not seem that way to people accustomed to Earth bandwidth, because current experience suggests people will use as much of the stuff as is available.

People also put up with some amazing inherent limitations, like watching TV on an iPhone.

Most ships won't need interplanetary comms, because they will be operating some region of orbital or local space, and can relay through local stations. Perhaps only deep space ships will carry such gear, and they are their own special beast anyway.

But the fact that high bandwidth interplanetary comms is not THAT hard or expensive is politically significant. As so often, technology has ruined or at least limited some rocketpunk era tropes, in this case 'seizing the relay station.'


It may have killed a trope or two, but it looks to me like we've opened up a few interesting possibilities.

I for one am not quite sure how a station or ship would deal with its comm laser module getting blinded by an enemy laserstar. Since Markus brought up that possibility I've been wondering how someone could engineer around that. You could deploy a thermal shield, and I suspect comm lasers would already do that as a matter of course. However, that could be overcome by a more powerful laser or one from the same direction as incoming transmissions, which could be countered with a better optical hood around the receiver aperture, ... etc.

Also a thought on data security (Jean Remy actually mentioned this in passing earlier). Since that hypothetical transmission beam we've been throwing around has a 90km radius after it's gone 1 AU, there isn't much to keep someone from listening in. If the intended receiver was out in the middle of deep space, it would be easy to catch the eavesdropper, but if it were happening in crowded planetary space, how do you tell who's listening in and who's just cruising by?

Luke Campbell:

Re: Comm jamming. My guess is that you would use directional information to discriminate between sources. Use a telescope, and only measure the signal from the pixels corresponding to the guy trying to communicate with you. A powerful enough laser in the same field of view could have diffraction spillover into the image of the talker, but this would require the laserstar to be nearly lined up with the talker and the receiver. Of course, a laserstar could also just burn the comm scope off, it were within range.

Re: Cooling. Yes, I assumed refrigeration down to liquid helium temperatures. I figure an extra 250 kg of helium liquifiers and cryostats and compressors and helium pumps is a small price to pay for a 50-fold increase in data rate, especially with cheap, abundant solar power. In any event, by the time orbital commerce becomes commonplace we'll probably have compact solid state chillers that can get us down to 4 K with less than a kg of mass.

Re: Comm Lasers as Weapons. I used Rick's parameters for a lasercom, which was stated as 100 W of power. A 100 W laser can be used at close focus in the near field range for industrial cutting, drilling, and welding. However, it is generally considered that you need three or more orders of magnitude more power in order to have a useful weapon.

Re: Shot Noise. On another note, I was thinking about shot noise. That 4 nW of 1 micron laser light corresponds to 20 billion photons per second. This alone limits you to no more than 20 GHz. If we assume that the laser transmits pulses that are either on or off, and we assume the absence of any background, then you need an average detection of 14 photons for a 1-in-a-million chance of a false negative (picking up zero photons during the "on" pulse). This reduces your data transfer rate to a bit more than 1 GHz. If we assume a 1 GHz bit rate (an average of 20 photons per pulse), you can set your lower limit of acceptance to 3 or more photons to take into account background or detector noise, and still achieve a bit error rate of 1-in-2.5 million.

Note that for laser light, shot noise puts a much more stringent limit on communication rate than thermal noise.


The general outlines of communications in the plausible mid-future look a bit like the Victorian era. Fast and reliable close to home, a little slower if you want to contact the next 'city' over, slower again but still reliable if you want to contact another 'country', and best of luck to you if you want to contact someone in the backbeyond. In this case city, country, and backbeyond are defined by both linear distance and orbit.

You could even end up with a Pony Express situation. Someone sets up a Planet Express torchship delivery system to carry high information-density packages or mail that requires high-security delivery... And then nine months later someone else sets up a tightbeam relay network that can handle multiple high-bandwidth high-security messages... And suddenly you've got a lot of cheap ponies on the market.

Except in this case the ponies have 45 gW/30 milligee legs.

From On Torchships comments (2010)


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.


The table below indicates the communication delay that occurs in a one-way transmission.

CircuitDistanceDelay Time
HF link (UK-NZ)~20,000 km0.07 s (67 ms)
Submarine cable(UK-NZ)~20,000 km0.07 s (67 ms)
Geosat Link (US-Aus)~80,000 km0.25 s
Earth-Moon384,000 km1.3 s
Earth-Mars55 - 378 million km3 - 21 minutes
Earth-Jupiter590 - 970 million km33 - 53 minutes
Earth-Pluto~5800 million km5 hours
Earth-Nearest Star~9.5 million million km4 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).


The speed-of-light delay from Mars to Earth means there's a long lag time between commands sent, commands executed, and confirmation. In the mission's early days, maneuvers were painfully slow and simple as the team learned how the rovers would respond. Later, the rovers got artificial intelligence upgrades to help determine interesting targets. This was helpful, because a lot can change on Mars in a few minutes — rampaging dust storms, loose soil, and other unexpected things can spoil the best-laid plans.

"Imagine driving your car to the grocery store with a light time delay like that. Nothing happens for somewhere between 4 and 20 minutes, and when it does happen, you don't know about it, because your rear window doesn't update for another 4 to 20 minutes," Maxwell says. To avoid excruciating waits, the team now programs Opportunity (and formerly Spirit) with a series of tasks, up to three days' worth over a weekend. After so many years, they're confident in the rovers' response times and capabilities, so they don't have to test every little command with exacting detail. The Curiosity mission will do that for some time, though.


     How exactly do you drive the one-ton Mars rover Curiosity, when the driver is, on average, 150 million miles away? With a one-way time delay of around 13 minutes, it certainly isn’t a matter of sitting down in front of a monitor and waggling a joystick…
     …In short, there are two ways that Curiosity can navigate the surface of Mars: NASA can transmit a series of specific commands, which the rover then dutifully carries out — or NASA can give Curiosity a target, and then trust the rover to autonomously find its own way there. In both cases, the commands are transmitted to Curiosity via NASA’s Deep Space Network — the worldwide network of big-dish antennae that NASA uses to communicate with spacecraft, and carry out some radio astronomy on the side.
     To decide which navigation method to use, NASA uses the Rover Sequencing and Visualization Program (RSVP), which is basically a Mars simulator. RSVP shows Curiosity’s current position on Mars, along with surface topology, obstacles (rocks), and so on. RSVP can then be used to plot a move (go forward 10 meters, turn 30 degrees right, go forward 3 meters) — or to pick an end point, which Curiosity will dutifully, autonomously navigate to. To safely navigate Mars, Curiosity uses its Hazcams (hazard avoidance cameras) to build a stereoscopic map of its environment, identifies which objects are too large to drive over, and then plots out a course to the end point.
     When Curiosity finishes its drive, it transmits a bunch of thumbnail images from its on-board cameras to NASA, which are then used to work out Curiosity’s exact location on Mars. This data is fed into RSVP, the next day’s movements are plotted, and so on and on.


     But the new stage that is opening up for the human drama will never shrink as the old one has done. We have abolished space here on the little Earth; we can never abolish the space that yawns between the stars. Once again, as in the days when Homer sang, we are face to face with immensity and must accept its grandeur and terror, its inspiring possibilities and its dreadful restraints. From a world that has become too small, we are moving out into one that will be forever too large, whose frontiers will recede from us always more swiftly than we can reach out toward them.

     Consider first the fairly modest solar, or planetary, distances which we are now preparing to assault. The very first Lunik made a substantial impression upon them, traveling more than two hundred million miles from Earth-six times the distance to Mars. When we have harnessed nuclear energy for space flight, the solar system will contract until it is little larger than the Earth today. The remotest of the planets will be perhaps no more than a week’s travel from Earth, while Mars and Venus will be only a few hours away.
     This achievement, which will be witnessed within a century, might appear to make even the solar system a comfortable, homely place, with such giant planets as Saturn and Jupiter playing much the same role in our thoughts as do Africa or Asia today. (Their qualitative differences of climate, atmosphere, and gravity, fundamental though they are, do not concern us at the moment.) To some extent this may be true, yet as soon as we pass beyond the orbit of the Moon, a mere quarter-million miles away, we will meet the first of the barriers that will sunder Earth from her scattered children.
     The marvelous telephone and television network that will soon emnesh the whole world, making all men neighbors, cannot be extended into space. It will never be possible to converse with anyone on another planet.

     Do not misunderstand this statement. Even with today’s radio equipment, the problem of sending speech to the other planets is almost trivial. But the messages will take minutes—sometimes hours—on their journey, because radio and light waves travel at the same limited speed of 186,000 miles a second. Twenty years from now you will be able to listen to a friend on Mars, but the words you hear will have left his mouth at least three minutes earlier, and your reply will take a corresponding time to reach him. In such circumstances, an exchange of verbal messages is possible—but not a conversation. Even in the case of the nearby Moon, the two-and-a-half second time lag will be annoying. At distances of more than a million miles, it will he intolerable.
     To a culture which has come to take instantaneous communication for granted, as part of the very structure of civilized life, this “time barrier” may have a profound psychological impact. It will be a perpetual reminder of universal laws and limitations against which not all our technology can ever prevail. For it seems as certain as anything can be that no signal—still less any material object—can ever travel faster than light.
     The velocity of light is the ultimate speed limit, being part of the very structure of space and time. Within the narrow confines of the solar system, it will not handicap us too severely, once we have accepted the delays in communication which it involves. At the worst, these will amount to eleven hours—the time it takes a radio signal to span the orbit of Pluto, the outermost planet. Between the three inner worlds Earth, Mars, and Venus, it will never be more than twenty minutes—not enough to interfere seriously with commerce or administration, but more than sufficient to shatter those personal links of sound or vision that can give us a sense of direct contact with friends on Earth, wherever they may be.

From SPACE, THE UNCONQUERABLE by Arthur C. Clarke (1962)

Not all the Relay Station's business was concerned with Earth, by any means. The interplanetary circuits passed through here: if Mars wished to call Venus, it was sometimes convenient to route messages through the Earth Relays. We listened to some of these messages … nearly all high-speed telegraphy, so they didn't mean anything to us. Because it takes several minutes for radio waves to bridge the gulf between even the nearest planets, you can't have conversation with someone on another world. (Except the Moon … and even there you have to put up with an annoying time-lag of nearly three seconds before you can get any answer.) The only speech that was coming over the Martian circuit was a talk beamed to Earth for re-broadcasting by a radio commentator. He was discussing local politics and the last season's crop. It all sounded rather dull…

From ISLANDS IN THE SKY by Arthur C. Clarke (1952)

     The safety of Earth was quite a responsibility, but it was really too big for one man to worry about. Whatever reason said, the vast imponderables of planetary politics were less of a burden than the little cares of everyday life. To a cosmic observer, it might have seemed very quaint that Sadler's greatest worry concerned one solitary human being. Would Jeannette ever forgive him, he wondered, for being away on their wedding anniversary? At least she would expect him to call her, and that was the one thing he dared not do. As far as his wife and his friends were concerned, he was still on Earth. There was no way of calling from the Moon without revealing his location, for the two-and-a-half-second time-lag would betray him at once.
     Central Intelligence could fix many things, but it could hardly speed up radio waves. It could deliver his anniversary present on time, as it had promised—but it couldn't tell Jeannette when he would be home again.
     And it couldn't change the fact that, to conceal his whereabouts, he had had to lie to his wife in the sacred name of Security.

     Jamieson tuned in the set, and rotated the antenna system toward Earth. There was a fair amount of noise from the solar background, for Earth was now almost in line with the sun, but the sheer power of the station made the message perfectly intelligible and there was no trace of fading.
     Steffanson was surprised to see that the tractor chronograph was over a second fast. Then he realized that it was set for that oddly christened hybrid, Lunar Greenwich Time. The signal he was listening to had just bridged the four-hundred-thousand-kilometer gulf from Earth. It was a chilling reminder of his remoteness from home.

From EARTHLIGHT by Arthur C. Clarke (1955)

(ed note: Aarn and Carlisle are 12 light-seconds apart)

     "Carlisle," said Aarn, and flipped up the switch. Carlisle's image appeared slowly on the screen.
     "Hello-hello-are you listening? I hope, since your Jupiter-dulled ears match your Jupiter-dulled wits, that you are. I'm about ten seconds away. You are sweeping in on line, and there is considerable anxiety among the forts. They've been trying to calculate your orbit, in at least two of them."
     "How did you know that?" snapped Aarn, and went on listening.
     "But haven't succeeded, since they can't allow for the effect of the collisions properly. Don't know your mass and all, nor how you will use your power, nor how great your power is. They think they'll be able to escape, most of 'em. They—eh—what? (Carlisle finally hears Aarn's question) Oh—we learned easily enough. We smuggled some very useful investigators (small robot spies) aboard. They haven't the new apparatus, by the way, and the Magyans here only mourn that they can't explore the whole fort. These darned things set up such a field, though, they are spotted the instant they move. The ones we're using now have been lying still now for nearly thirty-two hours. They came in along with the food supplies. We have some mighty ingenious and daring Magyans to thank for that. My idea of nerve. They made up three or four dozen crates of imitation food supply, and took them down a while ago, and planted them in Cantak, in a depot where a shipment of food for the forts was waiting. Made it and back without being spotted, thanks to the hubbub and general trouble down there, and now the investigators are loose on nearly all the forts." Carlisle paused for breath.
     "That's what I call the long-distance, nonstop, pollysyllabic, single-word speech," said Aarn, with a grin. "Fade off. I'm about to be busy. When you hear this, you'll know that merry voice of yours has been talking to a dead mike for twelve seconds." Aarn flipped his switch, and looked at the more-important scene ahead.

From THE MIGHTIEST MACHINE by John W. Campbell jr. (1934)


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.


(ed note: The meteorologist McDevitt is in a space station, the alien Dondragmer is 32 light-seconds away on the planet's surface. The Terrans are a bit over-proud of how they have a more advanced science than the aliens, but forget that meteorology of a new planet is more an art than a science)

     The meteorologist (McDevitt) obeyed without question. It would be the last time for many months that he would pay Easy that compliment. He began talking as he settled into the seat.
     “Dondragmer, you should have about nineteen hours of reduced visibility. The freezing fog should last for less than another hour; the temperature is going down
     …Near the end of McDevitt’s second sentence, long before the beginning of his message could have reached Dhrawn, a Mesklinite (Dondragmer) had approached the bridge pickup…
     …“Easy, or whoever is on watch, please get a special report to Barlennan. The temperature has gone up six degrees, to one hundred three, in the last few minutes, the ice has melted from the windows, and we are afloat.”
     Perhaps it was unkind for Dondragmer to have given his report in the human language. The time taken for translation might have eased the shock a trifle for McDevitt. The worst part, as the meteorologist said later, was realizing that his own prediction was on its way to Dhrawn and nothing could stop it. For a moment he had a wild notion of getting a ship and racing the radio waves to the planet so as to shadow them from the Kwembly’s receivers. The thought was only a flicker; only so much can be done in thirty-two seconds. Besides, none of the tenders then at the station was capable of faster-than-light flight. Most of them were used in servicing the shadow satellites.

From STAR LIGHT by Hal Clement (1970)

(ed note: here the "communication" and "reaction time" is more abstract. Due to relativistic travel, Our Hero's starship is fighting an enemy starship that basically comes from a time period with a hundred years more advanced technology. Yes, I know the starships in the novel travel FTL by "collapsar jump" but they have to enter collapsars at relativistic velocities.)

     "I can sum it up very briefly," the commodore said in a tight voice, "if only because we know so little.
     "Some ten seconds after we destroyed the enemy vessel, two objects, very small objects, struck the Anniversary amidships. By inference, since they were not detected and we know the limits of our detection apparatus, we know that they were moving in excess of nine-tenths of the speed of light. That is to say, more precisely, their velocity vector normal to the axis of the Anniversary was greater than nine-tenths of the speed of light. They slipped in behind the repeller fields."
     When the Anniversary is moving at relativistic speeds, it is designed to generate two powerful electromagnetic fields, one centered about five thousand kilometers from the ship and the other about ten thousand klicks away, both in line with the direction of motion of the ship. These fields are maintained by a "ramjet" effect, energy picked up from interstellar gas as we mosey along.
     Anything big enough to worry about hitting (that is, anything big enough to see with a strong magnifying glass) goes through the first field and comes out with a very strong negative charge all over its surface. As it enters the second field, it's repelled away from the path of the ship. If the object is too big to be pushed around this way, we can sense it at a greater distance and maneuver out of its way.
     "I shouldn't have to emphasize how formidable a weapon this is. When the Anniversary was struck, our rate of speed with respect to the enemy was such that we traveled our own length every ten-thousandth of a second. Further, we were jerking around erratically with a constantly changing and purely random lateral acceleration. Thus the objects that struck us must have been guided, not aimed. And the guidance system was self-contained, since there were no Taurans alive at the time they struck us. All of this in a package no larger than a small pebble.
     "Most of you are too young to remember the term future shock. Back in the seventies, some people felt that technological progress was so rapid that people, normal people, couldn't cope with it; that they wouldn't have time to get used to the present before the future was upon them. A man named Toffier coined the term future shock to describe this situation." The commodore could get pretty academic.
     "We're caught up in a physical situation that resembles this scholarly concept. The result has been disaster. Tragedy. And, as we discussed in our last meeting, there is no way to counter it. Relativity traps us in the enemy's past; relativity brings them from our future. We can only hope that next time, the situation will be reversed. And all we can do to help bring that about is try to get back to Stargate, and then to Earth, where specialists may be able to deduce something, some sort of counterweapon, from the nature of the damage.
     "Now we could attack the Tauran's portal planet from space and perhaps destroy the base without using you infantry. But I think there would be a very great risk involved. We might be shot down by whatever hit us today, and never return to Stargate with what I consider to be vital information. We could send a drone with a message detailing our assumptions about this new enemy weapon but that might be inadequate. And the Force would be that much further behind, technologically.
     "Accordingly, we have set a course that will take us around Yod-4, keeping the collapsar as much as possible between us and the Tauran base. We will avoid contact with the enemy and return to Stargate as quickly as possible."
     Incredibly, the commodore sat down and kneaded his temples. "All of you are at least squad or section leaders. Most of you have good combat records. And I hope that some of you will be rejoining the Force after your two years are up. Those of you who do will probably be made lieutenants, and face your first real command.
     "It is to these people I would like to speak for a few moments, not as your… as one of your commanders, but just as a senior officer and advisor.
     "One cannot make command decisions simply by assessing the tactical situation and going ahead with whatever course of action will do the most harm to the enemy with a minimum of death and damage to your own men and materiel. Modern warfare has become very complex, especially during the last century. Wars are won not by a simple series of battles won, but by a complex interrelationship among military victory, economic pressures, logistic maneuvering, access to the enemy's information, political postures-dozens, literally dozens of factors."
     I was hearing this, but the only thing that was getting through to my brain was that a third of our frends' lives had been snuffed out less than an hour before, and he was sitting up there giving us a lecture on military theory.
     "So sometimes you have to throw away a battle in order to help win the war. This is exactly what we are going to do
.      "This was not an easy decision. In fact, it was probably the hardest decision of my military career. Because, on the surface at least, it may. look like cowardice.
     "The logistic computer calculates that we have about a 62 percent chance of success, should we attempt to destroy the enemy base. Unfortunately, we would have only a 30 percent chance of survival—as some of the scenarios leading to success involve ramming the portal planet with the Anniversary at light speed."
     "I hope none of you ever has to face such a decision. When we get back to Stargate, I will in all probability be court-martialed for cowardice under fire. But I honestly believe that the information that may be gained from analysis of the damage to the Anniversary is more important than the destruction of this one Tauran base." He sat up straight.
     "More important than one soldier's career."

From THE FOREVER WAR by Joe Haldeman (1975)

Spark-gap Transmitter

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.


"And have him destroy or remove his equipment first? That would be pointless. No, we'll have to move him by subtlety, get him out on some plausible pretext and have experts check, copy and photograph every device in the place. After all, even a superman cannot send a sub-space radio call without equipment."

Rickman laughed harshly. "I suppose I'm always out of line but I still say kill him. In the first place I don't think any pretext will deceive him and, in the second, I think you're deluding yourselves. For example, and to quote from the text, you can build a non-directional sub-space radio transmitter with—again I quote—one solar cell, two Sharon compensator grids, one Larange tube and four short lengths of high resistance wire. I've no doubt our alien friend can make one with a good deal less."

From THE PRODIGAL SUN by Philip E. High (1964)

In the pictorial diagram:


(ed note: Jason dinAlt has been marooned on a colony planet that has fallen to a much lower technology level. And apparently a much lower I.Q. level as well. Anyway Jason has insinuated himself into the good graces of the local ruler, the Hertug. But Jason also has his own agenda.)

     “When will this be completed?” the Hertug asked, poking at the parts spread over Jason’s workbench.
     “Tomorrow morning, though I work all night, oh Hertug. But even before it is finished I have another gift for you, a way to improve your telegraph system.”
     “It needs no improvement! It is as it was in our forefathers’ days, and—”
     “I’m not going to change anything; forefathers always know best, I agree. I’ll just give you a new operating technique. Look at this—” and he held out one of the metal strips with the scribed wax coating. “Can you read the message?”
     “Of course, but it takes great powers of concentration, for it is a deep mystery.”
     “Not that deep; in one look I divined all its horrible simplicity.”
     “You blaspheme!”
     “Not really. Look here: that’s a B, isn’t it—two jiggles from the magic pendulum?”
     The Hertug counted on his fingers. “It is a B, you are correct. But how can you tell?”
     Jason concealed his scorn. “It was hard to figure out, but all things are as an open book to me. B is the second letter in the alphabet, so it is coded by two strokes. C is three—still easy; but you end up with Z, needing twenty-six bashes at the sending key, which is just a nonsensical waste of time. When all you have to do is modify your equipment slightly in order to send two different signals—let’s be original and call one a ‘dot’ and the other a ‘dash.’ Now, using these two signals, a short and a long impulse, we can transcribe every letter of the alphabet in a maximum of four increments. Understand?”
     “There is a buzzing in my head, and it is difficult to follow…”
     “Sleep on it. In the morning my invention will be finished, and at that time I will demonstrate my code.”
     The Hertug left, muttering to himself, and Jason finished the last windings on the armature for his new generator.

     “What do you call it?” the Hertug asked, walking around the tall, ornate wooden box.
     “This is an All Hail the Hertug Maker, a new source of worship, respect, and finance for Your Excellency. It is to be placed in the temple, or your local equivalent, where the public will pay for the privilege of doing you homage. Observe: I am a loyal subject who enters the temple. I give a donation to the priest and grasp this handle that projects from the side, and turn.” He began cranking lustily and the sound of turning gears and a growing whine came from the cabinet. “Now watch the top.”
     Projecting from the upper surface of the cabinet were two curved metal arms that ended in copper spheres separated by an air space. The Hertug gasped and recoiled as a blue spark snapped across the gap.
     “That will impress the peasants, won’t it?” Jason said. “Now—observe the sparks and notice their sequence. First three short sparks, then three long ones, then three short ones again.”
     He stopped cranking and handed the Hertug a clearly inscribed sheet of vellum, a doctored version of the standard interstellar code. “Notice. Three dots stand for H and three dashes signify A. Therefore as long as this handle is turned the machine sends out H.A.H. in code, signifying Huraoj al Heriug, All hail the Hertug! An impressive device that will keep the priests busy and out of mischief and your loyal followers entertained. While at the same time it will cry your praises with the voice of electricity, over and over, night and day.”
     The Hertug turned the handle and watched the sparks with glowing eyes. “It shall be unveiled in the temple tomorrow. But there are sacred designs that must be inscribed on it first. Perhaps some gold…”
     “Jewels too, the richer—looking the better. People aren’t going to pay to work a holy hand-organ unless it looks impressive.”
     Jason listened happily as the sparks crackled out. They might be saying H.A.H. in the local code, but it would be S.O.S. to an offworlder. And any spaceship with a decent receiver that entered the atmosphere of this planet should pick up the broad-spectrum radio waves from the spark gap. There might even be one hearing the message now, turning the loop of the direction finder, zeroing in on the signal. If he only had a receiver he could hear their answering message, but it didn’t matter, for shortly he would hear the roar of their rockets as they dropped on Appsala.

From DEATHWORLD 2 by Harry Harrison (1964)

      He pushed aside another cupboard door. Inside it were the odds and ends a man makes use of to repair the things about his house that he never notices until they go wrong. There was an assortment of wires, transistors, bolts, and similar stray items.
     "What now?" asked Bordman mildly.
     "I'm going to try to find out if there's anybody left alive over there. I'd have checked before if I'd known the colony existed. I can't prove they're all dead, but I may prove that somebody's still alive. It's barely two weeks' journey away from here. Odd that two colonies picked spots so near!"
     He picked over the oddments he'd selected.
     "Confound it!" Bordman said. "How can you check if somebody's alive some hundreds of miles away?"
     Huyghens threw a switch and took down a wall-panel, exposing electronic apparatus and circuits behind. He busied himself with it.
     "Ever think about hunting for a castaway?" he asked over his shoulder. "Here's a planet with some tens of millions of square miles on it. You know there's a ship down. You've no idea where. You assume the survivors have power—no civilized man will be without power very long, so long as he can smelt metals—but making a space-beacon calls for high-precision measurements and workmanship. It's not to be improvised. So what will your shipwrecked civilized man do, to guide a rescue-ship to the one or two square miles he occupies among some tens of millions on the planet?"
     "He's had to go primitive, to begin with," Huyghens explained. "He cooks his meat over a fire, and so on. He has to make a strictly primitive signal. It's all he can do without gauges and micrometers and special tools. But he can fill all the planet's atmosphere with a signal that searchers for him can't miss. You see?"
     Bordman thought irritably. He shook his head.
     "He'll make," said Huyghens, "a spark transmitter. He'll fix its output at the shortest frequency he can contrive, somewhere in the five-to-fifty-meter wave-band, but it will tune very broad—and it will be a plainly human signal. He'll start it broadcasting. Some of those frequencies will go all around the planet under the ionosphere. Any ship that comes in under the radio roof will pick up his signal, get a fix on it, move and get another fix, and then go straight to where the castaway is waiting placidly in a hand-braided hammock, sipping whatever sort of drink he's improvised out of the local vegetation."
     Bordman said grudgingly "Now that you mention it, of course…"
     "My space-phone picks up microwaves," said Huyghens. "I'm shifting a few elements to make it listen for longer stuff. It won't be efficient, but it will catch a distress-signal if one's in the air. I don't expect it, though."

From EXPLORATION TEAM by Murray Leinster (1956)

Morse Code

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.


      "We're picking up a beam from home," said Herndon anxiously. "But we can't make it out."

     Because the third planet of the sun Lani was being colonized from the second, inhabited world, communication with the colony's base was possible. A tight beam could span the distance, which was only light-minutes across at conjunction, and not much over a light-hour at opposition, as now. But the beam communication had been broken for the past few weeks, and shouldn't be possible again for some weeks more. The sun lay between. One wouldn't expect normal sound-and-picture transmission until the parent planet had moved past the scrambler-fields of Lani. But something had come through. It would be reasonable for it to be pretty much hash when it arrived.

     "They aren't sending words or pictures," said Herndon. "The beam is wobbly and we don't know what to make of it. It's a signal, all right, and on the regular frequency. But there are all sorts of stray noises and still in the midst of it there's some sort of signal we can't make out. It's like a whine, only it stutters. It's a broken-up sound of one pitch."
     Bordman rubbed his chin. He remembered a course in information theory just before he'd graduated from the Service Academy. Signals were made by pulses, pitch-changes, and frequency-variations. Information was what couldn't be predicted without information. And he remembered with gratitude a seminar on the history of communication, just before he'd gone out on his first field job as a Survey Candidate.
     "Hm," he said with a trace of self-consciousness. "Those noises, the stuttering ones. Would they be, on the whole, of no more than two different durations? Like—hm— Bzz bzz bzzzzzzz bzz?"
     He felt that he lost dignity by making such ribald sounds. But Herndon's face brightened.
     "That's it!" he said relievedly. "That's it! Only they're high-pitched like—" His voice went falsetto. "Bz bz bz bzzz bz bz." Bordman thought, we sound like two idiots. He said:
     "Record everything you get, and I'll try to decode it." He added, "Before there was voice communication there were signals by light and sound in groups of long and short units. They came in groups, to stand for letters, and things were spelled out. Of course there were larger groups which were words. Very crude system, but it worked when there was a lot of interference, as in the early days. If there's some emergency, your home world might try to get through the sun's scrambled-field that way."
     "Undoubtedly!" said Herndon, with even greater relief. "No question, that's it!"

     When he entered the office, Herndon sat listening to a literal hash of noises coming out of a speaker on his desk. The cryptic signal had been relayed to him, and a recorder stored it as it came. There were cacklings and squeals and moaning sounds, sputters and rumbles and growls. But behind the facade of confusion there was a tiny, interrupted, high-pitched noise. It was a monotone whining not to be confused with the random sounds accompanying it. Sometimes it faded almost to inaudibility, and sometimes it was sharp and clear. But it was a distinctive sound in itself, and it was made up of short whines and longer ones of two durations only.
     "I've put Riki at making a transcription of what we've got," said Herndon with relief as he saw Bordman. "She'll make short marks for the short sounds, and long ones for the long. I've told her to try to separate the groups. We've got a full half-hour of it, already."
     Bordman made an inspired guess.
     "I would expect it to be the same message repeated over and over," he said. He added, "And I think it would be decoded by guessing at the letters in two-letter and three-letter words, as clues to longer ones. That's quicker than statistical analysis of frequency."
     Herndon instantly pressed buttons under his phone-plate. He relayed the information to his sister, as if it were gospel. But it wasn't, Bordman thought. It's simply a trick remembered from boyhood, when I was interested in secret languages. My interest faded when I realized I had no secrets to record or transmit.
     Herndon turned from the phone-plate.
     "Riki says she's already learned to recognize some groups," he reported, "but thanks for the advice. Now what?"

     There was a stirring behind him; Riki Herndon had come silently into her brother's office. She looked pale. She put some papers down on the desk.
     "That's true," she said. "But while cycles sometimes cancel, sometimes they enhance each other. That's what's happening."
     Bordman scrambled to his feet, flushing. Herndon said sharply:
     "What? Where'd you get that stuff, Riki?"
     She nodded at the sheaf of papers she'd just laid down.
     "That's the news from home." She nodded again, to Bordman. "You were right. It was the same message, repeated over and over. And I decoded it like children decode each other's secret messages. I did that to Ken once. He was twelve, and I decoded his diary, and I remember how angry he was that I'd found out he didn't have any secrets."
     She tried to smile. But Herndon wasn't listening. He read swiftly. Bordman saw that the under sheets were rows of dots and dashes, painstakingly transcribed and then decoded. There were letters under each group of marks.
     Herndon was very white when he'd finished. He handed the sheet to Bordman. Riki's handwriting was precise and clear. Bordman read:
From CRITICAL DIFFERENCE aka SOLAR CONSTANT by Murray Leinster (1956)

Cole was listening carefully to the (Morse code) signals coming through from Pluto. "That," he decided, "sounds like Tad Nichols' fist. You can recognize that broken-down truck-horse trot of his on the key as far away as you can hear it."

"Is that what it is?" sighed Buck. "I thought it was static mushing him at first. What's he like?"

Like any really skilled operator, Cole had been sending Morse messages while he talked. Now he sat quiet waiting for the reply, glancing at the chronometer.

Nichols’ ragged signals were coming through — or pounding through. They were worse than usual, and at first Kendall and Cole couldn’t make them out. Then finally they got them in bursts. The man was excited, and his bad key-work made it worse.

“- Randing stopped. They got him I think. He said-th - ship as big - a - nsport. Said it wa - eaded my - ay. Neutrons - on instruments - he’s coming over the horizon - it’s huge - war ship I think - register - instru - neutrons - .” Abruptly the signals were blanked out completely.

Cole and Kendall sat frozen and stiff. Each looked at the other abruptly, then Kendall moved. From the receiver, he ripped out the recording coil, and instantly jammed it into the analyzer. He started it through once, then again, then again, at different tone settings, till he found a very shrill whine that seemed to clear up most of Nichols’ bad key work.

“T-247 - T-247 - Emergency. Emergency. Randing reports the - over his horizon. Huge - ip - reign manufacture. Almost spherical. Randing’s stopped. They got him I think. He said the ship was as big as a transport. Said it was headed my way. Neutrons - ont - gister - instruments. I think - is h - he’s coming over the horizon. It’s huge, and a war ship I think - register - instru ments - neutrons.”

From THE ULTIMATE WEAPON by John W. Campbell (1936)

     A gong sounded and they both jumped to their feet and raced madly into the Forlorn Hope. The ultra-receiver had come to life and the sounder was chattering insanely—someone was sending with terrific speed, but with perfect definition and spacing.
     "That's Brandon's fist—I'd know his style anywhere," Stevens shouted, as he seized notebook and pencil.
     "Tell me what it says, quick, Steve!" Nadia implored.
     "Can't talk—read it!" Stevens snapped. His hand was flying over the paper, racing to keep up with the screaming sounder.

From SPACEHOUNDS OF IPC by E. E. "Doc" Smith (1931)

(Captain) Kendall went to the main communication micro­phone and spoke. His voice went all over the Empress of Kolain from pilot room and cargo spaces to swimming pool and infirmary.

"Attention!" he said in a formal voice. "Attention to official orders!"

Kendall continued: "I believe that something never before tried is being attempted. I am forced against my better knowledge to believe that some agency is trying to make contact with us, a spaceship in flight! This is unknown in the annals of space flying and is, therefore, indicative of something important. It would not have been tried without preparations unless an emergency exists.

(ed note: this story was written before the invention of the maser, meaning radio communication across interplanetary distance was impossible.)

"However, the requirements of an officer of space do not include a knowledge of code, because of the lack of communication with the planets while in space. Therefore, I request that any person with a working knowledge of International Morse will please present himself to the nearest officer."

Minutes passed. Minutes during which the flashing lights continued.

Then the door of the bridge opened and Third Officer Jones entered with a thirteen-year-old youngster at his heels. The boy's eyes went wide at the sight of the instruments on the bridge, and he looked around in amazed interest.

"This is Timmy Harris," said Jones. "He knows code!"

"Go to it, Mr. Harris," said Kendall.

The boy swelled visibly. You could almost hear him thinking, "He called me 'mister'!"

Then he went to the table by the speaker and reached for pencil and paper. "It's code all right," he said. Then he winked at Jones. "He has a lousy fist!"

Timmy Harris began to write.

". . . course and head for Terra direct"—the beam faded for seconds—"Venusian fever and you will be quarantined. Calling CQ, calling CQ, calling CQ. Calling Empress of Kolain ... empowered us to contact you and convey ... message: You are requested to correct your course and head ... a plague of Venusian fever, and you ... Johnson of Interplanet has empowered us ... the following message: You are requested to correct your ... head for Terra direct. Calling CQ . . ."

"Does that hash make sense to you?" Jones asked of Kendall.

"Sure," smiled Kendall, "it is fairly plain. It tells us that Keg Johnson of Interplanet wants us to head for Terra direct because of a plague of Venusian fever that would cause us to stay in quarantine. That would ruin the (cargo of) line moss. Prepare to change course, Mr. Jones!"

"Who could it be?" Jones asked foolishly.

"There is only one outfit in the Solar System that could possibly think of a stunt like this. And that is Channing and Franks. This signal came from Venus Equilateral!"

"Wait a minute," said Timmy Harris. "Here's some more."

"As soon as this signal ... intelligible ... at right angles to your course for ten minutes. That will take ... out of ... beam and reflected ... will indicate to us ... left the area and know of our attempt."

"They're using a beam of some sort that indicates to them that we are on the other end but can't answer," Kendall said. "Mr. Jones, and Pilot Canton, ninety degrees north for ten minutes! Call the navigation officer to correct our course. I'll make the announcement to the passengers. Mr. Harris, you are given the freedom of the bridge for the remainder of the trip."

(ed note: it is the least he could do, since Timmy Harris just saved Interplanet several million dollars.)

Mr. Harris was overwhelmed. He'd learn plenty— and that would help him when he applied for training as a space officer; unless he decided to take a position with Venus Equilateral when he grew up.

From CALLING THE EMPRESS by George O. Smith (1943)

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.

Straight Key
Signal is "on" when knob is pressed and "off" when released. Length and timing of dots and dashes are controlled by the human operator. In other words it is a fancy on-off switch.
Semiautomatic Key (aka telegrapher's "bug")
Press paddle to the right with the thumb and it automatically generates a series of dots (length and timing are controlled by positioning the sliding weight at the rear). Press the paddle to the left with the knuckle of the index finger and it makes a single dash just like a conventional straight key. A mirror image bug is used by left-handed telegraphers. The Vibroplex contains no electronics, the series of dots is generated mechanically by an oscillating weight on a spring. The Vibroplex company never figured out a way to mechanically generate a series of dashes, which admittedly is a challenge (dash of three units, space of one unit, dash of three units, etc.).
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.
Iambic Paddles
Used with an electronic keyer to generate high-speed Morse code. The Vibroplex uses an oscillating weight to generate signals, the electronic keyer uses microprocessor. Pressing the right paddle generates a series of dashes, pressing the left paddle generates a series of dots, squeezing both paddles generates a dot-dash-dot-dash sequence. The action is reversed for left-handed telegraphers.
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.

Rufus Macquarie saw it all happen above the black ridgeline of the Brooks Range in Rufus operated a mine there. On clear nights he would drive his pickup truck to the top of a mountain that he and his men had spent the day hollowing out. He would take his telescope, a twelve-inch Cassegrain, out of the back of the truck and set it up on the summit and look at the stars. When he got ridiculously cold, he would retreat into the cab of his truck (he kept the engine running) and hold his hands over the heater vents until his fingers regained feeling. Then, as the rest of him warmed up, he would put those fingers to work communicating with friends, family, and strangers all over the world.

And off it.

After the moon blew up, and he convinced himself that what he was seeing was real, he fired up an app that showed the positions of various natural and man-made celestial bodies. He checked the position of the International Space Station. It happened to be swinging across the sky 260 miles above and 2,000 miles south of him.

He pulled a contraption onto his knee. He had made it in his little machine shop. It consisted of a telegraph key that looked to be about 150 years old, mounted on a contoured plastic block that strapped to his knee with hook-and-loop. He began to rattle off dots and dashes. A whip antenna was mounted to the bumper of his pickup truck, reaching for the stars.

Two hundred sixty miles above and two thousand miles south of him, the dots and dashes came out of a pair of cheap speakers zip-tied to a conduit in a crowded, can-shaped module that made up part of the International Space Station.

(ed note: Rufus' daughter Dinah is currently a mission specialist on the International Space Station (called "Izzy"))

Rufus, a die-hard ham radio enthusiast who still communicated in Morse code with a dwindling circle of old friends all over the world, had pointed out that radio transmission between the ground and Izzy was actually rather easy, given that it was line-of-sight (at least when Izzy happened to be passing overhead) and that the distance was nothing by ham radio standards. Since Dinah lived and worked in a robot workshop, surrounded by soldering gear and electronics workbenches, it had been a simple matter for her to assemble a small transceiver following specifications provided by her dad. Zip-tied to a bulkhead, it dangled above her workstation, making a dim static hiss that was easily drowned out by the normal background roar of the space station’s ventilation systems. Sometimes it would beep.

At any rate her focus was either on the screen or on the robots, and so it had been for many hours. Until a string of beeps came out of the hissing speaker zip-tied to the bulkhead, and her eyes went momentarily out of focus as her brain decoded the dots and dashes into a string of letters and numbers: her father’s call sign. “Not now, Pa,” she muttered, with a guilty daughter’s glance at the brass-and-oak telegraph key he had given her — a Victorian relic purchased at great price on eBay, during a bidding war that had placed Rufus into pitched battle against a host of science museums and interior decorators.


“Not now, Pa, I know the moon’s pretty, I’m right in the middle of debugging this method . . .”



And then she brought her face close to the window and twisted her neck to find the moon. She saw what used to be it. And the universe changed.

From SEVENEVES by Neal Stephenson (2015)

Communication Tips

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.

NATO Phonetic Alphabet

Space's Runic

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.

Brevity Words

This is only a short list. For a fuller list go here.

Directive to cease action/attack/event/mission
Let me know that you have received and understood this message
Directive to initiate a briefed attack sequence or maneuver
Yes, or permission granted.
Request for bearing and range to described point
Orbit about a specific point; ground track flown by tanker. Information call indicates a turning engagement about a specific location.
Request/comment regarding target aspect information.
To request or provide a response for a coded challenge.
Aircrew is operating without benefit of GCI/AWACS control.
{x} BENT
Identified system inoperative.
Prebriefed fuel state which is needed for recovery using prebriefed parameters.
No visual contact with friendly aircraft; opposite of term "VISUAL."
Directive/informational call that indicates aircraft will continue straight ahead at the merge and not turn with target/targets.
A radar/visual contact whose identity is unknown.
Request for target information as briefed/available.
To indicate the separation between portions of the messages. (To be used where there is no clear distinction between the text and other portions of the message).
BREAK {Up/Down/Right/Left}
Directive to perform an immediate maximum performance turn in the indicated direction. Assumes a defensive situation.
Term used to denote radio frequency is becoming saturated/degraded and briefer transmissions must follow.
BUGOUT {Direction}
Separation from that particular engagement/attack; no intent to reengage.
Change to channel ....... before proceeding.
Begin using briefed radio procedures to counter comm jamming.
Directive to briefly turn on exterior lights to enable visual acquisition.
Requested action is authorized (no engaged/support roles are assumed).
Ordnance release not authorized.
Ordnance release is authorized.
My version is ______. Is that correct?
Radar/IR contact at the stated position; should be in bearing, range, altitude (BRA), Bullseye, or geographic position format.
An error has been made in this transmission (message indicated). The correct version is ________.
Directive for the flight to maneuver to briefed positioning.
Proceed to alternate mission/base.
Maneuvering with the intent of achieving a kill. If no additional information is provided (bearing, range, etc.), ENGAGED implies visual/radar acquisition of target.
Proceed with your message.
GREEN {Direction}
Direction determined to be clearest of enemy air-to-air activity.
How well do you receive me?
Self-explanatory (use instead of “I repeat”).
Unpredictable maneuvers to negate a gun tracking solution.
Fuel state above Bingo at which separation/bugout/event termination should begin.
The spoken word for the distress signal. Lives in danger.
Is the spoken word for the distress relay signal.
No, or that is not correct, or I do not agree.
Aircrew does not have visual contact with the target/bandit; opposite of term "TALLY."
OFF {Direction}
Informative call indicating attack is terminated and maneuvering to the indicated direction.
My transmission is ended and I expect a response from you.
Conversation is ended and no response is expected.
The spoken word for the urgency signal. Trouble, but not mortal danger.
During long distress situations, communications can resume on a restricted basis. Communication is to be restricted to ship’s business or messages of a higher priority.
Repeat all of this message back to me exactly as received after I have given OVER. (Do not use the word “repeat”.)
I have received all of your last transmission.
I have received your message number ...
Target destroyed (air-to-air); weapons impact (air-to-ground).
I must pause for a few seconds or minutes, please wait.
Self-explanatory. (Do not use the word “repeat”.)
Is the spoken word for the safety signal.
Indicates that silence has been imposed on the frequency due to a distress situation.
Is the international expression to advise that a distress situation is in progress. This command comes from a vessel or coast station other than the station in distress.
Is the international expression for a distress cancellation.
Is the international expression to advise that a distress situation is in progress. The command comes from the ship in distress.
Request for an individual's tactical situation; response is normally "offensive," "defensive," or "neutral." May be suffixed by position and heading.
Turn off equipment indicated.
Sighting of a target/bandit; opposite of "NO JOY".
Hostile antiship missile
Check coding, check text with originator and send correct version.
{x} WELL
Described equipment is functioning properly.
Fire only;
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.
  1. As a request: Communication is difficult, please send each word twice.
  2. As information: Since communication is difficult, I will send each word twice.

"Mission Control, this is X-ray-Delta-One. At two-zero-fo-wer-fife, on-board fault prediction center in our niner-triple-zero computer showed Alpha Echo tree fife unit as probable failure within seventy-two hours. Request check your telemetry monitoring and suggest you review unit in your ship systems simulator. Also, confirm your approval our plan to go EVA and replace Alpha Echo tree fife unit prior to failure. Mission Control, this is X-ray-Delta-One, two-one-zero-tree transmission concluded."


"X-ray-Delta-One, this is Mission Control, acknowledging your two-one-zero-tree. We are reviewing telemetric information on our mission simulator and will advise.

"Roger your plan to go EVA and replace Alpha-Echo tree-fife unit prior to possible failure. We are working on test procedures for you to apply to faulty unit."

From 2001 A SPACE ODYSSEY by Sir Arthur C. Clarke

Dak was busy most of the time at the ship's communicator, apparently talking on a very tight beam for his hands constantly nursed the directional control like a gunner laying a gun under difficulties.

From DOUBLE STAR by Robert Heinlein, 1956

"Stand by to record a signal to Lieutenant Venizelos at Basilisk Control for immediate relay to Fleet HQ. Fleet scramble, no encryption. Priority One."

Heads turned, and Webster's swallow was clearly audible.

"Aye, aye, Ma'am. Standing by to record."

"`Mr. Venizelos, you will commandeer the first available Junction carrier to relay the following message to Fleet HQ. Message begins: Authentication code Lima-Mike-Echo-Niner-Seven-One. Case Zulu. I say again, Zulu, Zulu, Zulu. Message ends.'" She heard McKeon suck air between his teeth at her shoulder. "That is all, Mr. Webster," she said softly. "You may transmit at will." Webster said absolutely nothing for an instant, but when he replied, his voice was unnaturally steady.

"Aye, aye, Captain. Transmitting Case Zulu." There was another brief pause, then, "Case Zulu transmitted, Ma'am."

"Thank you." Honor wanted to lean back and draw a deep breath, but there was no time. The message she'd just ordered Webster to send and Venizelos to relay to Manticore was never sent in drills, not even in the most intense or realistic Fleet maneuvers. Case Zulu had one meaning, and one only: "Invasion Imminent."

From ON BASILISK STATION by David Weber, 1994

Memeweave: News/Stargate Project/Recontact/Broadcast (Read Only)
Source: CS Prince of Octania
Classification: WHITE (General Access)
Encryption: None
Distribution: Everywhere (News/Live)
As received at: Relay Station (Eliéra-Seléne L3)
Language: Eldraeic I
Time of transcript: 3953:10:04:9-01:30.2 (Imperial Standard Time); time set by relay
Light lag (at source): 0.432 p
Light lag (overall): 9.776 kp

WARNING: Source link is down, attempting to reestablish…
WARNING: Source link is down, attempting to reestablish…
WARNING: Source link is down, attempting to reestablish…

INFORMATIONAL: Link restored.

WARNING: New source routing address is within an unallocated/experimental range (system override).
WARNING: Experimental flag is set on routing protocol.
WARNING: Link fleetrelay-a/bigocty/tangle0 is not available; using fallback route.

— bridge transcript continues —

[1] …navigation systems restored. Starlocks are maintained, nominal errors. Sunlock is Finíällë.

[2] Drift?

[3] Commander? Communications are restored. Tangle is scrambled as expected, but we have a gate relay carrier. Auto-reestablish is running.

[2] Very well.

[1] This is ballpark, but estimate drift close to 60 kay spatial.

[2] And temporal?

[1] Frame lock was tight. No greater drift, certainly.

[2] Good. Get me a course for –

[4] Conn, Sensor Ops. Status change. We show three hot spots at twelve hundred kay, not under thrust, looks like they’re keeping gate-relative position. Designating them Target Vekar.

[2] Belay that course. Sensor Ops, give me –

[4] Conn, Sensor Ops, further status change. Target Vekar just lit off its drives. Acceleration’s still changing, but it’s compatible for going for a zero-zero with us if we hold position.

[2] Very well, Sensor Ops. Give them a lidar ping. Don’t go for a hull map, just let them know we’re here.

Put us on a heading to zero-zero with them at best time, as their course settles. Execute at two-thirds cruising.

[1] Course laid in.

[5] Executing at two-thirds cruising, aye.


[5] Two-thirds cruising established.

[6] Growler!

[2] Confirm target status.

[6] No status change on target. Steady illumination.

[2] That might not be a targeting laser. Comms, do a modulation check.

[3] Checking… yeah, I’m seeing pulses. It’s not in the standard bands, so the software missed it. Rerunning now.

Nothing on standard set, nothing on contact set. Ah, wait one. Checking the archives.

And we have a communication request. Old protocol. Watchvid abbrev stack, commercial-comm version, fourth revision, standard dated three-oh-one-eight.

[2] You’re surprised, Mr. Serquel? That’s got to be the latest version we have in common. Integrate, and put it up on two.

WARNING: Memeweave contains rebroadcast content received via obsolete IIP version (29 revisions in deficit) (system override).
WARNING: Memeweave contains rebroadcast content using obsolete trivid protocol flagged for discard (system override).
WARNING: Memeweave contains rebroadcast content with unverifiable embedded authentication keys.

[2] This is Flight Commander Miruna Dalael, Imperial Navy, aboard CS Prince of Octania. I believe you were expecting us? Over.

[3] Transmission lag 8.9 pulses.

[C] This is Flight Commander Adévis Amarens, Víëlle Star –

Correction. This is Flight Commander Adévis Amarens, Imperial Navy, aboard CVS Backup’s Pride. Welcome to Víëlle System. If you’ll take a course from us, there’s a party waiting for you when we reach Víëlle orbit. Over.

[2] Glad to, Flight Commander, and we’ll bring the beer. Dalael, clear.

Secure from alert stations. Stand down to condition three. And have my gig made ready for transit at intercept.

From OLD FRIENDS by Alistair Young (2015)



(ed note: Col. Hal Gascoigne is the sole occupant of Satellite Vehicle 1, in charge of a sizable portion of the United States nuclear arsenal. His orders are to stand by, and drop nuclear warheads as per coded commands. Unfortunately the stress and the solitude have driven him totally bat-poop insane.)

     The hammering of the teletype blanked it out. The noise was as loud as a pom-pom in the echoing metal cave. He got up and coasted across the deck to the machine, gliding in the gravity-free cabin with the ease of a man to whom free fall is almost second nature.
     The teletype was silent by the time he reached it, and at first the tape looked blank. He wiped the sweat out of his eyes. There was the message.


     He got out his copy of The Well-Tempered Pogo and checked the speeches of Grundoon the Beaver-Chile for the key letter-sequence on which the code was based. There weren’t very many choices. He had the clear in ten minutes.


     There it was. That was what he had been priming the bomb for. But there should have been earlier orders, giving him the go-ahead to prime. He began to rewind the paper.
     It was all blank.
     And Washington? Why would the Joint Chiefs of Staff order him to bomb Washington?

(ed note: For the record, Grundoon is a baby groundhog, not a beaver. And while there are many collections of Pogo comic strips, The Well-Tempered Pogo is not one of them.)

From KING OF THE HILL by James Blish (1955)

Public-key Cryptography

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:

[1] Encrypting messages
This allows anybody to send an encrypted message to you that only you can read.
[2] Digital signature
This allows you to send a message to everyone with a "signature" proving you were the one who sent it. It verifies your identity.

[1] 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.

[2] 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 [1] 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 [2] 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:

  1. Decrypt the encrypted hash with your public key
  2. Generate a hash of the plaintext document
  3. If the two hashes are identical, the digital signature is valid

Certificate Authority

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.

Automatic Identification System

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.

John Reiher: I had a thought. There's one component that never changes on any ship. It can be updated, but it can't be replaced, otherwise the Insurance companies will void your policies.

What is it?

The Vehicle Identification Number (VIN) Box. It's a transponder with a unique ID and call sign. It can be customized at time of purchase, but it is the "ship". Anything attached to it becomes the ship.

No VIN Box, no ship.

Two or more VIN Boxes on a single ship, you're breaking the law and you have to inactivate all but one of them.

This is something the Banking and Insurance companies would come up with. They need something that can be unique to each ship, and nothing is more unique than a government issued, sealed, black box VIN Box.

Winchell Chung

+John Reiher idea for a VIN Box is probably my favorite. It is very much the sort of thing that banking and insurance companies would come up with. And the requirement for renewability makes perfect sense. However it must always contain a legal ID, for liability purposes. Much like automobiles. If somebody crashes their car into a building or something else expensive, then flees the scene on foot, the car's license plate may be expired but it still allows the police and building owners to discover who is liable for the damages.

This also vaguely reminds me about ship transponders in the Traveller role playing game. They constantly broadcasts the ship's unique ID and location. Civilian starships are legally required to have transponders always turned on, unless there are extenuating situations. Such as pirate corsair ships in the area, using the transponder to home in on their prey.

Alistair “Cerebrate” Young Side note on registries and transponders: of course, the Worlds being a not-exactly-unified group of polities, actual requirements on these points vary widely.

Even leaving aside the anarchic Rim Free Zone (the entirety of whose admiralty law could be summed up as "try not to hit anything that might complain"), the Accord on the Law of Free Space leaves it at the minimal "you should have a certificate of registry and a transponder that will squawk it out when queried". Local regulations, on the other hand...

(The Empire, for example, which holds that sovereignty begins with the individual, will happily accept self-signed registries and doesn't require much transponding, although lacking functionality in this area may leave you restricted to operating VFR and staying outside all regions of controlled space.

The other end of this particular bell curve is the Hope Hegemony, which wants transponders willing to disgorge pretty much any information you can think of on demand, including your code-signed visa from the Hegemony Bureau of Navigation and remote-slave ackles for your starship, and legally defines any vessel without such as "debris, subject to salvage and/or destruction at discretion".)

(ed note: "ackles" is Access-Control List. "Remote-slave ackles" means to grant the Hope Hegemony government the ability to seize command of your ship and fly it by remote control, at their whim)

John Reiher

Also, I'm using a setting where while there are multiple governments, they do have treaties with each other and have agreed upon a common ship's registry system just to keep the confusion down and to prevent what you proposed: Smugglers with multiple transponders.

From private conversation on Google Plus (2015)

The Xinglong died stupid. Afterward, everyone knew she was one of thousands of small-time rock-hopping prospector ships. The Belt was lousy with them: five-or six-family operations that had scraped together enough for a down payment and set up operations. When it happened, they'd been three payments behind, and their bank—Consolidated Holdings and Investments—had put a lien on the ship. Which, common wisdom had it, was why they had disabled her transponder. Just honest folks with a rust bucket to call their own trying to keep flying.

If you were going to make a poster of the Belter's dream, it would have been the Xinglong.

The Scipio Africanus, a patrol destroyer, was due to head back down toward Mars at the end of its two-year tour of the Belt. They both headed for a captured cometary body a few hundred thousand kilometers from Chiron to top off their water.

When the prospecting ship first came in range, the Scipio saw a fast-moving ship running dark and headed more or less in their direction. The official Martian press releases all said that the Scipio had tried repeatedly to hail her. The OPA pirate casts all said it was crap and that no listening station in the Belt had heard anything like that. Everyone agreed that the Scipio had opened its point defense cannons and turned the prospecting ship into glowing slag.

(ed note: our Belter heroes have secretly escaped the destruction of the Martian battleship Donnager by flying away in one of the carried vessels, the Tachi. They are now trying to figure out what to do next.)

"Someone is going to investigate what happened to the Donnager." Holden said. "Martian ships are speeding to that spot as we speak. They'll already know the Tachi got away, because our transponder is blabbing our survival to the solar system at large."

"No it ain't," Alex said.

"Explain that, Mr. Kamal."

"This is a torpedo bomber. You think they want a nice transponder signal to lock on to when they're makin' runs on an enemy capital ship? Naw, there's a handy switch up in the cockpit that says 'transponder off.' I flipped it before we flew out. We're just another moving object out of a million like us."

Holden was silent for two long breaths.

"Alex, that may be the single greatest thing anyone has ever done, in the history of the universe," he said.

"But we can't land, Jim," Naomi said. "One, no port is going to let a ship with no transponder signal anywhere near them, and two, as soon as they make us out visually, the fact that we're a Martian warship will be hard to hide."

"Yep, that's the downside," Alex agreed.

From LEVIATHAN WAKES by "James S.A. Corey" (Daniel Abraham and Ty Franck) 2011.

(ed note: comment is about civilian starship Automatic Identification System in the game Traveller)

I think I'm going to add a line in the rules about a ship being considered "Active" when it either jumps into, or out of, a system.

Ships can still be sneaky—it just requires that they jump into a system at least 2-3 light seconds away from another vessel. If you want to be stealthy, jump into a system at very long range and take your time approaching your victim.


A ship's transponder will consider most ships as "Active", and if you detect a vessel that doesn't have it's transponder "on", then he's probably up to no good.

No transponder broadcast = Red Flag.

It's Imperial Law to keep your ship's transponder broadcasting at all times, but I look upon this like the speed limit on a real life freeway. Just as speed governors on cars are illegal but there's a speed limit, ship's are required to keep their transponders broadcasting at all times but can turn them "off".

The reason they can be turned "off" is probably because of pirate activity. But, finding a ship with a transponder not broadcasting is a sure-fire way to get boarded (and fined) by a system patrol boat.

Heck, the GM could take this another step and set up "transponder traps" in the same way we have "speed traps" here in the real world.

"Pysadi? Naw, can't stand that piddly TL world. They don't have an inter-system Navy, and sometimes pirates make a run through that system...but the Pysadians sure don't have a problem using one of their three ancient patrol cutters to board ya and fine ya for being safe and runnin' with your transponder down."

From comment to thread CLASSIC TRAVELLER SENSOR RULES (2006)

(ed note: comment is about civilian starship Automatic Identification System in the game Traveller)

I'm in agreement with most of what has been posted. In my Traveller universe, civilian transponders ...

  1. are mounted in the same container as the ship's black box recorder
  2. are hard wired to the ship's powerplant and supplimented by a battery backup
  3. are nearly impossible to access mechanically by design, due to location in the hull and interference from other equipment
  4. emit a code referencing ship's VIN and port of registry
  5. are electrically isolated from the ship's computer and require specialized (i.e. controlled) equipment to access electronically (e.g. anti-theft features on consumer products)

Military craft have transponders that can be manipulated at will to produce any number of emissions; from merchants to battleships to enemy IDs to nothing at all.

This included active duty scout craft, as altering an electronic signature is a piece of cake for a sensor platform. Reserve scout craft are normally retro fitted with civilian style transponders; but as an after-market modification, most Scout engineers know how to bypass them without breaking the seals.

From comment to thread TAMPERING WITH TRANSPONDER: ILLEGAL, YES... BUT HOW HARD? by Ran Targas (2006)

(ed note: comment is about civilian starship Automatic Identification System in the game Traveller)

Also don't forget that vessels can turn off their transponders at will. The result of those 'blank' periods depends on a whole host of factors. Turn off your transponder while inbound to Lunion and expect a full body cavity search. Have a customs inspection of your transponder's records reveal that you ran 'quiet' off Jasedipere during a corsair raid and you'll be credited for quick thinking.

It appears that 'tampering' with a transponder is much like everything else. It depends on who you know and why you want to do it.

From comment to thread TAMPERING WITH TRANSPONDER: ILLEGAL, YES... BUT HOW HARD? by Whipsnade (2006)

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