Signal loss - Tracking

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#1
Information for newbies - Why you lose a signal certain times of the day within a luna month outside the normal beam:

Although geostationary satellites are nominally fixed in the sky when seen from the Earth (i.e. Astra 2D @ 28.2°E), they do in fact wander slightly, tracing out a figure of 8 over 24 hour period. This movement is because geostationary satellite orbits cannot be made exactly circular with exactly 0° inclination. In time, even the most perfect geostationary orbit will become increasingly non-circular and inclined to the equatorial plane.

This is due to external gravitational disturbances resulting from the gravitational attraction of the sun and moon. The station keeping on the satellite is designed to periodically compensate for these disturbances by correcting the orbit. For current geostationary satellites this wander is kept less than +/-0.1° in both the N-S and E-W directions (+/-0.1° in a figure of 8).

In simple terms, if you were to stand within a circle on the ground and start to turn your body clockwise or anticlockwise, you will slowly move towards the edge of the circle and there will come a point where you will need to change your position slightly to keep within the circle.

You could invest in an automated tracking system to keep your dish pointing at the strongest signal from your selected satellite, then again, if you can afford an automated tacking system, it would be cheaper to invest in setting up cluster of antenni to compensate for the gravitational shift.

Hope this explains why at certain times of the day in good weather, you can lose a signal from a satellite. Don't forget, these sat's are around 35,000km's away, so moving the dish manually is not advised.
 

RedAltoGL

Now leaving, thanks all.
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#2
Surely the beamwidth of the dish is sufficient to cover this small amount of movement by the satellite?
regards Redmund
 
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#3
RedAltoGL said:
Surely the beamwidth of the dish is sufficient to cover this small amount of movement by the satellite?
regards Redmund
In theory, a large dish (3.8m or greater) with multiple feeds would redcuce the loss however, the chances of getting a good signal 24x7 is down to the size of the dish x accurate positioning. Within the official footprint, you wouldn't need to worry as the shift would be minimal, but outside the official footprint, the shift would be enhanced.

In simple terms, the circle you're standing in is larger in the official footprint and smaller outside the official footprint.
 
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#4
Yes, dbs satellites are subject to gradually increasing "wobble", Euler's laws (of mechanics) are used to minimise this. But, the effect on reception is negligible!
 
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#5
spiney said:
Yes, dbs satellites are subject to gradually increasing "wobble", Euler's laws (of mechanics) are used to minimise this. But, the effect on reception is negligible!
True, I have found that over the last couple of years in Marche Italy, the signal was getting lost on a more regular basis. To resolve the issue, I installed a larger dish with mutiple feeds.
 

PaulR

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#6
glantaff said:
In theory, a large dish (3.8m or greater) with multiple feeds would reduce the loss
I understand what you're saying, especially on the fringes of the normal reception areas, but as you go larger in dish size the beamwidth becomes less.

IIRC a 60cm dish has a beamwidth of about 3 degrees, which would take up any normal satellite wobble.

As you go larger and larger (outside the normal reception area) the beamwidth narrows, which makes it more difficult to align in the first place and also makes it more susceptible to the wobble that you're talking about.

PaulR
 

Likvid

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#7
This is important for fringe reception certainly, the 1.2M dish i installed in France recently for Thor reception was peaked when the satellite was placed in NOL, Nominal Orbit Location.

I could clearly see the difference on one transponder which was weaker than anyone else, TP 27 12.226GHz Vertical.

If i peaked it when it was in NOL i got almost 24/7 reception, it started to pixelize for an hour around 10PM to 11PM every night.

If i peaked the dish when the satellite was much further away from exact location i would get pixelation on the TP 27 12.226GHz Vertical transponder earlier, it started already at around 5 or 6 PM and continued to about 11PM and then the picture was steady again.

However this was the only transponder that was much weaker than everyone else, it was about 2-3dbW down at location in Nice, France compared to other transponders.

So this is very important.
 
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