True South Elevation

[viper5]

Trying to get my motor/dish set up. One thing I don't understand is why the elevation set at true south is so different from the actual true south sattelite elevation.

My true south sat is Nimiq 4 since it is located at 82W and my long is 81.1W. The dish elevation is to be set at 56.7 degrees per the sat finder instructions. My lat is 28.55 and I have that set as my motor elevation. According to the motor instructions, the true south position should be set for the dish at (30 degrees minus declination angle). My declination angle at this position is 4.8 so I set the dish to 25.2 degrees elevation. This doesn't make sense to me though because if true south is supposed to be at the top of the arc, then how is it that if it just moves over 1 degree from true south to the west (to locate nimiq 4), the elevation is set at 56.7 degrees which is much higher elevation. I was thinking it had to do with the skew making the dish come down a little, but it's only 1.8 degree skew since nimiq 4 is so close to true south position. This has really been bugging me. Thanks in advance.

 

[Lak7]

I take it you are using an Offset Ku Dish?
If so, the Dish is "looking" about 23 degrees higher that it appears.

Does your Motor have "Latitude" and "Elavation" scales?
If so, set the Latitude Scale to your Lat: 28.55 - in between 28 & 29
Set your Dish Elevation to 25.2 - or best Quality reading

I don't know if you will get any Quality reading from that Nimiq Sat, it's aimed at Canada.
You may need to use either 79 or 83 west as a "Temporary" True South Sat.
83 west has a few Strong Data TPs to Lock on to, you won't get channels, but the receiver will show a Quality reading.

 

[B.J.]

Trying to get my motor/dish set up. One thing I don't understand is why the elevation set at true south is so different from the actual true south sattelite elevation.

In actuality that aren't different at all. THey're exactly the same.

My true south sat is Nimiq 4 since it is located at 82W and my long is 81.1W. The dish elevation is to be set at 56.7 degrees per the sat finder instructions.

This is wrong. This angle should only be used for an Az/El mount, not for a motorized mount. This is the actual elevation of the sat, and the various angles produced by the motorized mount will give this same angle.

My lat is 28.55 and I have that set as my motor elevation.

Again wrong. You should set this on the "Latitude" scale, not the "Elevation" scale (if your motor has one). Elevation is roughly 90 minus your latitude, so the motor "elevation" would be approximately 90-28.55=61.45

According to the motor instructions, the true south position should be set for the dish at (30 degrees minus declination angle). My declination angle at this position is 4.8 so I set the dish to 25.2 degrees elevation. This doesn't make sense to me

What you are really trying to set here is your declination, ie that 4.8 deg. Most small dish motors have a bend in the shaft. Your motor's bend is 30 deg. That means it has a preset declination of 30 degrees, however all you need is 4.8. To get 4.8, you basically raise the dish elevation by that 25.2 deg, which results in a net 4.8.

NOW, if you take that motor elevation of 61.45 above, and subtract 4.8, you get 61.45-4.8= 56.65 , ie the 56.7 you mentioned above.

So you see, everything makes sense.

HOWEVER. Using the values you were given, you'll be off by more than a half degree on sats to the east or west. It is better to use a declination value of 4.2, and a latitude value of 29.15. If you do this, you will track much more accurately.

 

[viper5]

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[viper5]

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[viper5]

Shortly after posting I realized the answer to my question. I'm not sure if I posted it clearly. I was trying to understand why if I aim dish at true south, why the elevation of the motor is only 25 degrees, but for a satellite it's higher. I forgot to take into account that the true elevation is the (elevation of motor - 30 + (30 - 4.8)) or rather elevation of motor - declination.

Also, by motor elevation set to latitude, I was referring to the elevation of the unit, not the elevation markings. The latitude marking was on ~28.6. I just discovered a problem though. My elevation doesn't match my latitude (i.e. a setting of 28 lat will not equal 62 elevation on the other side). It's off by about 5 degrees. I didn't realize that until later today. So basically my motor tick marks are innacurate...

I think what you are saying at the end is that because my lat ends in a .55, it will be off by almost max amount. Why don't I just get the average of 28 and 29 lat position values so then I would keep 61.45 for elevation and 4.75 for declination angle yielding a 25.25 angle for dish. My reference is here:

Digiwave Satellite Wholesale Distributor - MPEG2 Digital Receivers, LNBFs, Digital Equipment, Stab Motor, Moteck Motor, Switch, CCTV Camera - Wholesale Satellite Distributor - Wholesale only

This isn't for my book, but I believe my book actually has slightly different values.

How you obtain a declination angle of 4.2 or change the lat position is beyond me. Please explain how changing my lat will make it track better. Also, what should I do about the whole tick marks aren't accurate issue?

 

[B.J.]

Shortly after posting I realized the answer to my question. I'm not sure if I posted it clearly. I was trying to understand why if I aim dish at true south, why the elevation of the motor is only 25 degrees, but for a satellite it's higher. I forgot to take into account that the true elevation is the (elevation of motor - 30 + (30 - 4.8)) or rather elevation of motor - declination.

Your motor elevation isn't 25 degrees, it's 61.45 {or about 60.85 if you use the modified values}. Elevation minus declination gives the actual elevation of the sat. Your "(elevation of motor - 30 + (30 - 4.8))" doesn't give anything meaningful that I can see. Not sure where that came from.

Also, by motor elevation set to latitude, I was referring to the elevation of the unit, not the elevation markings. The latitude marking was on ~28.6. I just discovered a problem though. My elevation doesn't match my latitude (i.e. a setting of 28 lat will not equal 62 elevation on the other side). It's off by about 5 degrees. I didn't realize that until later today. So basically my motor tick marks are innacurate...

Is it possible that you either have the little washers backward or are reading the angles from the wrong place on the washer? Ie, I'm not sure what motor you have, but you referenced a link to both the SG2100 and another motor, and the SG2100 has a little washer with a pointer.
See: http://eskerridge.com/bj/motorscaleind.jpg
which shows where the proper angle reading was for my motor. I've read posts from some people who have SG2100s with different style washers. If so, I'd recommend using an inclinometer before putting the dish on the motor, such as: http://eskerridge.com/bj/inclinometer.jpg (of course, have the motor on the pole... I don't think I did in the picture). Last time I did it, I used a digital level for more accuracy.

I think what you are saying at the end is that because my lat ends in a .55, it will be off by almost max amount. Why don't I just get the average of 28 and 29 lat position values so then I would keep 61.45 for elevation and 4.75 for declination angle yielding a 25.25 angle for dish. My reference is here:

Digiwave Satellite Wholesale Distributor - MPEG2 Digital Receivers, LNBFs, Digital Equipment, Stab Motor, Moteck Motor, Switch, CCTV Camera - Wholesale Satellite Distributor - Wholesale only

This isn't for my book, but I believe my book actually has slightly different values.

How you obtain a declination angle of 4.2 or change the lat position is beyond me. Please explain how changing my lat will make it track better. Also, what should I do about the whole tick marks aren't accurate issue?

No, the .55 didn't have anything to do with why I said you'd be off when you turn the dish to the east or west. The reason is that the tables in the digiwave link you gave is a relatively poor approximation that is based on the declination of a satellite to your south, which in your case, is the 4.8 you referred to. The problem is that satellites to your extreme east or west do not have a declination of 4.8, but instead it is closer to 4.2. Because of this, if you align the mount according to the digiwave method, you'll be off by about 0.6 degrees at the extremes. The better way, is to use what's referred to as modified declination, ie the 4.2 value. If you use this, however, then you'd be 0.6 degrees off on your south satellite if you used your actual 90-latitude value for motor elevation. So to compensate, you add 0.6 degrees to your latitude or subtract 0.6 from the motor elevation. This fixes the problem at the south satellite, and doesn't affect the sats to the east or west, because the elevation adjustment is orthogonal to, and has no effect on sats to the far east or west, and is proportionally adjusted automatically for sats between south and the extremes.
I have no idea why so many motors or dish mounts have instructions that use the old inaccurate method instead of the modified method. Only thing I can think of is that the calculation is easier.