TL9 interplanetary communication

[Anthony]

Radio communication in general is limited by the Shannon-Hartley theorem.

A more powerful transmitter multiplies signal by power. Since signal drops with the square of range, that multiplies range by the square root of power (in spaceships terms, +4 SM will give x10 range).

A directional transmitter multiplies effective signal by the inverse of its coverage -- i.e. if it only sends to 10% of the sky, signal strength is multiplied by 10. As coverage varies with the square of transmission angle, this means range is inversely proportional to transmission angle. Transmission angle cannot be less than (wavelength)/(antenna diameter) radians, so larger transmitters can give a tighter beam, increasing range (in spaceship terms, once above the minimum size where it's possible at all, +6 SM will give x10 range).

A more sensitive receiver increases the signal, but also increases environmental noise (there can be some internal noise in the receiver that is not affected). Typically this does not increase range.

A directional receiver multiplies environmental noise by coverage, allowing making use of a more sensitive receiver. As a directional receiver is generally also larger and thus inherently more sensitive, this mostly means you can multiply range by the inverse of the angular size. Note that it is possible to run multiple directional receivers off of a single antenna, in which case what matters is the angular size of each pixel in the receiver. Just like directional transmission, +6 SM will give x10 range.

[tshiggins]

I think you also need to keep in mind that, by the time we have permanent settlements on Mars (and expeditions to Main Belt asteroids), we'll have the ability to relay signals and won't have to brute-force them point-to-point.

A transportation system of two Aldrin Cycler stations could, should and almost certainly will include large antenna arrays that allow them to relay messages between Earth and Mars (and any nearby Inner System asteroids or other bases).

https://en.wikipedia.org/wiki/Mars_cycler

[Ulzgoroth]

Originally Posted by tshiggins

I think you also need to keep in mind that, by the time we have permanent settlements on Mars (and expeditions to Main Belt asteroids), we'll have the ability to relay signals and won't have to brute-force them point-to-point.

A transportation system of two Aldrin Cycler stations could, should and almost certainly will include large antenna arrays that allow them to relay messages between Earth and Mars (and any nearby Inner System asteroids or other bases).

https://en.wikipedia.org/wiki/Mars_cycler

How much does that reliably help? You get a decent savings when a cycler is near the midpoint of the planets, but neither cycler spends long there. So unless you want your interplanet communication to be very intermittent, that's a problem.

Though communication near opposition would require depending on relays regardless, I imagine.

[Anthony]

Originally Posted by tshiggins

I think you also need to keep in mind that, by the time we have permanent settlements on Mars (and expeditions to Main Belt asteroids), we'll have the ability to relay signals and won't have to brute-force them point-to-point.

Other than being able to route around the sun (it's problem when the sun is between Mars and the Earth) relaying signals isn't super valuable; the overall amount of hardware is somewhat smaller, but it's significantly less convenient to install and maintain.

[Varyon]

Originally Posted by Anthony

Other than being able to route around the sun (it's problem when the sun is between Mars and the Earth) relaying signals isn't super valuable; the overall amount of hardware is somewhat smaller, but it's significantly less convenient to install and maintain.

Yeah, I think the real benefit of relays comes into play when you're using them at a large scale, as then your ships don't need a very large communication's array, just one large enough to reach the nearest relay. But of course at that point you're talking about requiring more hardware, not less - you need a lot of relay stations if you want to be able to rely on your ship being close enough to communicate with one! Although with a lot of ships, you may reach the point where it's actually less overall hardware (although this will mean that not all ships can communicate at once), and of course with space travel the fact the ships get to save on mass may be well worth the extra hardware cost of having a bunch of relays.

[ehrbar]

The biggest trouble with the relay-through-spaceships idea is that Earth and Mars are moving, and many (non-constant-boost) orbits that get you from one body to the other are not even approximately on the straight-line path from where one currently is to where the other currently is.

In the case of Aldrin cyclers specifically, note they spend a majority of their actual travel time beyond Mars orbit. There's going to be relatively little time for any given cycler when the Earth-to-cycler distance and the Mars-to-cycler distance are both appreciably smaller than the full Earth-to-Mars distance.

[dcarson]

The situation in Venus Equilateral by George O. Smith where you have a large permanent relay 60 degrees leading Venus to relay around the Sun makes sense.

[Anthony]

Honestly, the easiest solution in a setting with significant space travel but the vast majority of humanity still on Earth is probably a ginormous radio telescope array on Earth (or somewhere in the Earth-Moon system, such as one of the Lunar Lagrange points, being outside the atmosphere has some benefits), and ships carry directional comms sufficient to talk to that array; two vessels and/or colonies, unless very close to one another, would probably route message through Earth. Solar interference would either just be something you put up with, or that's handled by a single relay station that's reliably not going to be blocked at the same time as Earth (probably at the earth/sun L3 or L5).

[tshiggins]

Originally Posted by Varyon

Yeah, I think the real benefit of relays comes into play when you're using them at a large scale, as then your ships don't need a very large communication's array, just one large enough to reach the nearest relay. But of course at that point you're talking about requiring more hardware, not less - you need a lot of relay stations if you want to be able to rely on your ship being close enough to communicate with one! Although with a lot of ships, you may reach the point where it's actually less overall hardware (although this will mean that not all ships can communicate at once), and of course with space travel the fact the ships get to save on mass may be well worth the extra hardware cost of having a bunch of relays.

This is what I was thinking about, I guess.
You'd want to put relays at just about every Lagrange Point or Trojan Point in any given Solar System, I suppose.

[Tyneras]

Once you have space manufacturing and don't need to pay to lift the material out of Earth's gravity well, and can probably have the finished satellite sent into it's orbit via a large gauss catapult, relay satellites become not too dissimilar to cell phone towers. They are absolutely everywhere, to the point not being in range of one is a surprise.

Of course, just because you can phone home, doesn't mean home can do anything helpful beyond give advice, or maybe record your final wishes and how you'd like your remains disposed of if you got yourself in a really bad orbit and it'll be a few decades or centuries before anyone can reasonably intercept.