Interstellar Probe Comms Link Design?

[Selfsim]

Ok, so following on from several responses in the "What happens if we discover Alpha Centaury simply does *not* have planets?" thread, I'd like to pose a question:
"What size receiver antenna would need to be used to receive an intelligible message from a probe, say, one light year distant? (For modelling purposes, I'd define 'intelligible' as being similar to say, the quality of messages Voyager 1 sent Earth, when it was ~120 AUs distant).

The purpose of this being, to quantify just how far away in technology development terms we really are, when it comes to attempting meaningful interstellar communications using RF technology. (Even though one light year is not really an interstellar distance for us ...).

So, to simplify the calculations, (and the model), say our probe has:

- the same transmitter power as Voyager 1, which was ~22 Watts (might increase this later on);
- X-band communications to be used (8.4 GHz);
- the same maximum downlink bit rate as Voyager (160/1400 bps);
- the same EIRP (Equivalent Isotropically Radiated Power) as Voyager 1 is capable of (~84.7 dBm). Interestingly, the Mars Reconnaisance Orbiter ends up being only about 0.8 dB different from this, even though it has 100 Watt transmit power! Mind you also, MRO's bitrate ends up being ~6Mbps ... but I think that's another story;
- the same receive signal-to-noise ratio as for the Voyager signal, when it was at 120 AUs from Earth;
- seeing as the Deep Space Network radio telescopes were used for Voyager, let's assume their general specs as well (with the exception of the required antenna aperture size);
- the best noise floor approximation I can come up with is ~ -175.8 dBm.

So, in other words, Voyager 1 is the probe, and it has made it to one light year distant, its power supply is still producing the same power as when it was commissioned, and we're trying to receive its signal. What size receiver antenna, (parabolic reflector), is needed?

Any takers ?

Regards

[antoniseb]

Holding all of the technologies constant, and just looking at dish size, to receive the same wattage from the source here, dish radius goes up linearly with distance. So since Voyager 1 is at about 1/500th of a light year, you'd need a dish 500 times larger than the ones currently in use for the deep space network... so, maybe 50 km diameters. You'd probably want them to be in orbit.

However, you should be able to get some improvement in signal to noise ratio by having a much narrower beam from the larger dish, resulting in the signal from the probe being against a much smaller piece of sky's background. I haven't thought through how to calculate that advantage, and whether something the size of the SKA and tuned to 8.4 GHz would be able to do the job.

Another thing I haven't thought through is the potential for natural processes in interstellar space to degrade long-wavelength signals. Does x-band get through unscathed? I'd have to do some research to know.

[Selfsim]

Thanks kindly for the response Antoniseb .. much appreciated.

I did some rough calculations yesterday which showed an extra ~56 dB of gain would be needed due to the extra path loss over 1 light year (~370.4 dB@8.4 GHz). At 8.4GHz, this equates to an antenna aperture diameter of 48.6 Kms .. (same as your approximation).

So also, (as per your 'heads up'), I had a look at the SKA technology. Looks like they're targetting:
- aperture diameter ~1.128 Kms;
- 100 MHz to 22 GHz (with upwards of 30GHz, beyond the year 2022);
- T_sys 20 - 30K
- point source sensitivity ~ 11 nanoJy @ 1.4 cm in 1 hr;
- most present technology issues to take 10 years to resolve eg: receiver cryo cooling;
- Cost is about $US1 billion

Amongst many issues with attempting to communicate with a remote probe, (not so far seriously under consideration), it looks to me like pointing the probe (big) antenna, whilst keeping it aligned with the receiver array would be a major issue because of the beamwidth (which would be extremely narrow) and the precision so demanded. The delays in detecting drift and subsequently realigning the probe, would mean that the probe would have to do this autonomously .. (and may also demand substantial fuel usage over the journey duration?).

It also looks like all future deep space missions will be moving towards using the 32 GHz K_a band, due primarily to a lack of spectrum availability in the 8.4 GHz X-band. MRO's K_a system was designed specifically as a proof of concept and the corresponding DSN BWG antenna subnet was upgraded for this purpose. (Not sure what implications this may have as far as extra noise/scattering by the interstellar medium - perhaps advantageous ??)

Whilst I notice some SETI justification is loosely cited in Wiki, it is only for intelligent ET signal detection and remote spectroscopy.

Still, at the end of the day, the receiver gain looks likely to not be around for what I'd guess to be at least 50 to 100 years(??) I think the issues of getting a working probe out to the light year distances however, dwarf the communications problems in overall magnitude, but there are physical limits (like antenna efficiency) which could, quite conceivably, impose unforeseen laws-of-diminshing-returns on the communications technologies.

It seems the advances in telescope technologies have computing advances at their core, so perhaps quantum computing might offer some kind of breakthroughs in this area (??)

Regards

[Van Rijn]

Ok, so following on from several responses in the "What happens if we discover Alpha Centaury simply does *not* have planets?" thread, I'd like to pose a question:
"What size receiver antenna would need to be used to receive an intelligible message from a probe, say, one light year distant? (For modelling purposes, I'd define 'intelligible' as being similar to say, the quality of messages Voyager 1 sent Earth, when it was ~120 AUs distant).

The purpose of this being, to quantify just how far away in technology development terms we really are, when it comes to attempting meaningful interstellar communications using RF technology. (Even though one light year is not really an interstellar distance for us ...).

So, to simplify the calculations, (and the model), say our probe has:

- the same transmitter power as Voyager 1, which was ~22 Watts (might increase this later on);

That seems very small to me. While I'd expect designers would want to minimize the mass of an interstellar probe, if they have the means to send it to another star in a reasonable time (decades) I would expect they would have better power systems and would be willing to invest in a larger spacecraft. My first cut thought would be something with an antenna diameter similar to Arecibo's, with a comparable transmitter. At least in that case, I believe it's been established that two Arecibo-like radio telescopes could communicate over several light years. (There have been some overstatements, claiming Arecibo could communicate with a twin across the galaxy, and while that's been shown to be wrong, it would apparently have little trouble if the twin was at a nearby star.)

Keep in mind that a space based telescope similar to Arecibo could be much less massive than the ground based one. The material of the reflector could be quite thin.

If you can get to another star in a reasonable time, I don't think communication will be a major problem.

[Van Rijn]

Amongst many issues with attempting to communicate with a remote probe, (not so far seriously under consideration), it looks to me like pointing the probe (big) antenna, whilst keeping it aligned with the receiver array would be a major issue because of the beamwidth (which would be extremely narrow) and the precision so demanded. The delays in detecting drift and subsequently realigning the probe, would mean that the probe would have to do this autonomously .. (and may also demand substantial fuel usage over the journey duration?).

I don't understand the alignment issue. I would think the antenna's alignment would be essentially fixed to point back at the solar system, and at those distances, the beamwidth should be substantial. I would think alignment would be one of the easier aspects of the mission. Also, I would expect acceleration of the spaceprobe to a significant fraction of the speed of light would require quite a lot of fuel use (and it would have to be an advanced nuclear rocket, unless perhaps it was a light sail design), but I don't see why adjusting an antenna would significantly affect fuel use.

Now, for a mission to another solar system, it probably would make sense to have one deep space "mothership" with a large antenna directed at the solar system and smaller antennas for in-system communication. Actual system exploration could be done by one or more smaller spacecraft similar to current interplanetary probes.

[Jeff Root]

I hesitated to reply to this thread because I wanted some
actual numbers. I'll go ahead without them...

Any interstellar probe intended to communicate with Earth
will have a power source more than 1000 times as powerful
as that of Voyager, probably more than 10,000 times, and
possibly more than 100,000 times as powerful.

It will have a larger radio reflector dish to provide a
narrower transmitted beam and stronger reception.

It will certainly operate autonomously. That aspect of
a probe could be designed and built today, no problem.
Keeping the narrow beam aimed at Earth is a minor
difficulty that could be done today. It would require an
infinitesimally tiny fraction of the total energy budget.

The transmission bitrate was the first thing I thought of
when I read the second sentence of the OP. The slower
the bitrate, the lower the power requirement. If you can
wait a year for an image to download, you can get away
with a very weak received signal. To ensure that the data
is usually received and accurately deciphered, it would be
re-sent several times, with priority given to the most
important data. So if bad weather at a receiver on Earth
means that part of an image is missed, it will be filled in
sometime during the next few weeks, months, or years.

-- Jeff, in Minneapolis

[Selfsim]

That seems very small to me. While I'd expect designers would want to minimize the mass of an interstellar probe, if they have the means to send it to another star in a reasonable time (decades) I would expect they would have better power systems and would be willing to invest in a larger spacecraft.

The size of a probe spacecraft would surely be driven by the mass/volume of: the fuel needed, the power delivery system and the science instrument package(?)

Voyager sets the design high water mark for deep space probe communications success. Even though technology has advanced since it was built, interestingly, the value of the main link design spec (EIRP), has remained unchanged in more modern probes which have more available power. Interesting.

As mentioned, MRO is pretty close to state-of-the-art for digital space comms. It has 100 Watt transmitters and a slightly smaller antenna, which results in about the same EIRP. The SNR of a received MRO signal is about 6 times better than Voyager's, and at 2.7 AUs, it can manage ~500 Kbps, compared with Voyager's 1.5 Kbps at 120 AUs .. I'm not sure if Voyager data can still be received at this bitrate .. probably not … (due to the inverse square digital information/distance law). Either way, the advances made in the MRO communications technology aren't huge steps forward when compared with the challenge light year distances impose on the design.

My first cut thought would be something with an antenna diameter similar to Arecibo's, with a comparable transmitter. At least in that case, I believe it's been established that two Arecibo-like radio telescopes could communicate over several light years. (There have been some overstatements, claiming Arecibo could communicate with a twin across the galaxy, and while that's been shown to be wrong, it would apparently have little trouble if the twin was at a nearby star.)

Keep in mind that a space based telescope similar to Arecibo could be much less massive than the ground based one. The material of the reflector could be quite thin.

Sure. I have doubts about the manageability and reliability of such a big antenna in space (of the size of Arecibo's 305 m). If it was made of lightweight, thin materials, I have doubts about the consistency of the steady-state radiation footprint pattern, its efficiency, and its ability to stay within the receiving station's link budget criteria. What happens to the link when the spacecraft has to move quickly, (for eg)? Remember Arecibo is embedded in a stable extinct sinkhole in the Earth … not free floating in space!
How such an antenna is planned on being deployed may also effect its end-goal performance too … eg: 'unfurling' following probe launch could result in having to use materials which are too flimsy for a long duration flight ..?.. or assembly by humans in LEO whilst resulting in a more rigid structure, requires having to actually build it in space (now how would that happen?)
How do you maintain the integrity of the antenna surface when it is expected to be bombarded with space debris and micro asteroid type particles over substantial periods of time? What would be the lifetime of the materials, relative to the voyage duration ? Would the materials 'make the distance' ? Many of the DSN antennae have fallen into a state of disrepair and require a lot of upkeep. How does this get done on our deep-space probe ? (If it doesn't, past Earth-based experience with big antennae has shown that they won't remain functional or reliable for very long, unmaintained).

Voyager has 3 axis stabilisation to maintain accurate pointing of its high gain antenna. This has required mass expulsion thruster systems which have already expended well over half of its available fuel for attitude control. Whilst other stabilisation methods have been devised, I think the trade-offs with each method compromises certain functionality of the probe. I have no doubts that trying to manage such a huge antenna, over what is likely to be a very long duration, would also come with major tradeoffs. The antenna could be so degraded by the time it gets to the 1 lyr distance, it may be no longer capable of doing the job !

The pointing issue is probably more relevant when the probe is closer to Earth. (It is still relevant when the probe is more distant … although this is less associated with comms per se). This is why secondary, low gain antennas are incorporated on present designs. So, the decision to use such a big, high gain antenna also comes with a need for big, low gain ones as well, (adding to the probe weight/fuel/power requirements). Redundant passive systems would be needed if the duration is long .. adding again to the need for more fuel.

The complexity builds from the details upwards. Eventually, the complexity can result in such unreliability that the idea can easily become not feasible. Engineering has its physics limits too .. not all design problems are necessarily solvable.

If you can get to another star in a reasonable time, I don't think communication will be a major problem.

I do think that deep space comms has its own critical path and physical limitations, quite separable from propulsion systems. I agree that power is the common area of overlap. Whilst such power systems presently exist on Earth, suggesting that such systems could be put into a deep-space probe, imposes a whole bunch of self-limiting constraints on the overall feasibility of such a venture.

I also think that the need to communicate effectively with a remote deep space observation platform, might be a lot closer in time, than the need to traverse the entire distance between Earth and a newly detected Earth-like exo-planet, too. The temptation to point to existing Earth-based systems to argue the feasibility of a deep-space mission, I think, also leads to false assumptions.

[Van Rijn]

The size of a probe spacecraft would surely be driven by the mass/volume of: the fuel needed, the power delivery system and the science instrument package(?)

An interstellar probe would have radically different issues than an interplanetary probe, the key issue being that it would have to travel at a significant fraction of the speed of light to reach nearby stars in a useful timespan. That requires advanced propulsion. The typical concepts I've seen proposed are

(1) A light sail design accelerated by large arrays of space based lasers. Some concepts have been ultra low mass (Starwisp, for example), but most are not.

(2) An advanced nuclear rocket, typically fusion or anti-matter/fusion hybrid. In these concepts, the rocket mass sets a minimum practical mass for the spacecraft - it would be quite a bit more massive than today's interplanetary spacecraft. And, if you're going to go that much trouble, it wouldn't make much sense to send it off without a decent communication system.

Voyager sets the design high water mark for deep space probe communications success. Even though technology has advanced since it was built, interestingly, the value of the main link design spec (EIRP), has remained unchanged in more modern probes which have more available power. Interesting.

What probes are you referring to? Outer system probes today don't have substantially greater energy generation than Voyager. However, the proposed JIMO mission (which would have had a nuclear reactor, and would have generated a lot more power) was to have a more powerful transmitter to support higher data rates. In any event, comparing probes intended for interplanetary exploration to ones intended for interstellar exploration is like comparing apples to oranges, given the very different missions, and mission requirements.

Sure. I have doubts about the manageability and reliability of such a big antenna in space (of the size of Arecibo's 305 m). If it was made of lightweight, thin materials, I have doubts about the consistency of the steady-state radiation footprint pattern, its efficiency, and its ability to stay within the receiving station's link budget criteria.

Apparently, the antennas for some current space satellites are on the order of a hundred meters in diameter, and are quite compact on launch.

What happens to the link when the spacecraft has to move quickly, (for eg)?

Why would the spacecraft have to move (accelerate) quickly?

How do you maintain the integrity of the antenna surface when it is expected to be bombarded with space debris and micro asteroid type particles over substantial periods of time?

I'd design the mission so that I wouldn't expect it to be bombarded with space debris. Among other things, there wouldn't be any particular reason the antenna would need to be flown around in a solar system where it would be most likely to encounter debris. Instead, as I mentioned in an earlier post, interplanetary spacecraft could relay messages to a "mothership" with the interstellar communication antenna.

Voyager has 3 axis stabilisation to maintain accurate pointing of its high gain antenna. This has required mass expulsion thruster systems which have already expended well over half of its available fuel for attitude control.

I believe Voyager used most of its fuel while it was near the planets, and I wouldn't expect an interstellar spacecraft to use chemical rockets for stabilization anyway (ion thrusters would probably be a better choice for minimizing the use of consumables) so I don't see this as a useful comparison.

The antenna could be so degraded by the time it gets to the 1 lyr distance, it may be no longer capable of doing the job !

It might not even be unfurled at that point (it might make more sense to do that after the acceleration, flight, and deceleration phases). But if it were, I'd expect there would be a thin shield in front of it in the direction of flight.

The pointing issue is probably more relevant when the probe is closer to Earth. (It is still relevant when the probe is more distant … although this is less associated with comms per se). This is why secondary, low gain antennas are incorporated on present designs. So, the decision to use such a big, high gain antenna also comes with a need for big, low gain ones as well, (adding to the probe weight/fuel/power requirements). Redundant passive systems would be needed if the duration is long .. adding again to the need for more fuel.

Again, there would already be a significant minimum mass for a fusion rocket. It's going to be much more massive than today's interplanetary probes, so there's no reason to force the whole thing into the mass of a present day interplanetary probe.

[Selfsim]

Well, I've been considering the content of both this thread, and the other one mentioned in my OP (which ended up, yet again, in the mire of opinion-based-non-science, as I suspected ).

I think there are a couple of further points worthy of being made for the sake of this record.

If one's goal is to create a feasible comms design … (which wasn't the purpose of this thread .. I was looking at the constraints .. not designs …) then I'd say that Project Icarus seems to have come up with a reasonable, integrated approach to analysing the feasibility of interstellar probe concepts, including the communications issue.

One of the principal scientists reviewing interstellar communications feasibility is Pat Galea. He contributed content to an article published in Feb 2012 entitled:

PROJECT ICARUS: THE INTERSTELLAR COMMUNICATION PROBLEM,
.. in which most conceivable issues, (also raised over the two threads), are discussed. Areas addressed are: radio, big transmitters, giant receivers and their location, data transfer/bandwidth, lasers, message relay and gravitational lensing.

As of the date of the article, none of these methods for overcoming the fundamental design problems, have reached definitive conclusions.

The article, (not actually written by Galea), concludes with the usual journalistic words of optimisim, even though the #1 fundamental design philosophy of Icarus is:

… to rely upon worst-case calculations as a means of capturing problem uncertainties and allowing a pessimistic assessment of the problem.

(I'll provide a link for this quote, in a yet-to-be created new thread).

Unknowingly, this is precisely one of the background philosophical positions motivating both my OP question and my responses on the other thread. It is a perfectly sound and reasonable position to adopt, when seriously considering engineering feasibility topics such as this one.

The content contained within the above article, I feel, gives a really good overview of the technical issues behind interstellar comms link design, and I'd recommend it as a good starting point for anyone wanting to look into the reality of what is a rather interesting topic.

Regards

[Van Rijn]

PROJECT ICARUS: THE INTERSTELLAR COMMUNICATION PROBLEM,
.. in which most conceivable issues, (also raised over the two threads), are discussed. Areas addressed are: radio, big transmitters, giant receivers and their location, data transfer/bandwidth, lasers, message relay and gravitational lensing.

As of the date of the article, none of these methods for overcoming the fundamental design problems, have reached definitive conclusions.

What do you mean by "fundamental design problems" that supposedly none of the methods can overcome and not having reached "definitive conclusions"? The article states that technological breakthroughs are not required for interstellar communication. The article points out that a radio system would be feasible, and discusses options of interest for future development for higher data rates, lower hardware mass, or lower power requirements.

[cjameshuff]

You have zero justification for your assumption that an interstellar probe would have the same EIRP as Voyager. The only thing you've shown is that planetary probes designed to operate at vastly shorter ranges are unsuitable for missions to other star systems.

The math clearly shows that a probe actually designed for interstellar communications, with a more reasonably sized transmitter, would not have problems communicating over interstellar distances. No new technological developments are necessary. We could talk to interstellar probes now, if we had a means to get them in place.

[Selfsim]

As of the date of the article, none of these methods for overcoming the fundamental design problems, have reached definitive conclusions.

What do you mean by "fundamental design problems" that supposedly none of the methods can overcome and not having reached "definitive conclusions"?

I didn't say that these methods could not overcome the design problems. I said that they haven't yet reached definitive conclusions.

As far as 'inconclusive' is concerned:
i) Big probe antennae:

We haven't yet established whether Icarus will have a large dish available for use as an antenna. As Icarus has a requirement to decelerate at the destination star system (a requirement that Daedalus did not have) it may be the case that we cannot rely on having this resource available to us.

… Ie: For further study. Inconclusive at present.

ii) Fresnel Zone antenna:

It looks like it's going to be too inefficient for our purposes right now, but just having done the research is a powerful contribution to our understanding of the options available to us.

Ruled out for the present? ('Not preferred at present' may be more accurate here?)

iii) Lasers:

While a radio antenna does have to be pointed accurately, the requirement for the laser is much more severe. The problem is known as Acquisition, Tracking and Pointing (ATP).
...
Now the problem for Icarus is that it's not clear whether transmitting an uplink laser beam from Earth will actually help the craft to aim its own laser. The distances involved are so large that the errors may swamp the benefits.

So we are looking into this, as well as other methods such as determining position and direction from analysis of pulsars. Initial calculations on the raw characteristics of laser communications, conducted with the assistance of our student designer Divya Shankar in India, show that the technology holds great promise for delivering respectable amounts of data across large distances.

This by no means, rules out lasers, (to the contrary, actually ... lasers actually look to be the best bet), none-the-less, the outcome appears to be: for further research (ie: not conclusive .. but holds a 'promise' .. )

iv) Location

By having multiple copies of the antenna array in different locations on Earth, weather effects can be mitigated, and a near-continuous link with the craft can be maintained. This might not be cost-effective for one mission, but if the network is being used to cover several missions (perhaps interplanetary craft as well) then it may begin to make economic sense.

Another inconclusive … 'may' = speculation.

Back to lasers .. Earth orbit or moon based .. Lagrange points ..

We are investigating all of these options to see which makes the most sense for Icarus.

Ie: for further investigation .. inconclusive.

v) Message Relay:

Unfortunately, when we built our mathematical model for a relay system using radio frequencies, it turned out that the number of relays required just to match the performance of the direct link is immense, even given very conservative assumptions about the antenna size and power available to each relay.
At this stage it appears that relays will not help the communications problem. However, that doesn't mean the relays will be abandoned as a concept. They may still have value for other purposes, such as scientific investigations, and there is a possibility that relays may be more economical for laser communications.

Ie: for further investigation .. inconclusive, and speculative.

vi) Grav Lensing:

Now there are a couple of significant challenges to getting this system to work.

First, we have to get a receiver craft out to 550 AU from the Sun. Currently, that's a tough problem, but the kind of civilization that can launch an Icarus-like interstellar probe will probably not find this too difficult.

A circular conclusion based on the premise that such a civilisation actually exists (??) For me, this statement doesn't really say anything of particular value as far as conclusions go.

Second, we have to hold the receiver craft exactly in line with the center of the Sun and the distant Icarus probe. The tolerance here is severe. The receiver cannot drift by more than a few meters off-track before it loses the signal completely.
...
The problem we have not tackled yet is the thrust requirements of the receiver craft. In order to maintain its lock on the signal, the craft at the focus will need to make adjustments to its position over time. If the distant Icarus probe is in orbit around the target star, then the receiver craft will need to be making a large amount of adjustment for a prolonged period of time. It is not yet clear whether this aspect of the system can be solved in the near term.
For now, we are keeping gravitational lensing as an interesting option that could potentially be tested using Icarus, but we are not going to specify it as the primary communications mechanism.

Grav lensing not to be specified for primary comms ...

The article states that technological breakthroughs are not required for interstellar communication.

Agreed that is what the article author quotes as being the views of the original Daedalus project from the 1970s. But clearly many enabling technological breakthroughs have happened since the 70s, and it could be argued that the statement cited in the article conclusion, (attributed to the Daedalus project), was in error, if it actually was made in the 70s.

And the study couldn't produce any such conclusions because of another one of their design philosophy tenets:

Extrapolations of current technology are to be of a linear type only and limited to a few decades hence.

The article points out that a radio system would be feasible, and discusses options of interest for future development for higher data rates, lower hardware mass, or lower power requirements.

I think Galea is saying that they need to investigate the outlined details further, before they reach that conclusion.

Please note, once again there are three ways of looking at this:
1) Feasible: (The optimistic conclusion);
2) Not feasible: The pessimistic conclusion;
3) Inconclusive … pending further investigative outcomes - this is where I'm coming from. Not (2) above. I offer this to clarify misunderstandings which may be going on here about this.

I'm of the view that engineering matters cannot be declared as being practically feasible, until the outcome of (i) a feasibility study (like Icarus' present state) is declared and; (ii) trials demonstrating the tolerances called for in the design, can be achieved. This point hasn't been reached yet, and in some of the cases above, this requires extensive trial outcomes.

Theoretical feasibility is a different matter. Clearly, it is theoretically feasible. (It wouldn't be admissible for Icarus, if it wasn't).

[Selfsim]

You have zero justification for your assumption that an interstellar probe would have the same EIRP as Voyager. The only thing you've shown is that planetary probes designed to operate at vastly shorter ranges are unsuitable for missions to other star systems.

From my recollection of my many words, I am unaware that I have assumed (or claimed) that an interstellar probe would necessarily have to have the same EIRP as Voyager. (If I have given that impression, then I am happy to retract the words which gave such an impression). I did observe that MRO and other interplanetary probes, have been designed with similar EIRPs, so I just used that as an example of how to do the calculations, and as a baseline figure (ie: using something which has been shown to work). I asked how big a receiving antenna would be required, to give some kind of quantifiable baseline about how far away in technology terms we might be, using RF technologies. Your objection to my using such a crude model is acknowledged .. but I wasn't making the universal assumption you imply. Clearly the discussion has yielded more information about other options. Whether they are practically feasible or not, is inconclusive to me.

As far as your observation of what I've shown .. yep .. and that's all I was attempting to do .. as a place to start.

The math clearly shows that a probe actually designed for interstellar communications, with a more reasonably sized transmitter, would not have problems communicating over interstellar distances. No new technological developments are necessary. We could talk to interstellar probes now, if we had a means to get them in place.

From a practical engineering perspective, I am not yet of that view. (See my prior post and the Icarus analysis). If the 'math' you are referring to, is being used in theoretical calculations .. well, I have no problems.
I remain open-minded about the need for technological developments, also. Any of the unknowns could result in niche technological developments, which could make all the difference.
Such developments could also happen irregardless of the interstellar comms needs. (After all, it seems this is what happened since the original Icarus statement from the 70s).

[Van Rijn]

I didn't say that these methods could not overcome the design problems. I said that they haven't yet reached definitive conclusions.

Definitive conclusions about what? Here's one definitive conclusion: Conventional radio would be sufficient for interstellar communication. It would not require technological breakthroughs, nor would there be radical hardware requirements. By the time we're actually ready to fly an interstellar probe, there may then be better choices, but it certainly is feasible.

As far as 'inconclusive' is concerned:
i) Big probe antennae:

We haven't yet established whether Icarus will have a large dish available for use as an antenna. As Icarus has a requirement to decelerate at the destination star system (a requirement that Daedalus did not have) it may be the case that we cannot rely on having this resource available to us.

… Ie: For further study. Inconclusive at present.

Please clarify. Are you claiming that quoted statement about the Icarus concept makes some general argument about interstellar communication? The Daedalus concept was for a fly-by mission, and as the article points out, in the Daedalus proposal part of the second stage engine was to be used as a 40 meter parabolic antenna after engine shut down. The Icarus concept, not a fly-by, wouldn't have that resource available. But, as mentioned earlier in thread, there are real spacecraft with 100 meter antennas.

iv) Location

By having multiple copies of the antenna array in different locations on Earth, weather effects can be mitigated, and a near-continuous link with the craft can be maintained. This might not be cost-effective for one mission, but if the network is being used to cover several missions (perhaps interplanetary craft as well) then it may begin to make economic sense.

Another inconclusive … 'may' = speculation.

What is your point? The quoted material is discussing relative costs of different antenna options, and has nothing to do with technical feasibility. Sorry, but it is sounding like you are quoting anything to repeat "inconclusive" and "speculation."

vi) Grav Lensing:

A circular conclusion based on the premise that such a civilisation actually exists (??) For me, this statement doesn't really say anything of particular value as far as conclusions go.

Not circular. That's referring to using the sun as a gravitational lens by spacecraft sent to 550 AU (a bit more is best) from the sun. A civilization that has built interstellar capable spacecraft could easily build spacecraft to reach 550 AU.

Agreed that is what the article author quotes as being the views of the original Daedalus project from the 1970s.

And follows it with "Today, this statement still holds good. We could, indeed, begin developing this system right now."

I'm of the view that engineering matters cannot be declared as being practically feasible, until the outcome of (i) a feasibility study (like Icarus' present state) is declared and; (ii) trials demonstrating the tolerances called for in the design, can be achieved. This point hasn't been reached yet, and in some of the cases above, this requires extensive trial outcomes.

"Practically feasible"? "Feasibility study"?

I'm wondering whether there is a conceptual issue here. Do you understand that propulsion is the key problem with interstellar spacecraft? Icarus is not a feasibility study, but a theoretical engineering study, because the assumed (and required) propulsion technology (a fusion rocket) does not yet exist. It may be centuries before there will be the technology and resources to build something like Icarus.

So, interstellar spacecraft that can reach other stars in less than a century currently are not practically feasible. If you want to talk about interstellar spacecraft feasibility, there's no need to discuss communication at all.

But we do have communication technology that could work over interstellar distances, and the mass requirements aren't unreasonable compared to the expected mass of a fusion rocket.

[Selfsim]

Definitive conclusions about what? Here's one definitive conclusion: Conventional radio would be sufficient for interstellar communication. It would not require technological breakthroughs, nor would there be radical hardware requirements. By the time we're actually ready to fly an interstellar probe, there may then be better choices, but it certainly is feasible.

I notice some variation of criteria here. I have specifically used the term 'practically feasible', and qualified it as meaning to allow useful, intelligible messages to be received from the probe.

I have no idea of what 'sufficient for interstellar communication' means .. I can only assume this differs from what I'm talking about (?) If I bring the comment back to my definition, then I still disagree with your view, until I see some evidence of a workable, quantified design which thus far, hasn't been forthcoming.

Please clarify. Are you claiming that quoted statement about the Icarus concept makes some general argument about interstellar communication? The Daedalus concept was for a fly-by mission, and as the article points out, in the Daedalus proposal part of the second stage engine was to be used as a 40 meter parabolic antenna after engine shut down. The Icarus concept, not a fly-by, wouldn't have that resource available. But, as mentioned earlier in thread, there are real spacecraft with 100 meter antennas.

... and those spacecraft aren't going anywhere even vaguely resembling the distances of an interstellar voyage! Just think about the differences from interstellar ...
- no massive ground array required;
- 'real-time' bidirectional interchange between the orbiting satellite and ground receivers;
- life expectency not needed to span mutiple decades;
- shorter link distances means way higher bandwith and way higher throughput efficiency;
- operating frequency choices due to not require prior knowledge of the intervening space 'column' composition (dust refraction, dispersion etc);
- link margins and manufacturing tolerances not so critical;
- no doppler downshift due to spacecraft radial velocities ... so no downshifting to risk proximity with atmospheric water absorption bands (in the case of ground based receivers) … or unknown itinerant space absorption within the RF comms bands;
- no need for high power supply output aboard the spacecraft, (100s to 100s KW), thereby lowering the significant spacecraft based wavetube amplifier magnet mass;
- no addtional link margins to compensate for pointing inaccuracies resulting from atmospheric refraction, (assuming ground-based receivers), spacecraft motion, and control limits of antenna pointing;
- no need for a surface reflector precision of better than ~ +/- 0.1mm, (ie: in order to maintain deep space link performance at X-band or above);
- no need for signal-relays as an alternate backup, etc;

Also interesting was why the Daedulus fly-by mission changed to decelerate, (ie: Icarus), thereby altering the availability of a potential antenna resource). Clearly a probe has to do more than just take some quick snapshots .. this means more data to transmit, which places other demands on the comms design (more throughput to maintain the same or better bit error performance, etc).

What is your point? The quoted material is discussing relative costs of different antenna options, and has nothing to do with technical feasibility. Sorry, but it is sounding like you are quoting anything to repeat "inconclusive" and "speculation."

Well, I am sorry but relative costs are part of engineering feasibility. Eg: if cost was not a constraint on the design, then why not put a receiver array on the moon? .. and one in orbit around the Sun ... and one at the outer edges of the solar system .. and anywhere else one's heart desires?

Now there are a couple of significant challenges to getting this system to work.

First, we have to get a receiver craft out to 550 AU from the Sun. Currently, that's a tough problem, but the kind of civilization that can launch an Icarus-like interstellar probe will probably not find this too difficult.

Not circular. That's referring to using the sun as a gravitational lens by spacecraft sent to 550 AU (a bit more is best) from the sun. A civilization that has built interstellar capable spacecraft could easily build spacecraft to reach 550 AU.

Of course its circular ... since we haven't constructed such a probe ourselves yet, how can we possibly judge the degree of difficulty for some other civilisation? How would we have any idea of the intellectual capabilities of other civilisations to come up with similar technologies? Its also circular because it assumes we know more about the conclusion than is currently available.

And follows it with "Today, this statement still holds good. We could, indeed, begin developing this system right now."

It sure does ... but that's not what the Icarus review has stated.
This (quoted) conclusion is speculative, and is not supported with a feasible design. If you disagree, can you please provide such a design? (Links to peer reviewed materials would be fine).

"Practically feasible"? "Feasibility study"?

I'm wondering whether there is a conceptual issue here. Do you understand that propulsion is the key problem with interstellar spacecraft? Icarus is not a feasibility study, but a theoretical engineering study, because the assumed (and required) propulsion technology (a fusion rocket) does not yet exist. It may be centuries before there will be the technology and resources to build something like Icarus.
So, interstellar spacecraft that can reach other stars in less than a century currently are not practically feasible. If you want to talk about interstellar spacecraft feasibility, there's no need to discuss communication at all.

The fundamental reason for building a probe is to report back to us, data gathered from that which it has surveyed (at say, ~ 4 lightyears distant). Its clear that the design of the probe is heavily influenced by the need to get to the sensory hardware to the destination AND the need to report back to us. The communications system design influences the propulsion system AND results in demands on the supporting technologies ... in order to achieve appropriate reliability, performance and precision over the designed probe lifetime. I think the conceptual issue here, is that the overall goal of an interstellar probe seems to have diappeared from sight due to the use of a heavy reductionist approach, resulting in a propulsion-only focus. There is a subset of solutions which might well solve the propulsion problem .. but do not result in solutions to the communications issues. Assuming that all interstellar communications issues can be resolved by solving propulsion matters, is certainly not clear from anything I've read about this.

But we do have communication technology that could work over interstellar distances, and the mass requirements aren't unreasonable compared to the expected mass of a fusion rocket.

.. a fusion propulsion system which does not exist yet? What if it never exists? Would the communications systems mass requirements be met by whatever propulsion system which does result? ..And what about the other issues (listed above), which go way beyond just propelled mass?

[Ara Pacis]

The fundamental reason for building a probe is to report back to us, data gathered from that which it has surveyed (at say, ~ 4 lightyears distant). Its clear that the design of the probe is heavily influenced by the need to get to the sensory hardware to the destination AND the need to report back to us. The communications system design influences the propulsion system AND results in demands on the supporting technologies ... in order to achieve appropriate reliability, performance and precision over the designed probe lifetime. I think the conceptual issue here, is that the overall goal of an interstellar probe seems to have diappeared from sight due to the use of a heavy reductionist approach, resulting in a propulsion-only focus. There is a subset of solutions which might well solve the propulsion problem .. but do not result in solutions to the communications issues. Assuming that all interstellar communications issues can be resolved by solving propulsion matters, is certainly not clear from anything I've read about this.

I have to agree with Selfsim here. Everything needs to be built around the data transmission system. If you can't hear it, then there's no sense sending it. I'd suggest two large dishes. One for use during transit and one for use after arrival. They could be the same size or the second could be even bigger if needed for higher throughput.

[cjameshuff]

I have to agree with Selfsim here. Everything needs to be built around the data transmission system. If you can't hear it, then there's no sense sending it. I'd suggest two large dishes. One for use during transit and one for use after arrival. They could be the same size or the second could be even bigger if needed for higher throughput.

Completely wrong. For interstellar probe, the overwhelming concern is the propulsion system. Communications is trivial in comparison...it has been demonstrated several times and in several different ways that present day systems (decades-old systems, in fact) could do it. A 100 meter 500 kilowatt 8415 MHz transmitter with 60% aperture efficiency (similar to Voyager, and likely a major underestimate for a real 100 meter dish) and a matching 100 meter receiver here in the solar system matches the signal Goldstone receives from Voyager at a distance of 10 light years. Use a 1 km receiver dish and the required power goes down to 5 kW. Use that with a 200 meter transmitter dish and 1 megawatt transmitter, and you can match Voyager's signal from a distance of nearly 300 light years. And Voyager 1's over 30 years old, it's not even close to the limits of what can be done now.

Selfsim's argument rests on the idea that the laws of physics might suddenly change when a communications link is used across interstellar distances, thus making anything that would actually reasonably be considered for interstellar communications "unproven". He's arguing from personal incredulity and ignoring the extremely well supported math that disagrees with him, instead engaging cherry picking and quote mining to support his position. It's a plain fact that present day conventional radio is entirely adequate for interstellar communication. Finding the optimal combination of radio/laser wavelength, power, and transmitter and receiver size would require extensive further study and would need to be considered together with the propulsion and other systems, but there is no reasonable basis for doubting that it could be done or even considering it to be one of the major difficulties.

[Ara Pacis]

Completely wrong. For interstellar probe, the overwhelming concern is the propulsion system. Communications is trivial in comparison...it has been demonstrated several times and in several different ways that present day systems (decades-old systems, in fact) could do it. A 100 meter 500 kilowatt 8415 MHz transmitter with 60% aperture efficiency (similar to Voyager, and likely a major underestimate for a real 100 meter dish) and a matching 100 meter receiver here in the solar system matches the signal Goldstone receives from Voyager at a distance of 10 light years. Use a 1 km receiver dish and the required power goes down to 5 kW. Use that with a 200 meter transmitter dish and 1 megawatt transmitter, and you can match Voyager's signal from a distance of nearly 300 light years. And Voyager 1's over 30 years old, it's not even close to the limits of what can be done now.

Selfsim's argument rests on the idea that the laws of physics might suddenly change when a communications link is used across interstellar distances, thus making anything that would actually reasonably be considered for interstellar communications "unproven". He's arguing from personal incredulity and ignoring the extremely well supported math that disagrees with him, instead engaging cherry picking and quote mining to support his position. It's a plain fact that present day conventional radio is entirely adequate for interstellar communication. Finding the optimal combination of radio/laser wavelength, power, and transmitter and receiver size would require extensive further study and would need to be considered together with the propulsion and other systems, but there is no reasonable basis for doubting that it could be done or even considering it to be one of the major difficulties.

I think you're reading too much into what I wrote. I only agree with the part I said I agree with, that solving the communications problem is a sine qua non for a probe. After you solve that, then you can worry about propulsion. If you believe you have communications solves, then great, move on to propulsion. That's all.

BTW, after scanning the thread again, I think that your post here is the first to support your claim with math.

[cjameshuff]

I think you're reading too much into what I wrote. I only agree with the part I said I agree with, that solving the communications problem is a sine qua non for a probe. After you solve that, then you can worry about propulsion. If you believe you have communications solves, then great, move on to propulsion. That's all.

Alright, communication of some sort is a prerequisite. But everything required for communications exists already. No new R&D is needed, it's just a matter of engineering to repeat what's already been done elsewhere. The reflector sizes I gave weren't even particularly speculative, in the mid-1990s we were launching Trumpet reconnaissance satellites with self-deploying 106 meter reflectors (some sources claim 150 meters), with single Titan 4 launches. And Hubble's 7 milliarcsecond pointing accuracy is good enough to get the center of the beam within 0.02 AU (10 light seconds) of its target from a distance of 10 light years. (and there's references to 0.3 milliarcsecond measurements, which might indicate what an upgraded tracking system could accomplish with Hubble's fine guidance sensors)

In comparison, the propulsion requirements are going to be many orders of magnitude beyond anything yet attempted, involving manipulating far larger amounts of power and expenditure of vast amounts of energy. If the probe is only sent at 2% c (218 years to Alpha Centauri), every 1 kg of probe has kinetic energy equivalent to 4.3 kt of TNT...or total conversion of 200 mg of matter. We can see some ways this might be done, but it's a problem of a completely different order from communications.

BTW, after scanning the thread again, I think that your post here is the first to support your claim with math.

There were bits and pieces scattered through this one and the thread this one split from. The scaling with size had been mentioned earlier, and detailed studies elsewhere have been referred to (Daedalus and Icarus, which clearly stated that communications did not require any new technology even in the 70s and that there were no fundamental obstacles to interstellar communication, despite Selfsim's own conclusions).

[Ara Pacis]

Alright, communication of some sort is a prerequisite. But everything required for communications exists already. No new R&D is needed, it's just a matter of engineering to repeat what's already been done elsewhere. The reflector sizes I gave weren't even particularly speculative, in the mid-1990s we were launching Trumpet reconnaissance satellites with self-deploying 106 meter reflectors (some sources claim 150 meters), with single Titan 4 launches. And Hubble's 7 milliarcsecond pointing accuracy is good enough to get the center of the beam within 0.02 AU (10 light seconds) of its target from a distance of 10 light years. (and there's references to 0.3 milliarcsecond measurements, which might indicate what an upgraded tracking system could accomplish with Hubble's fine guidance sensors)

In comparison, the propulsion requirements are going to be many orders of magnitude beyond anything yet attempted, involving manipulating far larger amounts of power and expenditure of vast amounts of energy. If the probe is only sent at 2% c (218 years to Alpha Centauri), every 1 kg of probe has kinetic energy equivalent to 4.3 kt of TNT...or total conversion of 200 g of matter. We can see some ways this might be done, but it's a problem of a completely different order from communications.




There were bits and pieces scattered through this one and the thread this one split from. The scaling with size had been mentioned earlier, and detailed studies elsewhere have been referred to (Daedalus and Icarus, which clearly stated that communications did not require any new technology even in the 70s and that there were no fundamental obstacles to interstellar communication, despite Selfsim's own conclusions).

Thanks. I didn't know what sizes and powers would be needed so I was looking for solid numbers.

[Selfsim]

Selfsim's argument rests on the idea that the laws of physics might suddenly change when a communications link is used across interstellar distances,

Absolute nonsense! ... I have never made that claim. Nor will I ever knowingly do so!

If you believe this to be so, then please provide us with evidence that I did!

He's arguing from personal incredulity and ignoring the extremely well supported math that disagrees with him, instead engaging cherry picking and quote mining to support his position.

Excuse me Sir/Madam .. your attempts at deliberately misinterpreting everything I've said are not appreciated. Kindly do not speak again on my behalf, as seen from your own perspective.

[cjameshuff]

Absolute nonsense! ... I have never made that claim. Nor will I ever knowingly do so!

If you believe this to be so, then please provide us with evidence that I did!

Your continued refusal to accept that interstellar communication is possible with present day technology, in spite of being shown otherwise, on the grounds that it hasn't been proven for interstellar communications. For those conclusions to be wrong would require those physical models to suddenly fail when applied to interstellar distances, which makes your position equivalent to a claim that the laws of physics might be inexplicably different when communicating between stars. Yes, it is nonsense! But, it is the argument you've made.

Excuse me Sir/Madam .. your attempts at deliberately misinterpreting everything I've said are not appreciated. Kindly do not speak again on my behalf, as seen from your own perspective.

Daedalus: "We do not need technological "breakthroughs" in this area; development of the system could begin now."

Icarus: "Today, this statement still holds good. We could, indeed, begin developing this system right now."

And you ignored this conclusion and selectively quoted and rather severely reinterpreted bits of the Icarus study to support your own claim that the studies were "inconclusive", to support your own assertion that interstellar communications wasn't possible. I'm not the one deliberately misinterpreting things, here.

[Van Rijn]

Selfsim, these were your OP questions:

Ok, so following on from several responses in the "What happens if we discover Alpha Centaury simply does *not* have planets?" thread, I'd like to pose a question:
"What size receiver antenna would need to be used to receive an intelligible message from a probe, say, one light year distant? (For modelling purposes, I'd define 'intelligible' as being similar to say, the quality of messages Voyager 1 sent Earth, when it was ~120 AUs distant).

The purpose of this being, to quantify just how far away in technology development terms we really are, when it comes to attempting meaningful interstellar communications using RF technology. (Even though one light year is not really an interstellar distance for us ...).

Do you agree that this has been answered?

[publiusr]

Large dishes are the key--its one of the reasons people are interested in the zombie sat deal. On page 20 of the Aug 20 2012 Av Week is a quote by Seamus Tuohy the space systems director at Draper labs "usually the antenna is OK...Large antennas drive the size of satellites and in turn rocket boosters because there is a limit on how much they can be folded for launch. The rule of thumb is 3:1"

This might not always be the case http://abyss.uoregon.edu/~js/space/lectures/lec25.html Take a look at the Spider Space Station Concept (1976).
http://www.nasa.gov/offices/oct/earl...spiderfab.html

Large structures are going to be needed at some point in time as craft move farther out.

[cjameshuff]

Large dishes are the key--its one of the reasons people are interested in the zombie sat deal. On page 20 of the Aug 20 2012 Av Week is a quote by Seamus Tuohy the space systems director at Draper labs "usually the antenna is OK...Large antennas drive the size of satellites and in turn rocket boosters because there is a limit on how much they can be folded for launch. The rule of thumb is 3:1"

That is just a rule of thumb, however. Here's the Trumpet satellite I mentioned:
http://www.globalsecurity.org/jhtml/...tstages.jpg|||
http://www.globalsecurity.org/space/systems/trumpet.htm

That's a 106 m "wrap-rib" unfurling dish that fits in a cylinder 16 m long and a bit over 4 m in diameter. Probably not cheap, but compared to the cost of sending something to another star system...

And that's self-deploying. If you're shipping up parts and materials for assembly in orbit...well, you can pretty much get as big as you want. And of course, laser communication can use much smaller reflectors...a Hubble sized transmitter would be reasonable.

[Ara Pacis]

What sort of signal would an interstellar comms link use? People have been talking about certain equipment having limits with regards to shelf life and operational lifetime. But would we need something that delicate, if delicate it is? Would we be using modulation of a carrier wave, spread spectrum or Ultra Wide Band or would we simply be using basic binary pulsing, like Morse Code (quinary)? Radiotelegraphy would seem to be the simplest and easiest to achieve at interstellar distances, wouldn't it?

[cjameshuff]

What sort of signal would an interstellar comms link use? People have been talking about certain equipment having limits with regards to shelf life and operational lifetime. But would we need something that delicate, if delicate it is? Would we be using modulation of a carrier wave, spread spectrum or Ultra Wide Band or would we simply be using basic binary pulsing, like Morse Code (quinary)? Radiotelegraphy would seem to be the simplest and easiest to achieve at interstellar distances, wouldn't it?

The Voyager probes use relatively simple modulation techniques. Biphase encoded digital data (essentially, combined with a clock to force frequent transitions to prevent loss of sync) modulated with BPSK (binary phase-shift keying). Just going to quadrature PSK would double the data rate with the same bandwidth, and there's higher orders of PSK that can further increase the data rate or decrease the required bandwidth or power. These do increase the probability of errors, but forward error correction codes can be used to protect data from these errors, still yielding a net benefit...and this isn't a particularly exotic modulation technique. Summarized...Voyager isn't particularly close to the theoretical limits of its link.

Frequency hopping and ultra-wideband would be sensitive to dispersion by ionized gasses...a signal transmitted at one frequency might be significantly delayed relative to a signal transmitted at another frequency. It should be possible to correct this at the receiver, however, or careful choice of frequency ranges could avoid it. These are mainly useful for terrestrial environments though...maybe they'd have application with multiple communication links between two systems.

Lifetime is a definite issue, but there's plenty of examples of electronic equipment that's built to last decades or has already done so. Building to operate for hundreds or thousands of years without maintenance would be an engineering challenge, but a relatively minor one in comparison to propulsion. Semiconductors mainly age while in operation, due to diffusion, electromigration, hot carrier injection, etc, and these aging processes can be greatly reduced by keeping current densities low, so spending most of the time in sleep mode would greatly increase longevity. Technologies developed for high temperature and high power use (silicon carbide, diamond semiconductors) could also be used to make particularly long-lived and robust control electronics.

You could also use vacuum electronics, which avoid much of the difficulties with diffusion or dielectric breakdown. Not old style thermionic tubes, but rather micro/nanoelectronic devices using field emission cathodes and fabricated using MEMS/integrated circuit techniques. This technology would require rather more development, though it is in current use (field emission displays were a competitor for LCD displays, and field emission electron guns are used in some electron microscopes).

At worst, you can just keep redundant equipment around. Shelf life is practically indefinite for most electronic components, and and there's alternatives for the those that do have a limited shelf life.