Orbital positions and footprints

Severn

Why is it that satellites which are mainly serving the British Isles, such as the Astra-2 cluster, are positioned so far to the east? Why not at a longitude which roughly bisects the UK and Ireland, somewhere west of Greenwich? And I notice that some serving Holland and Germany, say, are further west than Astra 2.

Obviously there must be good reasons. I'd love to know what they are.

Analoguesat

Generally speaking orbital slot have to be 3 degrees apart to prevent interference from the slot next door. For somewhere like Europe where the countries are close together it would not be possible to park all the satellites operating into France, Germany, Scandanavia , Eastern Europe the UK, Italy, Northern Africa etc in one slice of European sky.

All orbital slots are coordinated by the ITU to avoid mutual interference as far as possible, and later registrations have to be slotted in where there is free space. The UK slot was a reasonably late registration.

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Analoguesat

This is quite a good article about orbital slots and the problems with them:

_http://www.satellitetoday.com/via/features/Hot-Orbital-Slots-Is-There-Anything-Left_22108.html

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Severn

Thanks, Analoguesat, but that means there'd only be 120 slots. Surely there are a multitude more than 120 active geostationary satellites (never mind the dead ones)?

Did you mean 3 minutes? That would allow for more than 7000 slots and put them 30 or 40 km apart, if my maths is correct.

I wonder why the UK was late.

Severn

OK, I read the stuff you pointed to, and it shows that I don't know enough. But it does also talk about 5 or more satellites in the same slot.

Time I went to the library.

Analoguesat

No - 3 degrees (moving towards 2 degrees in places like America where the skies are very crowded)

The limiting factor is the acceptance angle of the dish on the house. Dishes dont just accept signals from "straight in front" of the the dish, you can also get usable reception from signals coming in from the side. A normal domestic dish accepts signals from about 2 degrees either side of its lnb arm. Therefore any signals transmitted off a closely located satellite transmitting into the same geographic region and using the same frequencies will cause awful interference.

Theres no problem where the birds are transmittinbg into different areas. Look at the European situation - this of course is generalising and making a lot of assumptions but is approximately true

5E - Europe
7E - Europe
9E - Europe (different frequency bands are used between the 4 slots)
10E - Europe

13E - Europe
16E - eastern Europe (same bands used - 3 degree seperation)
19E - Europe

21E - middle east
23E - Europe
26E - middle east (different target areas using the same freqs across this section of space)
28E - UK/Europe
30/31E - Turkey & Eastern Europe.
33E - Europe

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mad_elf

Thanks for the discussion. The article is quite interesting. However, I noticed that most of the time the talk is about slots without almost any discussion of the multiplexing techniques or generally speaking spectrum utilization techniques. For example, even if we had a neighboring satellite with different coding scheme and spectrum utilization technique it would be possible to have negligible interference.

I find the utilization techniques far more relevant to the future. Slowly we are moving towards high definition broadcast and more complex applications requiring more bandwidth. The services and applications are also merging and we see some 3.5G mobiles that can be used to make voice/video calls, watching TV, and browsing the net. So I guess to be able to increase the capacity of the system a more efficient utilization of the spectrum is essential.