B Communication Options With Future Deep Space Probes?

[Vanadium 50]

And that is just using RF technology

And a much more omnidirectional antenna.

I am amused that a giant planet sized iris shutter is sensible, but a radio? Now that's just crazy talk!

 

[Baluncore]

A planet sized mirror or metallic reflector, will not work near a pulsar because the eddy currents induced in the mirror by the pulsar rotation, will distort the mirror.

 

[sophiecentaur]

A planet sized mirror or metallic reflector, will not work near a pulsar because the eddy currents induced in the mirror by the pulsar rotation, will distort the mirror.

The cold light of reality. It would involve crude amplitude modulation of a very dirty signal.

But, if we're actually interested in getting some useful data across, we would probably be better with a lower power and some clever modulation and coding of a coherent source. I suspect there may be some mileage in using a high power microwave source - a compromise between beam width and possible noise / bandwidth performance.

 

[Devin-M]

I wasn't thinking it would necessarily need to be made of metal or have a physical shutter, like polarized 3d glasses which can rapidly change between clear and opaque.

Like this video:



"Each eye's glass contains a liquid crystal layer which has the property of becoming opaque when voltage is applied, being otherwise transparent."
https://en.wikipedia.org/wiki/Active_shutter_3D_system

 

[Baluncore]

"Each eye's glass contains a liquid crystal layer which has the property of becoming opaque when voltage is applied, being otherwise transparent."
https://en.wikipedia.org/wiki/Active_shutter_3D_system

A back-lighted LC (liquid crystal) shutter does not need to remain accurately aligned, but it does need a sandwich of five layers made up of two polarizers, two transparent sheet electrodes, and a liquid crystal filling. The transparent sheet electrodes cannot be superconducting because then they would become mirrors.

To minimise weight, the LC shutter will have a minimum volume of liquid, so will have a high capacitance and require a high drive current to change the control voltage. So it can be rolled or folded, the outer layers must not bond together.

Maybe you can use an LC shutter as a solar sail to accelerate, then to slow down on arrival.

 

[sophiecentaur]

One thing that could knock this on the head could be propagation delays and phase control over this vast shutter.
It would be Interferometry but the other way round. Whereas a massive synthesised radio telescope can be achieved with computers, a transmitter would need to be doing the same thing with power circuits, spread out over tens of thousands of km. hmmm?

 

[Vanadium 50]

This is the principle behind a Yagi antenna.

 

[Devin-M]

I used the Dalle 2 AI image generator algorithm to design “a megastructure in space that is capable of blocking the light from a neutron star when viewed from earth” and this is what it produced…

 

[sophiecentaur]

This is the principle behind a Yagi antenna.

Were you referring to my post about timing the feeds to parts of the reflector / mask?
A yagi is a passive device where the phases are set by only the geometry. What's required here is an active device and I think this introduces signal timing and matching all elements. Not trivial over thousands of km.

 

[Vanadium 50]

Sure, but it accomplishes the same thing, just with reflections rather than active delays on the feeds.

It's amusing that a planetary-sized mechanical iris is perfectly fine, but a planetary sized antenna? More crazy talk.

Ballpark, a 10 km antenna would have enough gain to make the signal as bright as the Pioneer missions with no more power, and if you could go to kilowatt-level power, you need only 1 km. Make it out of aluminized mylar and it weighs 100 tons.

This may be rocket science, but it's not Ringworld.

 

[sophiecentaur]

Sure, but it accomplishes the same thing, just with reflections rather than active delays on the feeds.

It's amusing that a planetary-sized mechanical iris is perfectly fine, but a planetary sized antenna? More crazy talk.

Ballpark, a 10 km antenna would have enough gain to make the signal as bright as the Pioneer missions with no more power, and if you could go to kilowatt-level power, you need only 1 km. Make it out of aluminized mylar and it weighs 100 tons.

This may be rocket science, but it's not Ringworld.

You seems to have missed my point here. The planned project would need to modulate the light from the star. The star light hitting the screen is uniform and continuous and the requirement would be to alter the level of the light output (reflected or in the shadow; it doesn't matter) from elements all over the disc. That means it's necessary to get a synchronised modulating signal to every part of the disc to drive any 'shutter'. Mis-timing of the modulating signal to any of the elements will blur the received pulse shape.
I'd suggest that a data rate would need to be at least tens of MB/s and the delays over hundreds of thousands of km would make this problematic. (It's not just a matter of 'detecting' the presence of the light modulator.)

BTW, a Yagi antenna is not a good example of a super directive antenna as there is only one driven element. There are synthesised arrays with multiple feeds and even multiple transmitters but, again, the transmitters need to have synchronised modulation as well mutually coherent RF carrier waves.

 

[tech99]

You seems to have missed my point here. The planned project would need to modulate the light from the star. The star light hitting the screen is uniform and continuous and the requirement would be to alter the level of the light output (reflected or in the shadow; it doesn't matter) from elements all over the disc. That means it's necessary to get a synchronised modulating signal to every part of the disc to drive any 'shutter'. Mis-timing of the modulating signal to any of the elements will blur the received pulse shape.
I'd suggest that a data rate would need to be at least tens of MB/s and the delays over hundreds of thousands of km would make this problematic. (It's not just a matter of 'detecting' the presence of the light modulator.)

BTW, a Yagi antenna is not a good example of a super directive antenna as there is only one driven element. There are synthesised arrays with multiple feeds and even multiple transmitters but, again, the transmitters need to have synchronised modulation as well mutually coherent RF carrier waves.

Can we launch repeater stations along the way? There is a particular advantage in having a relay station well away from Earth, as it means that the space probe is not looking at the warm, and hence noisy, Earth.

 

[sophiecentaur]

Can we launch repeater stations along the way? There is a particular advantage in having a relay station well away from Earth, as it means that the space probe is not looking at the warm, and hence noisy, Earth.

I don't know how effective relay links would be as there is nothing 'in the way' which is why relays are used for Earth systems.

But that doesn't answer my question /concern about the actual data rate that would be achievable with any imagined modulation system that any of these projects could use.

 

[berkeman]

I don't know how effective relay links would be as there is nothing 'in the way' which is why relays are used for Earth systems.

Repeater/relay links are also used for signal amplification. As long as there is a local source of energy, using a receiver/transmitter device at the relay location boosts the overall signal/noise ratio. Think about the undersea phone cables or other long communication links where powered repeaters are used...

https://en.wikipedia.org/wiki/Submarine_communications_cable#Submarine_cables_across_the_Pacific

The first trans-Pacific telephone cable was laid from Hawaii to Japan in 1964, with an extension from Guam to The Philippines.[21] Also in 1964, the Commonwealth Pacific Cable System (COMPAC), with 80 telephone channel capacity, opened for traffic from Sydney to Vancouver, and in 1967, the South East Asia Commonwealth (SEACOM) system, with 160 telephone channel capacity, opened for traffic. This system used microwave radio from Sydney to Cairns (Queensland), cable running from Cairns to Madang (Papua New Guinea), Guam, Hong Kong, Kota Kinabalu (capital of Sabah, Malaysia), Singapore, then overland by microwave radio to Kuala Lumpur. In 1991, the North Pacific Cable system was the first regenerative system (i.e., with repeaters) to completely cross the Pacific from the US mainland to Japan.

 

[sophiecentaur]

Repeater/relay links are also used for signal amplification.

Of course that's correct but there is always something 'in the way' with existing comms routes to make things worse than inverse square law. Cable loss is 'per metre' and all terrestrial routes involve obstacles (the horizon for example). I don't know what the 'absorption coefficient' of empty space is. I guess a signal passing through a nebula could be attenuated right across the band - there a certainly absorption lines.
What a nightmare, though, to plot a relay route across the galaxy.

The magic thing about the ISL is that you only lose 3dB when you double the distance.

 

[Baluncore]

Can we launch repeater stations along the way?

As the number of links in a repeater chain increases, the reliability of communication falls very rapidly.

The receiver near Earth does not need to be built until after the transmitter has been launched. Only the Earth end of the link can be serviced or upgraded later.

The data rate will depend on the phase accuracy of the transmit array or shutter.

 

[sophiecentaur]

“Depend on”? Could be one bit per 10s.

 

[collinsmark]

I don't think the problem is necessarily phase synchronization in this situation. The system could employ simple on-off keying (OOK) with Manchester encoding and your favorite, forward error correction (FEC) scheme thrown on top of that. The bitrate would be low, but c'mon, it probably would take several human lifetimes for the transmitter to reach its destination in the first place, so a slow bitrate doesn't seem that critical.

There are bigger problems than the modulation scheme. The iris would need to be gigantic, perhaps between planet sized and solar system sized. That's the real challenge.

I think a much better solution is for the transmitter to have its own power source (e.g., nuclear) if it's too far away from the star to gather energy via solar arrays. Then transmit with more conventional methods (e.g., RF). Unlike high-gain receivers, high-gain RF transmitters don't need to be big.

 

[Devin-M]

There are bigger problems than the modulation scheme. The iris would need to be gigantic, perhaps between planet sized and solar system sized. That's the real challenge.

Yes I agree. This reminds me of a couple more of the AI Art renderings I made using DALLE 2 with the following prompt:

“A megastructure near a neutron star that is capable of blocking the light of the neutron star when viewed from earth”

 

[Baluncore]

I don't think the problem is necessarily phase synchronization in this situation.

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.

 

[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.