TIREM Atmospheric Model
The Terrain Integrated Rough Earth Model (TIREM) adds fidelity to the calculation and dynamic modeling of point-to-point line-of-sight effects for link performance. It does this by taking into account the effect of irregular terrain, sea water, and nonline-of-sight effects. Among other things, TIREM predicts radio frequency propagation loss over irregular terrain and seawater. The maximum height for these models is 30 km.
This model supersedes all previous versions of the TIREM atmospheric model. You can use TIREM in conjunction with the STK's Communications and Radar capabilities. It enhances Radar computations and communications. It also enhances Communications by enabling it to predict radio frequency propagation loss over irregular terrain and seawater for ground-based and airborne transmitters and receivers. For more information on TIREM and its implementation in STK, see TIREM.
The computational frequency range for the TIREM model is from 1 MHz to 40 GHz. The model will not compute loss if the signal frequency is outside these limits.
| Parameter | Description |
|---|---|
| Surface Temperature | Specify the surface temperature along the path. |
| Surface Humidity | Specify the absolute humidity along the path. |
| Surface Conductivity (S/m) | Specify the effective conductivity (reciprocal of receptivity) of Earth along the path to a depth corresponding to the penetration depth appropriate for the frequency. |
| Surface Refractivity (N-Units) | Specify the surface refractivity of the atmosphere along the path. |
| Relative Permittivity | Specify the relative permittivity of Earth along the path to a depth corresponding to the penetration depth appropriate for the frequency (unitless). |
| Override available terrain resolution |
By default, STK samples terrain data at the available terrain resolution for use with TIREM. Select the "Override available terrain resolution" check box to specify a custom sample resolution and thus accommodate both long-range and short-range link budget calculations using TIREM. If the resolution you specify differs from the available terrain resolution, STK obtains samples through interpolation to meet the requirement. Enter your custom sample value in the Terrain Sample Res. field. |
| Polarization Type |
This is the transmitted signal polarization to use when computing the TIREM loss. Select either Vertical or Horizontal. For STK 12.4 and earlier, the default is Horizontal. For 12.5 and later, the default is Vertical. |
The TIREM model only applies when both the transmitter and the receiver are below 30,000 meters. The computation time is proportional to the terrain resolution, dynamic nature of the scenario, proximity to ground, and length of the simulation. AGI recommends using incremental steps and short initial scenario durations when using TIREM.
- The TIREM models are listed as an option only if you have a valid STK Pro, STK Premium (Air), STK Premium (Space), or STK Enterprise license. Also, you must install the TIREM model on the same machine as STK.
- The scenario must have terrain to use TIREM, and the terrain must be from a local terrain file and not from a Terrain Server.
- For TIREM, you can use any terrain format that STK can read. STK searches for terrain data for each object location. If the object moves out of the available terrain data, STK issues an error message that provides the object's time, latitude, and longitude when it crossed over the terrain's boundary.
Using terrain with scenario objects
Scenario objects must use terrain as the altitude reference.
- For a facility, select Use Terrain Data and set Height Above Ground to at least half a meter on the Basic Position properties page.
- For a vehicle, select Terrain as the Altitude Reference for vehicles and set the waypoint altitude to at least half a meter above terrain on the Basic Route properties page.
If you do not set the altitude reference to terrain for an object, STK will check the object height above ground (as determined by terrain) and issue a message in the message viewer each time the height drops below half a meter. TIREM will accept 0.5 meter height above ground, but the recommended minimum height is one meter above the ground level.
TIREM tutorial
For more information on using TIREM, see Using TIREM.