33.4. Atmosphere
The DIRSIG model uses an external atmospheric propagation program, know as MODTRAN, to predict atmospheric properties including path radiances, path transmission and sky radiances for a range of spectral resolutions. MODTRAN generates output in a look-up table in the form of an Atmospheric Database File (ADB). It contains all of the spectral information that DIRSIG will need for any geometry encountered during rendering. The input to the MODTRAN program is a text file known as the tape5 file. The <atmosphere> element provides a way for us to marshal all of the various input pieces in order to model the enviroment conditions. The following diagram shows the layout of the <atmosphere> element:
33.4.1. ADB
This is the atmospheric profile generated by MODTRAN. The XML markup for <adb> simply references the external file:
<adb>
<filename="example.adb">
</adb>
33.4.2. Tape5
The following set of XML examples show how we can externally reference a tape5 file, which is the input data for MODTRAN, or internally represent tape5 instructions within the tape5 element. The content of the CDATA section is a line for line copy of the tape5 file content:
<tape5 type="external">
<filename="mls.tp5">
</tape5>
or
<tape5 type="internal">
<![CDATA[
M 2 2 2 0 0 0 0 0 0 0 1 1 1 0.000 0.00
0 0 0 1 0 0 0.000 0.000 0.000 0.000 0.218
20.218 0.218 180.000 0.000 0.000 0.000 0
1 2 236 0
43.000 77.000 0.000 0.000 12.000 0.000 0.000 0.000
450 35050 100 2
0
]]>
</tape5>
33.4.3. Weather
The <weather> element references a weather file which stores meterological data as a function of time:
<weather type="external">
<filename="mls.wth">
</weather>
33.4.4. Skydome
The <skydome> element contains control parameters for the sky dome sampling values. This delta sampling can be tweaked to control how the sensor "sees" the sky.
<skydome angularunits="degrees">
<deltazenith>5.0</deltazenith>
<deltaazimuth>5.0</deltaazimuth>
</skydome>