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Brian May's New Horizons video (below) was released at about the same time that the New Horizions spacecraft passed 2014 MU69 Ultima Thule and includes a lot of technically meaningful graphics.

One item is the illustrating of downlink and uplink of data, shown as a narrow blue beam and wide green beam, respectively.

This answer to Function and nature of the RHC/LCH pair of cables to New Horizons's dish antenna secondary mirror? shows that the backside of the secondary mirror of the high gain antenna also has a small primary dish forming the medium gain antenna.

  1. Is it accurate that even out at Pluto and MU69 that New Horizons is using mostly its medium gain antenna for transmission, rather than its high gain antenna?
  2. If so, why? Returning the Pluto/Charon data took months, and it will be quite a while for MU69 data as well. Why not shorten it up and use the high gain antenna?

Downlink animation GIF (via New Horizons' medium gain antenna) from video

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Uplink animation GIF (via New Horizons' high gain antenna) from video

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Source: Spaceflight 101's New Horizons Spacecraft Overview borrowed from Function and nature of the RHC/LCH pair of cables to New Horizons's dish antenna secondary mirror? Images of dishes available in this answer.

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  • $\begingroup$ Why do you think they'd use the MGA, when the HGA points in the same direction and offers a much higher data rate? $\endgroup$ – Hobbes Jan 5 at 8:57
  • $\begingroup$ @Hobbes "Brian May's New Horizons video... includes a lot of technically meaningful graphics. One item is the illustrating of downlink and uplink of data, shown as a narrow blue beam and wide green beam, respectively... Is it accurate...? If so, why?" $\endgroup$ – uhoh Jan 5 at 9:19
  • $\begingroup$ You've taken a tiny tidbit (a visualization meant to look interesting/cool) and used that to come to a really improbable conclusion. I saw that video and instantly dismissed it as artistic license. Hence my question, what makes you think that video is accurate when that goes against basic logic? $\endgroup$ – Hobbes Jan 5 at 9:19
  • $\begingroup$ @Hobbes there is no conclusion here at all. I've asked if the animation is accurate. You are once again mischaracterizing my question. $\endgroup$ – uhoh Jan 5 at 9:21
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No. The mission is designed to use the HGA to downlink data. The MGA is used for uplinks, and for emergencies.

A highly efficient 2.1-meter parabolic reflector antenna (the HGA) is required to support the minimum 600 bps 36-AU post encounter return link.

The ability to command through the medium gain antenna (MGA) out to 50AU for the extended mission sets the MGA receive gain.

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To go beyond @Hobbes' answer, let's look at this quantitatively.

From the OP's linked Spaceflight 101 article, Wikipedia's New Horizons: Telecommunications and data handling this and this answer, and recall from various DESCANSO publications.

The advantage of specifying that New Horizions be able to receive instructions on its medium gain antenna out to 50 or even 100 AU rather than only its high gain antenna are several.

  • medium gain antenna has a beam half-width of over 4 degrees versus about 0.5 degrees for the high gain, meaning you don't need to tax the attitude control system so much to point at the Earth
  • If one of the pair (medium gain, high gain) antenna fails, the other can still receive instructions from Earth.

It turns out that the smaller receive antenna of New Horizions and (presumably) room-temperature front-end is compensated by the higher 1000x higher transmit power from DSN. (DSN dish gain appears in both link calculation so it cancels). Below is a very rough calculation and could easily be off by a half-dozen dB, but it's good enough to see why this works.

                               uplink                 downlink

 Transmit Power          ~20,000W  43 dBW          12W   11 dBW
 Transmit Gain                     73 dBi                42 dBi
 Receive  Gain                     25† (est.)            73 dBi
 path loss (43.4 AU, 8GHz)       -307 dB               -307 dB

 received power                  -166 dBW              -181 dBW
 receiver noise dBm/Hz   (300K)  -204           (20K)  -216 dBW

 S/N @ 1 Hz                        38 dB                 35 dB      ROUGHLY THE SAME!
 S/N @ 1 kHz                        8 dB                  5 dB      ROUGHLY THE SAME!    

 †estimated based on equation (https://space.stackexchange.com/a/24343/12102) and ratio to 2.1 m high gain.
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