gravity_field
#
This module contains a set of factory functions for setting up the gravitational potential models of celestial bodies in an environment. Below a short overview of aspects of some of the gravity field models in order to aid in properly selecting an choosing a model. Unlike most other environment model options in Tudat, there are multiple options for creating either a spherical harmonic gravity field, and a point mass gravity field:
Pointmass gravity field: defining the gravitational parameter manually (
central()
) or requiring the gravitational parameter to be extracted from Spice (central_spice()
).Spherical harmonic gravity field: defining all the settings manually (
spherical_harmonic()
), loading a predefined model for a solar system body (from_file_spherical_harmonic()
) or calculating the spherical harmonic coefficients (up to a given degree) based on an ellipsoidal homogeneous mass distribution (spherical_harmonic_triaxial_body()
)
Rigid body properties will always be created automatically when a body is endowed with a gravity field, as described below:
Pointmass gravity field: mass computed from gravitational parameter; zero inertia tensor, and center of mass at origin of bodyfixed frame
Spherical harmonic gravity field: mass computed from gravitational parameter, center of mass computed from degree 1 gravity field coefficients, inertia tensor as described in
spherical_harmonic()
Polyhedron gravity field: mass computed from gravitational parameter, center of mass and inertia tensor computed from homogeneous mas distribution inside body
References#
Functions#

Factory function for central gravity field settings object. 
Factory function to create central gravity field settings from Spice settings. 


Factory function for creating a spherical harmonics gravity field settings object. 

Factory function for spherical harmonics gravity field settings object from triaxial ellipsoid parameters, using the density to define the mass distribution. 
Factory function for spherical harmonics gravity field settings object from triaxial ellipsoid parameters, using the gravitational parameter to define the mass distribution.. 


Factory function to load a custom spherical harmonics gravity field settings from a file. 

Factory function for spherical harmonics gravity field settings of a predefined model. 

Factory function for creating a polyhedron gravity field settings object, using the gravitational parameter. 

Factory function for creating a polyhedron gravity field settings object, using the density. 
 central(gravitational_parameter: float) tudatpy.kernel.numerical_simulation.environment_setup.gravity_field.GravityFieldSettings #
Factory function for central gravity field settings object.
Factory function for settings object, defining a pointmass gravity field model with userdefined gravitational parameter \(\mu\). The gravitational potential is the defined as:
\[U(\mathbf{r})=\frac{\mu}{\mathbf{r}}\]with \(\mathbf{r}\) the position vector measured from the body’s center of mass.
 Parameters:
gravitational_parameter (float) – Gravitational parameter defining the pointmass gravity field.
 Returns:
Instance of the
GravityFieldSettings
derivedCentralGravityFieldSettings
class Return type:
Examples
In this example, we create
GravityFieldSettings
for Earth using a simple central gravity field model:# define parameters describing central gravity model gravitational_parameter = 3.986e14 # create gravity field settings body_settings.get( "Earth" ).gravity_field_settings = environment_setup.gravity_field.central( gravitational_parameter )
 central_spice() tudatpy.kernel.numerical_simulation.environment_setup.gravity_field.GravityFieldSettings #
Factory function to create central gravity field settings from Spice settings.
Factory function for settings object, defining a pointmass gravity field model. This function provides the same model as
central()
), but with gravitational parameter \(\mu\) from Spice. Returns:
Instance of the
GravityFieldSettings
class of gravity field typecentral_spice
 Return type:
Examples
In this example, we create
GravityFieldSettings
for Earth using a simple central gravity field model and data from Spice:# create gravity field settings body_settings.get( "Earth" ).gravity_field_settings = environment_setup.gravity_field.central_spice( )
 spherical_harmonic(gravitational_parameter: float, reference_radius: float, normalized_cosine_coefficients: numpy.ndarray[numpy.float64[m, n]], normalized_sine_coefficients: numpy.ndarray[numpy.float64[m, n]], associated_reference_frame: str) tudatpy.kernel.numerical_simulation.environment_setup.gravity_field.GravityFieldSettings #
Factory function for creating a spherical harmonics gravity field settings object.
Factory function for settings object, defining a gravity field model through spherical harmonic expansion. The
associated_reference_frame
must be the same frame ID as the target frame of the body’s rotation model. It represents the frame in which the spherical harmonic field is defined.The gravitational potential is the defined as:
\[U(\mathbf{r})=\sum_{l=0}^{l_{max}}\sum_{m=0}^{l}\mu\left(\frac{{R}^{l}}{r^{l+1}}\right)\bar{P}_{lm}(\sin\phi)\left(\bar{C}_{lm}\cos m\theta+\bar{S}_{lm}\sin m\theta\right)\]with \(\mathbf{r}\) the position vector of the evaluation point, measured from the body’s center of mass. The angles \(\phi\) and \(\theta\) are the bodyfixed latitude and longitude of the evaluation point, and \(\bar{P}_{lm}\) is the associated Legendre polynomial (at degree/order \(l/m\)).
For the spherical harmonic gravity field (including other spherical harmonic functions), the normalized mean moment of inertia must be set by the user, to allow an inertia tensor to be computed. This is done using the
scaled_mean_moment_of_inertia
attribute of theSphericalHarmonicsGravityFieldSettings
class, as in the example below# Add gravity field model settings to body of spherical harmonic type body_settings.get( "Mars" ).gravity_field = ... # Add setting for moment of inertia body_settings.get( "Mars" ).gravity_field.scaled_mean_moment_of_inertia = 0.365
This code snippet will automatically create a rigid body properties for Mars, with the inertia tensor computed from this value of 0.365 and the degree 2 gravity field coefficients. Note that, if gravity field variations are used for the body, timevariability of the degree 1 and 2 coefficients will be reflected in timevariability of the body’s center of mass and inertia tensor.
Note: Spherical harmonic coefficients used for this environment model must always be fully normalized. To normalize unnormalized spherical harmonic coefficients, see
normalize_spherical_harmonic_coefficients()
. Parameters:
gravitational_parameter (float) – Gravitational parameter \(\mu\) of gravity field.
reference_radius (float) – Reference radius \(R\) of spherical harmonic field expansion.
normalized_cosine_coefficients (numpy.ndarray) – Cosine spherical harmonic coefficients (geodesy normalized). Entry (i,j) denotes coefficient \(\bar{C}_{ij}\) at degree i and order j. As such, note that entry (0,0) of cosine coefficients should be equal to 1.
normalized_sine_coefficients (numpy.ndarray) – Sine spherical harmonic coefficients (geodesy normalized). Entry (i,j) denotes coefficient \(\bar{S}_{ij}\) at degree i and order j.
associated_reference_frame (str) – Identifier for bodyfixed reference frame with which the spherical harmonics coefficients are associated.
 Returns:
Instance of the
GravityFieldSettings
derivedSphericalHarmonicsGravityFieldSettings
class Return type:
Examples
In this example, we create
GravityFieldSettings
for Earth using a spherical harmonics gravity model:# Define the spherical harmonics gravity model gravitational_parameter = 3986004.415E+8 reference_radius = 6378136.3 # Normalized coefficients taken from https://cddis.nasa.gov/archive/egm96/general_info/egm96_to360.ascii # The above file is described in https://cddis.nasa.gov/archive/egm96/general_info/readme.egm96 normalized_cosine_coefficients = [ [1, 0, 0, 0], [0, 0, 0, 0], [0.484165371736E03, 0.186987635955E09, 0.243914352398E05, 0], [0.957254173792E06, 0.202998882184E05, 0.904627768605E06, 0.721072657057E06] ] normalized_sine_coefficients = [ [0, 0, 0, 0], [0, 0, 0, 0], [0, 0.119528012031E08, 0.140016683654E05, 0], [0, 0.248513158716E06, 0.619025944205E06, 0.141435626958E05] ] associated_reference_frame = "IAU_Earth" # Create the gravity field settings and add them to the body "Earth" body_settings.get( "Earth" ).gravity_field_settings = environment_setup.gravity_field.spherical_harmonic( gravitational_parameter, reference_radius, normalized_cosine_coefficients, normalized_sine_coefficients, associated_reference_frame )
 sh_triaxial_ellipsoid_from_density(axis_a: float, axis_b: float, axis_c: float, density: float, maximum_degree: int, maximum_order: int, associated_reference_frame: str, gravitational_constant: float = 6.67259e11) tudatpy.kernel.numerical_simulation.environment_setup.gravity_field.SphericalHarmonicsGravityFieldSettings #
Factory function for spherical harmonics gravity field settings object from triaxial ellipsoid parameters, using the density to define the mass distribution.
Factory function for settings object, defining a gravity field model through spherical harmonic expansion of a homogeneous triaxial ellipsoid, same as
spherical_harmonic
The constant mass distribution in the specified ellipsoid shape is expanded to obtain a spherical harmonic coefficient representation. Gravity fields from this setting object are expressed in normalized spherical harmonic coefficients. The constant mass distribution is defined by the density and gravitational constant (optional). The bodyfixed x, y and z axes are assumed to be along the A, B and C axes. This function implements the models of (see Balmino [1]). Parameters:
axis_a (float) – Dimension of largest axis of triaxial ellipsoid.
axis_b (float) – Dimension of intermediate axis of triaxial ellipsoid.
axis_c (float) – Dimension of smallest axis of triaxial ellipsoid.
density (float) – Density of ellipsoid.
maximum_degree (int) – Maximum degree of spherical harmonics expansion.
maximum_order (int) – Maximum order of spherical harmonics expansion.
associated_reference_frame (str) – Identifier for bodyfixed reference frame with which the spherical harmonics coefficients are associated.
gravitational_constant (float, default=physical_constants::GRAVITATIONAL_CONSTANT) – Gravitational constant G of the gravity field.
 Returns:
Instance of the
GravityFieldSettings
derivedSphericalHarmonicsGravityFieldSettings
class Return type:
Examples
In this example, we create
GravityFieldSettings
for Earth using the expansion of a homogeneous triaxial ellipsoid into a spherical harmonics gravity model:# Create the gravity field settings for Earth with Spherical Harmonics from a triaxial ellipsoid body_settings.get( "Earth" ).gravity_field_settings = environment_setup.gravity_field.spherical_harmonic_triaxial_ellipsoid_from_density( axis_a=6378171.88, axis_b=6378102.03, axis_c=6356752.24, density=5520, maximum_degree=50, maximum_order=50, associated_reference_frame="IAU_Earth" )
 sh_triaxial_ellipsoid_from_gravitational_parameter(axis_a: float, axis_b: float, axis_c: float, maximum_degree: int, maximum_order: int, associated_reference_frame: str, gravitational_parameter: float) tudatpy.kernel.numerical_simulation.environment_setup.gravity_field.SphericalHarmonicsGravityFieldSettings #
Factory function for spherical harmonics gravity field settings object from triaxial ellipsoid parameters, using the gravitational parameter to define the mass distribution..
Factory function for settings object, defining a gravity field model through spherical harmonic expansion of a homogeneous triaxial ellipsoid, same as
spherical_harmonic
The constant mass distribution in the specified ellipsoid shape is expanded to obtain a spherical harmonic coefficient representation. Gravity fields from this setting object are expressed in normalized spherical harmonic coefficients. The constant mass distribution is defined by the gravitational parameter. The bodyfixed x, y and z axes are assumed to be along the A, B and C axes. This function implements the models of (see Balmino [1]). Parameters:
axis_a (float) – Dimension of largest axis of triaxial ellipsoid.
axis_b (float) – Dimension of intermediate axis of triaxial ellipsoid.
axis_c (float) – Dimension of smallest axis of triaxial ellipsoid.
maximum_degree (int) – Maximum degree of spherical harmonics expansion.
maximum_order (int) – Maximum order of spherical harmonics expansion.
associated_reference_frame (str) – Identifier for bodyfixed reference frame with which the spherical harmonics coefficients are associated.
gravitational_parameter (float) – Gravitational parameter \(\mu\) of gravity field.
 Returns:
Instance of the
GravityFieldSettings
derivedSphericalHarmonicsGravityFieldSettings
class Return type:
 from_file_spherical_harmonic(file: str, maximum_degree: int, maximum_order: int, associated_reference_frame: str = '', gravitational_parameter_index: int = 0, reference_radius_index: int = 1) tudatpy.kernel.numerical_simulation.environment_setup.gravity_field.GravityFieldSettings #
Factory function to load a custom spherical harmonics gravity field settings from a file.
Factory function to load a custom spherical harmonics gravity field settings from a file. The file should contain fully normalized spherical harmonic coefficients. The associated gravitational paramerer and reference radius should be given in m^3/s^2 and m, respectively. The file format should be the same as that used for the files in the directories here. Specifically, the file should contain
The first line should be a series of text blocks (typically numerical data). Two of these blocks (by default the first and second one) should be the gravitational parameter and reference radius, respectively. The text block should be separated by spaces, tabs and/or commas
Each subsequent line should contain a set of spherical harmonic coefficients (first ordered in ascending order by degree, then in ascending order by order), where the first, second, third and fourth value of the line should be: degree \(l\), order \(m\), normalized cosine coefficient \(\bar{C}_{lm}\), normalized sine coefficient \(\bar{S}_{lm}\). Additional entries (for instance with coefficient uncertainties) are ignored.
 Parameters:
file (str) – Full file path and name where th gravity field file is located
maximum_degree (int) – Maximum degree of the coefficients that are to be loaded
maximum_order (int) – Maximum order of the coefficients that are to be loaded
associated_reference_frame (str, default = "") – Name of the bodyfixed reference frame to which the gravity field is to be fixed. If left empty, this reference frame will automatically be set to the bodyfixed frame defined by this body’s rotation (see rotation_model for specifying rotation models).
gravitational_parameter_index (int, default = 0) – Index of the values in the file header (first line of file) that contains the gravitational parameter
reference_radius_index (int, default = 1) – Index of the values in the file header (first line of file) that contains the reference radius
 Returns:
Instance of the
GravityFieldSettings
derivedSphericalHarmonicsGravityFieldSettings
class Return type:
Examples
In this example, we create
GravityFieldSettings
for the Moon using GGGRX spherical harmonics gravity model, up to degree and order 300:# Create the gravity field settings for Moon with Spherical Harmonics loaded in from a Spherical Harmonics file body_settings.get("Moon").gravity_field_settings = environment_setup.gravity_field.from_file_spherical_harmonic( r"...\.tudat\resource\gravity_models\Moon\gggrx_1200l_sha.tab", 300, 300)
 predefined_spherical_harmonic(predefined_model: tudatpy.kernel.numerical_simulation.environment_setup.gravity_field.PredefinedSphericalHarmonicsModel, maximum_degree: int = 1) tudatpy.kernel.numerical_simulation.environment_setup.gravity_field.GravityFieldSettings #
Factory function for spherical harmonics gravity field settings of a predefined model.
Factory function for spherical harmonics gravity field settings of a predefined model
 Parameters:
predefined_model (PredefinedSphericalHarmonicsModel) – Identified for gravity field model that is to be loaded
maximum_degree (int, default = 1) – Maximum degree and order to which the coefficients are to be loaded. If value is negative, all coefficients for the specified gravity field are loaded
 Returns:
Instance of the
GravityFieldSettings
derivedSphericalHarmonicsGravityFieldSettings
class Return type:
Examples
In this example, we create
GravityFieldSettings
for Earth using EGM96 spherical harmonics gravity model, up to degree and order 32:# Create the gravity field settings for Earth with Spherical Harmonics from a triaxial ellipsoid body_settings.get( "Earth" ).gravity_field_settings = environment_setup.gravity_field.predefined_spherical_harmonic( environment_setup.gravity_field.egm96, 32 )
 polyhedron_from_mu(gravitational_parameter: float, vertices_coordinates: numpy.ndarray[numpy.float64[m, n]], vertices_defining_each_facet: numpy.ndarray[numpy.int32[m, n]], associated_reference_frame: str, gravitational_constant: float = 6.67259e11) tudatpy.kernel.numerical_simulation.environment_setup.gravity_field.GravityFieldSettings #
Factory function for creating a polyhedron gravity field settings object, using the gravitational parameter.
Factory function for settings object, defining a gravity field model through a polyhedron. The
associated_reference_frame
must be the same frame ID as the target frame of the body’s rotation model. It represents the frame in which the polyhedron field is defined.The gravitational potential, acceleration, Laplacian of potential and Hessian of potential are computed according to Werner and Scheeres [2].
This function uses the gravitational parameter to define the gravity field. To instead use the density constant see
polyhedron_from_density()
. Since both models tend to be computationally intensive, it is recommended to use polyhedra with the lowest number of facets that allows meeting the desired accuracy. The number of facets of a polyhedron model can be reduced using any mesh processing software, for example PyMeshLab. Additionally, different functions to process a polyhedron are available in Polyhedron utilities. Parameters:
gravitational_parameter (float) – Gravitational parameter \(\mu\) of gravity field.
vertices_coordinates (numpy.ndarray) – Cartesian coordinates of each polyhedron vertex. Entry (i,j) denotes vertex i, coordinate j (one row per vertex, 3 columns).
vertices_defining_each_facet (numpy.ndarray) – Index (0 based) of the vertices constituting each facet. Entry (i,j) denotes facet i, and the jth vertex of the facet (one row per facet, 3 columns). In each row, the vertices’ indices should be ordered counterclockwise when seen from the outside of the polyhedron.
associated_reference_frame (str) – Identifier for bodyfixed reference frame with which the spherical harmonics coefficients are associated.
gravitational_constant (float, default=GRAVITATIONAL_CONSTANT) – Newton’s gravitational constant G, used to computed the density
 Returns:
Instance of the
GravityFieldSettings
derivedPolyhedronGravityFieldSettings
class Return type:
 polyhedron_from_density(density: float, vertices_coordinates: numpy.ndarray[numpy.float64[m, n]], vertices_defining_each_facet: numpy.ndarray[numpy.int32[m, n]], associated_reference_frame: str, gravitational_constant: float = 6.67259e11) tudatpy.kernel.numerical_simulation.environment_setup.gravity_field.GravityFieldSettings #
Factory function for creating a polyhedron gravity field settings object, using the density.
Factory function for settings object, defining a gravity field model through a polyhedron. The
associated_reference_frame
must be the same frame ID as the target frame of the body’s rotation model. It represents the frame in which the polyhedron field is defined.The gravitational potential, acceleration, Laplacian of potential and Hessian of potential are computed according to Werner and Scheeres [2].
This function uses the density to define the gravity field. To instead use the gravitational parameter see
polyhedron_from_mu()
. Parameters:
density (float, default=TUDAT_NAN) – Density of the polyhedron.
vertices_coordinates (numpy.ndarray) – Cartesian coordinates of each polyhedron vertex. Entry (i,j) denotes vertex i, coordinate j (one row per vertex, 3 columns).
vertices_defining_each_facet (numpy.ndarray) – Index (0 based) of the vertices constituting each facet. Entry (i,j) denotes facet i, and the jth vertex of the facet (one row per facet, 3 columns). In each row, the vertices’ indices should be ordered counterclockwise when seen from the outside of the polyhedron.
associated_reference_frame (str) – Identifier for bodyfixed reference frame with which the spherical harmonics coefficients are associated.
gravitational_constant (float, default=GRAVITATIONAL_CONSTANT) – Newton’s gravitational constant G, used to computed the gravitational parameter
 Returns:
Instance of the
GravityFieldSettings
derivedPolyhedronGravityFieldSettings
class Return type:
Enumerations#
Enumeration of gravity field types. 

Enumeration of predefined spherical harmonics models. 
 class GravityFieldType#
Enumeration of gravity field types.
Enumeration of gravity field types supported by tudat.
Members:
central_gravity :
central_spice_gravity :
spherical_harmonic_gravity :
polyhedron_gravity :
ring_gravity : No documentation found.
 property name#
 class PredefinedSphericalHarmonicsModel#
Enumeration of predefined spherical harmonics models.
Enumeration of predefined spherical harmonics models supported by tudat, for which thee coefficient files are automatically available (downloaded from here). The directory where these files are stored can be extracted using the
get_gravity_models_path()
function.Members:
egm96 :
Coefficients for EGM96 Earth gravity field up to degree and order 200, (see link )
ggm02c :
Coefficients for the combined GGM02 Earth gravity field up to degree and order 200, (see link )
ggm02s :
Coefficients for the GRACEonly GGM02 Earth gravity field up to degree and order 160, (see link )
goco05c :
Coefficients for the GOCO05c combined Earth gravity field up to degree and order 719, (see link )
glgm3150 :
Coefficients for the GLGM3150 Moon gravity field up to degree and order 150, (see link )
lpe200 :
Coefficients for the LPE200 Moon gravity field up to degree and order 200, (see link )
gggrx1200 : No documentation found.
jgmro120d :
Coefficients for the MRO120D Moon gravity field up to degree and order 120, (see link )
jgmess160a :
Coefficients for the MESS160A Moon gravity field up to degree and order 160, (see link )
shgj180u :
Coefficients for the SHGJ180U Moon gravity field up to degree and order 180, (see link )
 property name#
Classes#
Base class for providing settings for automatic gravity field model creation. 

GravityFieldSettings derived class defining settings of point mass gravity field. 

GravityFieldSettings derived class defining settings of spherical harmonic gravity field representation. 

GravityFieldSettings derived class defining settings of a polyhedron gravity field representation. 
 class GravityFieldSettings#
Base class for providing settings for automatic gravity field model creation.
This class is a functional base class for settings of gravity field models that require no information in addition to their type. Gravity field model classes requiring additional information must be created using an object derived from this class.
 property gravity_field_type#
readonly
Type of gravity field model that is to be created.
 Type:
 class CentralGravityFieldSettings#
GravityFieldSettings derived class defining settings of point mass gravity field.
Derived class of GravityFieldSettings for central gravity fields, which are defined by a single gravitational parameter.
 class SphericalHarmonicsGravityFieldSettings#
GravityFieldSettings derived class defining settings of spherical harmonic gravity field representation.
Derived class of GravityFieldSettings for gravity fields, which are defined by a spherical harmonic gravity field representation.
 property associated_reference_frame#
Identifier for bodyfixed reference frame with which the coefficients are associated.
 Type:
 property create_time_dependent_field#
Boolean that denotes whether the field should be created as timedependent (even if no variations are imposed initially).
 Type:
 property normalized_cosine_coefficients#
Cosine spherical harmonic coefficients (geodesy normalized). Entry (i,j) denotes coefficient at degree i and order j.
 Type:
 property normalized_sine_coefficients#
Sine spherical harmonic coefficients (geodesy normalized). Entry (i,j) denotes coefficient at degree i and order j.
 Type:
 property reference_radius#
readonly
Reference radius of spherical harmonic field expansion.
 Type:
 class PolyhedronGravityFieldSettings#
GravityFieldSettings derived class defining settings of a polyhedron gravity field representation.
Derived class of GravityFieldSettings for gravity fields, which are defined by a polyhedron gravity field representation.
 property associated_reference_frame#
Identifier for bodyfixed reference frame with which the vertices coordinates are associated.
 Type:
 property vertices_coordinates#
Cartesian coordinates of each polyhedron vertex. Entry (i,j) denotes vertex i, coordinate j (one row per vertex, 3 columns).
 Type:
 property vertices_defining_each_facet#
Index (0 based) of the vertices constituting each facet. Entry (i,j) denotes facet i, and the jth vertex of the facet (one row per facet, 3 columns). In each row, the vertices’ indices should be ordered counterclockwise when seen from the outside of the polyhedron.
 Type: