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In this question and answer pair I see that the ISS can maintain data links to earth through direct access to ground stations as well as through the TDRSS geostationary satellite network.

What kind of antennae are used on the ISS for these links? Are they directional? Do they have to continuously move to maintain a live link - for example in the case of the live streaming HDEV views? I'm wondering if they are phased arrays which can be steered (potentially very quickly) electronically, or a more conventional gimbaled dish, a little like the picture below.

I got excited when I found this image through a google search for space antenna GIFs, but it turns out to be something completely different. (also here)

enter image description here

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The US side of the ISS has a number of antennas to support its rather complicated comm system. Most visible are the two Ku-band High Gain Antennas which are 6 foot diameter azimuth/elevation gimbaled dish antennas mounted on the Z1 truss segment (near the center of the ISS truss). These antennas are sometimes referred to as SGANTs (Space-to-Ground ANTennas).

enter image description here

For most of its life, the ISS only had one SGANT. A second one was carried up on one of the later shuttle missions. This system carries the video and experiment data to the ground and would normally be gimbaling continuously to point at the selected geosynchronous comsat (i.e. TDRS).

You can see one tracking a TDRS satellite in this video (ignore the ammonia venting!)

The S-band system uses S-band Antenna Support Assemblies (SASAs) which contain a conical high gain antenna and an omnidirectional antenna. The SASAs can move in elevation and azimuth as well to point at the TDRS in use. They are located on the S1 and P1 truss sections, near the center of the truss. S-band is used for voice and data communications.

enter image description here

(NASA photo ISS021E033057, doesn't appear to be online at present)

There are also two Ultra High Frequency antennas on the ISS, primarily used to communicate with extravehicular activity crewmembers, formerly used to communicate with the space shuttle orbiter. They are non-gimbaling omni antennas.

My information on the Russian ISS comm system is very limited and possibly outdated but they have (or had) a system analogous to the S-band system called Regul that communicates with Russian ground stations and the former Luch satellite system. There is also a Russian UHF system used for space-to-space comm.

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    $\begingroup$ Very nice! With a Ku band wavelength of the order of 2cm, those SGANTs need to track better than 0.5 degree. If they point at the ground they have to slew up to 1.3 degrees per second, but towards TDRS, about 100x slower. Do you know if they do, or even can access ground stations? You mentioned it in your previous answer but I don't know if you meant the live streaming was sometimes direct to ground, or to TDRS and then ground. $\endgroup$
    – uhoh
    Jun 5, 2016 at 12:15
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    $\begingroup$ I am far from an ISS com expert but I'm reasonably sure that the SGANTs have never been used to communicate directly with the ground stations, both for the slew rate reason you mention, and because the TDRSS system pre-dates the ISS, so there would not have been a reason to. Is it possible? I don't know. $\endgroup$
    – Organic Marble
    Jun 5, 2016 at 12:18
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    $\begingroup$ I believe they alternate between the 2 Ku band systems at six month intervals or some such. Also kind of a random comment - there are only 2 TDRSS ground terminals, and they are both in White Sands, New Mexico. $\endgroup$
    – Organic Marble
    Jun 6, 2016 at 16:00
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    $\begingroup$ Great clip! I just moved it up in the answer out of the Russian section. $\endgroup$
    – Organic Marble
    May 15, 2017 at 19:59
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    $\begingroup$ @uhoh, yeah, very cool. Putting 'bishop' on the POA. $\endgroup$
    – Organic Marble
    Dec 7, 2020 at 3:18
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My small team at Dynacon in Toronto designed the software used by the gimbals to point the SGANTs (Space to Ground ANTennas); SPAR Aerospace's Montreal plant developed the SGANTs for GE (ISS comms subsystem contractor), who in turn were subcontractors to McDonnell Douglas (Work Package 3 contractor, IIRC, reporting to NASA's Johnson Space Centre). This was in the 1989-1993 time-frame.

The SGANT controller has 3 operating modes: slew, search and track. The first is open-loop: slew as fast as possible from one direction to another (basically as a TDRS (tracking and data relay satellite) sets, slew to where the next one is rising). Search is an open-loop spiral search pattern, looking for the strongest signal (i.e., the target TDRS), which takes a minute or two, after which a slew is commanded to where that signal was found. Track is a closed-loop mode, using signals from monopulse tracking sensors built into the SGANT horn to measure the pointing error wrt TDRS, which drives a feedback controller to minimize that pointing error. All pretty simple in principle, complicated by the gimbal's own internal dynamics (including friction), and the significant amount of flexibility and vibration of the ISS (including time-varying thermal distortion, which is what drives the need to search for TDRS at the start of each tracking session, as the ISS "bananas" in different directions as it heats up and cools down).

It is only used to track TDRS satellites, it does not have the slew-speed capability to track targets on Earth.

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    $\begingroup$ Great info, welcome to space! $\endgroup$
    – Organic Marble
    Mar 24, 2020 at 23:02
  • $\begingroup$ I started to try to wrap my head around what monopulse tracking is in answers to How exactly were the signals from quad dishes of Apollo Deep Space High-Gain Antenna or Russian B-529 processed for local control of tracking? I found out how classical conical scanning works in answers to Why is the reflector on this millimeter-wave antenna spinning? and looking at power differences from four slightly different directions can do something similar. $\endgroup$
    – uhoh
    Mar 24, 2020 at 23:12
  • $\begingroup$ But doesn't monopulse imply that there's something more going on, perhaps something helpful encoded in the transmission itself? $\endgroup$
    – uhoh
    Mar 24, 2020 at 23:13
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    $\begingroup$ The feed network in behind the horns is set up to combine the signals from all four horns to produce a "sum" signal (A+B+C+D), as well as two "difference" signals ((A+B)-(C+D)) and ((A+C)-(B+D)) --- the way it's done automatically minimizes calibration differences between the horns. $\endgroup$
    – Kieran A. Carroll
    Mar 26, 2020 at 1:41
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    $\begingroup$ The signal that are used for tracking are the two "difference divided by sum" signals, which (it turns out) are pretty much linearly dependent on the boresight pointing error in the two directions, out to the angle of the first null of the main lobe. So, a perfect signal for driving a feedback controller (because of the linearity) --- so long as the target is somewhere within the main lobe, of course. If you're off-pointed too far, then the tracking signal will tend to drive the controller farther away from the target, rather than closer to it. $\endgroup$
    – Kieran A. Carroll
    Mar 26, 2020 at 1:42

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