Third Body Gravity

In addition to earth gravity, you can model the effects of gravity from a third body on your satellite's orbit behavior. The Third Body Gravity table is ordered to reflect the maximum possible third body force that could be produced by each central body, when that body is located at its closest to the Earth. Bodies appearing higher in the list can produce a larger third body force than those appearing lower in the list. The actual contribution of a body, however, depends on the distance of that body from the earth during the satellite's time interval. For example, although Jupiter appears in the list right after the Sun and Moon, it is possible for Venus (next in the list) to produce a larger third body force when Venus is near its closest distance to the Earth if Jupiter is near its farthest distance from the Earth.

The ephemeris source for third bodies in HPOP will be inferred from the Gravitational Source setting. Earth, Moon, and Sun always use the Cb file setting for the ephemeris.The setting of JPL DE will cause the central body to use the DE file for its ephemeris (which for the outer planets is barycenter position).If the setting is Cb file, then the barycenter position and system mu value will be used for the outer planets unless the central body is the parent of the satellite central body, in which case the Gm value from the Cb file and the planet centered position will be used.

In general, the inclusion of solar and lunar third body gravity contributions for earth-orbiting satellites is sufficient for accuracy in all but the most demanding applications. The importance of including third bodies in modeling the force environment increases the farther the spacecraft is away from all central bodies (e.g., deep space spacecraft).

Use the editable grid cells in the Third Body Gravity table to select third body objects for consideration. To consider a body's gravitational pull in computations, set the following:

Option	Description
Name	The name of the body. STK populates the grid using all available central bodies.
Use	If this is checked, the gravitational pull from the selected body will be considered in force computations.
Source	The source from which the gravitational value is being supplied.: HPOP historical (Sun and Moon only) - hard-coded, uneditable value used in prior versions of HPOP. Cb file (all bodies) - gravitational value from the editable central body file shipped with STK. If the file contains a system value of the gravitation parameter (GM), the system value is used. The system value includes any moons the body may have. JPL DE (Sun, Moon, planets, and Pluto) - Gravitational Parameter used to compute JPL's DE 405 ephemerides. User Specified - enter the desired gravity value in the field provided. In general, the differences among these sources in gravitational values (other than User Specified) is likely to be computationally insignificant. However, the ability to select a source enables you to use values consistent with other software programs.
Gravity Value	The gravity value used by HPOP for third bodies is dependent on whether the body represents the entire planetary system or just the planet itself. For example, the value used for Jupiter as a third body for a Earth satellite is the barycentric value; the value used for Jupiter for a Europa satellite is the planetary value (since Europa is a child of Jupiter). If the Source (see above) is User Specified, enter the gravitational value that will be used in computations as a number greater than 0.0 in units of distance³/time².