Using Different Earth Models
Over the years, scientists have developed a number of different Earth models. By Earth model, we mean a description of the shape and size of the Earth (the ellipsoid), the orientation of the latitude and longitude coordinate system on the Earth (commonly called a datum), and the gravity model that goes with the shape. Commonly, you will hear descriptions such as WGS84, WGS72, etc.
By default, STK uses a WGS84 Earth model. Technically speaking, this means that it models the Earth’s shape as an oblate spheroid (a special case of the more generic tri-axial ellipsoid shape) using the shape parameters specified in the WGS84 definition. The default gravity model is also based on the parameters defined in the WGS84 definition. The datum is also defined using WGS84 definition. From time to time, users may need to use a different Earth model, usually for the purposes of comparing results with other tools that use a different model, or to accept inputs from tools using different models.
Key Parameters for Using a Different Earth Model
STK has the ability to use different Earth models, but only one at a time. At startup, STK reads the Earth.cb file located in STKData/CentralBodies/Earth to determine the nature of the Earth. This file describes the shape and size of the Earth as well as specifying the name of the associated gravity model file (*.grv) to use. The key parameters of interest for using a different Earth model are as follows:
• Gm - This is the gravitational constant times the Earth mass (in Nm2/kg). The default value is the WGS84 definition. This is the value used by the Two –Body, J2, and J4 propagators in STK.
• RefDistance - This is the equatorial radius of the oblate spheroid (in meters). The default value is the WGS84 definition. This is the value used by the J2 and J4 propagators in STK. As of STK v11.2, RefDistance is established from the GravityModel.
• MinRadius - This is the polar radius of the oblate spheroid (in meters). The default value is the WGS84 definition. As of STK v11.2, MinRadius is determined from Shape parameters.
• MaxAltitude - Height of the highest point on the Earth relative to the surface of the ellipsoid (in meters). This value is used for computing the surface area that must be searched when performing terrain mask calculations. While technically this value will be different depending on the Earth model you use, in practice it won’t matter and isn’t worth changing.
• GravityModel - This is the name of the file containing the gravity model parameters. The file is assumed to be in the same directory as the central body file and to have a .grv extension. All file names are case sensitive.
To use a different Earth model, copy the existing Earth.cb file and modify the parameters listed above to the desired values. Name your new file Earth.cb (don’t lose the old one, though; rename it to something like Earth.cb.original).
Note: You must restart STK (if already running) for the values in the new file to take effect.
Key Parameters in the Gravity Model File
The gravity model is assumed to be a spherical harmonic model. The key parameters in the gravity model file are as follows.
• Model - Name of the model used for display purposes in the STK user interface
• Degree - Number of degrees in the spherical harmonic model
• Order - Order of the spherical harmonic model
• Gm - The gravitational constant times the Earth mass (in Nm2/kg)
• RefDistance - The equatorial radius of the oblate spheroid (in meters); used as the reference distance in the gravity model by HPOP and STK's Astrogator capability
• Normalized - A flag indicating whether the coefficients to follow have been normalized or not; may be set to Yes or No
The corresponding coefficients for the spherical harmonic model are then supplied within the Begin/End Coefficients section of the file. Each line consists of the degree, order, and corresponding sectorial (cosine) and tesseral (sine) coefficients.
Additional gravity files are available in the STKData/CentralBodies/Earth directory for use by the HPOP and Astrogator propagators. It is often the case that users of these propagators want to use the standard WGS84 Earth model as far as the shape is concerned, but want to use a different gravity model. In this case, they may select the desired model (or build their own) and select when using these propagators.
The observant reader will by now have noticed that it appears that the Gm and RefDistance parameters appear to be redundant between the central body file and the gravity file. In some sense this is true. Historically, the Gm, J2, and J4 terms were specified in the central body file for use by the 2-body, J2, and J4 propagators. HPOP was originally hard-coded to use the JGM2 coefficients for the higher-order terms. As STK advanced, the J2 and J4 propagators were modified to get their values from the gravity file specified in the central body and HPOP was modified to use a gravity model file as an input to determine its gravity model coefficients. The Gm and Refdistance values in the gravity model file are not used by STK other than for computing gravitational accelerations in HPOP and
Astrogator.
