DiSEqC: Calculate Azimuth for Rotors in line with online references.

It appears that we lifted a bit too much from another available Azimuth calculation, which threw off our rotor positioning.

I've checked numerous online sources which indicate that this should be the correct calculation for Azimuth.

Fixes #8459, "meat" of the patch by Pieter Hoekstra.  Thanks.

mythtv/libs/libmythtv/diseqc.cpp

@@ -2010,8 +2010,9 @@ bool DiSEqCDevRotor::ExecuteUSALS(const DiSEqCDevSettings&, const DTVMultiplex&,
 
 double DiSEqCDevRotor::CalculateAzimuth(double angle) const
 {
-    // Equation lifted from VDR rotor plugin by
-    // Thomas Bergwinkl <Thomas.Bergwinkl@t-online.de>
+    // Azimuth Calculation references:
+    // http://engr.nmsu.edu/~etti/3_2/3_2e.html
+    // http://www.angelfire.com/trek/ismail/theory.html
 
     // Earth Station Latitude and Longitude in radians
     double P  = gCoreContext->GetSetting("Latitude",  "").toFloat() * TO_RADS;
@@ -2020,14 +2021,7 @@ double DiSEqCDevRotor::CalculateAzimuth(double angle) const
     // Satellite Longitude in radians
     double Us = angle * TO_RADS;
 
-    double az      = M_PI + atan( tan(Us - Ue) / sin(P) );
-    double x       = acos( cos(Us - Ue)  *cos(P) );
-    double el      = atan( (cos(x) - 0.1513) / sin(x) );
-    double tmp_a   = -cos(el)  *sin(az);
-    double tmp_b   = (sin(el)  *cos(P)) - (cos(el)  *sin(P)  *cos(az));
-    double azimuth = atan(tmp_a / tmp_b)  *TO_DEC;
-
-    return azimuth;
+    return TO_DEC * atan( tan(Us - Ue) / sin(P) );
 }
 
 double DiSEqCDevRotor::GetApproxAzimuth(void) const