? USALS accuracy ?

wejones · #1 (**permalink**) 08-30-2006, 01:25 PM

Re the question of which is best, USALS or DiseqC 1.2 , I was curious just how accurately the rotation angle is calculated by the USALS receivers.

I decided to compare, for my location,
(1) the angle used by my Fortec Lifetime (ie in the USALS menu, it calculates the angle and displays it),
(2) the angle predicted by the http://www.gaaps.com.tw/ web page someone posted a while back.
(3) my own calculation of what I think it SHOULD be.

I first did a quick calculation based on the basic type of alignment, (ie rotation axis parallel to earth's axis... ie no 0.6 degree adjustment). I got 57.7 for G10r. I then looked at what the Lifetime receiver calculated. It was 57.1 . I then looked at what the GAAPS web page calculated, and it was 57.2 .

My problem is, that I can't figure out why there is a difference between what I did, and what the receiver is calculating. The only likely difference is if they were using the so called "Modified" declination angle, which I was NOT using. Calculation of the rotation angle is a bit more complicated using this assumption, so I didn't attempt it, however I really don't think this is the reason for the difference, because this small adjustment "should" have little effect on the USALS angle, but I may be wrong. It is interesting that the difference is approximately the 0.6 degree tilt angle, however that tilt is largely orthogonal to the USALS rotation, so it shouldn't affect it nearly that much.

Anyway, does anyone have a link to the equations used by the USALS to determine what angle to go to? I was hoping that the GAAPS web page might have javascript calculations incorporated into the page, or have some explanation of how the calculations were performed, but it doesn't seem to.

rainman · #2 (**permalink**) 08-30-2006, 02:13 PM

check this site Bill some good reading on usals www.usals.info

wejones · #3 (**permalink**) 08-30-2006, 02:58 PM

Quote:

Originally Posted by rainman

check this site Bill some good reading on usals www.usals.info

Thanks. Didn't find any equations, but it was interesting.

elgemcdlf · #4 (**permalink**) 08-30-2006, 03:09 PM

Potentially it is the offset dish. If using a full dish perhaps your numbers are correct but with the offset and given the fact that the dish rotates at the same time the motor moves I would guess may be why the numbers do not align. When at 0 (south) the arm for the lnbf is perpendicular (concept) and the dish is somewhat oval shaped so I am assuming as the dish rolls out to a sat the actual prime focal point may change since we are no longer a vertical oval shape but now kinda laid on it's side.

With a full dish rollout of the dish would not matter as the dish is circular in shape. So far every one of these small dishes I have bought for Ku FTA reception is oval in shape. To check my assumption simply pick a sat in the middle between 123 and your south sat. Run the numbers are we .3 off this time? Due to shape in conjunction with my theory I would guess that the difference will be different for each sat based on distance from true south location and the difference will be an even spacing that should correspond with sat location in relation to true south

reho33 · #5 (**permalink**) 08-30-2006, 04:52 PM

Going back to an earlier post, someone said that if you setup USALS, go to ref (0) then punch in say AMC3 at 87W, move the whole assy to peak, and then go to another sat, it should be lined up fine. I am tempted to try that but I am satisfied with the motor as it is now and don't want to mess with it. Maybe I will try that system when it goes on the roof....

Vorg · #6 (**permalink**) 08-30-2006, 08:04 PM

Quote:

Originally Posted by reho33

Going back to an earlier post, someone said that if you setup USALS, go to ref (0) then punch in say AMC3 at 87W, move the whole assy to peak, and then go to another sat, it should be lined up fine. I am tempted to try that but I am satisfied with the motor as it is now and don't want to mess with it. Maybe I will try that system when it goes on the roof....

The problem with that idea is that there are ** 2 ** elevation settings (dish and motor) and an infinite number of combinations of those two settings for any ** 1 ** sat. But there is only 1 combination of settings that will follow the belt and be correct with all sats.

pmb1010 · #7 (**permalink**) 08-30-2006, 08:41 PM

that is actually *not* a problem.

Once set, USALS bases all satellite positon off your coordinates. The sats do not change, your coordinates dont change.

Diseqc 1.2 does not change that, either. Your motor's adjustment must be the right, and dish elevation to be correct - to track the arc. All Diseqc 1.2 does is allow a little "easier" setting adjustments via the "nudge" (debatable..., as posted above I can fine tune with USALS as easy as Diseqc 1.2) - but it sucks at getting the dish close to the sat in question. It's possible to nudge your self too far either way and get all messed up.

wejones · #8 (**permalink**) 10-29-2006, 11:05 AM

Quote:

Originally Posted by wejones

Re the question of which is best, USALS or DiseqC 1.2 , I was curious just how accurately the rotation angle is calculated by the USALS receivers.

I decided to compare, for my location,
(1) the angle used by my Fortec Lifetime (ie in the USALS menu, it calculates the angle and displays it),
(2) the angle predicted by the http://www.gaaps.com.tw/ web page someone posted a while back.
(3) my own calculation of what I think it SHOULD be.

I first did a quick calculation based on the basic type of alignment, (ie rotation axis parallel to earth's axis... ie no 0.6 degree adjustment). I got 57.7 for G10r. I then looked at what the Lifetime receiver calculated. It was 57.1 . I then looked at what the GAAPS web page calculated, and it was 57.2 .

My problem is, that I can't figure out why there is a difference between what I did, and what the receiver is calculating. The only likely difference is if they were using the so called "Modified" declination angle, which I was NOT using. Calculation of the rotation angle is a bit more complicated using this assumption, so I didn't attempt it, however I really don't think this is the reason for the difference, because this small adjustment "should" have little effect on the USALS angle, but I may be wrong. ........

Well, I finally came back to this, and figured out the calculation using the modified declination angle. It turns out that this WAS the difference. When done this way, I got 57.16 in the above example, which corresponds to within roundoff error between the GAAPS web page and what my Lifetime receiver calculates.

Anyway, now that I know how they calculated it, I put the calculation on my little on-line TVRO calculator page, ie:
http://wejones.ftdata.com/bjdishcalc2.htm
it has also been included in the more complicated TVCALC program:
http://wejones.ftdata.com/tvcalc06.zip { EDIT: corrected error in this after post, current file time stamp= 10/29/06 11:56AM }

Anyway, I'm a bit surprised that the calculation in the Fortec program uses the modified declination angle, since it is a popular simplification to ignore this, as does the Sadoun instruction pages.