5.4. Propagator Settings: Dependent Variables

By default, the DynamicsSimulator propagates the state of a body which uniquely defines its position and velocity. However, it is possible to define a number of dependent variables that are derived from such state. These dependent variables can be used as termination conditions as discussed in Propagator Settings: Termination Settings or saved for further post-processing. This page describes the dependent variables currently available and how these are retrieved from Tudat.

5.4.1. Saving dependent variables

The dependent variables are computed during the body propagation, thus the user must provide a list of dependent variables to save prior creating the DynamicsSimulator:

// Define list of dependent variables to save.
std::vector< boost::shared_ptr< SingleDependentVariableSaveSettings > > dependentVariablesList;

where dependentVariableList is populated as follows:

dependentVariablesList.push_back(
          boost::make_shared< SingleDependentVariableSaveSettings >( variableType , associatedBody , secondaryBody ) );

where:

• SingleDependentVariableSaveSettings

  Defines the derived class being used and must match with variableType.

• variableType

  Indicates the variable type and must match with one of the enum values in PropagationDependentVariables. A detailed description of how this is done is given in the next section.

• associatedBody

  Indicates to which body the saved dependent variables are associated.

• secondaryBody

  Optional argument that provides a secondary body that may be necessary to save the dependent variable. By default, this argument is empty.

Once the list of dependent variables to save has been populated, a DependentVariableSaveSettings object needs to be created and passed to PropagatorSettings:

// Create object with list of dependent variables
boost::shared_ptr< DependentVariableSaveSettings > dependentVariablesToSave =
      boost::make_shared< DependentVariableSaveSettings >( dependentVariablesList );

// Create propagation settings.
boost::shared_ptr< TranslationalStatePropagatorSettings< double > > propagatorSettings =
      boost::make_shared< TranslationalStatePropagatorSettings< double > >
      ( centralBodies, accelerationModelMap, bodiesToPropagate, systemInitialState,
        terminationSettings, propagator, dependentVariablesToSave );

Note

In the example above, the TranslationalStatePropagatorSettings derived class is used. Please note that any of the derived classes described in Propagator Settings: Basics can be used, as long as these support dependent variable saving.

5.4.2. Available dependent variables

The framework discussed in the previous section explains how the dependentVariablesList is populated and passed to the PropagatorSettings. The goal of this section is to list the available dependent variables and to explain how these are pushed to the dependentVariablesList.

class SingleDependentVariableSaveSettings

This base-class is a generic method to retrieve a large number of dependent variables that are not classified under a particular group. Variables are saved to the dependentVariablesList using the following code:

SingleDependentVariableSaveSettings( variableType, associatedBody, secondaryBody )

where:

• variableType

  PropagationDependentVariables variable that can take the following values:

  Variables returning a double

  • mach_number_dependent_variable
  • altitude_dependent_variable
  • airspeed_dependent_variable
  • local_density_dependent_variable
  • relative_speed_dependent_variable
  • relative_distance_dependent_variable
  • radiation_pressure_dependent_variable
  • total_aerodynamic_g_load_variable
  • stagnation_point_heat_flux_dependent_variable
  • local_temperature_dependent_variable
  • geodetic_latitude_dependent_variable
  • control_surface_deflection_dependent_variable
  • total_mass_rate_dependent_variables
  • periapsis_altitude_dependent_variable
  • total_torque_norm_dependent_variable

  Variables returning an Eigen::VectorXd

  • relative_position_dependent_variable
  • relative_velocity_dependent_variable
  • body_fixed_airspeed_based_velocity_variable
  • total_acceleration_norm_dependent_variable
  • total_acceleration_dependent_variable
  • aerodynamic_force_coefficients_dependent_variable
  • aerodynamic_moment_coefficients_dependent_variable
  • lvlh_to_inertial_frame_rotation_dependent_variable
  • rotation_matrix_to_body_fixed_frame_variable
  • total_torque_dependent_variable
  • single_torque_dependent_variable
  • single_torque_norm_dependent_variable
  • body_fixed_groundspeed_based_velocity_variable

class SingleAccelerationDependentVariableSaveSettings

This derived class is used to retrieve acceleration-related dependent variables. A large number of acceleration models are supported and both the acceleration-norm and the acceleration-vector can be saved. Variables are saved to the dependentVariablesList using the following code:

SingleAccelerationDependentVariableSaveSettings(
          accelerationModeType,
          bodyUndergoingAcceleration,
          bodyExertingAcceleration,
          useNorm )

where:

• accelerationModeType

  AvailableAcceleration variable that defines the type of acceleration that must be retrieved. It can take the following values:

  • undefined_acceleration
  • central_gravity
  • aerodynamic
  • cannon_ball_radiation_pressure
  • spherical_harmonic_gravity
  • mutual_spherical_harmonic_gravity
  • third_body_central_gravity
  • third_body_spherical_harmonic_gravity
  • third_body_mutual_spherical_harmonic_gravity
  • thrust_acceleration

• bodyUndergoingAcceleration

  std::string variable that indicates the body that experiences the acceleration that needs to be retrieved. Make sure that the body’s name is listed in BodyMap.

• bodyExertingAcceleration

  std::string variable that indicates the body that exerts the acceleration that needs to be retrieved on bodyUndergoingAcceleration. Make sure that the body’s name is listed in BodyMap.

• useNorm

  bool variable that indicates if the norm of the acceleration (true) or the acceleration vector (false) must be retrieved.

Warning

Make sure that the selected bodyExertingAcceleration is compatible with the accelerationModeType.

class IntermediateAerodynamicRotationVariableSaveSettings

This derived class is used to retrieve the rotation matrix between two desired frames. Variables are saved to the dependentVariablesList using the following code:

IntermediateAerodynamicRotationVariableSaveSettings(
              associatedBody, baseFrame, targetFrame )

where:

• associatedBody

  std::string variable that indicates the body for which a rotation matrix is to be saved. Make sure that the body’s name is listed in BodyMap.

• baseFrame

  AerodynamicsReferenceFrames variable indicates the frame from which the rotation is to be saved. The following frames are available:

  • inertial_frame
  • corotating_frame
  • vertical_frame
  • trajectory_frame
  • aerodynamic_frame
  • body_frame

• targetFrame

  AerodynamicsReferenceFrames variable indicates the frame to which the rotation is to be saved. The available frames are listed above.

class BodyAerodynamicAngleVariableSaveSettings

This derived class is used to retrieve a number of rotation angles. Variables are saved to the dependentVariablesList using the following code:

BodyAerodynamicAngleVariableSaveSettings(
              associatedBody, angle )

where:

• associatedBody

  std::string variable that indicates the body for which the angle is to be saved. Make sure that the body’s name is listed in BodyMap.

• angle

  AerodynamicsReferenceFrameAngles variable that provides the angle to be saved. The following angles can be saved using this method:

  • latitude_angle
  • longitude_angle
  • heading_angle
  • flight_path_angle
  • angle_of_attack
  • angle_of_sideslip
  • bank_angle