B Communication Options With Future Deep Space Probes?

[collinsmark]

Destructive interference will destroy the advantage of an aperture. If it takes 1 second for the modulation clock to spread across the elements of a flat iris, then the data rate will be less than 1 bit per second.

The data rate could be greatly increased by employing a parabolic iris, and distributing the modulation clock to each element from the focus. The modulation bandwidth would then be limited by the phase error of the parabolic iris surface, with receiver noise and bandwidth being a separate problem.

Or different elements of a flat iris could be phase compensated. In other words, a larger delay could be added to iris elements close to the clock source and smaller delays added to iris elements farther away from the clock source. From a far distance, all elements of the iris would appear to change state in-sync.

Such delays would involve a little latency. But again, such things are the least of our worries about such a monstrosity.

 

[sophiecentaur]

high-gain RF transmitters don't need to be big.

A large aperture is always necessary to make any array very directive and Tx RF power will always be limited. Information rate will be directly affected by the level of signal arriving at Earth so the array would need to be as big as possible in order to maximise data rate.
The same sums apply for Tx and Rx; SNR is affected by both.

 

[collinsmark]

A large aperture is always necessary to make any array very directive and Tx RF power will always be limited. Information rate will be directly affected by the level of signal arriving at Earth so the array would need to be as big as possible in order to maximise data rate.
The same sums apply for Tx and Rx; SNR is affected by both.

Fair enough. What you say is correct. Chalk it up to poor wording on my part.

What I should have said is that for a given link budget, and for practical reasons, it would be a lot easier to have a bigger Rx antenna on (or near) Earth and have a comparatively smaller Tx antenna on the probe that has to travel to a whole 'nother star system.

 

[sophiecentaur]

What I should have said is that for a given link budget, and for practical reasons, it would be a lot easier to have a bigger Rx antenna on (or near) Earth and have a comparatively smaller Tx antenna on the probe that has to travel to a whole 'nother star system.

I think we are not disagreeing. An Rx system can use fancy signal processing with multiple sub antennae. For Tx, the problem of getting powerful signals to be coherent would be much harder so there would be a real limit to the Tx gain available.

Bottom line for me is that the added complication of a shutter / reflector, using starlight sounds to be just too much effort. A serious Transmission system would probably need a way of multiplexing several / many high power devices. But there is a limit to how much you can double up your power (hybrid couplers etc) because practicalities limit the performance increase to below +3dB. I have a feeling that we're waiting for a 10MW solid state amplifier . . . .

Or what about using a number of independent lower power / low data rate links to get a useful overall rate?

 

[sophiecentaur]

Just imagine the content of communications at these vast differences.
Joke telling, for instance:
"Knock knock"
A hundred year delay
"Who's there?"
Another hundred years
"Your great great great grandfather"

 

[Devin-M]

It’s claimed in this article—

“a solar-lens telescope would be able to detect a 1 Watt laser coming from Proxima Centauri b, about 4 light-years away”

— by placing a telescope around 600AU from the sun & using the sun itself as a gravitational lens.

https://phys.org/news/2022-10-solar-gravitational-lens-humanity-powerful.html

 

[berkeman]

“a solar-lens telescope would be able to detect a 1 Watt laser coming from Proxima Centauri b, about 4 light-years away”

— by placing a telescope around 600AU from the sun & using the sun itself as a gravitational lens.

I wonder how you deal with that big bright thing right next to the image of your 1W laser... I didn't see that addressed in the short popular article -- is it addressed in the more detailed articles about this technique?

 

[Devin-M]

They cite a paper which says in the abstract:

“At the diffraction limit, the angular resolution is similar to that of a notional telescope with the diameter of the Sun, and the maximum light amplification is 8π^4GM⊙/(c^2λ), enough to detect a 1W laser on Proxima Centauri b pointed in the general direction of the Sun.”

https://academic.oup.com/mnras/arti...9/6695110?redirectedFrom=fulltext&login=false

 

[sophiecentaur]

“a solar-lens telescope would be able to detect a 1 Watt laser coming from Proxima Centauri b, about 4 light-years away”

Detecting is one thing; detecting modulation is an entirely different matter. There is really no point in discussing this without addressing the rate of information carried.

I wonder how you deal with that big bright thing right next to the image of your 1W laser... I didn't see that addressed in the short popular article -- is it addressed in the more detailed articles about this technique?

Yet again, SNR is not mentioned.

 

[tech99]

Maybe the Moon could be used as a zone plate to provide some gain and extend range - it is less noisy than the Sun.

 

[sophiecentaur]

Maybe the Moon could be used as a zone plate to provide some gain and extend range - it is less noisy than the Sun.

A zone plate has only half the area of a reflector.
What advantage(s) would it have? It’s flatness would need to be as accurate as a paraboloid for the same performance.
I think the Sun would not be much use.

 

[Devin-M]

There’s a pretty detailed description from 2:55 to 14:46 of a potential solar gravitational lensing mission concept:

 

[sophiecentaur]

I think the Sun would not be much use.

A bit too sweeping a statement but when a coronagraph is used (to shadow the imagers from the Sun), there will always be a Poisson /Fresnel spot appearing in the centre of the shadow. The Sun is very bright (magnitude -26), compared with the object we'd be imaging an object of very high magnitude - say 30? in the presence of the Sun's Fresnel spot.
There's the 'gain' of the solar lens fighting the Fresnel spot 'magnitude. Maybe a suitably shaped coronagraph could have a more diffuse Fresnel spot to reduce the effect of the extraneous light. The guy in the video seemed very enthusiastic and could have been over-egging his story by several orders of magnitude. Also, the cost could of JWST proportions +.

Also, the old SNR factor comes into play again; imaging / detecting is not the same as Signalling.

 

[Motore]

The guy in the video seemed very enthusiastic and could have been over-egging his story by several orders of magnitude. Also, the cost could of JWST proportions +.

https://www.nasa.gov/directorates/s...xel_Imaging_and_Spectroscopy_of_an_Exoplanet/

 

[Devin-M]

https://www.nasa.gov/directorates/s...xel_Imaging_and_Spectroscopy_of_an_Exoplanet/

“Even in the presence of the solar corona, the SNR is high enough that in 6 months of integration time one can reconstruct the exoplanet image with ~25 km-scale surface resolution, enough to see surface features and signs of habitability.”

 

[thewowsignal]

Discussion in this thread is based on the speed of the light. This of course assumes, that the speed of the light is the fastest way to transfer information. And of course it is. Today. Yesterday nobody realized, that our civilization's communication is limited to some 300k km per second. Tomorrow the situation may change, if only our understanding of the Universe expends. I think this forum is a very good proof, that it is not going to happen soon.

 

[russ_watters]

Discussion in this thread is based on the speed of the light. This of course assumes, that the speed of the light is the fastest way to transfer information. And of course it is. Today. Yesterday nobody realized, that our civilization's communication is limited to some 300k km per second. Tomorrow the situation may change, if only our understanding of the Universe expends. I think this forum is a very good proof, that it is not going to happen soon.

How can we have a conversation about a FTL communications strategy if we don't know of a FTL process upon which to base one? That's such a weird criticism. We work with what we have/know.