Skew angle.

[Black Hole]

I'm still working on how to calculate it.

I've worked out 18.2º, using the dishpointer figures for the location of Aberdeen and the direction of the satellite.

I'm not sure the formula I've derived is giving sensible results. Update: it is, see post 25.

 

[MikeSh]

I'm not sure the formula I've derived is giving sensible results.

If it gave you that 18.2 then it sounds like it's in the ballpark, which is good.

 

[Mac]

Thanks for the excellent find of the dishpointer site.
I put in my details and here's the result......

 

[Black Hole]

If it gave you that 18.2 then it sounds like it's in the ballpark, which is good.

Ballpark maybe, but it doesn't agree with dishpointer... and more importantly doesn't seem to track in the way I expect when I vary the inputs. I'll hold off publishing the details until I've found the problem (which might turn out to be an error in my expectations, but we'll see).

Thanks for the excellent find of the dishpointer site.

Beware that whatever skew you decide to set, there might be an existing twist built into the Sky dish LNB support.

 

[Black Hole]

I have confirmed my equations, but they give a skew of 19.3º for Mac's location rather than dishpointer's 11.4... and this is confirmed by the antenna pointing calculator on the SES website (SES are the operators of the Astra satellite) so I believe my method is correct.

So, why the difference from dishpointer? I think I would prefer to trust the horse's mouth, but the difference is great enough to demand an explanation.

Update: there are unsubstantiated reports of Astra 2E being pre-skewed 7.5º (google 'astra 2e "preskew"'). It would be good to find an authoritative source for transponder preskew figures.

 

[MikeSh]

there are unsubstantiated reports of Astra 2E being pre-skewed 7.5º

I thought we were served by 2 or 3 satellites, in which case having one with a different skew seems bonkers.
What am I missing here?

 

[Black Hole]

What am I missing here?

They must all have the same pre-skew, if they are all intended to be received by the same (fixed) dish assembly. Satelliteophiles with motorised steerable dishes also have to have motorised skew.

I assumed it would be impractical to broadcast with any pre-skew other than 0 (ie align V with the polar axis), and I suspect this is true, but these are not broadcasts – they're narrowcasts to a limited footprint, and there's no problem making the pre-skew whatever they want (I imagine there are technical reasons for this, or maybe just to polarise the consumers).

 

[Black Hole]

The situation re skew seems very confused, resulting from an absence of official data it seems. Found on a satellite forum:

Skew 2F,2G is different from skew 2E.....

So in fact Astra 2E at 28.5 east has no pre-skew of 7.5 degrees, as have 2F and 2G? Have you tested this?

 

[Black Hole]

I imagine there are technical reasons for this

Not wonderful English, but:

Some Eutelsat and Astra satellites, may not have their polarisations aligned with the equator. Errors of 3 deg and 7 deg are believed to apply. This minimised the polarisation adjustment required in the central region of the coverage area and reduced cross-polarisation interference during rain, since the bottom of falling rain drops tends to be horizontal.

 

[Black Hole]

I've found a technical document for Eutelsat containing a table of their satellite skews (PDF page indexes 94-95), of which some (including Astra 2E/2F/2G) are given as 0º but mostly 3.535º.


What gives? If the satellite skew is zero, why the difference between SES and Dishpointer?

 

[Black Hole]

I've found a technical document for Eutelsat containing a table of their satellite skews (PDF page indexes 94-95), of which some (including Astra 2E/2F/2G) are given as 0º but mostly 3.535º.

I now suspect the zero values in that table are only a place-holder, because the polarization is not specified. Perhaps Eutelsat have no responsibility for the Astra transponders, even though they're carried on Eutelsat platforms.

3.535º appears to be the datum. Quoting from p93-95 (as defined in the PDF, not the printed page number):

The linear polarization planes (defined as X and Y and orthogonal to each other) of most of the Eutelsat S.A. satellites are not parallel/orthogonal to the equatorial plane.

For historical reasons, the polarization planes of the satellites are inclined at an angle with respect to the equatorial plane. This angle is referred to as the polarization offset.

...

The reference X-polarization (horizontal) is defined as that polarization whose plane makes an angle of 93.535° in an anti-clockwise direction, looking towards the earth, about a reference vector with respect to a plane containing this vector and the pitch axis. The reference vector is defined as the vector from the satellite in the direction 0.21° towards West and 6.07° towards north in satellite coordinates. The reference Y-polarization (vertical) is defined as that polarization whose plane is orthogonal to the X polarization plane and the reference vector defined above. In other words the polarization offset angle of the EUTELSAT satellites is +3.535°, clock-wise when looking at the satellite from the earth, from anywhere on the meridian (in the northern hemisphere) corresponding to the orbital location of the satellite. In the southern hemisphere the polarization offset angle of the EUTELSAT satellites is +183.535°, clock-wise, from anywhere on the meridian corresponding to the orbital location of the satellite.

Note that, decoding this, it specifies what the "reference vector" is but not what the "pitch axis" is. OK, so we know what pitch is, its one of the three axes defining orientation in space, but I have not found a figure for it. Could it mean the pitch axis of the receiving antenna? I don't see how. Maybe, therefore, it's the pitch axis of the transmitting antenna, which will depend which way the beam is pointed.