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This question is inspired by an answer to this question, where it is stated that:

The real power of phased arrays is their ability to be steered electronically.

This set me wondering about the antenna on this satellite, Inmarsat 2 F1 enter image description here

I have been looking online for any description that recognises the antenna as a phased array without luck. However, just looking at the antenna it seems if there is no controlled phase relationship between the horns then at L-band their beams will be so large that most of the power from the antenna will miss the Earth. It appears to me that the horns must all be phase linked so that the effective antenna aperture is the size of the whole satellte Earth face and thus the beam will be much narrower.

Question: Does this count as a phased array or must the beam be electrically steerable once in orbit as well? Was it considered to be a phased array when it was launched back in 1990?

I'm aware some people may think this question is off topic, though I thought I'd start here as I'd be more likely to find someone with specific knowledge of the Inmarsat 2 satellites here.

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    $\begingroup$ Funny picture! The guy in green trousers with an uncovered beard almost hit the mirror with his head as he bent over. And please everyone do not sit down without watching your ass for those pointy things waiting for you down there. $\endgroup$
    – LocalFluff
    Commented Dec 4, 2015 at 20:12
  • $\begingroup$ Any particular reason this spacecraft is being assembled inside what appears to be an I Wanna Be the Guy level? $\endgroup$
    – Nathan Tuggy
    Commented Dec 5, 2015 at 23:15
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    $\begingroup$ It's an anechoic chamber en.wikipedia.org/wiki/Anechoic_chamber It's probably being prepped for testing rather than assembled. $\endgroup$
    – Organic Marble
    Commented Dec 6, 2015 at 2:43
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    $\begingroup$ I'm pretty sure that most "antenna people" would use the term "phased array" to refer to an array where the phase relationship is deterministic. The phase relationship could be fixed and the array mechanically steered, or it could be adjustable, and electronically steered, or somewhere in the middle, with the phase delays mechanically adjusted (e.g. change a length of coax). Any idea the frequency? (Wikipedia just says L-band is 1 to 2 GHz) - the size looks somewhere between 1.5 and 2 meters - we can calculate the footprint. $\endgroup$
    – uhoh
    Commented Jan 21, 2016 at 11:34
  • $\begingroup$ I see that the range 1530-1548 MHz is given in the Inmarsat paper extract that you posted in your answer. My rough application of 1.22 lambda / D for full Earth cover comes out at ~1.6m for 1540 MHz which looks plausible from the photo of the satellite. $\endgroup$
    – Puffin
    Commented Jan 24, 2016 at 18:33

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I think your question boils down to weather or not people associated with the design, fabrication or operation of Inmarsat 2F1 refer to the L-band transmit antenna specifically as a "phased array". I think we can all agree that the phases are specified and managed. Further, based on the article below (and others), I think they are designed to be fixed once the satellite is deployed. But I think the question is asking "Is this array antenna called a phased array antenna by those associated with the satellite?"

Since you are interested in a timely answer, I'm going to do a partial post right now. The amplitude sketch in the article's Figure 7 is actually not 100% correct. I think it's just an example. You'd use something like that (an Airy pattern) at the focal point of a reflector if you wanted to produce a flat beam. I'll see if I can reverse engineer a better hypothetical phase profile.

I see terms like "Beam Forming Network" and "Shaped Coverage Earth Beam", but not "phased array".

Boeing seems to be knee-deep in satellite shaped beam patents, as are other companies, but trying to go through all of that is a serious time sink.

Anyway, I found this article, and one more below for specifics:

Gambaruto, E.; Banks, D.K.; Krinsky, B., "INMARSAT second generation satellites for mobile communications" in Aerospace Applications Conference, 1989. Digest., 1989 IEEE , vol., no., pp.13 pp.-, 12-17 Feb. 1989 doi: 10.1109/AERO.1989.82422

I'll include snippets here. Most university libraries and many public libraries can obtain full IEEE journal articles for you.

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What is a Turnstile Cup Dipole? I'm glad you asked! The crossed dipole resembles a turnstile, and they are driven 90° out of phase to obtain circular polarization. There is a nice description of their advantages. Along with two figues and a paragraph, I've included the front pages from this 1973 MIT report at dtic.mil so you can see it says: "Approved for public release. Distribution unlimited."

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  • $\begingroup$ Thank you I can see you put a fair bit of thought into that answer. The clearest indication to me is in the Inmarsat paper: both the description of a beam forming network being used to excite the dipole array and also figure 8 showing the achieved to get better gain for users close to the Earth limb looking at the satellite at a low elevation. $\endgroup$
    – Puffin
    Commented Jan 24, 2016 at 18:20
  • $\begingroup$ It seems then that "phased array" was not used here, despite the obvious controlled phase relationships. Perhaps its the term is reserved for dynamically reconfigurable systems. $\endgroup$
    – Puffin
    Commented Jan 24, 2016 at 18:22
  • $\begingroup$ By the way, thank you for the bonus info on the turnstile cup dipole. Its now much clearer that their reddish colour in the photo are probably thick kapton aperture covers. I had previously wondered why part of the antenna was a bare metal colour as I had not really looked closely enough at it to see that this is a gap in the array of dipoles. $\endgroup$
    – Puffin
    Commented Jan 24, 2016 at 18:53
  • $\begingroup$ Oh, I see what you mean! I didn't understand the part about aperture covers at all, and the color was a mystery. I enjoy reading about the development of satellite technology - amazing things done within the limits of the technology of the era. Thanks for the question! $\endgroup$
    – uhoh
    Commented Jan 25, 2016 at 0:54

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