Coordinates transformation and falling body trajectory

Question

As it's well known, assuming the earth fixed and non rotating, the trajectory of a falling body with initial speed $v_0 = \{v_{0x},v_{0y},{v_{0z}}\}$ is contained in a plane. Assuming an observer in the origin of a fixed reference frame, he will measure three coordinates of the falling body: $P=\{\rho,\theta,\phi\}$. We can suppose to be $\rho$,distance, $\theta$, elevation and $\phi$, azimuth. How is it possible to find a reference frame in which the azimuth $\phi$ is zero? Thanks in advance.

Answer 1

The observer has to be in the plane of the motion. The surfaces where $\phi$ is constant are planes; the observer just has to orient her axes appropriately for one of these planes to be the plane of motion.