Atmospheric Absorption Models

ITU-R P676 Atmospheric Absorption Model

This model implements the ITU-R atmospheric absorption loss model. You can choose either version 9 or version 13, which is the latest update to the model. It supports the options for the fast approximate model, the seasonal/regional atmospheric model, and the option to add a site-specific atmospheric vertical profile. The maximum height for this model is 100 km.

The ITU model performs ray tracing on a propagation path. The intervening atmosphere is broken up into concentric shells, and computes line segments within each shell along the propagation path. The specific attenuation along each segment is computed and multiplied by the length of the segment. Finally, all segments are added to arrive at a total attenuation value.

The ITU recommendation specifies two different atmosphere models, an option for entering a site-specific atmospheric vertical profile, and two different specific attenuation calculations. These are controlled via the following options:

Option Description
Use Fast Approx. Method 1-350 GHz Uses an empirical curve-fit model valid over the frequency range 1-350 GHz. Otherwise, a highly accurate physics-based model is used, which accounts for the effects of each constituent spectral line (1-1000 GHz).
Use Section 2.2 Water Vapor Method Select this check box to implement the water vapor attenuation equations found in Section 2.2 of the ITU-R P676-13 model. This check box defaults to not selected, meaning that the model will implement Section 2.1 equations.
Atmospheric Data

Select one of the following options for determining barometric pressure, temperature, and water vapor density for the model:

  • Annual Global - This is a global reference atmosphere model.
  • Seasonal Regional - This uses season and latitude to determine the parameter values.
  • User Specified - Provide a file that contains measured vertical atmospheric profile data for pressure, temperature, and water vaopr density. The format for this file is specified in the section "User-specified file format" later in this topic. Selecting this option disables the Fast Approximation Method.

The Ansys Systems Tool Kit® (STK®) application only implements the instantaneous prediction methods in Version 13.

User-specified file format

This file contains data for the following quantities in the given units:

  • Altitude (m)
  • Pressure (Pa)
  • Temperature (K)
  • Water vapor density (KG/m3)

You must use the following fomatting for this file. The character "#" indicates that a line is a comment.

  1. The first non-comment line is a tag (ITUP676LocalProfiles) indicating the type of file and the file format version.
  2. The second non-comment line indicates the sampling mode. The only supported value is "SampleAndHold".
  3. The remaining non-comment lines specify the data for given altitudes in the following format:

    <altitude> <pressure> <temperature> <water vapor density>

Example file

#My latest file for vertical atmospheric profile data

ITUP676LocalProfiles v1

SampleAndHold

76.3285  101368   272.447  0.0034027
100.834  101063   272.357  0.00337976
200.149  99830.4  271.992  0.0032869
400.634  97372.2  271.245  0.00310048
800.493  92586.6  269.712  0.0027363
1622.08  83246.9  266.387  0.00204414
3211.48  67073.6  259.279  0.00102265
6412.67  42062.3  242.266  0.000157583
12794.9  17172.5  218.0    0.0
25519.3  2645.4   218.0    0.0
51399.7  58.9189  215.0    0.0
99457    0.04044  210.0    0.0