34.12. PPD: Generic Platform Positioning Data

The generic platform positioning data (PPD) file contains a series of records that indicate the platform's position and orientation as a function of time. This file is an XML formatted file which makes it easier and more reliable to parse and allows the data to be authored and viewed in other software packages.

Below is an example of an XML location/orientation data file for the "generic" platform model. This platform model is chosen by supplying the name generic in the type attribute.

The next element is the method is reserved for future expansion when the method and parameters used to generate specific platform positioning and pointing data (in the "Platform Edit" tool) will be stored here. For now, the type attribute for the method expects to see raw which is an free-style method that is used to describe any other means used to create the data (including hand generated data or data imported from other sources).

The optional uncertainty element contains a description of the uncertainty associated with each location and orientation parameter. At this time, the model only supports a zero meaned, normal distributed noise about the supplied data. For each parameter, the user can supply the variance for that normal distribution. At a future time, a mean value may be added that would model an offset in the measured values. The uncertainty element has spatialunits and angularunits attributes that define the units for the variance values and currently only meters and radians are supported, respectively. This feature can be enabled and disabled using the enabled attribute.

The position and orientation data is contained in a section named data. The rotationorder attribute is assigned a string that defines the order of the axis rotations. In this example, the string "zyx" indicates that the rotations will be applied in the order Z, then Y and then X.

Each entry contains a time variable which currently supports only "relative" seconds with respect to the main simulation time (absolute times will be supported in a future version). The xlocation, ylocation and zlocation variables store the XYZ location of the platform in meters with respect to the scene origin. The xrotation, yrotation and zrotation variables store the XYZ rotation angles about the respective axis in radians.

  
<platform type="generic>
    <method type="raw"></method>

    <uncertainty enabled="1" spatialunits="meters" angularunits="radians">
        <xlocation type="normal">
            <variance>0.1</variance> 
        </xlocation> 
        <ylocation type="normal"> 
            <variance>0.1</variance>
        </ylocation> 
        <zlocation type="normal">
            <variance>0.2</variance>
        </zlocation>
        <xrotation type="normal">
            <variance>0.01</variance>
        </xrotation>
        <yrotation type="normal">
            <variance>0.01</variance>
        </yrotation>
        <zrotation type="normal">
            <variance>0.01</variance>
        </zrotation>
    </uncertainty>

    <data rotationorder="zyx" angletype="absolute" spatialunits="meters" angularunits="radians">
        <entry>
            <datetime type="relative">0.000</datetime>
            <xlocation>0</xlocation>
            <ylocation>0</ylocation>
            <zlocation>681000</zlocation>
            <xrotation>0.00000</xrotation>
            <yrotation>0.00000</yrotation>
            <zrotation>0.00000</zrotation>
        </entry>
        <entry>
            <datetime type="relative">0.485765</datetime>
            <xlocation>-764.373</xlocation>
            <ylocation>3613.804</ylocation>
            <zlocation>680999.034</zlocation>
            <xrotation>-0.000351813</xrotation>
            <yrotation>-0.001122422</yrotation>
            <zrotation>+0.000002780</zrotation>
        </entry>
    </data>
</platform>
  
  

This axis rotations are based on a right-handed coordinate system that is oriented such that the +Z axis is up (normal to the earth) and the +Y axis is along the nominal/default direction of the platform. The following figures illustrate the resulting sign conventions when using an aircraft as the reference platform:

In Figure 34-2, a positive X rotation is results in a "pitch up" or "nose up" rotation.

In Figure 34-3 (plane heading out of the page), a positive Y rotation is results in a "right wing down" rotation.

In Figure 34-3, a positive Z rotation is results in a "counter clock-wise turn" or "left yaw" rotation.

To learn more about the Generic Platform model, consult the Improved Platform Modeling chapter.