[linux-dvb] USALS Algorithm

Andrew Hakman andrew.hakman at gmail.com
Tue Aug 30 11:22:25 CEST 2005

Has anyone else noticed that the USALS algorithms being used in both
the vdr-rotor plugin and MythTV aren't accurate for moderate distances
away from south (or north)? Has anyone else tried to get the real
algorithm from STAB? I've e-mailed them a few times and they seem
reluctant to release the algorithm - they have a good system, but it's
pretty f'in useless without the correct algorithm. What exactly are
they trying to protect?

I haven't worked out all the trig in the formula being used in vdr or
myth, but it generates almost identical numbers (the differences are
in the thousandths of degrees which is irrelevant anyways) to a much
simpler algorithm I found being used in the DiseqcU plugin for ProgDVB
which uses very simple trig, the diamater of the earth, and the height
of the satellites to calculate the rotation angle. The further away
from south (or north) you calculate the angle for, the more you have
to re-correct back towards south (or north). I haven't put a lot of
thought into it, but a first guess at the problem I think could have
to do with the angle present at the motor shaft. If anyone else has
also noticed this and put some thought into what might be wrong, I'd
sure like to get this figured out.

As an example, for the furthest east satellite I can see just above
the horizon, DiseqcU and the algorithm used in vdr-rotor and myth
gives an angle of  75.46 degrees, whereas a real USALS receiver
configured with the same reference location and same satellite
location gives a motor angle of 70.4 (which considering how much
further west I have to go from 75.46 to actually get that satellite is
probably correct).

Andrew Hakman

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