Interstellar / planetary space communication

[LB5KE]

I got a little bored so I decided to do some calculations regarding space communication. All calculations are on 435MHz.

You should be able to talk between the space station and earth with two 5 watt handholds on each side even if the ISS is 2000km away and a an elevation is less than 10 degrees.

The path loss to a geostationary satellite 36000 km away is 176 db or more. You now need a small beam in both ends and a 50watt mobile transceiver.

The distance is much larger than you might think


So what about the moon? Well the path loss is 197db so you can still use a 50w FM mobile
in both ends and both stations need a long yagi.

What about larger EME stations? If they can hear their own echo using CW they are capable
of communicate to a distance of up to 600 million Km. This enables the user to communicate
with an equal station on Mars, Asteroids, Venus or Mercury. Big-gun EME stations could
reach Saturn.

If you want to communicate to Pluto when the planet is the furthest away from the earth,
And the path loss is 280db you need a 20meter parabolic dish in both ends and 1500watts.

You could do the same using simple  5 watt handholds both standing in the focal point of
a areceibo like dish.

In order to talk 4.3 light years away to the closest star Alpha Centauri, you need to overcome
a path loss of 358 db. You need a arecibo class dish both on earth and on Alpha Centauri system
and 200kw transmitter.

A week ago a planet with a similar mass and size to our own planet Earth,
likely similarly composed of rocks and iron, has been spotted orbiting a star some 700 light years away in the constellation Cygnus, scientists announced Wednesday.

This planet has a path loss off  -401.6 db path loss @ 435Mhz so if there is aliens on the planet you need a  500 megawatt transmitter ( if it's possible to build one i am not sure). And an antenna equivalent to a 1km dish in both ends.

So, if some feels like criticising the SETI project. Remember how short the interstellar distance is for radio and how unlikely it is to hear anything unless some on purpose tries to contact us with a very large system, close by.

[KB1WTY]

"What about larger EME stations? If they can hear their own echo using CW they are capable of communicate to a distance of up to 600 million Km."

What am I missing? The moon's about 400,000 km away, so the round trip needed for hearing your own signal would be less than a million km. Why would a station that can communicate at a 1 million km distance necessarily be capable of communicating at 600 million?

[KA4POL]

A week ago a planet with a similar mass and size to our own planet Earth,
likely similarly composed of rocks and iron, has been spotted orbiting a star some 700 light years away in the constellation Cygnus, scientists announced Wednesday.

I'd quickly forget about communication to that one. The orbital period is 8.5 hours. Now calculate how close it is to the star it orbits. All equipment on that planet has been fried 

[LB5KE]

"What about larger EME stations? If they can hear their own echo using CW they are capable of communicate to a distance of up to 600 million Km."

What am I missing? The moon's about 400,000 km away, so the round trip needed for hearing your own signal would be less than a million km. Why would a station that can communicate at a 1 million km distance necessarily be capable of communicating at 600 million?

First the signal need to travel towards the moon, it hits the moon, however only around 7% of the signal is reflected. The signal that travels from the moon is spread out it all directions. Typical path loss is around 261db at 70cm. in an EME situation.
The path loss for 435MHz at 600million Km is -260.8 db in Free space.

[KB1WTY]

"First the signal need to travel towards the moon, it hits the moon, however only around 7% of the signal is reflected. The signal that travels from the moon is spread out it all directions. Typical path loss is around 261db at 70cm. in an EME situation.
The path loss for 435MHz at 600million Km is -260.8 db in Free space."

Ahh, understood, thanks!

[WB6BYU]

But we are still getting data back from Voyager, out well beyond Pluto (whether you count
it as a planet or not), which is using a much smaller antenna and far less power (only a
few watts IIRC, due to the very weak sunlight available for the solar cells.)

[K2OWK]

Does anyone realize that it would take 4.3 years for a signal to get to Alpha Centauri and another 4.3 years to receive an answer using our radio transmission techniques. That's 8.6 years round trip. Long wait to have a rag chew, unless a deep space transmission is developed that that uses warp technology (Star Trek).

73s

K2OWK

[LB5KE]

But we are still getting data back from Voyager, out well beyond Pluto (whether you count
it as a planet or not), which is using a much smaller antenna and far less power (only a
few watts IIRC, due to the very weak sunlight available for the solar cells.)

Signals from Voyager 1 take over 17 hours, it's powered by a Radioisotope thermoelectric generator. It puts out around 20 watts in to a 3.7 meter dish at 8GHz, 3.7 meters 8GHz 23 watt. It will probably work until 2025. Using NASA's own data their receiver sensitivity is around -160dbm. Very good numbers, i guess they uses nitrogen cooled receivers and very slowbit FSK.

A typical eme station using cw has around -144dbm sensitivity.

[LB5KE]

During the early 1960s, the Jet Propulsion Laboratory developed a maser to provide ultra-low-noise amplification of S-band microwave signals received from deep space probes. This maser used deeply refrigerated hydrogen to chill the amplifier down to a temperature of four kelvin. This gave such a low noise figure that the Mariner IV space probe could send still pictures from Mars back to the Earth even though the output power of its radio transmitter was only 15 watts, and hence the total signal power received was only -169 decibels with respect to a milliwatt (dBm).

(From "masers" Wikipedia)

[KE7TMA]

Does anyone realize that it would take 4.3 years for a signal to get to Alpha Centauri and another 4.3 years to receive an answer using our radio transmission techniques. That's 8.6 years round trip. Long wait to have a rag chew, unless a deep space transmission is developed that that uses warp technology (Star Trek).

73s

K2OWK

I think any species sufficiently advanced to have developed long-range interstellar travel is going to be using entanglement to communicate.  The nice thing is that this technology is not science fiction, it is working in the lab right now.

[KB1WTY]

I think any species sufficiently advanced to have developed long-range interstellar travel is going to be using entanglement to communicate.  The nice thing is that this technology is not science fiction, it is working in the lab right now.

Using entanglement to communicate faster than the speed of light allows is ruled out by the no-communication theorem.

See http://lesswrong.com/lw/q2/spooky_action_at_a_distance_the_nocommunication/

[KE7TMA]

I think any species sufficiently advanced to have developed long-range interstellar travel is going to be using entanglement to communicate.  The nice thing is that this technology is not science fiction, it is working in the lab right now.

Using entanglement to communicate faster than the speed of light allows is ruled out by the no-communication theorem.

See http://lesswrong.com/lw/q2/spooky_action_at_a_distance_the_nocommunication/

Certainly these species are using a much better model of physics than our slapdash thing.

[KB1WTY]

Certainly these species are using a much better model of physics than our slapdash thing.

The problem isn't that faster-than-light communication is outside of our present knowledge, it seems like it's outside of what's possible for very good theoretical reasons that we actually do know about (including various paradoxes as a result of the space-time equivalence inherent in relativity). Our understanding *could* turn out to be wrong, but there are a lot of clues that seem to make the case that the universe conspires to keep us from traveling or communicating faster than the speed of light.

[KE7TMA]

Certainly these species are using a much better model of physics than our slapdash thing.

The problem isn't that faster-than-light communication is outside of our present knowledge, it seems like it's outside of what's possible for very good theoretical reasons that we actually do know about (including various paradoxes as a result of the space-time equivalence inherent in relativity). Our understanding *could* turn out to be wrong, but there are a lot of clues that seem to make the case that the universe conspires to keep us from traveling or communicating faster than the speed of light.

I guess my point is that, with new developments things that used to be theoretically impossible become possible.  It is hubris to imagine that we have reached anything more than a very rough estimate of how a small subset of things work, and with further refinements or a total rework, thing that were impossible become possible as has happened before.  This is why I don't put much stock in estimates to what impossible, because these estimates only apply within our current framework and can not possibly take into account future developments.  Newton couldn't judge what was possible here in the early 21st century and we can hardly do the same for some theoretical interstellar-travel capable species.

[KB1WTY]

Sure, at one point it seemed like heavier-than-air flight was impossible, but it's now possible. So does that mean that, in the same vein, time travel (which is a natural result of faster-than-light travel) might be one day possible, too? Well, maybe, but it would certainly create some paradoxes (like the grandfather paradox) which seem like they shouldn't be realizable.