Using Topography Data for Central Bodies Other than the Earth

Topography data, which describe the actual surface of a body as opposed to an idealized surface such an ellipsoid, are typically specified in terms of a grid of height offsets from a reference surface. There are two common types of reference surfaces which are used: idealized ellipsoidal surfaces and equipotential surfaces (analogous to the geoid for the Earth). In the case of Earth topography, most data are referenced to the geoid and the differences among modern geoid definitions are small. The same is not true for topography data sets for other bodies, where a lack of agreement on the shape of reference surfaces can lead to potentially improper use of data sets.

When an ellipsoidal reference surface has been used, the definition of the idealized surface may be supplied with the topography information; alternatively, it can be implied or given in a reference document. In any case, the proper application of the topography data to subsequent analyses requires that the surface be properly represented relative to the center of mass of the body, which means that the ellipsoidal reference shape used for the topography data should match the shape of the central body as used by STK. The central body shapes used by STK are defined in the *.cb files located in each central body-specific directory under <STK install folder>\STKData\CentralBodies. The reference shape may be specified to be a sphere, oblate spheroid, or tri-axial ellipsoid as in the following examples:

Shape	Sphere
MajorAxis	1737400.0
MiddleAxis	1737400.0
MinorAxis	1737400.0
Shape	OblateSpheroid
MajorAxis	1738000.0
MiddleAxis	1737400.0
MinorAxis	1737400.0
Shape	TriaxialEllipsoid
MajorAxis	1738000.0
MiddleAxis	1737500.0
MinorAxis	1737400.0

Changes to the shape of central bodies must be made prior to starting STK.

When an equipotential surface has been used as the reference for the topography information, the problem is more difficult. Each equipotential surface is computed based on a particular gravity field solution and a specific angular velocity of the body. To overcome the difficulty imposed by the use of an equipotential reference surface to the problem of determining the three dimensional coordinates of a surface point, the difference between the equipotential surface and an idealized ellipsoidal surface is often provided as a grid of height offsets (the topography of the equipotential surface). STK does not currently support these types of data sets for central bodies other than the Earth. Sometimes, however, an equivalent data set will also be produced which contains radius information as opposed to height offsets where the radius is measured from the center of mass of the body. STK does support the use of radius data using the Planetary Data System format.