Re: daily cyclic signal change


geojohnt@...
 

Hello Nigel,

I would say that that the diurnal variation we see is more to do with the state of the troposphere and space weather than the figure of 8 movement of our satellites.
When the Sun is on 'our side of the planet' our polarised signals can become slightly twisted as they pass through the various 'excited layers.'

Commercial TV satellites are quite tightly controlled by the satellite operators - but of course their 'movement' is not that important for domestic satellite TV reception with 45 cm receiving dishes - more  important for the service uplinks to the satellite.

Just look at the inclination of MSG-1 and MSG-2 - though they are spin stabilised, not three axis.
They will take some tracking from the ground station control dishs.

On our GEO visits to EUMETSAT we went to the satellite ground station at Ursingen and standing under the giant Meteosat receive/control dishes, they would make a loud clunk every now and again as they tracked the slight movement of the satellite.

You are absolutely right, it's amazing how the geostationary satellite 'stay in the same place.'
One forgets(?) they are actually moving - as the Earth is. 

Regarding our 'dish alignment,' most of us use domestic satellite TV dishes which I would say in their original construction are impossible to align to 100% accuracy.
And as for the Triax 1 m dish - the less said about that, the better.

Several members have designed a sort of Vernier lever system to adjust both elevation and azimuth to very good accuracy.


Regards,
John




-----Original Message-----
From: nigel <nigel@...>
To: MSG-1@groups.io
Sent: Thu, 6 May 2021 8:48
Subject: Re: [MSG-1] daily cyclic signal change

Hi John,
Good to hear from you, also.
Thanks for your thoughts and yes, I am seeing a "diurnal variation" - i.e. the same pattern repeating each day, but I do not know if it is due to the earths rotation or the satellite describing a figure of 8 ( or 0 - thanks for the reminder of your animations, Ernst! ) about its nominal position.
I  hadn't really thought about what is required to keep a geostationary satellite fixed in position, in relation to a location on earth, when we are both spinning in space. My mind boggles at the maths involved, let alone the technology required. Harmut's comment about the saving in fuel really started me thinking !!
As for getting the alignment within 0.5 degree - I would think that must involve a very precise mounting arrangement - not possible with the bog standard commercial setup. I did plan to try and improve my dish's mounting and adjustment system, but that is still at "the design stage".
More googling and research is needed, I'm sure.
Regards
Nigel


On 05/05/2021 17:35, geojohnt via groups.io wrote:
Hello Nigel,

Good to hear from you again.

Firstly TP 1 has been 'doing something' in the last few days.
You will have seen that Ian and I reported a few days ago that our SNR has increased considerably - but it wasn't to last.
Back to normal a day or so later.
Though I did see another brief increase again yesterday afternoon.

By daily variation, do you mean at 'set' times/periods - your graphs appear to show this.
I too have a variation in SNR during the day - diurnal variation - due to that state of the atmosphere/troposphere.
I wrote an article about this for the GEO Quarterly a few years ago.

I always wondered about whether 'we' would see signal variations due to a satellite's figure of 8 movements but dismissed this as the movements were 'very small.'
However looking at EUMETSAT's dish off pointing graphs as posted by Ernst, you can lose 1 dB 'very easily' with an 0.5 degree dish error when using larger dishes.

Goodness, is it possibly to align a dish to 0.5 degree accuracy or less? 

Best wishes,
John.

 

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