atm_layers

pycraf.atm.atm_layers(freq_grid, profile_func, heights=None)

Calculate physical parameters for atmospheric layers to be used with atten_slant_annex1 and other functions of the atm sub-package.

This can be used to cache layer-profile data. Since it is only dependent on frequency, one can re-use it to save computing time when doing batch jobs (e.g., atmospheric dampening for each pixel in a map).

Parameters:

freq_gridQuantity

Frequencies at which to calculate the layer properties [GHz]

profile_funcfunc

A height profile function having the same signature as profile_standard

heightsQuantity [km], optional

Layer heights (above ground); see Notes [km]

Returns:

atm_layers_cachedict

Pre-computed physical parameters for each atmopheric layer to be used by functions raytrace_path, path_endpoint, and atten_slant_annex1. It contains the following keys:

• “space_i” : int

  Layer (edge) index of the top atmospheric layer, before space begins. (See Notes.)

• “max_i” : int

  Layer (edge) index of the outermost layer (can be in space). (See Notes.)

• “freq_grid” : ndarray (float)

  Frequencies at which to calculate the layer properties [GHz]

• “heights” : ndarray (float)

  Layer heights above ground (lower edges) [km]

• “radii” : ndarray (float)

  Layer distances to Earth center (lower edges) [km]

• “temp” : ndarray (float)

  Layer temperatures (at layer mid point) [K]

• “press” : ndarray (float)

  Layer pressure (at layer mid point) [hPa]

• “press_w” : ndarray (float)

  Layer water pressure (at layer mid point) [hPa]

• “ref_index” : ndarray (float)

  Layer refractive index (at layer mid point) [dimless]

• “atten_dry_db” : ndarray (float)

  Layer specific attenuation (dry; at layer mid point) [dB/km]

• “atten_wet_db” : ndarray (float)

  Layer specific attenuation (wet; at layer mid point) [dB/km]

• “atten_db” : ndarray (float)

  Layer specific attenuation (total; at layer mid point) [dB/km]

Notes

One should usually stick to the default layer heights (as defined in ITU-R Rec. P.676-11) to ensure consistency with other software/results.

The layer heights are actually the layer edges, while the physical quantities are calculated at the mid-points of the layers and only up to the maximal height of the atmospheric profile function, i.e.:

>>> import numpy as np
>>> from pycraf import atm
>>> from astropy import units as u
>>>
>>> atm_layers_cache = atm.atm_layers(1 * u.GHz, atm.profile_standard)
>>> len(atm_layers_cache['heights'])
908
>>> len(atm_layers_cache['temp'])
901

This is also stored in the space_i variable:

>>> atm_layers_cache['space_i']
900

while the index of the last layer edge is stored in max_i:

>>> atm_layers_cache['max_i']
907

In contrast to P.676 we add outer-space layers (scaled for LEO, moon, solar-sys and deep-space “heights”) to allow correct path determination, e.g. for satellites. Of course, these don’t add attenuation (and have refractivity One).