STK Settings for Central Bodies
This topic provides technical notes concerning the following settings for central bodies in the Ansys Systems Tool Kit® (STK®) application.
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 <Install Dir>\STKData\CentralBodies\<central body name>. For example, Earth’s configuration is contained in <Install Dir>\STKData\CentralBodies\Earth\Earth.cb while the Moon’s configuration is in <Install Dir>\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. The STK application 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 software 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 software does not consider prolate shapes, as no major body of the solar system is prolate.
For the STK application, 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 <Install Dir>\STKData\CentralBodies\ directory (e.g., <Install Dir>\STKData\CentralBodies\Earth\Earth.cb, <Install Dir>\STKData\CentralBodies\Moon\Moon.cb, etc.).
Central body ephemerides
The STK application models ephemerides for central bodies using both a JPL Developmental Ephemeris (DE) file and a set of JPL SPICE files. Such files contain tables of position and velocity of celestial bodies 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).
For more information on how STK automatically loads SPICE files, click here.
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).
Special modeling for Mars planet-centered ephemerides
Before the release of JPL Developmental Ephemerides (DE) 430, the Mars planet-center location was considered to be the same as the Mars barycenter location, since the Martian moons Phobos and Deimos are so tiny relative to Mars. Beginning with DE 430, however, the Mars planet-centered location has been modeled as separate from its barycenter. The Mars planet-centered ephemerides are no longer included in the SPICE deNNN.bsp file and instead must be obtained from a Mars-specific SPICE file (e.g., mar097.bsp).
The mar097.bsp ephemeris for the planet Mars is a high-frequency oscillation (seven cycles in about three days) of about 5-6 cm amplitude with a 15-16 cm offset. The bsp file requires quite a large amount of disk space to represent the Mars planet center to this level of accuracy at this frequency content. Most users of AGI products, however, do not need this level of accuracy in the location of Mars and would benefit from the use of a file with a smaller disk footprint.
Thus, AGI has chosen to model the Mars planet-center location as its barycenter in the planets.bsp file that ships with its products. Users wishing to model Mars more precisely, as provided by mar097.bsp, can simply override the installed ephemerides by placing mar097.bsp (or other spice file containing Mars 499 ephemerides) into C:\ProgramData\AGI\STK_ODTK 13\Spice. While both this file and the installed file will provide ephemerides for use by the SPICE toolkit, STK software will use the ephemerides in ProgramData file in preference to the installed file's ephemerides because the ProgramData file was loaded later. This is consistent with the SPICE toolkit behavior for resolving ephemeris time overlaps.
Reverting to an older DE version
AGI products use two forms of DE files:
- plneph.NNN
- planets.bsp
You should keep these two files in sync so that bodies in the solar system are modeled consistently.
If you need to use a different DE version than the current version in the install, then proceed following these steps:
- Switch the planetary ephemeris file that the STK application uses. In the application, this setting exists in the _Default.ap file in your Config/Defaults folder. Use a text editor to search for plneph and set the version appropriately. For example, to use 421, set plneph.421 rather than the installed version plneph.440.
- Copy the corresponding deNNN.bsp file to C:\ProgramData\AGI\STK_ODTK 13\Spice. These files are available from the JPL/NAIF website (de405.bsp, de421.bsp, de430.bsp, de440.bsp). Because files loaded from this folder location occur after the files are loaded from the install area, the application will use these files, in preference to the installed files, for providing ephemeris locations using the SPICE toolkit. This is consistent with the SPICE toolkit behavior for resolving ephemeris time overlaps. For example, to use 421, copy de421.bsp to C:\ProgramData\AGI\STK_ODTK 13\Spice.
- Copy the text file spice_data_notes.tpc from C:\Program Files\AGI\STK_ODTK 13\STKData\Spice to C:\ProgramData\AGI\STK_ODTK 13\Spice. Change the attributes on the copied file to no longer be ReadOnly, and then edit the text file to update the DE version. This indicates to the software which DE version is in use.