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This is something I was thinking about right now when this beta was opened ... perfect!

In radio astronomy, there is an technique which is called very long baseline interferometry. The idea is to combine a number of telescopes to resemble a somewhat larger non-existing telescope. The intended purpose is usually described as gaining a higher angular resolution when observing the sky.

Now imagine that you do something similar with a number of dishes somewhere in deep space for the purpose of picking up signals from Earth. Compared to that, current deep space missions such as New Horizons use a fairly large single dish for receiving commands from Earth. Imagine, you instead have a number of small spacecrafts with very small dishes but at high distance to Earth. Imagine that those small space crafts are somehow capable of performing very long baseline interferometry and are supposed to receive commands from Earth.

My question is, could this array of small dishes with the small spacecrafts be used to "substitute" a single bigger dish in this context? Or are an improved angular resolution and picking up signals over long distances somewhat different problems?

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The problem with very distant communications is one of power, not "resolution" (which doesn't quite make sense in that context anyway). So you could have a whole array of receivers, but they're all going to be receiving the same (small) amount of power, and since the assumption is that one receiver is not enough to close a link, any number of receivers is not going to help you.

Now, let's say a signle receiver just barely doesn't close a link. Something like you see a coherent signal, but there is too much noise for it to be usable. In that case, if you have an array of receivers like this, it's possible you could work some communications wizardry and lower the error rate in the signal (if all receivers in your array were receiving at a slightly different phase, for instance), but that goes beyond my understanding of signal processing.

(Although I use "receivers" in the answer, the same holds for "transmitters", since it's the same problem, but in reverse).

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  • $\begingroup$ I think it could be about 'noise' and some 'communications wizardry'. What I do not see is how the physics behind an 'improved resolution' can (or cant) kick in here ... If I understand your answer correctly, an extended baseline with small dishes will not do the trick for what I want? $\endgroup$ – s-m-e Jul 24 '13 at 22:43
  • $\begingroup$ @ernestopheles - Yes, you understand correctly. I am asserting that 'improved resolution' does not help you here, unless you were trying to locate a signal that is strong enough to be seen by individual antennae. The "big dish" you want to get rid of is necessary to receive enough RF energy to pull the signal down. $\endgroup$ – user29 Jul 25 '13 at 0:14
  • $\begingroup$ On the other hand adequate dispersal may reduce the necessity of having the transmitter antenna on outward-bound craft pointed exactly at Earth. That dispersal though would need to encompass a huge volume! $\endgroup$ – Everyone Aug 30 '13 at 7:52
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Interferometry is used to calculate distance. In VLBI they use multiple telescopes so that they can check the arrival time of the signal (like how GPS works).

From the wiki:

In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth. The distance between the radio telescopes is then calculated using the time difference between the arrivals of the radio signal at different telescopes. This allows observations of an object that are made simultaneously by many radio telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes.

The key part is emulating; as in it gives the effect of using a large telescope. I think that is where the core of your question is coming from.

Using small dishes on several deep space satellites may not work because they just may not be able to amplify the signal enough to receive anything. Also, VLBI would require that the time the signal arrives and the position of the receiver are known with great precision. Probes flying out into space at different speeds will have different drifts in their atomic clocks, as well as problems in determining the exact position (within millimetres) of each other.

If you're going to launch multiple satellites to improve deep space communication then it may be better/simpler to launch them so that they act as repeaters for the signal.

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  • $\begingroup$ "effect of using a large telescope [...] core of your question" - Right, it is. I am looking into this issue because of a mission concept, which requires high precision timing and relative positioning for the payload anyway, so consider those problems 'solved'. The last remaining elephant in the room is a big dish, which I want to eliminate. So the emulating bit in your wiki quote is actually interesting. $\endgroup$ – s-m-e Jul 23 '13 at 17:58
  • $\begingroup$ @ernestopheles, With a VLA telescope you can get the receive the data from the probe on earth just fine; however getting the probe to receive the data from earth is the hard part? I say use a super power laser to send light deep into space where the probe is. Modulate the data by using a mirror galvanometer to deflect the beam away from the probe as a way of sending pulses. $\endgroup$ – user39 Jul 24 '13 at 3:14
  • $\begingroup$ Lasers are interesting in this context, too, but I am specifically asking for radio communication here :-) So, yes, the hard part is receiving commands in space. I need a definite rationale, why this could or could not work. Bounty is off in 22 hours, btw. $\endgroup$ – s-m-e Jul 24 '13 at 22:53
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It should work - the challenge you would have is positioning the dishes accurately on that baseline, or at least knowing how far off the baseline they are at any moment, in order to calculate the relative phase of an incoming waveform.

On Earth, their positions can be measured very accurately, and they tend to stay where they are. In space, they will each have their own orbit and be affected by impact from debris in space, solar wind and other particles etc.

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  • $\begingroup$ I am sure that VLBI works in space. The question is more whether it makes sense for picking up distant signals rather than improving the resolution of imaging ... ? $\endgroup$ – s-m-e Jul 16 '13 at 23:24
  • $\begingroup$ Well, it does improve the resolution - you get improvement from extending the baseline, and from adding more receivers. $\endgroup$ – Rory Alsop Jul 18 '13 at 22:07
  • $\begingroup$ 'improvement from extending the baseline' - that's the point. How could this be useful for picking up a signal in terms of communication? Bounty is off in 22 hours, btw :-) $\endgroup$ – s-m-e Jul 24 '13 at 22:36

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