The default Earth central body in STK is a high-fidelity model. The shape is based on the WGS84 ellipsoid and the rotation definition is based on the ICRF/FK5IAU76 theories. However, if you want a simplified Earth, all you have to do is make the Earth spherical and reference an alternate rotation file.
How to make the Earth spherical:
1. Find the Earth central body file (Earth.cb), located in the STK install directory: <STK install folder>\STKData\CentralBodies\Earth\Earth.cb .
2. The file may be read-only on install; if so, you will need to make it read-write to save your edits.
3. If you do not have permissions to change files in the install directory, simply copy the CentralBodies\Earth folder to the <User Area>\CentralBodies\Earth folder and edit the Earth.cb file in the new location. This will supersede the install folder version.
4. Open the file in a text editor and comment out the WGS 84 section:
# Shape values are the WGS84 ellipsoid
# ShapeName WGS84
# Shape OblateSpheroid
# MajorAxis 6.378137E6
# MinorAxis 6.378137E6
# MinorAxis 6.35675231424E6
5. Remove the comment marks (#) for the Spherical Earth section:
# Shape values are for a simple spherical Earth approximation
ShapeName SphericalEarth
Shape Sphere
Radius 6.378137E6
Another way to do this is to set Earth's MinorAxis to the same value as the MajorAxis. You could use the value 6.378137E6, to agree with the radius of WGS84. Setting the Sphere Shape or setting MajorAxis and MinorAxis to the same value in the Earth.cb file creates a spherical Earth model that STK will apply to all shape-related computations, including grazing angle.
How to reference a rotation file:
Note: This capability was added to STK in version 9.2.1, so you must have version 9.2.1 or newer to do this.
1. In the Earth central body file, there should be a section for SpinData. If this section does not exist, add the following lines after the end of the AstroDefinition section:
BEGIN SpinData
# Uncommenting the RotationDefinitionFile line will override the use of ICRF/FK5IAU76 theories
# and instead use the (lower fidelity) model defined by the specified file.
#
RotationDefinitionFile EarthAttitude2000.rot
END SpinData
This section enables you to override the ICRF/FK5IAU76 theories for rotation and use a different model specified by a rotation file.
2. Remove comment marks for the RotationDefinitionFile line.
3. Reference the appropriate rotation file.
• The EarthAttitude2000 rotation file uses coefficients from the IAU 2000 parameter set. It is a low-fidelity model that does not account for nutation and uses simplistic precession.
• You can remove the precession in the EarthAttitude2000 rotation file by setting the SpinAxisRightAscension and SpinAxisDeclination rates to zero.
Attached to this knowledge article are the following files:
• The EarthAttitude2000.rot file
• An Earth.cb file that is set to spherical and includes the simplified rotation parameters
