The φ angle is the angle from the positive *y*-axis
toward the positive *z*-axis, to the vector’s
orthogonal projection onto the *yz* plane. The φ
angle is between 0 and 360 degrees. The θ angle is the angle
from the *x*-axis toward the *yz* plane,
to the vector itself. The θ angle is between 0 and 180 degrees.

The figure illustrates φ and θ for a vector that
appears as a green solid line. The coordinate system is relative to
the center of a uniform linear array, whose elements appear as blue
circles.

The coordinate transformations between φ/θ and *az/el* are
described by the following equations