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I saw this image in Popular Science, one of the example photos from the book by astronaut/photographer/astrophotographer(?) Donald Pettit. It's beautiful and impressive not only from a color/composition perspective, but also from a purely engineering perspective.

I noticed there is a fixed dish antenna, so I've included a cropped/magnified/rotated view of it for detailed examination. I'm wondering what it would be for, since it does not appear to have any, or at least much steerability.

What is it for? Which way does it point? Roughy what wavelength/frequency might it operate?

enter image description here

enter image description here

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    $\begingroup$ Kudos to you for noticing a spare part simply due to a "lack of ability to swivel" in a picture where 99.5% of people would just say "neat"! I certainly did not notice that at first glance hah. $\endgroup$
    – Magic Octopus Urn
    Jun 29, 2018 at 20:29

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That's a spare part - a spare Space to Ground Antenna dish for the Ku band. It's mounted on an External Stowage Platform. It was brought up on STS-127.

From the STS-127 Press Kit Mission Objectives (p 17) (emphasis mine):

Install Integrated Cargo Carrier (ICC) to Payload Orbital Replacement Unit Accommodation (POA)

  • Remove and replace six Port 6 batteries (spacewalk)
  • Transfer PM-2, Linear Drive Unit (LDU) and Space-to-Ground Antenna (SGANT) to External Stowage Platform-3 (spacewalk)

Here's a shot of it in the payload bay.

enter image description here

And here are the transfer ops in progress

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All those other ORUs (Orbital Replacement Units) you see mounted on those platforms are spares as well. The last group of shuttle flights brought up many external spares to cover failures of parts that were too large for the remaining logistics vehicles.

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    $\begingroup$ Beat me to it. I was also reading about this other SGANT spare on STS-132. $\endgroup$
    – called2voyage ♦
    May 12, 2017 at 13:35
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    $\begingroup$ Yeah, there are a whole bunch of spare parts stuck to the outside of the ISS. $\endgroup$
    – Organic Marble
    May 12, 2017 at 13:37
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    $\begingroup$ Planning for lack of space shuttle to carry up big parts. $\endgroup$
    – Organic Marble
    May 12, 2017 at 13:41
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    $\begingroup$ Yep, now I'm wondering how good of condition spares stay when not in use. $\endgroup$
    – called2voyage ♦
    May 12, 2017 at 13:42
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    $\begingroup$ I was the lead shuttle robotics instructor to the STS-127 crew, so I'm fairly familiar with that ORU :) One of my favorite missions. $\endgroup$
    – Organic Marble
    May 12, 2017 at 15:02
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To complement: The SGANT is the antenna that transmits high data rate signals, including many up/down video feeds, from/to the ISS. There is a total of 3 antennas: 2 at the end of booms on the Z1 trust, and the spare that you have identified. The active SGANT transmits/receives data by pointing to a TDRS satellite continuously for a portion of an orbit (~120 degrees), then transitions to a different TDRS via ephemeris and a spiral search pattern. They have been designed and built in Canada by SPAR/MDA, same with the ISS S-band antennas, and sold to L3-C (all ISS communications) and then Boeing. As far as I know, the SGANT is the only way to exchange video with the ISS. Hope this helps!

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  • $\begingroup$ Where did you go? Come back to Space SE $\endgroup$
    – Muze
    Oct 2, 2018 at 18:34

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