8.2. Technical Description

8.2.1. Automatic modifications to the tape5 file

The easiest way to determine the DIRSIG/MODTRAN interaction is to go to the tape5 editor GUI. Under "List", there is a pull-down menu, in which the user can select the cards viewed in the window. There are three options: Required cards, which must be filled in for MODTRAN to run at all, "All cards", which lists all cards and associated sub cards, and "DIRSIG cards". If "DIRSIG cards" is chosen, it will list all cards that DIRSIG does NOT alter. It should be noted that only card1, card1A, and card2 remain. This is because DIRSIG will change the geometry (card3) and the Spectral information (Card4) as is pertains to the DIRSIG configuration file.

So, the user must supply the information in Card1, Card1A, and Card2. This is where all of the atmospheric customization occurs. Its a very important step, because after that, DIRSIG does not "know" anything about the atmosphere. It derives all of the values used for rending from the tape7.scn files. As said before, these cards control all of the parameters for the modes in which MODTRAN runs, as well as the parameters dictating aerosol, water vapor, cloud, and other species' effects on the atmosphere.

What DIRSIG does change in the tape5 file is the geometric information (where it looks). This, again, is Card3. There are a variety of options for how to convey where MODTRAN looks. All of this geometry calculation is handled in DIRSIG, so the user does not need to worry about any more than precisely entering the correct data in the DIRSIG configuration file. (This would be the time of day, time of year, latitude and longitude, GMT offset. DIRSIG takes all this information and calculates where MODTRAN should look.)

If the user wishes to run MODTRAN directly, and bypass DIRSIG, the information for all of the geometric conventions are in the MODTRAN manual.

8.2.2. The Atmospheric Database

Now that DIRSIG has read in the tape5 file, what does it do with it? In short, it creates a look-up-table of output values. It loops through a set of look angles (zenith and azimuth), runs MODTRAN for that position, extracts the data from the tape7.scn file, and stores it in the Atmospheric Database File (ADB). The following sections will describe how DIRSIG uses this ADB.

The next question to answer is, what is the convention for the geometry of the ADB? The ADB is made up of 3 main sections. The first is the Source Paths. This contains the exo-atmospheric solar and lunar irradiance, as well as transmissions for both (four columns). This is the amount of energy that reaches the exterior of the atmosphere. The transmission is the transmission through the entire atmosphere along the source-target path.

The second section is known as the Sensor Path section. This has a set of spectral blocks (a list of spectral data) which is the result of the MODTRAN "sensor" looking down from the perspective of the DIRSIG sensor. In this section, the thermal and solar path radiance is stored, along with the transmission from ground to sensor. This is intended to sample the upwelling radiance of the simulated scene. The upwelled radiance is defined as the light energy scattered off of the atmosphere and towards the sensor. This also includes energy emitted directly from the atmosphere, which would be in the form of thermal radiation. The geometry for this section is very simple. Looking down, DIRSIG samples at a set interval, a number of zenith angles about the target. So, in the file is seen a list of spectral blocks with varying zenith angles.

The final section is known as the downwelled section. This also has a set of spectral blocks, however, this time, the sensor is looking up. Downwelled radiance is defined as the amount of energy that gets scattered off of the sky, and travels down to strike the target. This section is similar to the previous one, but it samples not only in the zenith dimension, but the azimuth dimension as well. It, too, records and stores the solar and thermal radiance, but it leaves out the transmission term (which is accounted for in the Source Paths section). The sampling protocol for this sampling is independent of the scene or array size. There are 12 azimuth angles, and 6 zenith angles that are sampled, for a total of 72 distinct look-angles (which corresponds to 72 different spectral blocks.)

8.2.3. How DIRSIG uses the Atmospheric Database

Now, what is the purpose of this ADB? As stated in section Atmospheric Modeling Overview, this file holds all of the atmospheric data that DIRSIG needs to render a scene. In this section, we give a brief overview of how DIRSIG renders an image, and how it uses this ADB to do so. The first thing DIRSIG takes into account is the downwelling radiance. Consider a basic framing array imaging system. (See Figure 8-1.) Imagine a ray originating from the sensor, passing through the focal plane, and striking a spot on the ground. (This spot will be a projection of a pixel on the focal plane onto the ground.) DIRSIG will then send out rays from this spot to sample the downwelling sky radiance (the sky dome). It calculates how much spectral energy reaches that pixel. It does this by referencing the ADB, specifically, the downwelled section. Each ray will have a specific look-angle. DIRSIG interpolates the value for each ray from the surrounding look angles in the ADB.

Using properties of the material, (reflectance/emission, temperature, etc...) and the previously mentioned downwelled radiance, DIRSIG calculates how much energy leaves the target pixel towards the sensor. It combines this value with the upwelled radiance ( the amount of energy scattering off of the atmosphere towards the sensor ) to calculate the total sensor-reaching radiance. Like the downwelled case, the values for the upwelling radiance are interpolated from the values found in the adb (see Section 34.1.3). DIRSIG follows this scheme of calculations for each pixel in the scene.