ATAN2(b1,b2)
ATAN2 function calculates the arc tangent of the two variables b1 and b2. It is similar to calculating the arc tangent of b2 / b1, except that the signs of both arguments are used to determine the quadrant of the result.
ATAN2(0.5,1.0) equals 1.107149.
ATAN2(-0.5,2.0) equals 1.815775.