1.3. The Environment During Propagation

Each Body object and its constituent members is updated to the current state and time automatically during the numerical propagation. We stress that only those models that are relevant for a given propagation are updated every time step (this is handled automatically, without user intervention). Some time-dependent properties of the body are set in the environment models themselves. Others are updated and stored directly in the Body object. Below is a full list of (possibly) time varying environment models, and how to retrieve them from a Body object during propagation.

The current translational state

Retrieved direct from a Body object directly, with the getState function, as Cartesian elements. Note that this state is always in the global frame origin and orientation (note that separate getPosition and getVelocity functions are also available.

The current rotational state

Retrieved direct from a Body object directly. The current orientation is defined by the body’s rotation model, and is retrieved as a quaternion. To get the quaternion to transform a vector from inertial (e.g. with global frame orientation) to the body-fixed frame, the getCurrentRotationToLocalFrame can be used. The inverse rotation can be obtained from getCurrentRotationToGlobalFrame. The time-derivative of the orientation is provided in two formulations (with equivalent information content): the angular velocity vector, and the time derivative of the rotation matrix. The angular velocity vector, in inertial and body-fixed coordinates, is obtained from the getCurrentAngularVelocityVectorInGlobalFrame and getCurrentAngularVelocityVectorInLocalFrame functions respectively. The time-derivative of the rotation matrix from inertial to body-fixed frame is given by getCurrentRotationMatrixDerivativeToLocalFrame, while the derivative of the inverse rotation is taken from getCurrentRotationMatrixDerivativeToGlobalFrame

The current body mass

Retrieved direct from a Body object directly, with the getBodyMass function.

Spherical harmonic gravity field coefficients

These coefficients may be time variable (set by using one or more GravityFieldVariationSettings). The cosine and sine coefficients can be retrieved from a Body object through its gravity field model. A piece of example code on retrieving these coefficients is given below for the case of Earth:

NamedBodyMap bodyMap = ....

std::shared_ptr< SphericalHarmonicsGravityField > sphericalHarmonicsGravityField =
    std::dynamic_pointer_cast< SphericalHarmonicsGravityField >( bodyMap.at( "Earth" ) );

if( sphericalHarmonicsGravityField == nullptr )
{
    throw std::runtime_error( "Error when retrieving spherical harmonic coefficients: Earth does not have a SphericalHarmonicsGravityField" );
}

Eigen::MatrixXd cosineCoefficients = sphericalHarmonicsGravityField->getCosineCoefficients( );
Eigen::MatrixXd sineCoefficients = sphericalHarmonicsGravityField->getSineCoefficients( );

Note the check to see if the dynamic_pointer_cast was succesfull. These checks are very helpful in ascertaining the reason for a crashing program.

Curent flight conditions

The FlightConditions class, and its derived class AtmosphericFlightConditions stores data relating to altitude, flight angles, atmospheric properties, etc. Follow the links for their detailed description. The FlightConditions base class pointer is obtained from the getFlightConditions function of a Body object.

Note

As a user, you will typically not access these variables directly. Important examples of cases where users can explicitly access them, are custom aerodynamic or thrust guidance.