# ARTS built-in documentation server

## Workspace Method ppath_stepRefractionBasic

### Description

```Calculates a propagation path step, considering refraction by a
basic approach.

Refraction is taken into account by probably the simplest approach
possible. The path is treated to consist of piece-wise geometric
steps. A geometric path step is calculated from each point by
using the local line-of-sight. Snell's law for spherical symmetry
is used for 1D to determine the zenith angle at the new point.
For 2D and 3D, the zenith angle is calculated using the average
gradient of the refractive index between the two points. For 3D,
the azimuth angle is treated in the same way as the zenith one.

The maximum length of each ray tracing step is given by the WSV
ppath_lraytrace. The length will never exceed the given maximum,
but it can be smaller. The ray tracing steps are only used to
determine the path. Points to describe the path are included as
for ppath_stepGeometric, this including the functionality of
ppath_lmax.

```

Authors: Patrick Eriksson

### Synopsis

 ppath_stepRefractionBasic( ppath_step, refr_index_air_agenda, atmosphere_dim, p_grid, lat_grid, lon_grid, z_field, t_field, vmr_field, refellipsoid, z_surface, f_grid, ppath_lmax, ppath_lraytrace )

### Variables

 OUT+IN ppath_step (Ppath) A propagation path step. IN refr_index_air_agenda (Agenda) Agenda calculating the refractive index of air. IN atmosphere_dim (Index) The atmospheric dimensionality (1-3). IN p_grid (Vector) The pressure grid. IN lat_grid (Vector) The latitude grid. IN lon_grid (Vector) The longitude grid. IN z_field (Tensor3) The field of geometrical altitudes. IN t_field (Tensor3) The field of atmospheric temperatures. IN vmr_field (Tensor4) VMR field. IN refellipsoid (Vector) Reference ellipsoid. IN z_surface (Matrix) The surface altitude. IN f_grid (Vector) The frequency grid for monochromatic pencil beam calculations. IN ppath_lmax (Numeric) Maximum length between points describing propagation paths. IN ppath_lraytrace (Numeric) Maximum length of ray tracing steps when determining propagation paths.