Gravity Model Files
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By default, FreeFlyer uses the "EGM96.potential" data file for modeling the geopotential of Earth and point masses for modeling any other celestial objects included in a Spacecraft's Force Model, but custom files can be specified for Earth or any other celestial body using the approaches discussed on the Zonals and Tesserals page of the Setting up the Force Model section of the Spacecraft Propagation Guide.
The format used for FreeFlyer's gravity model files consists of a header and the body. The header of the potential file includes values for the physical properties of the celestial body (Gravity Constant (mu), Body Radius, Reciprocal Flattening). These values are used when propagating the Spacecraft using a gravitational potential field file (see Zonals and Tesserals). For other calculations that depend on the physical properties of a celestial body, such as Spacecraft.Height or propagation of a Spacecraft with a point mass gravitational model, the celestial body physical properties are accessed using syntax such as "Earth.Mu".
A sample header (from the EGM96 potential file included with the installation) is shown below, as well as the first line of the body. This sample can be used as a starting point when creating your own gravity model files using custom gravitational field data.
The following gravity models are provided with FreeFlyer: