CelestialBody

Celestial body orientation model

The orientation of built-in celestial bodies is modeled using high fidelity theories on a per-body basis. The orientation of Earth is modeled using IAU-1976/FK5. The orientation of the Moon is modeled using lunar librations from the DE file. The orientation of Neptune is modeled using IAU-2002. The remaining built-in celestial body orientations are modeled using data published by the IAU/IAG in "Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000".

The orientation of a custom CelestialBody is modeled by providing three angles and their rates based on IAU/IAG conventions. The figure below illustrates the angles. The angles αo, δo, and W, are respectively the SpinAxisRAConstant, SpinAxisDECConstant, and RotationConstant. The angular rates are respectively SpinAxisRARate, SpinAxisDECRate, and RotationRate. All angles are referenced to the X-Y plane of the ICRF axis system. The constant values SpinAxisRAConstant, SpinAxisDECConstant, and RotationConstant are defined to be the values at the epoch defined in OrientationEpoch.

Below is an example illustrating how to configure a CelestialBody according to the IAU 2006 recommended values for Vesta. Note the orientation epoch typically used by the IAU is 01 Jan 2000 12:00:00.00.000 TDB and this must be converted to A1ModJulian which can easily be performed using the Spacecraft Orbit dialog box.

          Create Asteroid Vesta
Vesta.CentralBody         = Sun
%  Note that currently the only available
%  format for OrientationEpoch is A1ModJulian
Vesta.OrientationEpoch    = 21544.99962789878  
Vesta.SpinAxisRAConstant  = 301.9
Vesta.SpinAxisRARate      = 0.9
Vesta.SpinAxisDECConstant = 90.9
Vesta.SpinAxisDECRate     = 0.0
Vesta.RotationConstant    = 292.9
Vesta.RotationRate        = 1617.332776
        

Note: The orientation models available for Earth and Luna have additional fields for configuration. Earth has an additional field called NutationUpdateInterval that controls the update frequency for the Nutation matrix. For high fidelity applications, NutationUpdateInterval should be set to zero. The RotationDataSource field for Earth and Luna defines the theory used for the rotation of those bodies. Currently, only FK5IAU1980 and DE are available for Earth and Luna respectively and the field is displayed for information purposes only.

Setting colors on orbits of celestial bodies

GMAT allows you to assign colors to orbits of celestial bodies that are drawn in the OrbitView graphics display windows. GMAT also allows you to assign colors to perturbing celestial body orbital trajectories drawn during iterative processes such as differential correction or optimization. The CelestialBody object's OrbitColor and TargetColor fields are used to assign colors to both orbital and perturbing trajectories. See the Fields section for description of these two fields. Also see Color documentation for discussion and examples on how to set colors on a celestial body.

Configuring orbit ephemerides

The ephemerides for built-in celestial bodies is specified by the SolarSystem.EphemerisSource field and the same source is used for all built-in bodies. Ephemerides for a custom CelestialBody are provided by SPICE files. Archives of available SPICE files can be found at the JPL NAIF site and the Solar System Dynamics site . JPL provides utilities to create custom SPICE files in the event existing kernels don't satisfy requirements for your application. To create custom SPICE kernels, see the documentation provided by JPL. The list of NAIF Ids for celestial bodies is located here.

Note that the DE files model the barycenter of planetary systems. So for Jupiter, when using DE405 for example, you are modeling Jupiter's location as the barycenter of the Jovian system. SPICE kernels differentiate the barycenter of a planetary system from the location of the individual bodies. So when using SPICE to model Jupiter, you are modeling the location of Jupiter using Jupiter's center of mass.

To specify the SPICE kernels for a custom CelestialBody, use the NAIFId, CentralBody, and SourceFileName fields. GMAT is distributed with an SPK file for CERES which has NAIF ID 2000001. Here is how to configure a CelestialBody to use the CERES SPICE ephemeris data.

          Create CelestialBody Ceres
Ceres.CentralBody = Sun
Ceres.SourceFilename = '../data/planetary_ephem/spk/ceres_1900_2100.bsp'
        

Note: GMAT currently only supports a single ephemeris model for custom bodies (SPICE) and this is set using PosVelSource field. The default for PosVelSource is SPICE and it is not necessary to configure this field in the current version of GMAT.

Configuring physical properties

GMAT models all celestial bodies as spherical ellipsoids. To define the physical properties use the Flattening, EquatorialRadius, and Mu fields.

Configuring for event location

GMAT's event location subsystem (consisting of ContactLocator and EclipseLocator) uses celestial body definitions from the SPICE toolkit. Properites such as radius, flattening, ephemeris, and orientation must be configured separately for use with the event locators.

CelestialBody shape and orientation are configured via SPICE PCK files, loaded from two sources in the following order:

Data loaded last takes precedence over data loaded first, if there is a conflict. Note that because the SPICE kernel pool is shared for the entire run, a PCK file loaded for Pluto may override data loaded by Sun, if the file contains conflicting data. Note that this order isn't absolute—coordinate systems that with an SPK-defined origin load differently, for example. To determine the exact load order, see the GmatLog.txt file.

The body-fixed frame for a CelestialBody is defined on the Orientation tab by the SpiceFrameId and SpiceFrameKernelFile fields. The SpiceFrameId contains the SPICE ID for the body-fixed frame, which may be built-in or defined via external FK files. External FK files can be loaded by adding them to the SpiceFrameKernelFile list for each body. These files are loaded just after PlanetarySpiceKernelName for each body. The list of built-in frames is available as an appendix in the SPICE documentation. GMAT's default frames are: