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does anyone have any suggestions for wired communications over long distances on the Lunar surface? The maximum length would need to be around 1km. Distributors like Gore and Axon make great lightweight cables, but they are designed for short (<10m) distances.

If this is just a pipe dream, I suppose I'll resort to wireless communication, which is no fun.

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    $\begingroup$ Optical fiber is another option to consider, though the conditions (temperature swing, radiation damage) is a problem. There's also free-space optical links. $\endgroup$ – uhoh May 2 at 3:51
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    $\begingroup$ What data rate are you looking for? And how resilient does the cable have to be: will it be moved regularly, or will it be stationary? $\endgroup$ – Hobbes May 2 at 6:17
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    $\begingroup$ DSL may be used up to 1km and more. Only two wires are needed like a telephone cable. To save weight aluminium instead of copper may be used. The wire should not be too thin, about 0.4 mm diameter at least for copper. Of course a power source is needed at both ends of the cable. $\endgroup$ – Uwe May 2 at 11:20
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    $\begingroup$ @Hobbes Data rates between 500Mbps - 1Gbps. The wires will be stationary. Additionally, I could bury them in a thin cover (~5cm) of lunar soil, which may provide some protection. $\endgroup$ – Spuds May 2 at 19:06
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    $\begingroup$ 500kbps - 1Mbps over 1 km seems possible using a twisted wire pair and DSL. For equal bitrates in both directions SDSL. $\endgroup$ – Uwe May 2 at 20:19
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A lot more of this has to do with the physical nature of the cable than the signal protocol, in that you get the best cable you can and then adjust your signalling method to move as much data as physics will allow.

The classic solution for 1km distances is RS422 though various alternatives exist, including all those intended for two wire telephone use that include things like adaptive speed changes based on noise. You can also consider optic fibre as they may come in lighter than copper and avoid having a 1km long antenna.

The bigger questions are about how the cable will be used. Will it be run out by a human in a space suit or from a fixed length drum by robot. Will it be pre terminated on earth for a specific length and purpose or in situ by someone in a suit (only two wires, PLEASE!). Does it need to survive multiple day/night cycles or just long enough to do a single test/experiment. Will it be deployed by being dragged out across the rocks or unrolled from a moving spool. Will it need to be re-used/retermineted in place. Does it need a tight bending radius to fit in the stowage space (most optic fibres do not like being tightly coiled).

Once you know some of the answers above and how much weight you have you can work out what sort of protocol will work on the cable you got. Shielding is great but will add a lot of weight, more cores will give you more data but add complication in any connections (two wires=one right one wrong way, 8 gives almost 100 wrong ways) . Light plastic insulation will get more capability in mass allowance but may fall apart under UV and cold. If length matters above all else you can go single conductor and earth return for maximum length for a given weight of copper.

If doing this as a basic project a good starting point is 8 core ethernet cable because it a is cheap, well understood with lots of off the shelf hardware. Probably running RS422 with some cores left over for power.

If looking deeper (and have deeper pockets) the optic fibre tethers used for under water devices would be the starting point since they are dealing with similar challenges and have some off the shelf hardware/data/research papers/failures to learn from.

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    $\begingroup$ I doubt that a single conductor and earth return will work with Lunar dry regolith. Without digging a grounding rod very deep into bedrock there will be isolation instead of low resistance ground. Think about grounding within Atacama desert. Ground resistance on Earth may vary between 2 and up to 8000 Ωm. $\endgroup$ – Uwe May 2 at 14:55
  • $\begingroup$ I still worry about radiation damage to the optical fiber. Whether polymer or glass, it doesn't have to darken much for a 1 km thickness of the stuff to become uselessly attenuating. And so I've just asked How well would optical fiber work on the Moon or Mars? $\endgroup$ – uhoh May 3 at 0:33
  • $\begingroup$ @uwe, agree with earth(moon?) return being the wrong solution for the moon (and possibly everywhere else) but it does have the singular virtue of needing just one conductor so has a place on the list. Would not personally want anything to do with a plan trying to make it work though. . $\endgroup$ – GremlinWranger May 3 at 5:56
  • $\begingroup$ RS422 over 1 km is limited to about 100 kBit/s but the questioneer needs 500kbps - 1Mbps. Repeaters at every 100 m would be required. $\endgroup$ – Uwe May 3 at 9:06
  • $\begingroup$ @Uwe, hmm data rate was not there when I started the answer. Still do-able but would require the entire chain to be tuned/tested as a complete system and starts to look more like a custom Ethernet implementation than RS422. $\endgroup$ – GremlinWranger May 3 at 11:19

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