RF Environment Properties

Environmental factors can affect the performance of a communications link or a radar system. The RF Environment properties page enables you to apply the models listed below to your analyses. You can set these properties at all three levels: scenario, platform (facilities, places, targets, and all vehicles except satellites), and subobject (transmitter, receiver, radar, and antenna). There is a priority ordering for scenario/platform/subobject to delineate which environment model takes precedence when you enable a specific model type (atmospheric absorption, tropospheric scintillation, etc.) on more than one object or subobject.

In the Properties browser, under RF, select "Environment" to display the following environment properties tabs:

These are all available at the scenario, platform, and subobject levels.The properties on the tabs are the same for a scenario, platform, or subobject, except for the properties on the Environmental Data tab.

Environment models in STK have frequency/wavelength limitations, as specified in the following table:

Environment Model Frequency/Wavelength Limitations
Model Type Frequency/Wavelength range
ITU-R P.676 Atmosphere 1 GHz to 1000 GHz
Simple Satcom Atmosphere 1 GHz to 350 GHz
TIREM Atmosphere 1 MHz to 40 GHz
VOACAP Atmosphere 2 MHz to 30 MHz
Beer-Bouguer-Lambert Law Atmosphere (Laser)

Approximately 300 nm to 1600 nm (IR, visible, and ultraviolet), which is a frequency range of 300 GHZ to 30,000 THz
MODTRAN Atmosphere (Laser) 30 GHz to 1500 THz
MODTRAN-derived Lookup Table Atmosphere (Laser) 10.714 THz to 1071.4 THz
ITU-R P.840 Cloud & Fog 1 GHz to 200 GHz
ITU-R P531-13 Ionospheric Fading 100 MHz to 12 GHz
CCIR 1982 Rain 1 GHz to 1,000 GHz (recommended only up to 55 GHz)
CCIR 1983 Rain 1 GHz to 1,000 GHz (recommended only up to 55 GHz)
Crane 1985 Rain 1 GHz to 100 GHz
ITU-R P.618 Rain 1 GHz to 55 GHz
ITU-R P.618 Tropospheric Scintillation 4 GHz to 20 GHz
ITU-R P1814 Tropospheric Scintillation (Laser)

Approximately 300 nm to 1600 nm (IR, visible, and ultraviolet), which is a frequency range of 300 GHZ to 30,000 THz
Two Ray Urban/Terrestrial The frequency range is scenario dependent. The link range has to be more than . The dependence of range on the wavelength (λ) indirectly places a limit on the valid frequency.
Urban Propagation Urban/Terrestrial 100 MHz to 7 GHz

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 Appears in... Description
Active Interference Environment Scenario Select a CommSystem object to use in interference constraints. The scenario must contain at least one CommSystem object, with participating transmitters, receivers, and interference sources defined. If you make no selection here, STK will automatically use the first CommSystem in the scenario.
Earth Temperature Scenario

This is the 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 Scenario This is the percent outage for the global rain model. STK only uses it in calculating contour graphics for transmitters, to determine the rain level calculated for RIP and Flux Density types.
Total Propagation Calculation All levels Select the Use ITU-R P618 Section 2.5 check box to enable the Rain model and Cloud & Fog model concurrently. The total propagation loss is not a standard summation of each propagation model; STK computes it 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 the frequency is greater than ~18GHz and at low elevation angles.
Magnetic North Pole Scenario Specify the latitude and longitude of the magnetic north pole for your analysis.
Local Rain Data Platform

Select this option to override the global-level rain model, if one is enabled. Select the Enable check box and specify the following parameters:

  • Rain Height - Maximum atmospheric height at which rain is present.
  • Rain Rate for 0.01% of Time - Amount of rain per unit of time.

Use local rain data to model real-world conditions at a particular time or when more accurate rain data is available — based on local rain statistics — than through the application of the global statistics.

You can choose whether to specify Local Rain Data on just one end or on both ends of the communications link (transmit and receive).

Rain, and clouds & fog

You can select both a rain model and a clouds & fog model. They are not mutually exclusive. For example, you can model both rain and clouds & fog loss when the rain clouds have another layer of clouds above the rain. In this example, the rain cloud altitude is up to 4 km, and above it there is a cloud layer from 5 km to 7 km.

Rain model

When enabled, STK uses the rain model 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 increases with increasing frequency. The loss also increases with decreasing ground elevation angle due to a greater path distance through the portion of the atmosphere where rain occurs. Rain also causes an increase in the antenna noise temperature.

The rain models 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 models divide the world up into different rain regions, each with its associated rainfall rates and probabilities.

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, follow these steps:

  1. On the Rain, Cloud & Fog tab on the RF -Environment properties page, select the Use check box in the Rain Model panel. Then click the ellipsis under Rain Loss Model and select a global rain model:
  2. Specify the Surface Temperature that the model will use.
  3. Edit any model-specific parameters.

You can enable or disable the rain model for an individual receiver or radar. For a receiver, open its Basic Definition properties page and select the Model Specs tab. For a radar, open its Basic Definition properties page, select the Receiver tab, and select the Specs tab. In these locations, there is a Rain Model panel with a Use check box that you can select or clear to enable or disable the model for that object. The panel also enables you to adjust the Outage Percent value.

Outage values and range depend on the rain model. 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 you select the Rain option as part of the System Temperature calculation.

STK computes rain loss for objects on the ground and for aircraft below the specified rain height. While the receiver controls the rain model usage, it does not need to be on the ground if the transmitter is. Generate a Link Budget detailed report to view the rain margin values.

Does the signal pass through the atmosphere?

To determine whether or not a signal passes through the atmosphere, STK checks the altitudes of the transmitter and receiver 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, STK does not attenuate the signal.

The attenuation due to rain is a function of several factors: frequency, elevation angle, location, and outage percentage. STK accounts for variations in each of these 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 situations in which both objects are below the isothermal height by computing the projected surface distance, based on the two objects' 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 STK disables the model 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 a clouds and fog model, select the Use check box on the Clouds and Fog Model panel and select the latest ITU-R P840 model, which implements the recommendation for attenuation due to clouds and fog. For details on selecting the model parameters, click the link below.

Atmospheric absorption models

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

Urban and terrestrial models

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

Tropospheric scintillation

This part of the ITU-R P618 model accounts for rapid fluctuations of the signal due to tropospheric scintillation fade. The model will also select whether to compute deep fade or shallow fade, depending on the link elevation angle.

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

Custom loss plugin models

You can add up to three custom loss plugin scripts on the Custom Models tab. For each script, select the Use check box and enter the path and file name or click ... to browse for the file. To disable a script, clear the Use check box.

Each script file must have a different name with distinct internal function names.

STK does not automatically reload the plugin script after you make changes to it. To reload a script, click Reload.

For information on scripting in STK, see Engine Plugin Scripts. 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 from \CodeSamples\Extend\PluginScripts; this template has the same format as a custom loss plugin script. You can find examples in MATLAB or VBScript.

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