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INTRO TO STK MODELS
For STK/6.0, STK/Professional (STK/PRO), and STK/Advanced Visualization Option
(STK/Advanced VO) - Estimated Completion Time: 40 min
What You Will Do
In this exercise you will learn how to:

Select the desired model file for use in a scenario

Review the .mdl file in a text editor

Use the AGI Modeler tool to help identify model components

Scale the size of a model

Use attach points and steerable elements
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Convert a model from a different format to STK .mdl format

Review and download updated model files from AGI’s website.
Model Files
You may enhance your STK graphical output by using an appropriate model of the
spacecraft, missile, vehicle, or ground station in your scenario. STK offers nearly 500
detailed 3-D models from which the user can choose. In this class you will create a very
simple STK scenario and then manipulate the model used in this scenario.
Create a Simple STK Scenario
We will open a new STK scenario and populate it with the Hubble space telescope.
1. Open a new STK scenario (
). We will just use the default time period.
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2. Use the Satellite Database to insert the Hubble space telescope (Insert ->
Satellite from Database, search on Common Name of Hubble).
3. In the 3D Graphics Window zoom in until the Hubble’s name and marker are
clear. Holding down the Shift key, double-click on the Hubble’s marker. This
should put Hubble at the center of the 3D Graphics window.
You should observe that the model in the 3D Graphics window does look like the
Hubble Space Telescope. When a satellite is imported from the satellite database,
the appropriate model (when available) is automatically imported into the scenario.
Note
If the Orbit Wizard is used to create a satellite, a generic satellite model will be used as the
default model. The desired model can then be assigned by accessing the 3D Graphics > Model page of the satellite’s properties.
Overview of a .mdl file
The 3D Hubble model that you see in the 3D graphics file is an STK proprietary file
format, a .mdl file. This file format is a text file and a text editor should be used to
modify it. Open up the Hubble model file (hubble.mdl) in a text editor and look at the
following sections.
1. Using Windows Explorer, go to the following directory:
C:\Program Files\AGI\STK 6.0\STKData\VO\Models\Space\Civil.
2. Double click on hubble.txt
You will notice several sections in the .mdl file.

Header – Information about who built the model, when it was built, and some
information about the internal structure of the model, for instance textures,
articulations, and dimensions.
 Components – Components are the pieces of the model. In components you would
see
Articulations – movable model parts
Attach points – a point used to attach a sensor beam
Pointable elements – model part that can track a scenario object
 Root – The master component which refers to all other components in the file.
Articulations can also be found here.
Note
The units in a model file are specified in meters. Models can be built in any unit, but the
Root component in the file assumes they are in meters.
The different components of the Hubble model are comprised of numerous polygons.
The triple columns of numbers that you see in the text editor represent the x. y and z
components of each point of the model. Once defined, these points are later
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referenced (by row number, starting at 0) to build polygons. For example, an entry of
3 1 6 0 prefaced by the header Polys would indicate that a 3-point polygon is being
created using points 1, 6 and 0 from the previous listing of points. While a model can
be created directly in a text editor, this is quite complicated and is rarely the
approach used. Most model files are converted from different model file formats. This
will be discussed in a later section.
Modeler (MDE)
Modeler is a stand alone tool that comes with STK and can be started from the
Windows Start menu under Program Files-> Satellite Tool Kit-> Modeler. You
might use Modeler to accomplish the following:



View a model
Test an articulation
Identify model components
Bring up Modeler and perform the following steps to identify model components:
1. Open the Hubble model (hubble.mdl) from the C:\Program Files\AGI\STK
6.0\STKData\VO\Models\Space\Civil directory.
2. Zoom in closer to the model using VO-like mouse movements.
3. Ctrl-right click on a side panel of the model. You should see it flash red and the
message window will display the component name, number of places it appears
in the file, and number of triangles that compose it.
4. Refer back to the .mdl file to find the references to this component.
Try selecting some of the other components on the Hubble model and look for their
references in the .mdl file. Close Modeler when finished.
Model Scale
If you observe the model of the Hubble in the 3D Graphics Window you will notice
that its size is quite exaggerated. The size of models in STK is scalable.
1. Select Hubble in the Object Browser.
2. Open Hubble’s Properties Browser (
).
3. Go to the 3D Graphics -> Model page.
4. In the Model section adjust the Scale of the model. Click Apply and observe the
change in size in the 3D Graphics Window. You may have to zoom significantly
in or out to see the new model.
5. When finished reset the scale to 0 and click OK. We will need to model scale to
be set to 0 for some future calculations to be correct.
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The model scale is logarithmic. If the size is increased from 1 to 2, it does not double but
rather increases by a factor of 10. If actual size is desired, the scale should be set to zero.
Attach Points
Some model files offer attach points. These provide locations on the model to which
a sensor can be attached. (Smoke trails can also be attached to attach points. This
requires the setting on an environment variable. For details on this topic, review the
FAQ section of AGI’s website at www.agi.com under Support/ Product Support/ FAQ.
Specifically, FAQs 11204 and 10462 address this subject.)
When placed on an attach point, the sensor is moved both visually and
mathematically, assuming the appropriate properties are adjusted. If the following
steps are accomplished, the calculations to the sensor no longer are based from the
center of the model, but rather from the location of the sensor on the attach point.
Return to the Hubble .mdl file already open in your text editor. Search on the term
AttachPoints and observe that Hubble contains two attach points. Each corresponds
to one of the two high gain antennas on the Hubble model. We will now attach a
sensor to one of these attach points.
1. Highlight Hubble in the Object Browser.
2. From the Object Catalog (
), insert a Sensor (
).
Observe in the 3D Graphics window that this sensor emanates from the center of
Hubble. We will move it to an attach point.
3. Open the Properties Browser (
) of the Sensor.
4. Go to the 3D Graphics -> Vertex Offset page. Uncheck Inherit from Parent
Object.
5. Check Use Attach Point and select AP12_HGA1-000000. Click Apply.
6. Go to the sensor’s Basic -> Location page. Select 3D Model as the Location
Type. This will mathematically move the sensor to the attach point specified in
the Vertex Offset section. (It is this step that requires a 0 scale size. If the scale
is other than 0, the offset will be incorrectly computed.)
7. Click OK to dismiss the sensor’s properties.
Now reset the animation (
) and observe the Hubble in the 3D Graphics Window.
The sensor is no longer attached to the center of Hubble. Instead, the sensor is
attached to one of Hubble’s high gain antennas. However, our model does not look
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quite right. The boresight of the high gain antenna is pointing up, yet the sensor is
pointing down. This will be solved with articulations. We will address articulations in
the next section, and we will correct this inconsistency.
Pointable Elements
Some models contain pointable elements. These are pieces of the models (often
solar panels or antennas) which may be steered so that they are always pointing at a
particular object.
Return to the hubble.mdl file in the text editor. Search on the term Pointing. The first
occurrence of this term will reveal a list of model components which can be pointed.
In this case the entire Hubble telescope, both high gain antennas and both solar
arrays may be pointed toward a target.
Subsequent occurrences of this term will give specifics for each pointable element,
indicating about which axis it may be rotated. We will now point Hubble’s solar
arrays toward the sun.
1. Highlight Hubble in the Object Browser.
2. Open Hubble’s Properties Browser (
).
3. Go to the 3D Graphics -> Model Pointing page.
4. From the list of Pointable Elements select Port_Solar_Panel-000000 and then
select Sun from the list of Available Targets.
5. Do the same for the Starb_Solar_Panel-000000.
6. Go to the 3D Graphics -> Vector page.
7. Turn on Hubble’s Sun Vector by checking its Show box. This will provide a
reference for us so we can determine if the solar panels are, in fact, pointing
toward the sun.
8. Click OK to dismiss Hubble’s properties.
Return to the 3D Graphics Window. Reset the animation (
) and observe the
orientation of the solar panels as you animate. Are they pointing toward the sun?
Since the solar arrays may only rotate about one axis, they will not be perfectly
aligned with the sun vector. Still, you should clearly see the arrays rotating so that
they obtain as much sun exposure as possible.
Specifics on how to add a new pointable element to an existing model file can be
found in the FAQ section of AGI’s website. This is available at www.agi.com under
Support/ Product Support/ FAQ. Specifically, FAQ 11223 addresses this topic.
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Model building tools
The Hubble model that we have been using is an example of a model file that comes
with the STK installation. However, sometimes a user must build a model file for
their particular project because an existing file does not already exist. There are
many excellent model building tools on the market. Here are a few of the tools our
customers have said they use.
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LightWave
3-D Studio Max
Alias/WaveFront Maya
CAD packages (AutoCAD Mechanical Desktop)
Power Animator
Softimage
Houdini
Solidworks
Rhino
This is just a subset of the available tools. Regardless of the tool used to build a
model file, the file must first be converted to LightWave LWO format in order to
ultimately be converted into STK’s proprietary .mdl format.
Converting Models
The primary tool that AGI currently uses to convert other model file formats to .mdl is
LwConvert. This can be downloaded from AGI’s website at www.agi.com under
Support/ Downloads / Software Downloads.
LwConvert
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

Converts a LightWave file to an STK model file.
Takes an .lwo file as input and saves an .mdl file as output.
Model type needs to be specified to apply the correct axis rotation.
Extensive help in the Hints button.
If you use any of the tools listed above (aside from LightWave), you will want to
convert your model to the LightWave format so that it can then be converted to an
.mdl format using AGI’s LightWave Converter. Here are some tools that can convert
other formats to the LightWave format:


Viewpoint Interchange by Viewpoint Digital http://www.viewpoint.com
PolyTrans V2 by Okino Computer Graphics http://www.okino.com/conv/conv.ht
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The AGI website FAQ section offers detailed instruction on how to convert model
files. This FAQ can be found at www.agi.com under Support/ Product Support/ FAQ.
Specifically, FAQs 11134, 10456 and 10453 provide details on converting models
and working with model formats other than .mdl.
Obtaining Updated Models
AGI continually updates and improves the existing collection of model files available
to the user community. These updates occur on a daily basis. STK users may
access the latest models via the AGI website. These can be accessed at
www.agi.om under Support/ Downloads/ Models, Videos and Animations.