Scenario RF Environment Properties

A number of environmental factors can affect the performance of a communications link or a radar system. A scenario's RF Environment properties page enables you to apply the following models to your analyses. When enabled, the models will affect all RF phenomena in the scenario except where overridden or varied on the individual STK object.

Environmental data
Rain, clouds and fog
Atmospheric Absorption
Urban and terrestrial
Tropospheric scintillation
Ionospheric Propagation Fading Loss Model
Custom models

Because of limitations to the range of numbers that can be represented internally by STK, values in logarithmic space are limited to a maximum of 3000 dB. Thus, if one or more RF propagation models compute a loss of 3000 dB, the total propagation loss will also be capped at 3000 dB. This is a special case representing full attenuation, and the resulting modeling error is negligible.

Environmental Data

Option	Description
Active Interference Environment Selection	Select a CommSystem object to use in interference constraints. At least one CommSystem object must be present, in which the participating transmitters, receivers and interferers are defined. If no selection is made here, the first CommSystem in the scenario is automatically used.
Earth Temperature	Global Earth temperature that receivers inherit by default. You can override this value for a specific receiver by specifying an Earth Temperature value on the Receiver's System Noise Temperature properties page.
Contour Rain Outage Percent	The percent outage for the global rain model, which is used only in calculating contour graphics for transmitters. This value is used to determine the rain level calculated for RIP and Flux Density types of contour graphics.
Total Propagation Calculation	Enabling the Use ITU-R P618 Section 2.5 checkbox, allows the Rain model and Cloud & Fog model to be enabled concurrently. The total propagation loss will not be a standard summation of each propagation model, but will be computed as defined in section 2.5 of the ITU-R P618 Specification. Computing the total propagation loss as defined in ITU-R P618 section 2.5 is recommended when frequency is greater than ~18GHz and at low elevation angles.

Rain and Clouds & Fog

Rain Model

Rain models are used to estimate the amount of degradation (or fading) of the signal when passing through rain. The degradation is primarily due to absorption by the water molecules and is a function of frequency and elevation angle. Generally speaking, the rain loss will increase with increasing frequency. The loss will also increase with decreasing ground elevation angle due to a greater path distance through the portion of the atmosphere where rain occurs. The rain will also cause an increase in the antenna noise temperature.

The rain models used in STK Communications and STK Radar are global annual statistical models. The annual rainfall rate and probability of the rate for a particular location are determined from historical measurements. In general, the world is divided up into different rain regions, each with its associated rainfall rates and probabilities.

Using a Rain Model

To use a rain model in your analysis of a signal, the signal must pass through the atmosphere. For more information, see Does the Signal Pass Through the Atmosphere?.

To use a rain model for a Receiver or Radar:

Select Use Rain Model on the Rain & Cloud & Fog tab on the Scenario's RF Environment properties page, and then select a global Rain Model Type:

Adjust the Surface Temperature used by the model.
Enabling a global Rain Model will automatically enable the Receiver's or Radar's Use Rain Model and an Outage Percent value. To enable/disable the Rain Model for an individual Receiver or Radar, open the Receiver's Basic Definition properties page or the Radar's Basic System properties page. Enable or disable Use Rain Model appropriately. Enabling Use Rain Model will allow you to adjust the Outage value.

If the Rain Model fields are grayed out, then the global Rain Model is not enabled on the Scenario's RF Environment properties page.

Outage values and range are rain model-dependent. All calculations include the appropriate rain margin values based on the frequency, elevation angle and location of the ground segment. STK also includes the rain noise contribution in the calculation if the Rain option is selected as part of the System Temperature calculation.

If both the transmitter and receiver are located above the rain height threshold, the rain loss is zero.

Rain loss is computed for objects on the ground and for aircraft below the specified rain height. While rain model usage is controlled by the receiver, the receiver does not need to be on the ground as long as the transmitter is. The rain margin values can be examined by generating a Link Budget- Detailed report.

We recommend that you identify which rain models are necessary for your particular analysis or for yielding results comparable to those produced by other analyses.

Overriding Rain Height, Rain Rate, Surface Temperature

For objects that can reside within the atmosphere (Facility, Place, Target, GroundVehicle, Aircraft, Ship, Missile, and LaunchVehicle), the rain height, rain rate, and surface temperature can be overridden. To override a value, enable Local Rain Data on the object's RF Atmosphere properties page, and adjust the Rain Height, Rain Rate (for 0.01% of time), and Surface Temperature.

Does the Signal Pass Through the Atmosphere?

To detect whether a signal passes through the atmosphere, the altitudes of the transmitter and receiver are checked to see if one is greater than 50 km and the other is below the maximum altitude specified on the Scenario's RF Environment page. If either check fails, the signal is not attenuated.

The attenuation due to rain is a function of several factors, namely the frequency, elevation angle, location, and outage percentage. Variations in each of these are accounted for within the Communications module at each time step. The rain models within STK are meant for earth to space communications where one object is always below the isothermal height and the other is well above. However, STK adapts the models to work for the case when both objects are below the isothermal height. STK will perform this by computing the projected surface distance based on the two object locations.

In the case of satellite-to-satellite communication at very low grazing altitudes, the signal passes through the atmosphere; however, the rain model won't attenuate the signal since the model is disabled under this condition.

Ippolito, Louis J., Jr., Radiowave Propagation in Satellite Communications, New York: Van Nostrand Reinhold (1986), Ch. 4-5.

Clouds and Fog Models

To use the clouds and fog model, select Use on the Rain & Clouds & Fog tab, and select the latest ITU-R P840 model, which implements the recommendation for attenuation due to clouds and fog:

This model implements the ITU-R P840-6 recommendation for attenuation due to clouds and fog.

Atmospheric Absorption Models

To use an atmospheric absorption model in a link analysis, select Use on the Atmospheric Absorption tab, and select one of the following available models:

Urban and Terrestrial Models

To use an Urban and Terrestrial model in a link analysis, select Use on the Urban & Terrestrial tab, and select one of the following available models:

Tropospheric Scintillation

This part of the ITU-R P618 model takes account of the rapid fluctuations of the signal due to tropospheric scintillation fade. You also have the option to compute deep fade.

To use the tropospheric scintillation model, select Use on the Tropo Scintillation tab and select the latest ITU-R P618 model:

Ionospheric Propagation Fading Loss Model

To use this model or an alternate AP data file, select Use on the Iono Fading tab. To use an alternate model, you must also select Use alternate AP data file. For more information, see Ionospheric Propagation Loss Model.

Custom Loss Plugin Models

Up to three custom loss plugin scripts can be added on the Custom Models tab. For each script to be added, select Use and enter the path and file name or use the ellipsis (...) button to browse for the file. To disable a script, clear Use.

Each script file must have a different file name with different internal function names. Do not make three copies of the same file and use them without editing the file name and function names.

The plugin script is not automatically reloaded after you make changes to it. To reload the script, click Reload.

For information on scripting in STK, see the Engine Plugin Scripts topic in the Programming Interface Help, which is accessible from the Help menu. For instructions on the setup and use of plugin points, see Plugin Scripts. To develop a custom script from a sample template, select an Absorption Loss Model plugin script (which has the same format as a Custom Loss plugin script), in MATLAB, Perl, or VBScript, from \CodeSamples\Extend\PluginScripts.

You can use a custom loss model even when both link terminals are on central bodies other than Earth.