Satellite Signals

[Kate]

By email


ASTRA 2E AND 2F SIGNALS AT THEIR STRONGEST FOR THE YEAR
By email

It's not entirely a coincidence that the winter solstice, Christmas and the strongest satellite signals on the UK-only beams are at the same point of the calendar. Whilst viewers in the UK and near continent enjoy good reception all year round, those in fringe areas of Europe trying to view BBC & ITV suffer seasonal variations. Probably due to the attitude of the satellite's solar panels, signals peak at this time of year. From February onwards, levels decrease, with a low point in the hot months. Whilst in part this is caused by an increase in background electronic noise from the ground and your own device in warm weather, the rest is down to the satellite's orbit.
On the ground, this means that many who lost their BBC in February have found pictures returning over the past three months. Sadly, the gain is temporary.

[russell]

Kate,
Perhaps you should tell us who this came from and what his qualifications are?

Probably due to the attitude of the satellite's solar panels, signals peak at this time of year. From February onwards, levels decrease, with a low point in the hot months. Whilst in part this is caused by an increase in background electronic noise from the ground and your own device in warm weather, the rest is down to the satellite's orbit.

I'm afraid that this is uninformed speculation.

Russell.

[lonesome paddy]

Kate,
Perhaps you should tell us who this came from and what his qualifications are?

Probably due to the attitude of the satellite's solar panels, signals peak at this time of year. From February onwards, levels decrease, with a low point in the hot months. Whilst in part this is caused by an increase in background electronic noise from the ground and your own device in warm weather, the rest is down to the satellite's orbit.

I'm afraid that this is uninformed speculation.

Russell.[/quote

Uniformed speculation as opposed to what ? the truth !!
Is the poster wrong if so what is the reason

[Davidsimon]

Kate,
Perhaps you should tell us who this came from and what his qualifications are?

Probably due to the attitude of the satellite's solar panels, signals peak at this time of year. From February onwards, levels decrease, with a low point in the hot months. Whilst in part this is caused by an increase in background electronic noise from the ground and your own device in warm weather, the rest is down to the satellite's orbit.

I'm afraid that this is uninformed speculation.

Russell.

It may be anecdotal but that is the way it seemed to pan out last year at least in Southern Spain.In mid Summer quit a few channels pixalated only to come back clear later in the year.

[Kate]

I believe it is actually from a Spanish resident ex pat who took it from a regional English speakers forum. They didn't claim to be expert or qualified, and weren't selling anything - just passing this on for our general interest....and discussion.

[russell]

First, apologies to Kate. No criticism intended. It is just that the more a false statement is repeated on the Internet, the more it becomes accepted as fact.

Uniformed speculation as opposed to what ? the truth !!
Is the poster wrong if so what is the reason

Yes, uninformed speculation as opposed to facts.
Yes the poster is wrong.

"Probably due to the attitude of the satellite's solar panels, signals peak at this time of year." Wrong, there are control systems on the satellite that keep the solar panels pointing in the correct direction. The radiated power received from the sun is not polarised so their attitude has no effect on their performance. Furthermore the TWTs that feed the power to the antennas have a feedback system that keeps the output power constant to within about 0.1 dB.

"From February onwards, levels decrease, with a low point in the hot months." Reception varies as a result of a number of factors but, as far as I can tell, nobody has made proper long term measurements of signal strength that can be analysed. Instead we get vague reports that the picture quality (an undefined quantity) has gone up or down by 10% or "we had a good picture today". Unfortunately, since my retirement, I no longer have access to measurement facilities.

" Whilst in part this is caused by an increase in background electronic noise from the ground and your own device in warm weather" This is true but the effect is totally insignificant, particularly in southern Europe where the satellite elevation is high. The thermal noise power received from the ground is so low as to be insignificant. The variation with temperature is even lower as the noise power is proportional to the absolute temperature (look up Nyquist noise in any communication theory textbook) which varies very little between summer and winter.

"the rest is down to the satellite's orbit" There is some truth in this although there are other factors. The satellite is not truly geostationary. It's orbit is affected by the Moon, the Sun and, to a lesser extent, other celestial bodies. However the satellite is not left to drift about that would produce chaos with all the other satellites around. It is equipped with small rocket motors that are used to maintain it's position within strict limits in accordance with international agreements made initially at WARC-71 and updated periodically since then. This is the reason for the limited life of these satellites, they run out of rocket fuel.
They are allowed to drift around within a defined box and corrected by firing the rockets when necessary. There is certainly not a consistent annual pattern and the size of the box is such that it should remain well within the beamwidth of any dish being used here.

Some other things that can cause a degradation in performance:
Variations in apparent sky temperature due to different atmospheric conditions (not to be confused with air temperature).
Rain attenuation (not necessarily at ground level).
Poor dish alignment such that the "box" containing the satellite is not centred on the beam of the dish (more of a problem with big dishes and likely to cause a monthly cycle).

Russell.

[Allan]

"the rest is down to the satellite's orbit" There is some truth in this although there are other factors. The satellite is not truly geostationary. It's orbit is affected by the Moon, the Sun and, to a lesser extent, other celestial bodies. However the satellite is not left to drift about that would produce chaos with all the other satellites around. It is equipped with small rocket motors that are used to maintain it's position within strict limits in accordance with international agreements made initially at WARC-71 and updated periodically since then. This is the reason for the limited life of these satellites, they run out of rocket fuel.
They are allowed to drift around within a defined box and corrected by firing the rockets when necessary. There is certainly not a consistent annual pattern and the size of the box is such that it should remain well within the beam width of any dish being used here.

Russell.

Interesting post Russell but I have to say, the effect of the satellite moving in its box confuses me. SES publishes a table of centre of box events that appear to happen daily https://extranet.ses.com/18723856/center-of-box so it seems improbable that the satellites orbit can have any bearing on seasonal fluctuations.

I get further confused by the fact that the box is roughly 100km in diameter so if the angles don't change then movement within the box would have the effect of moving the receiver the same distance in the opposite direction. Clearly it doesn't have this effect so what is happening.

Apologies to anyone that finds this too geeky but it is a Geek forum

[Kate]

Geeky is good. Gives me a chance to glaze!

[russell]

Interesting post Russell but I have to say, the effect of the satellite moving in its box confuses me. SES publishes a table of centre of box events that appear to happen daily https://extranet.ses.com/18723856/center-of-box so it seems improbable that the satellites orbit can have any bearing on seasonal fluctuations.

That's right. As I said the satellite should remain well within the beamwidth of any dish used here. When I said that there is some truth in the statement I meant in principle not in degree. It is correct that there is no seasonal variation.

I get further confused by the fact that the box is roughly 100km in diameter so if the angles don't change then movement within the box would have the effect of moving the receiver the same distance in the opposite direction. Clearly it doesn't have this effect so what is happening.

It depends on how the alignment of the dish on the satellite is maintained. If it is tracked so that it remains pointing at the same point on the Earth's surface then the only effect is that of it changing it's position with respect to the receiving dish beam. The received EIRP remains constant.
Let's put some numbers on it.
If the receiving dish is aligned when the satellite is at the centre of the box and the box diameter is 100 km then the maximum deviation from the centre of the beam is atan(50/38232) or 0.075° (distance calculated from Millas but doesn't vary much). That amount of movement will give about 0.06 dB reduction of signal with a 1 m dish, or 0.2 dB for a 3 m dish. Hardly enough to give any noticeable picture degredation.
Now if the dish was aligned when the satellite was at one edge of the box and it moves to the other edge that would be an error of 0.15° and give a signal loss of 0.12 dB for a 1 m dish and 0.8 dB with a 3 m dish so where big dishes are needed it could be noticeable although much less so than changes in atmospheric conditions.
Thinking about possible causes for seasonal variations; and here we're getting into the realms of speculation not fact, I wonder how stable the ground is where people have installed big dishes. If the ground moves as a result of changing moisture content how much could it move the dish? I can't answer that not having experience of civil engineering but a movement of only 0.3° from the optimum angular position for a 3 m dish will halve the received signal.

Russell.