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It would seem to me that a phased array antenna would be great for spacecrafts since it removes the need to turn the entire spacecraft around in order to point the radio dish towards Earth. Many spacecrafts already have to turn to point their instruments, solar panels and radiation shield in the right direction, so getting rid of antenna pointing should be helpful for both spacecraft and operations design. An interplanetary probe like New Horizons could've been able to send data while making observations and more importantly always been able to receive a last minute command. Station keeping costs could be lowered for orbiting satellites, including the ISS. So why do spacecrafts still carry a fixed dish instead of a phased array?

I note that the 285 kg and 90 Watt Mercury Magnetospheric Orbiter, a planned JAXA Mercury orbiter, will have a phased array for communicating with Earth at 160 Gbit/year, and Mercury is about 90 to 210 million km from Earth. So cost, mass, size, power requirements, data rate and distances don't seem to be obstacles for phased arrays in space. So what is the problem with them? Or are they maybe already taking over?

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    $\begingroup$ My guess would be gain. A parabolic dish gives a large amount of amplification to your signal (20-30 dB?). Replicating that signal with a phased array means you need 1000 times the power. $\endgroup$
    – Hobbes
    Dec 2, 2015 at 7:51
  • $\begingroup$ @Hobbes Yes, 20-30 dB gain in the UHF or low SHF region (few gigahertz) is almost trivial with a parabolic dish antenna. It probably wouldn't be too hard, even in spaceflight, to push that toward 40 dB. As long as the antenna can be aimed accurately, that's a huge boost. $\endgroup$
    – user
    Dec 2, 2015 at 13:48
  • $\begingroup$ 160 Gbit/year is a little over 5 kbits/second. Even at the distance of Mercury that doesn't sound quite as impressive, does it? Rosetta managed about 28 kbits/second. $\endgroup$
    – user
    Dec 2, 2015 at 13:49
  • $\begingroup$ @Hobbes But little MMO is supposed to do it at Mercury. And I'm sure several other spacecrafts, including commercials, are doing it already. Does it work well or does it work bad, and for what main reasons? $\endgroup$
    – LocalFluff
    Dec 2, 2015 at 13:50
  • $\begingroup$ @MichaelKjörling 160 gigabits for 90 W on the yearlong primary mission certainly seems enough, for such a cheap tiny spacecraft to investigate the magnetic environment of Mercury. Sounds scalable in distance, gain, power and many things. I don't see the limit, I'm asking for it. $\endgroup$
    – LocalFluff
    Dec 2, 2015 at 13:56

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To address the last part of your question ".. or are they already taking over" phased arrays have been used in commercial communications and remote sensing satellites for a while already though I don't think they are "taking over" as such.

These examples aren't the interplanetary use you have suggested but they do serve to demonstrate that the technology is well established for satellite use.

Inmarsat 4 uses a phase array for its main L band receive and transmit antenna: enter image description here

The Synthetic Aperture Radar is closely related to a phased array. The Sentinel 1 satellite uses a phased array as the basis for its C-band SAR payload: enter image description here

I suspect the costs of the solution have been higher in the past, perhaps improving now with wider use and that this has probably driven the trade off for science missions to date.

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Phased arrays have mathematical limits, so you may still need to have gimbals or orient the array. In particular they suffer from decreased gain off-boresight. The more elements to the antenna, the tighter the beam, but the worse the off-boresight issues. NASA has studied these issues, and there are solutions (such as having multiple arrays). In the example from that paper, they needed 8 separate antennae, each a phased array. That's a lot of mass and power, though it certainly has its advantages!

The real power of phased arrays is their ability to be steered electronically. The best use of this is in situations where one needs to scan an area quickly, but those situations rarely arise in spacecraft communications.

There are several satellites which communicate with a phased array, but from what I have been able to tell, they're usually either there because they needed the array for another purpose (such as SAR imaging), or because they needed to rapidly switch between multiple communication channels with different craft without involving a physical gimbal movement.

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  • $\begingroup$ Thank you, your response has provoked some more thoughts about phased arrays, It was a bit too lengthy to add here so I've created a new question for it here: space.stackexchange.com/questions/12874/… $\endgroup$
    – Puffin
    Dec 4, 2015 at 19:50

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