Modified Declination Settings

cyberham

SatelliteGuys Master
Original poster
Jun 16, 2010
5,041
3,699
Halfmoon Bay, British Columbia
Can anybody point me to a good link for the procedure to use modified declination settings with USALS? The links I've found on the site are broken.

Since I'm starting from scratch at a new latitude I may as well do it right. Unfortunately the declination gauge numbers on the dish are worn off so that will add a challenge. And the bolts need rust dissolver before I can start.

My motor manual for my 49.5 degrees latitude says I should use (30 - 7.25) = 22.75 degrees declination.

Sent from my SM-G990W using Tapatalk
 
... to use modified declination settings with USALS ...

You might even want to set not your 'normal' latitude in your receiver, but the 'modified' latitude, for more accurate USALS calculations.
From theory, it would be slightly better (but hardly noticeably, in my practical experience). See USALS Notebook

By the way, notice that the chart that you linked mentions modified elevation angles, but gives modified latitude angles for your motor axis setting.
The ( motor tube bend angle 30 - dish declination offset angle ) = 22.75 degrees would be dish elevation scale angle. (So you are setting your declination to an elevation angle, so to say... ;) )

When you have difficulty finding the correct dish elevation, remember the clarke belt elevation angle due south for lat=49.5 would be 33.24 degrees. So with taking the dish offset angle into account, you could set the dish elevation using the angle of the dish face.

Greetz,
A33
 
All of this detail will be interesting to follow if I ever get a shiny new dish with readable elevation numbers.

Fortunately, my motor elevation numbers are rusty but still readable. So I will begin by setting the elevation angle of the motor precisely for the manual-specified 40.5 degrees at this latitude. This is my only reference. With the motor assembly aimed as due south as I can determine, I will USALS motor to my due south satellite (AMC21 at 125W assuming PBS is still there). This is 1.5 degrees azimuth to my west. I will vary the dish elevation (and motor-assembly azimuth a tiny amount) as necessary to find this satellite since I have no readable numbers for dish elevation.

Once I receive signal from 125W, I can then motor to each west/east extreme looking for those satellites. There is hardly anything on Ku to my west: maybe Reuters on 129W if it's still there or any known strong data transponder. To my east, I will try for SES2 at 87W. I will vary the dish elevation (varying the declination) only as necessary to find these. Then I'll motor back to 125W and only vary motor angle elevation to peak it. Back and forth to improve as necessary.

The main thing is to not do this during the peak of the day's heat since a Zen-like calm and patience accompanied with a bit of luck is required when doing this setup especially without visible scale numbers. At least I don't have tree obstacles or dish imperfections to worry about. The dish is a former Shaw pay TV dish and has battleship quality.

ADDED: I may not have enough Zen for all of above. I think I'm going to mount the dish with no motor to start. This is a lot simpler than beginning with a motor. Then I can learn about my new sky and familiarize myself with all active receivable transponders on the satellites.
 
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All of this detail will be interesting to follow if I ever get a shiny new dish with readable elevation numbers.
As I wrote, you can measure dish elevation at the dish face (while taking into account the dish offset angle, which can be calculated as well, even if it has a non-flat dish face).
And scales of motors or dishes might not be perfect at all, so using an inclinometer(-app) is not bad at all.

So I will begin by setting the elevation angle of the motor precisely for the manual-specified 40.5 degrees at this latitude.
Elevation 40.5 would not be the modified motor elevation angle for lat=49.5, but the traditional one.
Modified motor elevation would be about 39.85.
So I don't understand why you ask for the modified declination offset angles, but would not want to start with the modified motor elevation angle? That would be rather sense-free, I would say.


Setting up a motor with USALS is not really difficult, as you can use USALS to exactly align to due south.
Most difficult would be to have motor-zero -- dish -- LNB-arm exactly in line, I would say.
If you can find 125W that way, you have already success as a quasi fixed setup; and without worry you can take your time for finetuning the motor settings (if still needed, with the modified settings)...

Greetz,
A33
 
...you can measure dish elevation at the dish face...
Assuming my dish offset is 24 degrees, then I would adjust for a dish face angle of 33.24 - 24 = 9.24 degrees up from vertical. This is good to know.
Elevation 40.5 would not be the modified motor elevation angle for lat=49.5, but the traditional one.
Modified motor elevation would be about 39.85....
Yes, I agree. I will start with 39.85 degrees modified motor elevation.
Setting up a motor with USALS is not really difficult...
It's not difficult in theory perhaps where we can calculate to the 1/100 of a degree. In the real world, with all the factors involved, and when using a dish without numerical markings, it is difficult. But that's the fun! The harder it is, the better you feel when you get it right.
 
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I found the link to the modified declination settings:
Modified Declination Settings Chart

BTW.
I always thought the modified declination value at LAT=65 was the only evident fault of this chart,
but looking at the modified "elevation" values at LAT=51 to 54, it is clear that those are not very accurate either (and consequently their modified declination angles also).

From lat=51 to lat=54, the added amount in degrees should be diminishing from .64 to .61 (the correctly added values for LAT=50 and LAT=55). I checked, with my own modified motor angles calculator (0-180 fit approach).
The fault is just a few hundredths in degrees, but why have a chart with two decimals precision and then have such inaccuracies?
So all in all, the chart seems not two decimals accurate at all.

Thankfully, the differences are not great; and using any sort of modified motor angles is still better than following the traditional motor angles.
Maybe someday I'll test which modified motor angles calculation method I like best: the 0-180 degree fit, the 0-90 degree fit, the 0-horizon fit, or the 0--two-thirds-of-horizon fit, or something inbetween; and make a chart of my preferred method...

Greetz,
A33
 
Keep up your research. If not you, who?

Moving the 1-metre dish by hand, I easily found and scanned in 129W, 125W, 123W, 117W, 111W and I saw a glimpse of 103W before the sun began burning my legs and I had to quit. These sats popped in with decent qualities. After setting elevation for 125W, I hardly changed it for the rest for these tests. Same for the LNB skew.

So I'm ready to move forward with the motor. I know some receivable transponders and I perfected my system of using my smartphone to remotely view the signal qualities at the dish.

Sent from my SM-G990W using Tapatalk
 
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I easily found and scanned in ...125W, ...
...
So I'm ready to move forward with the motor.
To already get an indication of the offset angle of the dish, you could measure the dish face angle while fixed at 125W!
(Elevation angle towards the satellite is 33.23, so practically the same as to due south.)
That could help for your uncertainties about the dish elevation scale.


Keep up your research. If not you, who?
You are a flatterer! :)
I recently managed to develop an exact multifeed skew angle calculation method, which was really a challenge. It needed to incorporate three elaborate angle calculations; which took me quite some time to think through. I still have to put the calculations in a spreadsheet, and test them in practice.

Apart from that, I want to finish a bunch of (new) equations for measuring and calculating the geometry of parabolic dishes, no matter if prime focus, offset, or multifeed dishes.
For the modified motor angles, there are practical methods already, that are about accurate enough, I would say. So though theoretically very interesting, not a real priority for me.
Same is valid for a calculator for designing the actuator triangle and the zone of maximum precision, etc. I find there seems to be not much interest in that subject (as there was not much interest for USALS-like calculations of actuator lengths, and pulse counts, for actuator setups)....

I like to use my brains for analyzing such problems, though!

Succes with the next steps in setting up your motor,

greetz,
A33.
 
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To already get an indication of the offset angle of the dish, you could measure the dish face angle while fixed at 125W!
(Elevation angle towards the satellite is 33.23, so practically the same as to due south.)
That could help for your uncertainties about the dish elevation scale.
I did exactly that. I never even looked at the scale markings on the dish. I used my smartphone angle app and set the dish face for just under 10 degrees up from horizontal assuming about a 24 degree offset for the dish. With that elevation, I simply scanned the azimuth a bit and 125W popped in so fast I actually said: "That's too easy!". And all the PBS transponders scanned in. I will use the same method once I de-rust and install the motor.
 
After installing the motor, I went through the adjustment procedure two complete times. After motoring to my strongest "due south" satellite (125W), I peaked the signal by adjusting the motor assembly azimuth and motor elevation. I then motored to my most extreme satellite (87W) and peaked by adjusting dish elevation (motor declination). I went back and forth between 125W and 87W and repeated this several times.

The end result is I've got pretty good reception from all satellites between 139W and 78.8W. I checked as far east as 65W but I'm not in the footprint of any satellite east of 78.8W. However I only use USALS to find satellites. Once I find them, I always use manual diseqc commands to peak the signal of each satellite then save the locations into motor memories.

The reason for doing two complete times is I discovered midway during the first procedure that I had to slightly adjust the LNB arm to place the LNB in the best location. This added maybe 1 dB to readings so it was important.
 
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