Why do GEO satellites like the one shown in the picture below need different types of antenna? Is there a reference library or website where we can get more information on different antenna types, suppliers and their suitability for particular orbit and usage.
For this satellite here is my impression:
- Its an old satellite, just going on the diagram reproduction quality. The way they have done things here is still used in more modern satellites, though there are now even more options.
- There are two customer frequency bands on this satellite, C band and Ku band. Each has an uplink and a downlink and so there are four distinct frequencies: 4GHz, 6GHz, 11GHz and 14GHz
- Very often the customer uplink (i.e. receive on the satellite) will be at a higher frequency than the downlink (i.e. the transmit band on the satellite. Hence C band is up at 6GHz and down at 4GHz. Ku band is up at 14GHz (in this case) and down at 11GHz (though Ku band has several variants, world wide)
- You can see that the two large dishes are C band are one for receive and one for transmit. They could both point at the same patch of planet Earth though they might easily be shaped differently. If we knew how large the antennas are then we could make a rough estimate of the spot size.
- The two Ku band antennas appear to be used for both transmit and receive at the same time which suggests that there are two different geographic customer traffic regions on the Earth.
- The two global C band horns at 4 and 6 GHz are there so that larger Earth terminals (this could mean big 10m + things) can communicate through the satellite even though they are outside of the C-band spots. There will probably be some switching on board that allows these terminals to reach some or all of the C-band spot region (e.g. uplink to the C band global horn, downlink through a spot)
- It is quite possible that the feeds for the spot antennas define several adjacent spots on the ground (as per the link in the comment that uhoh provided).
- It is quite possible that there is some inter-connectivity between the customer areas at C-band and Ku-band, its just not visible on the exterior of the satellite.
- The service module (aka: platform, bus, spacecraft) has telemetry and commands at some unspecified frequency, probably C or S band, with an omni-directional antenna.
Lastly, if you know which satellite this is we can look it up and the chances are (if it isn't too old) that there may be a map of the spotbeams somewhere online.