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Dish alignment explanation

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Posts: 18
Aug 23rd, 2007 at 11:27am  
I am in Mauritius (indian Ocean) and want to point my 12 ft c band to true North. I have some problems with
https://www.satsig.net/maps/satellite-dish-pointing-mauritius.htm It gives the following:
Latitude = -20.1436, Longitude = 57.5491
Lat = 20 degrees, 8.6 minutes South
Long = 57 degrees, 32.9 minutes East
If you want to point a satellite TV dish at the Satellite at = 57 East orbit longitude

Dish elevation= 66.4, Azimuth= 20.8 (magnetic compass), Polarisation= -1.5

Polar mounts only: Main angle= 20.6, Downward tilt= 3.0, Motor drive sideways angle= -0.6

From Satellite Alignment Soft(SAA)
I get this without the magnetic declination

57,0°E - NSS 703 Az 358.621 ° EL 66.288
adding 18 deg for magnetic declination would give me 358.621+18 = 16.621 ( magnetic compass) anyone to explain why satsig gives (20.8 instead of 16.21 or 16.8)
For elevation it's ok 90 - 20.6 - 3= 66.4 (or 90- 23.6 = 66.4)
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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Aug 23rd, 2007 at 12:04pm  
The magnetic compass azimuth angle is approximate as a result of my earth's magnetic field model.  It has errors of several degrees in areas where the magnetic deviation varies significantly from place to place (e.g. north Canada, south and south eastern Indian ocean) .  It interpolates across 30 deg lat/long grid squares so it does not respond to rapid local variations.  I don't even try to estimate the magnetic deviation in polar regions beyond 60 deg north or south latitude.

If you have a polar mount drive your motor just 0.6 deg to the left from the centre position, stop the motor, and then swing the entire antenna and head unit (by hand) till you find the 57 deg satellite. You will have set your head unit fixed to due north.

If you have an az-el mount then set the elevation to 66.4 degrees and swing the dish boldly around sideways between north and north east (magnetic compass) and you should find the satellite on the first swing.  Precision in azimuth is not critical.

See here on how to use a compass

Best regards, Eric.
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« Last Edit: Oct 14th, 2007 at 5:01pm by Admin1 »  
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