Diseqc 1.2/USALS motor query

[lelandv]

Having read BGonaSTICK's excellent tutorial, and have successfully installed my Diseqc 1.2 motor, I've found a small (not major) issue that is intriguing me, and was wondering if someone might be able to explain it.

The actual installation and alignment across the whole arc was relatively painless, and everything aligns perfectly across the sky all the way from 40W to 45E, so I'm well chuffed with that.

I did get a little confused when I managed to get it aligned right, and tracking using Diseqc 1.2 commands from the receiver. When I switched it to USALS mode, however, and having entered my lat/long coordinates, it placed every single satellite exactly 4 degrees off in azimuth. Elevation was still correct for every satellite though as it still tracked through the arc correctly, and didn't skew across the horizon line.

I found this rather odd and at first decided to myself that I'd just use the Diseqc 1.2 mode on the receiver and then I figured that if USALS said that every satellite was 4 degrees east of where it actually was, as an experiment I compensated for this by simply adding 4 degrees east to my input longitude. Well.. 'lo and behold, it worked perfectly, so can now simply "goto" any satellite or azimuth that I want.

My query is simply why would USALS calculation be exactly 4 degrees off? It couldn't really be a misalignment of the dish otherwise there would be skew errors at the extreme east and west limits of the arc. Exact coordinates were cross-referenced from both ordinance survey and GPS. (Actual longitude is 0.5W, but I had to enter 3.5E to compensate for the offset).

Just curious as to what may be the cause of this. It's not a problem since it's tracking perfectly.

Thanks

Leland

[BGonaSTICK]

That's an interesting finding, and I don't know the answer - but my suggestion would have been to try what you've already found to work - 'falsifying' your location.

Neither GPS or OS maps are guaranteed to be accurate. GPS uses a different reference point (not the Greenwich Meridian, apparently) and if you were using the magnetic north indicated on an OS map, it can be out of date if the map is also past it's sell-by date. Neither of these errors would amount to anything like 4 degrees though.

I don't think the Skew would be sufficiently out with an azimuth error of only 4 degrees to make a blind bit of difference to your reception, but with a 'real' error of 4 degrees, you'd certainly expect the arc tracking to be sufficiently poor to cause you reception problems.

Did you need to make an initial correction to your motor position when setting up on your due-south satellite, i.e. did you compensate for any difference between your longitude and the longitude of your nearest-south satellite?

[lelandv]

Did you need to make an initial correction to your motor position when setting up on your due-south satellite, i.e. did you compensate for any difference between your longitude and the longitude of your nearest-south satellite?

Not for the initial manual alignment.. since I'm actually at 0.5W and the south satellite (Thor 2/3) is at 1W, there was hardly much of an offset correction needed.

Used true north as the compass reference for alignment, but then again I wonder if that might explain the 4 degree variance for USALS... wonder what the reference datum was from OS... might explain a couple of things since our mag variation here is 4degW. :?


Leland

[lelandv]

DOH!

ops:

Silly me... turns out that I DIDN'T take into account magnetic variation, hence the 4 degree offset. So taking it into account in the USALS point of origin solved the problem.



Leland

[BGonaSTICK]

OK, good news. Magnetic variation obviously depends on where you are, but is maybe three or four degrees across most of the UK.

I'm very surprised that you managed to get so far East and West with a 4 degree error introduced at setup, but who cares if it's all sorted now?