Tutorials 4.4 Propagator

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What is a propagator?

We must differentiate a numerical integrator and a numerical propagator ...

A numerical integrator is a low level block allowing to integrate differential equations, not only those linked to a trajectory. So, if you want to use this integrator level, you will have to define your own equations, specially the second term which can be complex when you have to consider a lot of perturbation terms in a trajectory extrapolation.

That is why PATRIUS proposes a higher level object allowing to propagate an orbit (and more generally a trajectory). This object already have internaly some differential kind of equations (cartesian form, equinoctial form, ...) and has some very powerful mechanism to add perturbation force models as we could see in further tutorials by using the addForceModel() method.

The basic philosophy is then:

  1. to create a numerical propagator giving it an associated numerical integrator
  2. to give it initial conditions via a SpacecraftState object (including at least an orbit but also possibly a mass model and an attitude).
  3. optionaly to specify which kind of equations and which frame to use (by default the ones consistent with the initial orbit definition).
  4. at last to call for the propagate() method up to a certain absolute date. It will return a new SpacecraftState object.

Code examples:

With a step handler

A step handler (here a fixed step handler) is the functionnality allowing to extract some data along the numerical propagation. This mechanism is done thanks to an internal interpolation and do not modify the precision of the numerical integration.

For example, if you have a RK4 integrator with a time integration step of 10s and if you want to extract data every 5s, it will not interfere on the 10s steps (meaning that it will not reduce the time integration step to 5s).

Code example: Numerical propagation with a fixed step handler

With a STOP event

Thanks to a "G-stop" functionnality included into numerical integrators, it is possible to add events to the propagator. This one will then automatically detect when the event occured and will execute the action associated with the event (a STOP action in the first example below). A large amount of predefined events are already available with PATRIUS. Here, the example will treat of an altitude event. The second example is more or less the same kind of example than the previous one but, in that case, we added a custom event. This is done using an internal class in order to get all in the same method but, practically, it is recommended to define this custom event in a specific class for a better readability.

Code examples:

With attitude laws

The way to consider a single attitude law is relatively easy as we have just to create an attitude law (in the code below, a LOF law) and call for the setAttitudeProvider() method. For more explanations about creating attitude laws, see specific tutorials.

To add an attitude sequence in a numerical propagator is just a bit more complex than just adding a single attitude law:

  • first, we have to define an attitude sequence. In the code below, we considered a sequence only with two switches and two laws but it can be actually more sophisticated. For more explanations about the way to build attitude sequences, see specific tutorials.
  • then, we must add the sequence to the propagator by using the setAttitudeProvider() method.
propagator.setAttitudeProvider(seqAtt);
  • at last, we must not forget to tell to the sequence that a propagator is "looking" at it by calling the registerSwitchEvents() method
seqAtt.registerSwitchEvents(propagator);

Code examples:

With a mass model

A mass model allows, within a propagation, to take into account mass variations (typically due to maneuvers). This is done thanks to the Assembly construction (see also specific tutorials on it). Anyway, when using such MassModel, it is mandatory :

  • to add it to the initial SpacecraftState
  • to feed the additional equations to the propagator by using the (setMassProviderEquation()) method.

Code example: Numerical propagation with a mass model

Using force models

If nothing specific is requested, a numerical propagator will only deal with a Keplerian motion using the central term of the potential got from the initial orbit. If we want to add other force models, we will have to use the addForceModel() method.

Using a potential field

In the first example below, we will give the way to use a ICGEM potential field with Droziner equations up to 8x8 degrees. Up to now, it is not possible simply to select a specific potential field if several ones are available. For such functionnality, we can use the utility method given at the end of this tutorial.

Code example: Numerical propagation with a specific potential field

Using a drag model

In the second example, we will take into account a drag model and have to do both things:

  • adding to the vehicle (i.e. the Assembly) the AeroSphereProperty
  • creating a force model including this aerodynamic drag model plus an atmospheric model and adding it to the propagator

Code example: Numerical propagation with a drag model

Using a drag and lift model

A bit like the previous example, we are going to see how to use a drag and a lift model. The way to do it is almost the same:

  • adding to the vehicle (i.e. the Assembly) the AeroGlobalProperty) (rather than the AeroSphereProperty)
  • creating a force model including this aerodynamic lift+drag model plus an atmospheric model and adding it to the propagator

Nevertheless, an another important point is mandatory: as we need to know which is the direction of the lift (perpendicular to the drag which is opposite to the relative velocity), we must add an attitude law to the Spacecraft as it is done below by using the setAttitudeProvider() method.

The only difference between both example is due to the change of the atmospheric model.

Code examples:

Using a SRP model

In this example, we will take into account a Solar Radiative Pressure (SRP) model and have to do both things:

Code example:

With maneuvers

Impulsive maneuvers

Here we consider we will just add impulsive maneuvers. A way to do it is to consider such maneuvers as events. So, we will use the addEventDetector() method.

Nevertheless, a very important point to consider is the fact that, due to a maneuver, the mass of the spacecraft will be modified: so, it is mandatory to define a SpacecraftState with a MassModel (see specific tutorial).

Code example: Numerical propagation with an impulsive maneuver

Continuous maneuvers

Here we will not use the principle of adding an event to the propagator but adding a force model, considering the thrust is a supplementary force. To do that, we will use the addForceModel() method. For more details in the way to define a continuous thrust maneuver, see the specific items of the user manuel, the JavaDoc or specific tutorials.

As for the previous tutorial about impulsive maneuvers, it is mandatory to define a SpacecraftState with a MassModel.

At last, if the thrust direction is defined in the vehicle frame, it is also mandatory to define an attitude law (or a sequence) to be able to know the actual direction of the thrust in the integration frame.

Code example: Numerical propagation with a continuous maneuver

Sequence of maneuvers

Here we will use a sequence of maneuvers. This sequence will include both impulsive and continuous maneuvers (see specific tutorials to know how to do it). The main point to know is the fact that, once the maneuver sequence has been built, we do not have to pass it to the propagator ... but we must pass the propagator to the sequence using the applyTo() method !

And, as for the two previous tutorials, do not forget to define a MassModel and eventually an attitude law ...

Code example: Numerical propagation with a sequence of maneuvers

Orbital increment maneuvers

Since V4.4, it is possible to define maneuvers with orbital increment as input:

  • on semi-major axis
  • on eccentricity (and semi-major axis eventually)
  • on inclination (and semi-major axis eventually)

In the Following example, we will propagate such maneuvers included in a sequence …

Code example: Numerical propagation with orbital increments

Used Velocity increment (DV)

Since V4.4, method can be used to follow the used velocity increment (ΔV) all along the trajectory.

Code example: Numerical propagation with used DV

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