The fields of S:
---
r_planet : The radius of the (spherical) planet
n_funct  : Function handle to refractive index function
n_params : Parameters to refractive index function
l_step   : Ray tracing step length


The fields of P:
---
alpha  : Angle in cylindrical coordinate system
length : The length from transmitter along the path 
n      : Refractive index
r      : Radius in cylindrical coordinate system
theta  : Local zenith angle of propagation
x      : x of Cartesian coordinate system (transmitter at x=0)
y      : y of Cartesian coordinate system