STK Settings for Central Bodies
This topic provides technical notes concerning the following settings for central bodies in STK.
For additional technical notes on coordinate systems, see Central Body Reference Frames.
Central body configuration settings
Each central body has its own configuration file that defaults many of its parameters. The central body configuration files are named <central body name>.cb and are located at <STK install folder>\STKData\CentralBodies\<central body name>. For example, Earth’s configuration is contained in <STK install folder>\STKData\CentralBodies\Earth\Earth.cb while the Moon’s configuration is in <STK install folder>\STKData\CentralBodies\Moon\Moon.cb. Editing these files to change parameter settings is recommended for advanced users only.
| Settings | Description |
|---|---|
| Gm | Represents the product of the universal gravitational constant G and the central body mass. |
| SystemGm | Represents the product of the universal gravitational constant G and the central body system mass. The system consists of a planet and its moons. |
| GravityModel |
This is the default gravity model for the central body. You can use this with the Twobody, J2Perturbation, and J4Perturbation propagators. You can also use it when you need gravity values for element conversions (e.g., Cartesian to Keplerian elements). For consistency, the Gm value for the central body should match the Gm value for its default gravity model. STK stores gravity models in the same directory as the central body *.cb file; they use the file extension .grv. |
| Shape | These are the values associated with the central body shape. Parameters correspond to a sphere, oblate spheroid, or triaxial ellipsoid. |
| PathGenerator | These are the propagators available for use with objects having this central body. |
| EphemerisData | Choose from the possible sources of the ephemeris for this central body: |
| SpinData | This is the source of the attitude definition. Usually, the attitude is specified using the algorithm and data from Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and satellites: 2015, B.A. Archinal et al., Celest. Mech Dyn Astr 130 (22), 2018. The data itself is contained in a rotational coefficients file (with file extension .rot) sitting in the same directory as the central body *.cb file. Earth uses the ICRF theory instead to define its attitude. |
Central body shape
STK models central bodies as having one of the following shapes:
- a triaxial ellipsoid, with the three semiaxes have different lengths
- an oblate spheroid shape, where the Z axis is the axis of symmetry and also the minor axis
- a sphere, where all semiaxes have the same length
STK does not consider prolate shapes, as no major body of the solar system is prolate.
For STK, AGI obtains the size and shape for each central body from from Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and satellites: 2015, B.A. Archinal et al., Celest. Mech Dyn Astr 130 (22), 2018. Values for each central body are specified in the central body configuration file (file extension .cb) in the \STKData\CentralBodies\ directory (e.g., \STKData\CentralBodies\Earth\Earth.cb, \STKData\CentralBodies\Moon\Moon.cb, etc.).
Central body ephemerides
For STK, AGI obtains the ephemerides for most central bodies from a Developmental Ephemeris (DE) file or a JPL SPICE file. Such files contain tables of position and velocity over a large span of time.
The Developmental Ephemeris is generated from a numerical integration of the solar system, using barycentric positions for the outer planets and including effects from general relativity. The ephemerides are made available to the public from a JPL website. AGI has collected the ephemerides for the period 1950-2150 and packaged it into a JPL DE file, a particular binary format that allows software (publicly available from JPL) to extract and interpolate the data. By default, version 440 (i.e., DE440) dated March 2021 is used. It contains the then-best ephemerides of the major planets, the Moon, and Sun and updates the attitude of the Moon over the older DE versions.
JPL also makes available ephemerides in another format called SPICE. There are SPICE files that contain the same developmental ephemerides as are normally shipped in a DE file. In addition, JPL makes available ephemerides for a wide range of celestial bodies in the solar system. These ephemerides are usually developed during JPL mission planning and operations. Time spans for different bodies can vary widely; moreover, there may be several versions of ephemerides for the same celestial body developed at different times (and possibly incorporating different sets of data). For example, the ephemerides for the Jovian system was updated and improved during the Galileo mission.
STK reads and interpolates SPICE files using SPICE software available from JPL. SPICE files are provided that not only report the barycenter location of each planet (as appropriate) but also the location of each planetary center, unlike a DE file that contains only barycenters of the outer planets. AGI has incorporated those parts of SPICE dealing with ephemeris interpolation into the STK software and has collected the best ephemerides for many of the larger bodies of the solar system into a few SPICE files. JPL does not publish a solar system SPICE file of its own.
For all central bodies shipped with AGI software, you can configure them to use the ephemerides from SPICE files. However, the older DE file format provides better time precision when requesting interpolated ephemeris. Thus, the Earth, Moon, and Sun are configured to use the older DE file format as their ephemeris source; all other bodies use ephemerides contained in SPICE files.
All central body ephemerides span the period 1990–2099 (at least).
A discussion of the numerical differences of using SPICE and DE files is in the document Differences Resulting from Using SPICE-based JPL DE421 Planetary Ephemerides (PDF).