raytrace_path

pycraf.atm.raytrace_path(elevation, obs_alt, atm_layers_cache, max_arc_length=<Quantity 180. deg>, max_path_length=<Quantity 1000. km>)

Calculate the propagation path geometry through atmosphere by ray-tracing.

For several tasks related to the atm sub-package, it is necessary to determine the exact path geometry of a propagating ray through Earth’s atmosphere. One example would be the application of the atten_slant_annex1 function to calculate the total attenuation along a (slant) path, where the length of a ray within each of the discrete layers of an atmospheric model must be multiplied with the specific attenuation of the layers.

Parameters:

elevationQuantity, scalar

(Apparent) elevation of source/target as seen from observer [deg]

obs_altQuantity, scalar

Height of observer above sea-level [km]

atm_layers_cachedict

Pre-computed physical parameters for each atmopheric layer as returned by the atm_layers function.

max_path_lengthQuantity, scalar

Maximal length of path before stopping the ray-tracing [km]

(default: 1000 km; useful for terrestrial paths)

max_arc_lengthQuantity, scalar

Maximal arc-length (true angular distance between observer and source/ target) of path before stopping ray-tracing [deg]

(default: 180 deg; useful for terrestrial paths)

Returns:

path_paramsrecarray

Geometric parameters of each piece of the paths (i.e., per layer) as determined by the ray-tracing algorithm. It has the following fields:

• a_n : float

  Length of path through the layer during iteration n. [km]

• r_n : float

  Distance to Earth’s center after iteration n. [km]

• h_n : float

  Height above Earth’s surface after iteration n. [km]

• x_n/y_n : float

  Cartesian coordinates of path after iteration n. [km]

  The reference is Earth’s center. Starting point is (0, r_E + obs_alt).

• alpha_n : float

  Exit angle of path after going through the layer (relative to surface normal vector) after iteration n. [km]

• beta_n : float

  Entry angle of path into the layer (relative to surface normal vector) at beginning of iteration n. [km]

• delta_n : float

  Angular distance of path position after iteration n w.r.t. starting point. [deg]

  The polar coordinates (r_n, delta_n) are directly associated with the cartesian coordinates (x_n, y_n).

• layer_idx : int

  Index of the layer, through which the path went during each step. This could be used to query the physical parameters from the atm layers cache (see atm_layers).

• layer_edge_left_idx : int

  Index of the layer edge to the left (associated with the entry point) of current step.

• layer_edge_right_idx : int

  Index of the layer edge to the right (associated with the exit point) of current step.

refractionQuantity, scalar

Refraction angle (i.e., the total “bending” angle of the ray) [deg]

is_space_pathbool

Whether the paths ends outside of Earth’s atmosphere (in terms of the atmospheric profile function used).

Notes

Terrain heights are neglected by the atm sub-package. All heights are w.r.t. the flat (spherical) Earth (with a radius of 6371 km).

The first row in the path_params array is the starting point of the path and has valid entries only for the r_n, h_n, and x_n/y_n parameters. There are n+1 layer edges and n layers. The edge indices start from zero, while the layer indices start from one.