note: This is an old-style @uhoh question meaning that it is a bit on the long side. In this case I think it really needs to be since the question is a bit complex; I'm trying to understand how all these components interact.
I've included many images in order to save the time of readers each doing their own image search.
Spaceflight 101's article Inmarsat 5-F4 Satellite Overview contains a lot of helpful discussion and excellent images. It says for example:
The four Inmarsat-5 satellites are essentially carbon copies, using the same satellite platform and featuring identical payloads. Based on Boeing’s BSS-702HP satellite platform, each weighs in at around 6,100 Kilograms and hosts 89 Ka-Band transponders, supporting high user download speeds of 50Mbps and uplink of 5Mbps.
I think I can get to approximately 89 Ka-Band transponders by adding up all of the feed horns arranged as focal plane arrays for the three side-mounted dish reflectors. And when I look at illustrations of the fully deployed satellite and estimate the geometry, the width divided by distance to dish is a similar ratio to the diameter of the earth divided by the GEO distance. Here's my arithmetic:
above x2: Photo: Inmarsat from Spaceflight 101.
The problem with my arithmetic is that there is a little bit of a dish peeking out at the bottom, and I don't know what to do with it. If you look at the next image, it seems to suggest a second set of 3 dishes, with perhaps dozens of more transponders:
above: Artist’s concept of a Global Xpress satellite. Credit: Inmarsat. From Spaceflight Now.
Also, the article goes on to say:
Inmarsat 5-F4 hosts two transmit and two receive apertures, and six steerable spot beams deliver a high-degree of flexibility to respond to changing demands and direct additional capacity where it is needed. The high-capacity spot beams are fed by twelve 130-Watt traveling-wave tube amplifiers while the regular communication beams use bent-pipe repeaters and 60:48 TWTAs.
Question 1: I thought I could get 89 transponders by counting the feed horns on one side, but there are suggestions that there is a symmetric array on the other side. Is one uplink and the other downlink? If not, how does the arithmetic work?
Question 2: I don't see any evidence of phased arrays (though that doesn't mean they aren't there) so I'm wondering how the six steerable spot beams are generated!
Is this done the good ole' fashioned way? Is it done using articulated, movable feed horns, or perhaps tilting dishes?
above x2: cropped from Spaceflight 101 Boeing images below:
above: Inmarsat 5-F1 (F4 Identical) – Photo: Boeing. From Spaceflight 101
above: Inmarsat-5 F1 & F2 – Photo: Boeing. Cropped from Spaceflight 101. Original image is huge!
above: Inmarsat-5 Coverage Map – Credit: Inmarsat. From Spacelfight 101.
above: Screen shot from Inmarsat-5: Overview Brief for Arctic Aug 2014, pre F4, but showing layout of individual beams.