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How to calculate Satellite Skew

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May 3rd, 2013 at 3:32am  
Does Anyone know how to calculate The Satellite Skew, Not The Lnb Or Dish Skew? I asked Intelsat and they referred me here. Please advise. Thanks
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« Last Edit: Mar 3rd, 2015 at 6:59pm by Admin1 »  
 
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Eric Johnston
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Reply #1 - May 3rd, 2013 at 9:39pm  
Satellites normally have their linear polarisation planes aligned with the equator and the earth's polar axis.

Very few satellites might have offsets of a few degrees.  This was done in the case of early Eutelsat satellites to minimise the earth station (LNB or dish) skew for customers in the middle of the service area, which was not on the same longitude as the satellite.  There is a slight benefit in reduced cross-pol isolation in rain, where tilted polarisation is more affected by the flat horizontal underside of water rain drops.

Skew is the same thing as polarisation adjustment angle.  

I am not aware of any satellites with other polarisation angles, although you could make the satellite polarisation adjustable if there was a need for that added complexity.

So there no need to ever calculate the satellite skew.

You do need to calculate the skew (polarisation adjustment angle) for earth station LNB or complete dish rotation, but you said that was not your question.

If you use my dish pointing calculator at http://www.satsig.net/maps/lat-long-finder.htm the polarisation anjsutment ange is given for earth station antennas.  A positive + angle means a clockwise rotation adjustment amount as with you behind the dish and facing forwards towards the satellite in the sky.

Your polarisation starting position is the nominal named polarisation set with reference to the local horizon.  Vertical polarisation is when the half dipole pin in the LNB waveguide is vertical. Like so:
...

Start with whatever polarisation name you want, then apply the polarisation adjustment amount.  On a transmit antenna check the accuracy with the VSAT hub who will measure your cross-pol isolation and maybe talk you into the exact null, about +/- 1 deg wide. Accuracy is important.  An inclinometer put sideways across the LNB/BUC is suggested for initial set up, excepting the Prodelin and other dish designs with a giant circular polarisation scale at the rear.

Best regards, Eric.
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« Last Edit: Aug 17th, 2015 at 1:51pm by Admin1 »  
 
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Reply #2 - May 3rd, 2013 at 11:42pm  
Thank you for the reply and the knowledge Eric. However, how would you calculate this satellite skew if any. Here's a little back drop on this question. I am a Field Engineer and I commission martime VSAT systems. Well one of the antenna systems mean screen displays a satellite skew setting and a mechanical skew setting. I totally understand the mechanical offset when performing CPI testing, but the satellite skew value, which is currently set to "0" was throwing me off. I even contact IntelSat and no one, I mean no one knew anything about what I was asking. I then contact SES Americom and one Earth Station Tech suggest try here. GO figure huh?
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Eric Johnston
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Reply #3 - May 4th, 2013 at 8:25am  
As far as I am aware all satellites have been constructed with a zero polarisation skew value, except some Eutelsat satellites which had value of 3.5 deg and Astra with 7 deg.  No sure what sign you should use and not sure which exactly which satellites it applies to.

I repeat, virtually all satellites are built with their linear polarisations aligned with the equator and the earth's polar axis.  There is no calculation to be done.

On a dynamic mobile mount, there may be an initial mechanical polarisation offset angle to initially calibrate the polarisation.  This might apply if the mechanical polarisation is set out of true, such as to make the BUC or LNB cable loop fit within the radome cover.  With the antenna drive switched off and all at rest, if the polarisation is then actually horizontal or vertical I would leave the screen setting at zero.  If you find when it works that the polariation is 90 deg wrong, maybe you need to input +90 or -90 deg.  Do whatever is necessary so that in service readouts make sense.

When the antenna moves location, the software will use GPS and recalculate the polarisation angle and adjust accordingly, assuming it is properly zeroed initially.

Best regards, Eric.
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« Last Edit: May 4th, 2013 at 12:44pm by Eric Johnston »  
 
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