I just saw some footage of the ISS in which a dish antenna could be seen, constantly rotating.

The dish looked more-or-less parabolic, meaning that it is presumably sending/receiving along a fairly confined path. The central axis of the parabola (and what appeared to be a bit of terminal equipment at its focus) was inclined something like 45 degrees to its axis of rotation, and was rotating at something like 30RPM.

It would seem to suggest that the antenna was essentially sweeping a hollow cone. If used for ranging or detection, it seemed as though it would not be covering a large volume inside the swept cone.

Is that what was actually happening? For what purpose?

  • $\begingroup$ I wanted to point out that the antenna referred to in this question (and in the answer) was decommissioned in August 2016 (according to Wikipedia) "On 13 January 2018, the trunk section and ISS-RapidScat re-entered Earth's atmosphere and were destroyed as planned" -- Now there is one usually prominently visible in NASA's live stream from the ISS which appears to be similar, but I have no idea what it does. $\endgroup$ – Daniel F Aug 24 '18 at 22:57

You probably mean RapidScat. It is a microwave scatterometer that measures near-surface wind speed and direction.

rotating beam

Here's RapidScat in action, installed on Columbus module's External Payloads Facility (CEPF), as seen from one of ISS external cams:

                                     enter image description here

  • $\begingroup$ Wikipedia claims that "NASA formally terminated operations on 28 November 2016". But it's still spinning. Is that because there's no off switch? $\endgroup$ – djsadinoff Nov 13 '17 at 10:22
  • $\begingroup$ Why do you think it's still spinning? $\endgroup$ – Mark Adler Nov 13 '17 at 18:47
  • $\begingroup$ because the NASA Live feed is showing it. ( or something that looks very similar) youtube.com/watch?v=RtU_mdL2vBM&feature=youtu.be $\endgroup$ – djsadinoff Nov 13 '17 at 23:42
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    $\begingroup$ ...and I have apparently been suckered into thinking that it was still spinning by the "live" tag on that video. It's a tape after all, so that's tape from 2016 showing RapidScat still alive. Never mind. $\endgroup$ – djsadinoff Nov 14 '17 at 19:55

The constantly rotating antenna is a good sign that the AE-35 unit is working correctly, and that the ISS astronauts should therefore avoid any unnecessary EVAs regardless of the advice of their onboard computer.

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    $\begingroup$ Somebody upvoted this, and this is not April 1st. O tempora, oh mores. $\endgroup$ – Deer Hunter Apr 14 '15 at 21:21
  • $\begingroup$ In fact it looks like I got two upvotes and five downvotes so far. I bet the people on the ISS think this is funny. I bet they're laughing right now if they have internet up there in low orbit. ;) $\endgroup$ – dodgethesteamroller Apr 15 '15 at 0:12
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    $\begingroup$ Thanks for not deleting the best joke I've ever made on this site, everyone. Despite the party-pooping downvoters. $\endgroup$ – dodgethesteamroller May 29 '15 at 20:29
  • $\begingroup$ news.avclub.com/… $\endgroup$ – uhoh Apr 4 '18 at 7:12
  • $\begingroup$ I'm imagining a super-advanced 2130 society that has a rotating antenna on all modules still, simply because "dodgethesteamroller" stated it was the perfect debugging module. Hundreds die, because they hired a man who relied on an ancient stackexchange DB dump to decide design requirements. $\endgroup$ – Magic Octopus Urn Jun 27 '18 at 20:41

As said above, RapidScat measured wind speed and relative wind direction. Scatterometers relate the reflected power (backscatter coefficient in the picture below) with the relative wind direction with a cosine dependency.

enter image description here

This means that there is an ambiguity when resolving the direction, as up to four different directions translate into the same backscatter. However, the amplitude also depends on the incidence angle and on the polarization (VV or HH in the picture above). Scatterometers address the ambiguity problem by using (almost) simultaneous observations of the same area using different incidence/azimuth observation angles and/or polarizations. By combining the results, they can resolve the ambiguity.

This is why Rapidscat rotated. This architecture is known as pencil beam.

enter image description here

  • $\begingroup$ Checking references 1, 3, and 4 in Wikipedia's ISS-RapidScat will show that the scattering is off of waves, not the wind, but that results in a somewhat similar situation; you can best probe the ocean surface wind when the radar beam is parallel to the direction of the wind, but it's because the reflectivity of the waves is maximum in that direction because they tend to be perpendicular to the wind. $\endgroup$ – uhoh Sep 23 '20 at 16:06
  • $\begingroup$ I think if you add that to your other answer it will be better. $\endgroup$ – uhoh Sep 23 '20 at 16:08
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    $\begingroup$ Indeed the scattering comes from the sea waves. In fact the backscatter comes from capillary waves over the ocean surface, which are produced by the local wind (the magnitude aiming to retrieve). This phenomena is known as Bragg resonance. $\endgroup$ – danipascual Sep 23 '20 at 16:12

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