ODTK Settings for Central Bodies

This topic provides technical notes concerning the following settings for central bodies in the Ansys Orbit Determination Tool Kit (ODTK�) 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. AGI recommends that only advanced users edit these files to change parameter settings.

Settings Description
Gm This is the product of the universal gravitational constant G and the central body mass.
SystemGm This is the product of the universal gravitational constant G and the central body system mass. The system consists of a planet and its moons.
GravityModel

Use this default gravity model with the central body with the Twobody, J2Perturbation, and J4Perturbation propagators. You can also use it for 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.

Gravity models are stored in the same directory as the central body (*.cb) file; they use file extension .grv.

Shape Includes the following 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 one of these sources of the ephemeris for the central body:
  • JplDE uses JPL DE file (only for major planets, Sun, Moon).
  • JplSpice uses SPICE files using bodyID given by JplSpiceID
  • File uses an external file (*.e or *.pe).
  • Analytic uses a slowly varying two-body algorithm.
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: 2009, B.A. Archinal et al. (2011), Celest. Mech Dyn Astr 109 (2), 101-135 (DOI: 10.1007/s10569-010-9320-4). 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

Central bodies are modeled as having one of the following ellipsoid shapes: a triaxial ellipsoid (the three semiaxes have different lengths); an oblate spheroid shape, where the Z axis is the axis of symmetry and also the minor axis; or a sphere, where all semiaxes have the same length. Prolate shapes are not considered; no major body of the solar system is prolate.

The size and shape for each central body is obtained from �Report of the IAU/IAG Working Group on cartographic coordinates and rotational elements: 2009,� B.A. Archinal et al., Celest. Mech Dyn Astr 109 (2), 101-135 (DOI: 10.1007/s10569-010-9320-4). Values for each central body are specified in the central body configuration file, a file with 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 ODTK 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 1960-2060 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. The default is version 440 (i.e., DE440), dated March 2021. It contains the then-best ephemerides of the major planets, the Moon, and Sun, and includes major improvements to the orbits of Jupiter, Saturn, and Pluto.

JPL also makes available ephemerides in another format called SPICE. There are SPICE files that contain the same developmental ephemerides as 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.

SPICE files are read and interpolated using SPICE software available from JPL. The SPICE toolkit natively reports the position of planetary centers, rather than the barycenters of planetary systems from a DE file. AGI has incorporated those parts of SPICE dealing with ephemeris interpolation into its software, and collected the best ephemerides for many of the larger bodies of the solar system, as well as 19 major asteroids, into a set of SPICE files. JPL does not publish a solar system SPICE file of its own.

Currently, all central bodies shipped with AGI software could be configured to use the ephemerides from SPICE files. However, the older DE file format (DE405/DE403) 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 (except for Ceres) use ephemerides contained in SPICE files. Ceres uses a simple analytic formula, which is essentially a two-body formula where the elements are modeled as drifting linearly in time.

All central body ephemerides span the period from January 1,1990, to December 23, 2099 (at a minimum), with additional historic data extending to January 1, 1950. For a discussion of the numerical differences of using SPICE and DE files, see SPICE-based 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:

  1. Switch the planetary ephemeris file that the ODTK 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.
  2. 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.
  3. 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.