One problem with nano-satellites and generally any small spacecraft is that you can't fit a powerful power source and antenna - their usability outside vicinity of 'adult' satellites or Earth is thus limited.

Is it possible for a whole swarm of, say, cubesats somewhere in the Asteroid Belt, each staying in range of communication of several neighbors, to synchronize data to send and mutual positions, and amplify the signal by broadcasting together - and similarly extracting uplink data from whatever they 'hear' with their small receiver antennas, through communicating, comparing results and finding signal in the mass of noise each receives? Or does a phased array require a physical link between the segments?

  • $\begingroup$ I like this question as it brings up parallels such as the VLA: public.nrao.edu/visit/very-large-array $\endgroup$ – fred_dot_u Oct 9 '18 at 20:45
  • $\begingroup$ @fred_dot_u thanks for the suggestion! It got me thinking and I always welcome an opportunity to procrastinate. $\endgroup$ – uhoh Oct 10 '18 at 4:58
  • $\begingroup$ Thanks for the accept, but people could have more to offer on this topic. Maybe let this question go around the Earth a few times before accepting? $\endgroup$ – uhoh Oct 10 '18 at 6:27
  • $\begingroup$ But the question is: will that whole swarm of, say, cubesats somewhere in the Asteroid Belt have a total mass comparable to a single spacecraft there? If you need to launch much more mass of nano-probes to get the phased antenna working, is there any advantage? $\endgroup$ – Uwe Oct 10 '18 at 7:45

Partial answers, remaining alert for potential caveats in the question:

This is is a really interesting question! While resolution of a radio telescope array benefits from a huge baseline, the total receiving area (and received power) is just the sum of the effective apertures. Thus the name Square Kilometer Array refers to the sum of the dish areas, not the footprint on the Earth. For a single solid dish, gain scales with size squared and resolution with inverse size, so we tend to link improvements in gain with improvements in theoretical resolution. But for an array of small antennas, they are somewhat decoupled.

So say 2,000 cubes with effective apertures of 10 cm each could be thought of as providing a roughly similar area as a 3.7 m Voyager-like dish antenna.

But now you have the electronic thermal noise from 2,000 front-ends.

On the ground, to improve signal to noise (S/N) ratio for weak signals received from deep space, dishes like some of the larger ones of the Deep Space Network use cooled front-ends. For more on that, see the cool images (pun intended) and links in Why doesn't thermal radio emission from a DSN “hot dish” completely swamp the benefits of a cold LNA?.

But your cubes could use cold front ends as well, if they had some mechanism to radiatively cool without getting heat from the Sun, as discussed in Did any of Voyagers' receivers' front ends take advantage of the “cold of space” to lower noise? so the "collective noise" of 2,000 separate front-ends could be managed as well.

No physical connection is needed. Using ALMA as an example this time, once the signal is amplified above electronics noise, it's down-converted to a few GHz and digitized with a surprisingly low-resolution ADC. See Why are the ALMA receivers' ADCs only 3-bits? and these signals containing mostly noise with a trace of signal, now in digital form, are collected at a central correlator via fiber optic cables, as is of course as done at the SKA as well.

So to maintain the analogy, the swarm could use 10 cm telescopes and free space optical connections rather than fiber. No physical connection necessary.

For transmitting, if each of the cubes transmitss 500 milliWatts, that's a kiloWatt of total power!

If they are somehow able to know each others' relative positions accurately via timing from the nearest-neighbor optical link round-trip delays, they could encode phase when relaying their received signals, and phase themselves properly during transmit to emulate a giant phased array. See When is a phased array antenna not a phased array?

This does not provide the same gain as a solid dish of the same baseline, and that's because a sparse array will put a lot of energy into a plethora of weak side-lobes. Nonetheless there is some gain available by phasing the sparse array. That plus the power gain from having 2,000 separate power sources and transmitters, will likely allow good reception at Earth.

Also note that if the cubes are strung out in the ecliptic, then you only have a long baseline (and narrow beam width) in one direction. You'd have to incline them and play with the nodes and phasings to get a decent baseline and beam width perpendicular to the ecliptic.

So far I can't think of any reason why this couldn't be done with current, or at least currently discussed technology.

except: to note that the math needed to combine the signals (transmit or receive) needs a hefty numerical implementation along with accurate positions information of each member in the swarm and the site on Earth. While it's absolutely possible that the calculation could be distributed amongst processors throughout the swarm, the details of how to implement that in this context are beyond me and a single Stack Exchange answer. Have a look at the ALMA correlator for example (1, 2, 3, 4, 5, 6, 7) which is "space-like" in that the altitude is so high and pressure so low that conventional spinning hard drives don't work (no air cushion for read/write head) and cooling is a challenge.

This application would not need something as big as the correlator because the swarm is a one-pixel telescope in that it only needs to "image" it's target point on Earth, not a wide field like SKA, VLA, ALMA, etc.

And of course this swarm network (swarmwork? netswarm?) would all be maintained at a high level of security and integrity by using blockchain (humor).

note: this is 2012. The correlator has gone through one or more substantial upgrades since then, but I can't find a newer, embeddable video.

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    $\begingroup$ Joke about the blockchain; I can see it used as means of maintaining a map of the scanned area and distribution of the swarm, and its stored records used in planning the behavior - plus sent to Earth as a periodic telemetry/science report. I didn't say 'a swarm somewhere in the asteroid belt' for no reason. Each block consisting of entirety of swarm activity over past couple hours, assignment of tasks (like coordinator of 'phased' broadcast) through a deterministic algorithm basing on the state of the swarm, same for choosing targets for analysis. Thousands of asteroids visited per year. $\endgroup$ – SF. Oct 10 '18 at 6:29
  • $\begingroup$ This is really, really cool. Mining bitcoin in the asteroid belt ;-) (more humor) $\endgroup$ – uhoh Oct 10 '18 at 6:33
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    $\begingroup$ Bitcoin is just one application, which utilizes the "common agreement ledger" part of the technology, but uses a completely pointless, bogus task for the "choosing the best", optimized for tunable difficulty but producing a completely useless product (a hash with a long row of zeroes on the start). There are blockchains that use better, more useful tasks - e.g. protein folding. In space, where no participant has any incentive to falsify the results, the task of 'collect best data' seems sensible, and computationally light, plus implies vicinity of an interesting target. $\endgroup$ – SF. Oct 10 '18 at 16:31
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    $\begingroup$ ...that way the significance of the two parts is switched - the scientific discovery process becomes the means of 'mining' and the accompanying ledger is just a bonus. $\endgroup$ – SF. Oct 10 '18 at 16:32

Phased arrays only work if the distance between the elements in the array is known to within a fraction of a wavelength. That's difficult to achieve with an orbiting swarm (where distances keep changing). You'd have to constantly measure the distance and direction between all of the spacecraft, and keep adjusting the timing of each wave.

For comparison, LISA is the first mission to achieve that sort of accuracy between spacecraft. This is a billion-dollar mission.

  • $\begingroup$ If 1 GHz is used as frequency, the wavelength is about 0.3 m (one light foot). So distances should be known to within some millimeters. $\endgroup$ – Uwe Oct 10 '18 at 9:02

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