>>>>Just varying either the x or y axis gives hugely different heading results (including at 'steep' angles Nan)
>>>>
>>>
>>>
>>>

>>>  // Calculate compass heading
>>>    var compassHeading = Math.atan(Vx / Vy);
>>>

>>>
>>>Have you tried replacing the Math.atan()) by the Math.atan2() ?
>>
>>No, but I will. My trig's too rusty to guess what the effect might be :-}
>>
>>>http://www.w3schools.com/jsref/jsref_atan2.asp
>
>
>Haven't tested this in javascript but
>
>You know that the sign of the tangent is positive in quadrants I and III, negative in quandrants II and IV
>
>In the example below for 225 degrees ( = 180 + 45)
>
>The angle returned by atan() is 45 degrees
>The angle returned by atan2() is 225 degrees
>
>Atan does not know the exact quadrant whilst atan2 does since you pass both sine and cosine (here both are negative)
>

>class Test_atan
>	{
>		internal static void Go()
>		{
>			// 180 + 45= 225 degrees
>			double rads =  Math.PI + Math.PI / 4.0; 
>
>			double sin = Math.Sin(rads);
>			double cos = Math.Cos(rads);
>
>			double atanDegrees = RadianToDegree(Math.Atan(sin / cos));
>			double atan2Degrees = RadianToDegree(Math.Atan2(sin, cos));
>
>			Console.WriteLine("{0}  {1} ", atanDegrees, atan2Degrees);
>
>		}
>		static double DegreeToRadian(double degrees)
>		{
>			return Math.PI * Modulo(degrees, 360.00) / 180.0;
>
>		}
>		static double RadianToDegree(double rads)
>		{
>			return Modulo(rads, Math.PI * 2) * (180.0 / Math.PI);
>		}
>		static double Modulo(double dividend, double divisor)
>		{
>			return (dividend % divisor)
>				+ (Math.Sign(dividend) * Math.Sign(divisor) >= 0 ? 0 : divisor);
>		}
>	}
>

Hmm. Firstly I screwed the atan2() function - I had Math.atan(Vx / Vy) instead of Math.atan2(Vx ,Vy).

With that correction the heading does seems to remain fairly constant regardless of beta and gamma values. But changes on the x axis still result in the skip :-{
H