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enter image description here Source: Chinese language blogpost

The NASA Spaceflight article Queqiao relay satellite launched ahead of Chang’e-4 lunar mission describes the communications relay satellite that will be used to communicate with Chang'e-4, which will be on the far side of the Moon and therefore out of direct communications with Earth.

This tweet says:

And the 448 kg Queqiao Chang'e-4 relay sat before launch

and shows the images below, which show a folded dish antenna that looks to me to be much larger than I would have expected. There's an element of risk using a folded dish instead of a rigid one, and so this must have some substantial advantage.

Will this large dish be used to communicate with Earth, or Chang'e-4, or both? The reason I ask is that since the satellite in orbit around the Moon will be visible from a station on Earth for many hours at a time, a huge data rate would not be necessary, unless it were live video and couldn't be buffered. In that case though, how would it pick up signals from Chang'e-4 at the same time?

Question: Why does Queqiao (Chang'e-4 relay satellite) have such a large dish antenna? Will it be the largest dish ever on or near the Moon?

enter image description here

enter image description here

Below x2: Apollo 15 lunar Rover and Service Module. As @Hobbes points out the Apollo Service Module used four 78 cm dishes for Earth communication, and the Apollo Lunar Rover's antenna had a similar area. With advances in error correction low noise front ends and ground reception, it's surprising to see a much larger antenna used now. From here and here.

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    $\begingroup$ it's far larger than the Apollo CSM high-gain antenna (4x 78 cm dishes, ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720012253.pdf). $\endgroup$
    – Hobbes
    Commented May 21, 2018 at 11:08
  • $\begingroup$ @Hobbes that document is jam-packed with goodies, thank you! Based on your comment I've added a bit. $\endgroup$
    – uhoh
    Commented May 21, 2018 at 12:22
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    $\begingroup$ As far as I can tell, it will be significantly larger than any dish ever sent to the moon, even including gravity assists for interplanetary missions (Mariner-10 or Nozomi possibly contenders for runner-up). Certainly there have been larger antennae, (eg. whip antenna) but nothing that would count as a dish. $\endgroup$
    – Jack
    Commented May 22, 2018 at 14:22
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    $\begingroup$ As for why, I can't work out. I'm sure it has something to do with the different frequency bands used - X-band for comms to lunar surface, S-band back to Earth - but I don't understand enough to make a judgement. Also maybe worthy of note - the rover will apparently be controlled in (near) real time, so data integrity may be critical. $\endgroup$
    – Jack
    Commented May 22, 2018 at 14:26
  • $\begingroup$ @Jack consider adding a partial answer? You can always add more information as it becomes available. $\endgroup$
    – uhoh
    Commented May 23, 2018 at 7:14

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Several factors contribute to needing a large antenna:

  • it is used to communicate with the rover, which weighs only 100 kg so can't carry a large dish
  • it'll be in orbit at the Earth-Moon L2 Lagrangian Point, a distance of 60,000 km from the Moon.
  • the lander and rover use omnidirectional antennas (i.e. with low gain) to receive data

These factors are all part of the link budget for the communications between rover, lander, relay and Earth.

Data relay capacity:

It provides four 256 kBps X-band links between itself and the lander/rover and one 2 MBps S-band link towards earth.

It looks like the dish will be used for both X-band and S-band. The relay can communicate with both rover and lander simultaneously.

I think the following specification means the satellite can relay information simultaneously, which suggests both Earth and Moon are in the field of view of the antenna:

The relay satellite should be able to transmit data to Earth and relay with the lander and the rover simultaneously.

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  • $\begingroup$ How much does the rover weigh on the Moon? Oh, it doesn't matter ;-) $\endgroup$
    – uhoh
    Commented May 23, 2018 at 12:05
  • $\begingroup$ cool stuff here: gbtimes.com/… probably has an additional answer for this question as well. $\endgroup$
    – uhoh
    Commented May 23, 2018 at 12:07
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    $\begingroup$ fyi that link also explicitly states "This is largest communication antenna ever used in deep space exploration, according to Xinhua, which also described it as a 'large umbrella'. NASA's Galileo Jupiter orbiter carried a 4.6 m diameter high-gain antenna, but this failed to fully deploy." $\endgroup$
    – uhoh
    Commented May 23, 2018 at 12:14
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    $\begingroup$ Agreed. The dish size is driven by the link budget. (distance + antenna gain of all antennas involved) $\endgroup$
    – Hobbes
    Commented May 23, 2018 at 13:15
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    $\begingroup$ Here's a paper that may have more information, but it's paywalled: link.springer.com/article/10.1007%2Fs11431-016-9034-6 $\endgroup$
    – Hobbes
    Commented May 23, 2018 at 15:15

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