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The NASA Spaceflight article Arianespace press onward with dual-passenger Ariane 5 launch shows a photo of Intelsat 39 during testing. The dish reflectors are strangely shaped:

  1. rather than being round or square, their edges are irregular, like the coast line of an island
  2. the surface contours also appear bent and irregular, rather than having a pure parabolic shape.

and they are densely covered in little white dots, at least during this testing phase.

Question: Why the strange shape (especially the irregular edges), and what are all of those little white dots?


Intelsat 39 Intelsat 39

Intelsat 39 Intelsat 39

Image source and credit: Intelsat

Intelsat 39


Here's another example of "white dots", these kinds of images are common.

enter image description here

Source

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  • $\begingroup$ Different but related question: How do commercial broadcast satellites in GEO produce such carefully shaped signal footprints? $\endgroup$ – uhoh Aug 6 '19 at 22:37
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    $\begingroup$ I'm pretty sure the irregular shape is for beam shaping. They're contoured to specifically cover specific regions and not waste transmission power on empty patches like oceans. While also just a guess, the white dots are very reminiscent of a phased array system or they could be sensors to measure and tune the system after deployment. $\endgroup$ – Dragongeek Aug 7 '19 at 0:34
  • $\begingroup$ @Dragongeek thanks! That's a lot of sensors to put on a passive reflector. I think there's a single feed horn that faces each one of these reflectors, no? What would be tuned in that case? $\endgroup$ – uhoh Aug 7 '19 at 1:23
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    $\begingroup$ @amI I'm pretty sure the large non-flat things are reflectors, illuminated by feed horns or antenna arrays at the "top" of the satellite (facing towards us and not so visible). Low res images here and here. It's possible they are using an electronic array instead of horns to illuminate the reflectors, but I don't think these big things are arrays. It would be handy to have a source one way or the other though. $\endgroup$ – uhoh Aug 8 '19 at 11:37
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    $\begingroup$ Contouring the antenna edges doesn’t save transmit power. The beam from the feed horn is going somewhere, whether or not the reflector is there. Rather, it’s used to reduce transmitter interference on the ground (confining the beam to a licensed area) and interference in the uplink. $\endgroup$ – Bob Jacobsen Dec 28 '19 at 2:38
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The irregular shape of the reflector is to shape the beam coming from feed in such way that it is tailored to match the contours of geographic region of service area. Here's a short paper on shaped reflectors of INSAT-4A.

"Development of dual gridded reflector antennas for INSAT-4A satellite"

https://books.google.co.in/books?id=drIVnRVDQIgC&pg=PA33#v=onepage&q&f=false

Here's a quote from blog by EADS Astrium's Head of Engineering

Satellite operators cannot afford to waste this limited power as it is beamed back to Earth. Over two thirds of the planet is covered in water and population densities vary greatly, so telecommunication signals need to be focused on distinct geographical areas.

Composites provide a great deal of flexibility in the manufacturing process, enabling us to shape reflectors so that they can focus on specific countries or continents.

There is some preliminary work underway on reconfigurable reflectors as well.

https://www.largespace.de/products-services/antenna-reflectors/#1513173698506-e9146f28-933b

Those tiny dots on reflector surface are for photogrammetry to validate the shape of reflector. In following video at 3 min. 23 sec. Negar Feher of SSL briefly explains their purpose.

…and all those little dots you see on there are for photogrammetry; they’re used to make sure that the surface is actually exactly as designed, so we have these cameras that take pictures of these dots to make sure that it was built according to the design.

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    $\begingroup$ Bingo! Jackpot! Thank you very much! $\endgroup$ – uhoh 8 hours ago
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This answer responds to a part of the question.

1) For both the last two pictures, the satellites, Intelsat 39 and Skymuster, are geostationary communications satellites. We are looking at the Earth pointing face. The images are not especially clear but, approximately the antennas in the centre of the picture are mounted on the Earth face and the outer antennas are more likely set back and deployed off the side of the satellite after separation from the launch vehicle.

2) Each antenna is fed by a horn, or in some cases by a horn with an intermediate subreflector. Its possible that there could be one or several horns for a given reflector, each pointing off it from a different direction to get a different beam on the ground. It is also possible (but not definite) that there could be a phase relationship between the various horns, i.e. a phased array. Its also possible that the horns could be replaced with an array of patch antennas, in which case the phased array relationship is essential. These horns (etc) are facing away from us in the photos, either mounted on a central tower abouve the Earth face or mounted on the East/West walls so as to view the side mounted antennas.

3) The irregular shape of the antennas is indeed to help shape the beam and save power. It is possible this way to put more power over high traffic areas and to guarantee an isolation zone, e.g. so as not to spill over into a neighbouring country.

4) The little white dots. I don't know, I have often wondered. Its seems unlikely that they are sensors as there would have to be a mass of wiring to them and they appear to be little more than bits of tape. They have been visible in photos like this for decades so I don't think bluetooth sensors are in the game. I suspect they are more likely there for measuring the antenna surface properties. I've not seen them on late stage integration photos and suspect they are removed for flight.

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  • $\begingroup$ I've often wondered if they were similar to tracking dots in that imaging from several angles could be used to verify the correct surface figure preflight. Otherwise it's hard to tell if a given undulation is correct or from a dropped happer. $\endgroup$ – uhoh 9 hours ago
  • $\begingroup$ it seems so! $\endgroup$ – uhoh 8 hours ago

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