Re: how to use compass -- The Astra 2D Forum

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Subject: Re: how to use compass

Author:
Outsider

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Date Posted: 13:19:43 11/28/03 Fri
In reply to: peter smith 's message, "how to use compass" on 10:50:18 11/28/03 Fri

>Can anyone please tell me how to find astra 2d (28,2
>east) using a simple compass? I assume I should first
>point to the magnetic North, and then (??) count the
>degrees to the (right-hand) side until 28,2? Magnetic
>North being zero degrees. Is that right? Thank you.

Yes, North is zero degrees on compass dial but otherwise the thinking was a bit flawed. The satellite positions are given in relation to Greenwich meridian (0 deg.), East or West, and the orbital position of Astra 2D is 28.2 degrees East. But this alone tells nothing about the compass reading. You must know your own location (longitude and latitude) and then calculate the compass reading. This can be done using e.g. the program SMWLink that can be freely download here: http://www.smw.se/smwlink/smwlink.htm . The result got is however in relation to true North, not the compass reading. If you want to get an accurate compass reading you must apply a correction called magnetic variation, it can be positive or negative depending on the location and is got from special maps. Lets take an example: Assume you are in Helsinki, latitude 60N and longitude 25E and you want to know the compass reading for Astra 2D at 28.2E. SMWLink gives azimuth angle 176 degrees and also the elevation angle 21.7 degrees. Magnetic variation in Helsinki is appr. +5 degrees, so you must subtract this value from 176 degrees and you'll get 171 degrees that can be put on the compass dial. Of course, if the compass has a scale 0..60 instead of 0..360 the reading must be divided by 6. So the satellite that was in position 28.2E is seen in compass direction 171 degrees, i.e. 9 degrees to the East from (magnetic) South.

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