Printing and processing the results

The results of the numerical propagation are stored in the propagation results, which can be used to perform further analysis, and retrieve the details of the propagation process. In addition, there are a number of processing steps and terminal outputs that Tudat can perform before, during and after the propagation. Although such settings can be provided directly when creating propagator settings (for instance, when calling the translational() function), it is adviced to modify these settings afterwards:

propagator_settings = propagator.translational( ... )
console_print_settings = propagator_settings.print_settings
post_processing_settings = propagator_settings.processing_settings

Here, the resulting settings object for console printing and post-processing are of type PropagationPrintSettings and PropagatorProcessingSettings (with derived class SingleArcPropagatorProcessingSettings for single-arc propagation), respectively. Below, the options available in both these settings objects are described in more detail.

Automatic processing

By default, Tudat will save the propagated states and dependent variables at each time step, and return these the user after the propagation is finished. Below, a number of additions/extensions to this behaviour are described, where the propagated states that are returned are used and/or processed before being returned to the user.

Updating ephemerides (and other environment models)

Tudat can be set up to automatically use the results of the numerical propagation to reset properties in the environment upon the successful completion of the propagation. This option can be toggled using the boolean attribute set_integrated_result of the PropagatorProcessingSettings class:

propagator_settings = propagator.translational( ... )
propagator_settings.processing_settings.set_integrated_result = True

Specifically, this will result in:

  • Translational dynamics: use the numerical results to reset the ephemeris of the body with (using the default) a 6-th order Lagrange interpolation scheme to create a tabulated ephemeris

  • Rotational dynamics: use the numerical results to reset the rotational ephemeris of the body with (using the default) a 6-th order Lagrange interpolation scheme to create a tabulated rotation model

  • Mass dynamics: use the numerical results to reset the mass function of the body with (using the default) a 6-th order Lagrange interpolation scheme

  • Multi-type dynamics: automatically processes all of the constituent dynamics as listed above

  • Custom dynamics: no action

To be able to perform the reset of the ephemeris (or other environment model), the existing ephemeris must be of the suitable type. This means that it must either be a tabulated ephemeris already (in which case the tabulation is reset) or the body must contain no ephemeris (in which case one is created on the fly when creating the dynamics simulator.

In cases where a user wants to use a non-tabulated ephemeris for a body, but still use the functionality described here to reset the ephemeris later on, the ephemeris type of the body can be redefined using tabulated_from_existing() when defining the body settings. In essence, this converts any ephemeris into a tabulated ephemeris, where the tabulated ephemeris is populated by states from the original (non-tabulated) ephemeris.

For specific applications, most notably a state estimation, a user may want the numerical solution to only be used to reset the environment, while not needing access to the numerical results directly. To enable this behavior, the boolean attribute clear_solution of the PropagatorProcessingSettings class is provided.

propagator_settings = propagator.translational( ... )
propagator_settings.processing_settings.set_integrated_result = True
propagator_settings.processing_settings.clear_solution = True

When set to true, the numerical results of the propagation are completely deleted after the propagation is performed. When this option is selected, the numerical results ‘live on’ only in the environment models that have been reset, but are no longer available from the propagation results. This option may be attractive when memory usage of the application is a concern.

Reduced saving cadence

By definition, Tudat saves and returns the state and dependent variables at every full step of the numerical integration. For long numerical integrations, this can result in excessively long data structures being stored in memory, potentially leading to issues. Options are provided to modify the cadence at which data is saved, using the results_save_frequency_in_steps and results_save_frequency_in_seconds attributes of the PropagatorProcessingSettings class, which allow the results to be saved only every X steps, or every Y seconds (of time in the simulation). For instance, using:

propagator_settings = propagator.translational( ... )
propagator_settings.processing_settings.results_save_frequency_in_steps = 3

The results are only saved every third time step. By using

propagator_settings = propagator.translational( ... )
propagator_settings.processing_settings.results_save_frequency_in_steps = 3
propagator_settings.processing_settings.results_save_frequency_in_seconds = 60.0

the results are saved every third time step or every 60 seconds of time in the simulation, whichever one occurs first since the previous saved data point.

Multi- and hybrid-arc considerations

For the multi- and hybrid arc propagation, the setting of the numerical results in the environment, and the clearing of the numerical solution (as described above, is always consistent between all the arcs. As a result, these settings in the constituent single-arc propagation settings will be overridden by the settings in the multi- or hybrid-arc propagation settings. Objects of type MultiArcPropagatorProcessingSettings or HybridArcPropagatorProcessingSettings are automatically created and stored in the propagator settings when creating multi- or hybrid-arc propagator settings, and can be retrieved similarly as for the single-arc settings:

multiarc_propagator_settings = propagator.multi_arc( ... )
post_processing_settings = propagator_settings.processing_settings

To reset the dynamics of a body with the results of a multi-arc propagation (e.g. if the set_integrated_result option is set to true), the ephemeris of this body must be a multi-arc ephemeris. If the body has no ephemeris before propagation, one is created on the fly when creating the dynamics simulator. In cases where a user wants to use a single-arc ephemeris for a body, but still use the functionality described here to reset the ephemeris from multi-arc results later on, the ephemeris type can be forced to multi-arc by using the make_multi_arc_ephemeris attribute of the EphemerisSettings when defining the body settings. For example, to reset the ephemeris of the Earth from a multi-arc propagation result, the following can be used to permit this:

# Create body settings
body_settings = environment_setup.get_default_body_settings( ... )
body_settings.get("Earth").ephemeris_settings.make_multi_arc_ephemeris = True

The cadence at which data is saved during the propagation (see above) may vary per arc. The list of single-arc settings can be retrieved from the results as follows:

multiarc_propagator_settings = propagator.multi_arc( ... )
single_arc_processing_settings = multiarc_propagator_settings.single_arc_settings

Settings to print data to the console (see below) is also defined seperately per arc. Additional options for multi- and hybrid-arc propagation are provided below

Console Output

Tudat also provides a range of options on information to be printed to the console during the process of the propagation. These settings are specified through a PropagationPrintSettings object, which can be retried from single-arc propagator settings through:

propagator_settings = propagator.translational( ... )
console_print_settings = propagator_settings.print_settings

A full list of print options is provide in the API documentation. Typical examples of information that can be printed to the console are:

  • The indices in the full dependent variable vector (print_dependent_variable_indices; see Dependent Variables) where each separate dependent variable is stored, with a brief text description of the associated dependent variable (printed before the propagation starts)

  • The current time and state can be printed during the propagation (state_print_interval), at a simulation time interval specified by the user

  • Total runtime, number of function evaluations of the state derivative, and the reason for the termination of the propagation (printed after the propagation is finished; see print_propagation_clock_time, print_number_of_function_evaluations and print_termination_reason)

In most cases, the separate print settings (as attributes of the PropagationPrintSettings class) are defined by a boolean (print this information: yes or no). For specific cases, such as the interval at which information should be printed to the console during a propagation, are to be provided as a floating point value. To enable all console printing that can be defined by a boolean, the enable_all_printing() function can be used. To disable all console printing, us the disable_all_printing() function.

An example of defining console output is:

propagator_settings = propagator.translational( ... )
console_print_settings = propagator_settings.print_settings
console_print_settings.print_state_indices = True
console_print_settings.print_dependent_variable_indices = True
console_print_settings.print_propagation_clock_time = True
console_print_settings.print_termination_reason = True
console_print_settings.print_number_of_function_evaluations = True

which will result in the following terminal output (for a specific script propagating dynamics of Delfi C-3 w.r.t. Earth):

===============  STARTING SINGLE-ARC PROPAGATION  ===============

[Vector entries], content description
[0:5], Translational state of body Delfi-C3 w.r.t. Earth

[Vector entries], content description
[0:2], Total acceleration in inertial frame of Delfi-C3
[3:8], Kepler elements of Delfi-C3 w.r.t. Earth

Total Number of Function Evaluations: 43201
Total propagation clock time: 2.94223 seconds
Termination reason: Propagation successful; termination condition exceeded


Multi- and hybrid-arc console output

For the multi- and hybrid arc simulations, the console output is specified in its constituent single-arc propagation settings where, in principle, these settings can be different for each arc, and are processed independently. However, a number of additional options are available for printing output to the console for multi- and hybrid-arc propagation, in the MultiArcPropagatorProcessingSettings and HybridArcPropagatorProcessingSettings classes:

  • For the multi- and hybrid arc propagation, there is an option to ensure identical print settings for each arc (see set_consistent_print_settings)

  • For the multi-arc propagation, there is an option to automatically suppress all output for all arcs except the first arc (see print_first_arc_only) This is typically used in cases where the settings for each arc are largely identical

  • For the hybrid-arc propagation, the constituent single- and multi-arc settings can be independently modified. These settings can be extracted from the single_arc_settings and multi_arc_settings attributes.