Do antennae on the ISS have to constantly move to maintain data links?

Question

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)

Organic Marble · Accepted Answer · 2020-11-14T20:31:11.240

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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).

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.

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My information on the Russian ISS comm system is very limited and possibly outdated but they have (or had) a system analogous to the S-band system called Regul that communicates with Russian ground stations and the former Luch satellite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

edited Nov 14 '20 at 20:31

answered Jun 05 '16 at 11:49

Organic Marble

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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. – uhoh Jun 05 '16 at 12:15
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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. – Organic Marble Jun 05 '16 at 12:18
OK got it. So in general, streaming data is most likely always (can I say that?) via TDRS. Really nice photo!! The PDF is really helpful (I like the image on page 41, not sure if it's a real photo or just an illustration) - thanks! – uhoh Jun 05 '16 at 12:32
I think that's accurate. It may be possible to send video on s-band system but that is not normal ops. The intent was video and payload data on Ku, voice and housekeeping data on s-band, but it may not always be used strictly like that. – Organic Marble Jun 05 '16 at 12:36
If they are both active, one could possibly acquire the next TDRS satellite and establish a link before the other looses contact with the current TDRS and so continuous streaming might be possible without breaks. But I'm guessing they'd rather leave one as a standby unit and not "wear it out". – uhoh Jun 05 '16 at 12:41
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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. – Organic Marble Jun 06 '16 at 16:00
I see ground stations listed Guam and Goddard as well in https://en.wikipedia.org/wiki/Tracking_and_Data_Relay_Satellite_System#Ground_segment but I don't know the accuracy of that article. I looked at the individual satellites in Wikipedia but gave up, they have almost all been moved around a lot. Tomorrow I'll check the TLEs, but it seems some of those locations for active satellites are not even accessible from White Sands. – uhoh Jun 06 '16 at 18:10
Didn't know about Guam, interesting. – Organic Marble Jun 06 '16 at 18:15
I hope you don't mind; I just ran across this video and got excited and wanted to put it somewhere! – uhoh May 15 '17 at 19:20
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Great clip! I just moved it up in the answer out of the Russian section. – Organic Marble May 15 '17 at 19:59
striking animation in this video found in your recent question, (lower volume before playing) https://youtu.be/d6lBmmxScsI?t=166 – uhoh Dec 07 '20 at 03:15
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@uhoh, yeah, very cool. Putting 'bishop' on the POA. – Organic Marble Dec 07 '20 at 03:18
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My acronymology is DOA; what is POA? – uhoh Dec 07 '20 at 03:19
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@uhoh it's a 2nd level acronym. Payload ORU Adapter, ORU = orbital replacement unit. It's basically one of the "grippers" from the station arm mounted on the "railroad car" so a component can be placed there temporarily. The arm puts the 2nd grapple fixture on the payload into the POA, the POA grapples it, then the arm releases. – Organic Marble Dec 07 '20 at 03:21
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3D bishop sighting: https://geekandsundry.com/a-history-of-three-dimensional-chess/ (but I can't tell what piece he's moving there) – uhoh Dec 07 '20 at 03:25

score 10 · Answer 2 · edited Mar 25 '20 at 00:58

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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.

edited Mar 25 '20 at 00:58

answered Mar 24 '20 at 22:51

Kieran A. Carroll

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Great info, welcome to space! – Organic Marble Mar 24 '20 at 23:02
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. – uhoh Mar 24 '20 at 23:12
But doesn't monopulse imply that there's something more going on, perhaps something helpful encoded in the transmission itself? – uhoh Mar 24 '20 at 23:13
fyi I've just asked How badly does the ISS “banana” or flex under the constantly changing conditions of solar heating? – uhoh Mar 24 '20 at 23:21
There's a decent overview of monopulse trackign in this wiki page: https://en.wikipedia.org/wiki/Monopulse_radar. Basically, it involves pointing two antennas in slightly different directions, and comparing the signals from the two of them. When the two signals have the same amplitude, the target is exactly half-way between the two (modulo differences in calibration between the two antennas). – Kieran A. Carroll Mar 26 '20 at 01:30
The SPAR monopulse design was pretty sophisticated, in the sense of a clever design that accomplishes the trackign function as well as the main comms-link function, both using the same hardware. The focal plane of the dish antenna has four horns arranged in a square pattern, call them A and B on the top row and C and D in the bottom row. – Kieran A. Carroll Mar 26 '20 at 01:41
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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. – Kieran A. Carroll Mar 26 '20 at 01:41
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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. – Kieran A. Carroll Mar 26 '20 at 01:42
That's why SPAR designed the system to have a "Search" mode, so that they could get the boresight pointed close-enough to the target before turning on Track mode, so that the tracking signal would be within its linear range. – Kieran A. Carroll Mar 26 '20 at 01:42
(BTW, MD's work package was actually WP2, not WP3.) – Kieran A. Carroll Mar 26 '20 at 01:42

Do antennae on the ISS have to constantly move to maintain data links?

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