3.4. Propagator Settings: Termination Settings

The PropagationTerminationSettings are a key parameter in the propagation of a body’s orbit, since these will determine the computational time and the size of the output file. Depending on the application, the user may want to end the body propagation according to different criteria. Currently, the following PropagationTerminationSettings are offered in Tudat:

  • Termination once a certain time has passed.
  • Termination once a certain CPU time is reached.
  • Termination once a dependent variable meets a certain criterion.
  • Termination once a user-defined function returns true.
  • Termination once multiple criteria are met.

The different types of PropagationTerminationSettings are implemented by means of derived classes and custom constructors, as detailed in this page.

class PropagationTerminationSettings

Base class for the termination settings of the propagation. Different options are implemented in the derived classes below.

class PropagationTimeTerminationSettings

As the name suggests, these settings will cause the propagation to terminate after a certain time has passed. Please note that the simulator will finish the final time-step, which may cause the termination time to be slightly overpassed. The constructor is:

PropagationTimeTerminationSettings( terminationTime, terminateExactlyOnFinalCondition )

where:

  • terminationTime

    double that dictates the time that must pass before the simulation is terminated.

  • terminateExactlyOnFinalCondition

    bool that determines if the integrator will either stop exactly on the final condition (true), or if the integrator will terminate on the first step where it is violated (false), which is the default value.

class PropagationCPUTimeTerminationSettings

You may want to make sure that the propagation does not exceed a certain computation time. In this case, you can easily set the termination settings as follows:

PropagationCPUTimeTerminationSettings( cpuTerminationTime )

where cpuTerminationTime is the maximum allowed CPU time, after which propagation should be stopped.

class PropagationDependentVariableTerminationSettings

A more powerful method is the use of a DependentVariable to terminate a propagation. Using this class will terminate the propagation once the chosen dependent variable meets a certain criterion. To do so, the user needs to first create the dependent variable that will dictate the termination of the propagation:

std::shared_ptr< SingleDependentVariableSaveSettings > terminationDependentVariable =
      std::make_shared< SingleDependentVariableSaveSettings >(
          altitude_dependent_variable, "Vehicle", "Earth" );

where in this example the variable created is an altitude_dependent_variable which provides the altitude of Vehicle with respect to Earth.

The versatility in the use of PropagationDependentVariableTerminationSettings to terminate a simulation lies in the large number of dependent variable types available in Tudat. The user is referred to Propagator Settings: Dependent Variables for a full list of the available dependent variables as well as their use.

The constructor for this derived class is:

PropagationDependentVariableTerminationSettings( terminationDependentVariable,
                                                 limitValue,
                                                 useAsLowerLimit,
                                                 terminateExactlyOnFinalCondition )

where:

  • terminationDependentVariable

    std::shared_ptr< SingleDependentVariableSaveSettings > contains the dependent variable used for termination, in this example: altitude_dependent_variable as declared above.

  • limitValue

    double that provides the lower-limit (or higher-limit) that the terminationDependentVariable can take before terminating the propagation.

  • useAsLowerLimit

    bool that dictates whether the limitValue is used as a lower-limit (true) or as a higher-limit (false).

  • terminateExactlyOnFinalCondition

    bool that determines if the integrator will either stop exactly on the final condition (true), or if the integrator will terminate on the first step where it is violated (false), which is the default value.

class PropagationCustomTerminationSettings

With this class, you can set a custom function that based on some internal calculations will return whether to stop propagation. The function should take the current time as input (hence a double) and output a boolean (i.e., bool). This boolean should be true when the propagation has to be stopped and false otherwise. The constructor looks like this:

PropagationCustomTerminationSettings( checkStopCondition )

where checkStopCondition is the only input and is of type std::function< bool( const double ) >.

Tip

In case your custom function requires more inputs (e.g., it may depend on the position of the spacecraft or other variables that are not the current time), you can use std::bind to add more inputs.

As an example, the case where the state of the spacecraft is added as an input is shown below:

std::function< Eigen::Vector6d( ) > spacecraftStateFunction =
        std::bind( &Body::getState, bodyMap.at( "Satellite" ) );
std::shared_ptr< PropagationTerminationSettings > terminationSettings =
        std::make_shared< PropagationCustomTerminationSettings >(
            std::bind( &customTerminationFunction, std::placeholders::_1, spacecraftStateFunction ) );
class PropagationHybridTerminationSettings

It may be possible that the user desires to terminate a propagation according several criteria, where such criteria may or may not be fulfilled simulataneously. The constructor for this derived class is:

PropagationHybridTerminationSettings( terminationSettingsList,
                                      fulFillSingleCondition,
                                      terminateExactlyOnFinalCondition )

where:

  • terminationSettingsList

    std::vector< std::shared_ptr< PropagationTerminationSettings > > where each of its elements contains derived classes of PropagationTerminationSettings. The desired PropagationTerminationSettings can be added by using the push_back method of std::vector, where the pushed elements are objects of the classes discussed above.

  • fulFillSingleCondition

    bool that determines whether the propagation terminates once a single condition is met (true) or whether all conditions must be met (false).

  • terminateExactlyOnFinalCondition

    bool that determines if the integrator will either stop exactly on the final condition (true), or if the integrator will terminate on the first step where it is violated (false), which is the default value.

Note

For both PropagationCPUTimeTerminationSettings and PropagationCustomTerminationSettings the termination cannot be set to occur exactly on the final condition.