== Using EM.Terrano as an Asymptotic Field Solver ==
Like every other electromagnetic solver, [[EM.Terrano]]'s SBR ray tracer requires an excitation source and one or more observables for the generation of simulation data. [[EM.Terrano]] offers several types of sources and observables for a SBR simulation. You already learned about the transmitter set as a source and the receiver set as an observable. You can mix and match different source types and observable types depending on the requirements of your modeling problem.
The available source types in [[EM.Terrano]] are:
{| class="wikitable"
Click on each type to learn more about it in the [[Glossary of EM.Cube's Simulation Observables & Graph Types]].
When you define a far-field observable in [[EM.Terrano]], a collection of invisible, isotropic receivers are placed on the surface of a large sphere that encircles your propagation scene and all of its geometric objects. These receivers are placed uniformly on the spherical surface at a spacing that is determined by your specified angular resolutions. In most cases, you need to define angular resolutions of at least 1° or smaller. Note that this is different than the transmitter rays' angular resolution. You may have a large number of transmitted rays but not enough receivers to compute the effective radiation pattern at all azimuth and elevation angles. Also keep in mind that with 1° Theta and Phi angle increments, you will have a total of 181 × 361 = 65,341 spherically placed receivers in your scene.
{{Note| Computing radiation patterns using [[EM.Terrano]]'s SBR solver typically takes much longer computation times than using [[EM.Cube]]'s other computational modules.}}
<table>
<tr>
<td> [[Image:SBR pattern.png|thumb|450px540px|Computed 3D radiation pattern of two vertical short dipole radiators placed 1m apart in the free space at 1GHz.]] </td>
</tr>
</table>