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I have a story set on a habitable but otherwise unpopulated alien world with a breathable atmosphere in which two opposing groups of survivors of a crash each try to seize control of an orbiting mothership.

One of these two groups hides out in jungle and mountain hideouts and uses 3D printers contained within hidden lander craft to build equipment and other resources, with the help of a limited supply of onboard raw materials such as refined metals and rare earths provided for that purpose.

In order to achieve their aims, these people hiding out in the jungle - let's call them 'rebels' - want to build a long-range, ad hoc radio network to communicate with each other, using a limited number of components printed on board these landers.

It's my understanding that most worlds with an atmosphere have an ionosphere, and that the ionosphere can be used to 'bounce' a radio signal beyond the visible horizon to a distant receiver. But I'm not certain how reliable that actually is, depending, I guess, on the amount of power available to a radio transmitter.

In order to avoid being captured, I imagined these rebels would likely rely on portable radio equipment that might otherwise be triangulated and then captured by their enemies, and also, possibly, on ad hoc radio repeaters placed at very high points (trees, mountains) in order to further extend their radio network. They might (or so I imagined) sometimes carry these repeaters with them, as they explore further away, leaving them behind at high points during their exploring to use as relays back to base. And, also, because the limits to their manufacturing capabilities mean they only have a limited number of repeaters.

I'm not remotely an expert on radio technology, and I'm sure there must be numerous flaws in all this I need to address.

It occurred to me to wonder whether 3D printing batteries might be feasible, to power the radios, or whether such things might be hand-cranked in the absence of batteries. Batteries, or perhaps some form of jury-rigged solar panel, would presumably be necessary for relays and/or repeaters, since these would be most often left on their own.

I also wondered about how they might avoid detection, and whether some form of encryption might be possible, although it occurred to me so long as they had agreed-on code words, that might not be necessary.

Any help, suggestions, gratefully received. Over and out.

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  • $\begingroup$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. $\endgroup$
    – Community Bot
    Dec 24, 2023 at 9:42
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    $\begingroup$ "Depending, I guess, on the amount of power available to a radio transmitter": The amount of power needed for long-distance communications depends on the wavelengths used. For longwave transmission, one or two megawatt trasmitters were once common. While there are but few longwave stations still alive, longwave time signals are still in use, for example the well-known (in Europe) 50 kW German DCF77. On the other hand, 100 W is aplenty for communicating between America and Australia on shortwave. $\endgroup$
    – AlexP
    Dec 24, 2023 at 12:09
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    $\begingroup$ Many ham operators regularly use less than 5 Watts for contacts, including across oceans. $\endgroup$
    – Jon Custer
    Dec 24, 2023 at 16:28
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    $\begingroup$ The question exhibits a blind spot I've seen in several stories featuring advanced but limited 3D printers (specifically thinking of the Bobiverse novels), particularly the suggestion that they might avoid batteries because the printers couldn't make them. One of the most useful applications of 3D printing is production of tools and equipment, not final products. If it's reasonably capable, they can certainly use it to make what they need to manufacture batteries...maybe not the most sophisticated ones available, but good enough that they don't have to resort to hand cranks. $\endgroup$ Dec 24, 2023 at 21:05

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The major challenge is manufacturing of the radio modules

Current 3d printing technology is rather limited. 3d printers are not magic machines that you can press "print" and have finished technology come out of it. At the moment, the absolute top-of-the-line machines that go for seven plus figures are room-sized machines that can produce highly detailed small parts out of a single (or multiple types) of polymer and there are other machines in a similar price/scale segment that can do the same but print metal instead.

Regardless, even if we assume that future 3d printers can do advanced multi-material printing and even print macro-scale passive electrical components (wiring/pcbs, resistors, capacitors, inductors), there are two categories of components that I don't think will be readily printable:

  • Semiconductor elements: Transistors, ICs (computer chips), solar panels, etc. Producing something like a CPU today takes literally a planet-wide supply chain and manufacturing systems that, in total, are probably the size of small cities and employ people appropriately. The idea that in any near future setting this process will have been miniaturized enough that a generalist machine you'd have in a landing craft can do this makes me highly skeptical (unless there's been a major gray-goo breakthrough or something). Maybe capital ships or large dedicated repair ships have the capacity to produce ICs, but for everyone else, the complexity/utility balance would just be too poor.

  • Energy-containing elements: 3D printing today manipulates the energy state in the material being printed. Classic FDM printing heats plastic to melt it and then cools it in the desired shape while something like metal SLS uses a laser to melt metal powder into a desired shape. I think that despite massive leaps forward in 3d printing tech, this will remain something of a constant and this means that printing things that are chemically or thermally reactive like explosives or energy-dense batteries will probably be a big no-no, and these will be manufactured en-mass in traditional specialized assembly facilities.

Fortunately, both of these issues wouldn't be unknown to the people of this future, and are rather easy to counter by simply bringing stocks of general-purpose items that can be installed into otherwise 3d-printed technology.

In particular, ICs have basically no weight and volume, so I think it's totally reasonable that every fabrication kit or emergency supply dump in a setting like this would have ten thousand or so cheap multi-purpose programmable ICs in a shoe-box-sized container exactly for this purpose.

If I were prepping such a kit today, my first instinct would be to pack a bunch of ESP32 chips. They're about as big as two grains of rice each, have enough compute to do basically any small task, require very little power, natively do wifi/bluetooth, have very generous GPIO and communication protocol support, and cost like less that $2 a piece.

Your future tech people probably have access to $2 chips that are orders of magnitude better, so basically have all the capabilities of a modern laptop or so in a similar sized package.

Once you have small chips like these, then solving this radio issue wouldn't be very difficult.

How I would do it today

If I were a guerilla movement and I wanted to set up a communication network on a "virgin" planet that doesn't already have comms satellites or a cell network, here are a bunch of decisions that I'd make when deciding on network topology and structure:

Mesh vs Centralized

In a mesh network, you have many different nodes that only 'talk' to their close neighbors. This is useful, especially in a hostile environment, because radio-direction-finding is a trivial task. If you set up a large radio mast that regularly transmits, the enemy will know where you are basically instantly. Today's militaries even have dedicated weapons that are basically fire-and-forget systems that automatically home in on radio emission sources and blow them up. Instead, an approach where you have many hundreds or thousands of small transmitters is desirable, because this offers robustness (a single node failing or being destroyed isn't a big issue) and it denies the enemy the juicy target of a large radio mast.

Analog vs Digital

When most people think of radio, they think of analog radio: people talking with crunchy voices and saying over along with NATO-alphabet words. The primary advantage of analog radio is that it's very simple, but with your 3d printers and stockpile of basic computer chips, the difficulty difference is basically gone. It is also not stealthy, at all. By switching to digital, you can use many more operational security techniques and operate a much safer system in a hostile environment. Specifically:

  • Encryption: Digital radio is trivial to encrypt, and encryption today is good enough that it's basically a solved issue. You could even set it up in such a way that using the radio requires the operator to punch in a decryption key periodically so that even if the enemy captures your physical radio, they still can't use it, and even if they capture your radio and your operator, you make sure to change the decryption key every day so they get cut out that way.

  • Packet Radio: Instead of continuously transmitting while someone is talking, instead send what are essentially text messages as primary communication, or if needed, recorded audio messages. This way, the entire message can be compressed into a radio-chirp that's only a couple milliseconds long. At the receiving end, this message then gets "unpacked". This makes it significantly harder for the enemy to locate transmitters, because (compared to continuous talking), they need to listen all the time for very short radio "chirps" that are then passed around the mesh network. This would make it difficult to separate unmanned mesh nodes from ones that are being carried and make locating via radio much more difficult.

  • Frequency Hopping: This is actually how modern military radios maintain security (to a degree). Basically, the radios constantly hop from one frequency to another in a way that looks random from the outside. From the outside, this looks like white noise or pure randomness, but to someone who "knows" the pattern beforehand, it is trivial to tune into the right frequencies at the right time.

  • Low Power: There's always background radiation which causes noise. A mesh network can keep power low enough that it becomes essentially impossible to detect at range because of the inverse-square properties of radio waves. Even the best radio-direction finder would only be able to detect a couple close nodes on the mesh network.

Physical construction

I'd make each mesh-node module a cheap and semi-disposable element, that can just be scattered through the forest at random or even dropped by a drone or something. Today we have LoRa Products like this one, which costs only like 30 bucks and can do most of what you'd need it to do. It's a digital packet radio with an individual range of maybe 2km in good conditions and ready for mesh-networking.

LoRa Module

Assuming that these future people have generic chips that can be programmed to have this function and a stockpile of coin-cell sized batteries to power each module for a month, they could print up bottle-cap-sized enclosures that hold the chips, some extra components like antenna, and have a place to mount the battery all in a watertight case. Then they can just walk around the forest, tossing them all over the place.

If they really want to get tricky, they could even make these mesh-nodes mobile by attaching them to local wildlife by printing them with bristles so they get caught in fur or hiding them in food so that the wildlife eats them and carries them around to really confuse the enemy's tracking and intelligence efforts.

Conclusion

It should be possible for the rebels to craft a secure and encrypted stealthy mesh network with only what they have on hand--a prepared group/individual can do this today. I suggest you learn a bit more about radio fundamentals and operational security, as this would probably help you a lot.

There are plenty of decent youtube videos that cover this specific topic too: if you can ignore the politics, there are some "prepper" or "survivalist"-types that make in-depth content about how to setup a radio communication system in a hostile environment after the apocalypse.

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  • $\begingroup$ Fantastic answer! $\endgroup$ Dec 24, 2023 at 15:37
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You are severely underestimating the effort and the amount of trials it takes to explore an unknown world, let alone building relay stations on it.

To build a station you want to be sure the place is suitable, and you can't do that with a couple days or hours stay.

Together with that you don't want to fully supply every scout sent to explore with the necessaire to build a relay station: they can't travel fully loaded and if they die along the way you have lost your products, wasting scarce resources.

You are better off having portable repeaters which you deploy as you need.

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    $\begingroup$ A shortwave transmitter can be set up basically anywhere, except possibly in a cave, in a canyon or some sort of deep valley. And even vacuum tube shortwave transmitters are definitely man-portable. $\endgroup$
    – AlexP
    Dec 24, 2023 at 12:12
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    $\begingroup$ They've been stuck on the planet for some years now, in the story, with the known arrival of another ship on the horizon: the story partly revolves around who gets to that new ship first. So not just a couple of days. Thanks for this! $\endgroup$ Dec 24, 2023 at 15:38

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