Laser Environment Properties
Environmental factors can affect the performance of a laser communications link. The Laser 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, topospheric scintillation, etc.) on more than one object or subobject.
In the properties browser, under Laser, select "Environment" to display the following environment properties tabs:
Systems Tool Kit® (STK®) laser environment models have frequency limitations you need to be aware of.
Because of limitations to the range of numbers that can be represented internally by the STK application, 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.
Atmospheric loss
To model atmospheric loss, select the Enable check box on the Atmospheric Loss tab and select one of the following models:
- Beer-Bouguer-Lambert Law
- MODTRAN-derived LookupTable (frequency range 10.714 THz - 1071.4 THz) - See the description following this list.
- MODTRAN (frequency range 30 GHz - 1500 THz) - This model only appears as an atmospheric loss option if you preinstall the MODTRAN application and have a valid license for it. You must purchase and obtain the MODTRAN application through Spectral Sciences, Inc. If you have the MODTRAN software installed and licensed prior to starting the STK application, then at startup the STK application will load the MODTRAN DLL and the MODTRAN propagation model will appear in the list of available atmospheric models. Click the MODTRAN link above to learn more about the model and about setting input and output parameters for the model.
MODTRAN-derived Lookup Table option
The MODTRAN-derived Lookup Table propagation model is available whether or not you install the MODTRAN application. It is a simplified model for which a few key MODTRAN inputs were swept over a reasonable range to produce a multidimensional lookup table. The STK application uses this table in the MODTRAN-derived Lookup Table propagation model. At each time step, STK processing takes the user inputs and the dynamic geometry parameters to determine the transmittance via a table lookup. It then uses this transmittance to compute the atmosphere loss. When you choose this model, enter the following input parameters:
| Parameter | Description |
|---|---|
| Aerosol Model | Select one of the aerosol models available from the drop-down menu. |
| Visibility | To set the visibility, specify the greatest distance a black object located near the ground can be seen and recognized against a bright background. |
| Relative Humidity | Specify the percentage of relative humidity. |
| Surface Temperature | Specify the temperature at the surface of the Earth in the units given. |
Tropospheric scintillation loss
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 the tropospheric scintillation model, select the Use check box on the Tropospheric Scintillation Loss tab and select the latest ITU-R P618 model: