Creating a simplified Earth in STK


QuestionHow to I simplify the Earth in STK?
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, it can be done!  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. Open the file in a text editor and set the MinorAxis to the same value as the MajorAxis (e.g., 6.378137E6 to agree with the radius of WGS84)
Setting the MajorAxis and MinorAxis to be the same value in the Earth.cb file creates a spherical Earth model which will be applied 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 allows the user to override the ICRF/FK5IAU76 theories for rotation and use a different model specified by a rotation file. 
  1. Uncomment the RotationDefinitionFile line
  2. 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.
    • The precession can also be removed in the EarthAttitude2000 rotation file by setting the SpinAxisRightAscension and SpinAxisDeclination rates to 0.
Attached to this knowledge article:
  • The EarthAttitude2000.rot file
  • An Earth.cb file that is set to spherical and includes the simplified rotation parameters.
Resource File 1
Resource File 2