26.3. Tutorials
26.3.1. Creating data for a static platform
The purpose of this tutorial is to demonstrate how to use the program to create a simple platform description that locates the platform and points it in a desired direction. This description will consist of a single location/orientation data, which means that the platform will be fixed (or static) as a function of time. This is applicable for situations where the instrument is fixed on a tripod, or if the user wishes to model a single data aquisition from an airborne or space based platform.
To begin, start the "Platform Editor" tool from the command line:
prompt> platform_edit
This will start the tool with an empty description of the platform as a function of time (see Figure Figure 26-2).
The first step is to create a single location/orientation entry in the platform description. This is accomplished by selecting the Add entry option from the Edit menu. This will create a new row in the table as shown in in Figure Figure 26-3.
The next step is to select this entry so that the program will now which entry to modify for the next step. This is accomplished by clicking on this row in the left most column on the label "1" (not the cell under the "Time" header). This will highlight the entire row as shown in Figure Figure 26-4.
The next step is to use the "Static Positioning Wizard" to configure the platform location and orientation. This tool is available via Static position wizard ... option in the Tools menu. This will open a dialog window shown in Figure 26-5
The "Static Positioning Wizard" tool allows the user to easily configure the "Generic Platform Model" which describes the platform using platform location and absolute axis rotations, exclusively. The tool allows the user to use platform and target relative geometries in a similar manner used by the old DIRSIG3 platform model.
In this example, we assume that we know where the platform is located and we desire it to point at a specific location (referred to as the "target location"). This allows us to utilize the "Specify platform location and target location" option in the "Method" box at the top of the window. In other situations, the user may know the platform relative orientation angles that point the platform at the target, or the target relative angles that point towards the location of the platform. These options are available via the other choices in the "Method" boxs, and the sign conventions for the relevant angles are shown at the bottom of the window.
For this example, we will place the platform the location 100, 0, 100 and look at the origin of the scene. The units of these locations are relative to the scene coordinate system and always in meters regardless of the units of the scene being used (many older DIRSIG scenes have native units other than meters). The completed dialog is shown in Figure 26-6
Click the OK button and the tool will modify the location and rotation values in the selected row to reflect the desired pointing geometry (see Figure Figure 26-7). In this case, we can recognize that the "X Rotation" angle is 0.78539816 radians which corresponds to the 45 degree declination angle created from the described pointing description. Similarly, the 1.5707963 radian "Z rotation" angle corresponds to a +90 degrees rotation (CCW from +Y) so that the platform points down the -X axis (back towards the origin from +100 m down the X axis).
26.3.2. Creating data for a moving platform
The purpose of this tutorial is to demonstrate how to use the program to create a simple platform description that moves the platform at a given speed and direction with respect to a given ground location. In this tutorial, we will create a simple set of position data
The tool to help the user create this data is accessed through the Dynamic position wizard ... option in the Tools menu. This will open a dialog window like the one in Figure 26-8:
In this example, we will simulate an airborne platform that is flying at an altitude of 1 km (1,000 m) and at a speed of 100 m/s. The platform will fly due North, which is a heading of 0 degrees and the flight will be 1 second in duration.
The Time Delta parameter describes how many intermediate way points will be computed within the flight. If you specify a Time Delta that is the same as the Total Time, the tool will generate a way point for the start of the flight (relative time of 0.0 seconds) and the end of the flight (relative time of 1.0 seconds). If you specify a smaller Time Delta, then intermediate way points will be computed based on that time delta. If the flight is straight and level, then there is not a difference between having only two waypoints and several thousand way points (since intermediate times are always interpolated from the known way points). However, if you want to impose platform deviations from the straight and level flight line, then using a smaller Time Delta will provide you intermediate way points to modify. For this example, we will specify a Time Delta of 0.1 seconds which should result in 11 total waypoints at 0.1 seconds from 0.0 to 1.0 seconds inclusively.
The Reference Location allows the user to position the flight line over a location in the scene. For this example, we want to fly over the scene origin (0,0) midway through the flight, so you should select the Specify a mid-point location and set the Location fields to 0.0 and 0.0.
The completed flight data generation parameters should now match those shown in Figure 26-9:
Click the OK button and let the tool generate the flight data. The resulting data should resemble the data illustrated in Figure 26-10:
Most airborne platforms do not fly straight and level so the current position and orientation data is not realistic. The platform editor has an Add noise ... tool under the Tools menu. In this example, we will add some noise to the orientation of the platform. Open the Add noise ... tool, select the Add Gaussian noise to rotation data and enter 0.01 degrees. The completed dialog window is shown in Figure 26-11:
Click the OK button and let the tool add the noise to the previously generated flight data. The resulting data should resemble the data illustrated in Figure 26-12. Note that the tool always displays the axis rotation/orientation angles in radians.
Figure 26-12. The main window containing the generated flight data with the addition of rotation/orientation noise.
There is also a "translation" tool that will allow the user to shift the position data. For this example, we want to shift the flight line to an altitude of 2 km (2,000 m). Open the Translate data ... tool in the Tools menu. Set the Z Translation to 1000 which is the required translation from the current value of 1,000 meters to 2,000 meters. The X Translation and Y Translation values can remain 0 since only a vertical translation is desired. The completed dialog window is shown in Figure 26-13:
Figure 26-13. The "Translate Data" dialog window containing the addition of rotation/orientation noise.
Click the OK button and let the tool translate the Z position of the platform. The resulting data should resemble the data illustrated in Figure 26-14:
Figure 26-14. The main window containing the generated flight data after the data has been shifted to a higher altitude.
The data can be saved to a Generic Platform XML File file by selecting the Save ... option from the File menu.