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Suppose that in our Solar System in the future will be two stealth ships (small cross section/low temperature signature/radar absorbent material). Still visible for human eye and detectable when moving at full speed or engaging in combat.

To communicate between each other they use encrypted tight-beam laser link. Mostly they keep radio silence and avoid communication unless it is absolutely necessary.

The question is how they could communicate not knowing each other's exact position? Suppose there is no possibility for fixed-point relay station or making schedule (stealth ships observe the Earth space fleet and its colonies).

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    $\begingroup$ There's no such thing as a stealth spaceship: projectrho.com/public_html/rocket/spacewardetect.php#nostealth While a ship may be able to go dark while it's not accelerating at all, its position can be tracked perfectly from its last course change. $\endgroup$
    – Mike Scott
    Jan 8, 2016 at 16:34
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    $\begingroup$ @MikeScott One side of this is that space is really really really big, and you'd have to be looking in the right place to see the burn. We've had large things like asteroids get pretty close before seeing them, because we weren't looking there. Once the enemy saw you that would be another matter, but first they'd have to get lucky. $\endgroup$
    – AndyD273
    Jan 8, 2016 at 16:37
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    $\begingroup$ The problem is that any kind of maneuvering is visible with passive sensors at tens, if not thousands of AU distance. "The Space Shuttle's much weaker main engines could be detected past the orbit of Pluto. The Space Shuttle's manoeuvering thrusters could be seen as far as the asteroid belt." $\endgroup$ Jan 8, 2016 at 16:39
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    $\begingroup$ @Draco18s I wonder though, if the shuttle was thrusting toward you, so the engines were pointing away and blocked by the body of the ship, and maybe even baffled to better hide the exhaust, and you weren't looking directly at it, would it be as visible. Also, if you did all your thrusting way out and mostly coasted in, then you could go a long way without any maneuvering burns at all. $\endgroup$
    – AndyD273
    Jan 8, 2016 at 16:43
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    $\begingroup$ The funny thing about hot expanding gasses is that they're expanding. The ship would be hiding the main glow of the torch, but the cloud of hot gasses expanding outward would still be picked up by infra red. $\endgroup$ Jan 8, 2016 at 16:45

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There is no solution within the parameters you've described. Laser communications require the beam to be aimed precisely at the receiver. If you have no way of knowing where that receiver is then you can't communicate with them.

The best way for these ships to communicate would be to drop multiple stealthed buoys and transmit their messages to them. The two possible approaches are either that

  • The buoys know about one another and share messages, which is more dangerous if one is found, but gives ships the option of getting the latest message from any buoy they connect to

OR

  • The ships could send the message to each buoy they are in range of, but you couldn't be certain of getting the latest message without checking each buoy in turn, which I think is the bigger danger, as the ships would be transmitting for longer, or maybe even have to move in order to get in range of some devices.

In either scenario the messages would obviously be encrypted in case the buoy is found by the enemy.

Since the buoys are passive in nature the chances of them being found, or their communications being intercepted are very low. (they don't transmit any signals and lasers don't count as they are aimed at a particular location and are undetectable unless you make contact with the beam).

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  • $\begingroup$ Passive buoyus as mail boxes are ok. Should they contain the location of other buoys/message with new buoys placed by second ship? $\endgroup$
    – light
    Jan 8, 2016 at 16:39
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    $\begingroup$ If you went with buoys, I'd just have very precise locations picked out ahead of time to place them, and then communicate with the buoy via laser. That way the buoy isn't sending off any kind of radio signal, encrypted or not. If an enemy detected a buoy, then they'd know that several ships were around and would start actively looking. $\endgroup$
    – AndyD273
    Jan 8, 2016 at 16:46
  • $\begingroup$ This is a really smart approach. Also, if ultra-high secrecy is called for, and people are worried the bouys are compormized, they can always reveal their location so that the other ship can talk to them directly. It'd spoil a very important secret for a stealth craft (their position and velocity), but if some communication is simply too important to risk sending through bouys, it might be worth the exposure of that information. Until something's that important, they can just communicate through the relay bouys. $\endgroup$
    – Cort Ammon
    Jan 9, 2016 at 0:51
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The thing with laser is that you won't be able to detect it unless you're right in the beam, or you see reflected light off of dust and other things. That is to say, it would be harder to detect than your stealth ships unless it was pointed directly at you.

So if you knew roughly where the other ship was going to be at a certain time, you could do a laser sweep of the area.

The ship that you are communicating with could then detect the origin of the beam, and use that to aim its own communication laser. When the first ship sees the communication request, it would stop sweeping and more precisely aim its laser, allowing communication to proceed.

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As noted in some comments, stealth in space is difficult. As noted in some other comments, space is big. I'm assuming that the stealth ships have a good chance to avoid being detected if the defense has no contact, and that the defenses can maintain contact once there was a detection.

  • Assume that the two ships are in close proximity and know exactly where they are. They establish two-way communication by laser.
  • Before one of the ships starts to maneuver, it informs the other ship about the course change. It waits long enough for twice the required message lag.
  • After the maneuver, it informs the other ship about the new course.
  • If a ship receives a warning message, it must not maneuver until it receives a confirmation message (and possibly sends a handshake back and forth).
  • If a ship receives a warning message after sending a warning message itself, there are predetermined rules which of the two ships cancels the maneuver.
  • If a ship is forced to maneuver to avoid detection (generally a bad idea, see above), it will go to a predetermined course and wait for a message.

Note that when I'm talking about predetermined priorities and evasion courses, that can always be changed by a message exchange.

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When the ships initially depart from base, their orbital parameters are set, and while drifting unpowered their positions will be known to each other and their base with a high degree of precision. Space probes like Voyager and New Horizon can travel for years or even decades and radio telescopes can be pointed with a high degree of precision to pick up their signals, even though their transmissions now are coming in with the power of a Christmas tree light bulb. So the two ships and the base can all communicate with high degrees of precision so long as nothing changes and the ships are in ballistic trajectories.

Once a ship makes a manoeuvre, it will be visible to everyone by its heat emissions and exhaust plume. The other ship and the base can see it equally well, and simply track the burn and calculate the new orbital trajectory. The ship and the base might have to use a slightly wider beam arpeture to ensure they account for possible errors, but once contact is made, the ship which changed trajectories can send a "handshake" signal which helps correct for any errors. Once again, the trajectories can be plotted with a high degree of accuracy so long as ships travel in ballistic trajectories.

To simplify things, ships might be told not to transmit to each other, but to the base, since the base is probably a space station, asteroid or moon which is easy to identify, and which has much larger and better optical equipment (much like the Deep Space Tracking network on Earth follows current space probes). The downside of this is messages ship to ship will take considerably longer, and this is a single point of failure which could cripple the fleet.

Of course since stealth is not possible in space, everyone will have seen the initial and subsequent burns, so know something is on a particular orbit, but with stealth and laser comms, won't know what exactly is out there.

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