Package org.orekit.attitudes

Class AttitudesSequence

java.lang.Object

org.orekit.attitudes.AttitudesSequence

All Implemented Interfaces:

Serializable, AttitudeProvider

public class AttitudesSequence
extends Object
implements AttitudeProvider

This classes manages a sequence of different attitude providers that are activated in turn according to switching events.

Only one attitude provider in the sequence is in an active state. When one of the switch event associated with the active provider occurs, the active provider becomes the one specified with the event. A simple example is a provider for the sun lighted part of the orbit and another provider for the eclipse time. When the sun lighted provider is active, the eclipse entry event is checked and when it occurs the eclipse provider is activated. When the eclipse provider is active, the eclipse exit event is checked and when it occurs the sun lighted provider is activated again. This sequence is a simple loop.

An active attitude provider may have several switch events and next provider settings, leading to different activation patterns depending on which events are triggered first. An example of this feature is handling switches to safe mode if some contingency condition is met, in addition to the nominal switches that correspond to proper operations. Another example is handling of maneuver mode.

Note that this attitude provider is stateful, it keeps in memory the sequence of active underlying providers with their switch dates and the transitions from one provider to the other. This implies that this provider should not be shared among different propagators at the same time, each propagator should use its own instance of this provider.

The sequence kept in memory is reset when resetActiveProvider(AttitudeProvider) is called, and only the specify provider is kept. The sequence is also partially reset each time a propagation starts. If a new propagation is started after a first propagation has been run, all the already computed switches that occur after propagation start for forward propagation or before propagation start for backward propagation will be erased. New switches will be computed and applied properly according to the new propagation settings. The already computed switches that are not in covered are kept in memory. This implies that if a propagation is interrupted and restarted in the same direction, then attitude switches will remain in place, ensuring that even if the interruption occurred in the middle of an attitude transition the second propagation will properly complete the transition that was started by the first propagator.

Since:

5.1

Author:

Luc Maisonobe

See Also:

Serialized Form

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static interface	AttitudesSequence.SwitchHandler	Interface for attitude switch notifications.

Constructor Summary

Constructors

Constructor	Description
AttitudesSequence()	Constructor for an initially empty sequence.

Method Summary

Modifier and Type	Method	Description
<T extends EventDetector> void	addSwitchingCondition(AttitudeProvider past, AttitudeProvider future, T switchEvent, boolean switchOnIncrease, boolean switchOnDecrease, double transitionTime, AngularDerivativesFilter transitionFilter, AttitudesSequence.SwitchHandler handler)	Add a switching condition between two attitude providers.
<T extends RealFieldElement<T>> FieldAttitude<T>	getAttitude(FieldPVCoordinatesProvider<T> pvProv, FieldAbsoluteDate<T> date, Frame frame)	Compute the attitude corresponding to an orbital state.
Attitude	getAttitude(PVCoordinatesProvider pvProv, AbsoluteDate date, Frame frame)	Compute the attitude corresponding to an orbital state.
<T extends RealFieldElement<T>> void	registerSwitchEvents(Field<T> field, FieldPropagator<T> propagator)	Register all wrapped switch events to the propagator.
void	registerSwitchEvents(Propagator propagator)	Register all wrapped switch events to the propagator.
void	resetActiveProvider(AttitudeProvider provider)	Reset the active provider.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

AttitudesSequence

public AttitudesSequence()

Constructor for an initially empty sequence.

Method Detail

resetActiveProvider

public void resetActiveProvider(AttitudeProvider provider)

Reset the active provider.

Calling this method clears all already seen switch history, so it should not be used during the propagation itself, it is intended to be used only at start

Parameters:

provider - provider to activate

registerSwitchEvents

public void registerSwitchEvents(Propagator propagator)

Register all wrapped switch events to the propagator.

This method must be called once before propagation, after the switching conditions have been set up by calls to addSwitchingCondition.

Parameters:

propagator - propagator that will handle the events

registerSwitchEvents

public <T extends RealFieldElement<T>> void registerSwitchEvents(Field<T> field,
                                                                 FieldPropagator<T> propagator)

Register all wrapped switch events to the propagator.

This method must be called once before propagation, after the switching conditions have been set up by calls to addSwitchingCondition.

Type Parameters:

T - type of the field elements

Parameters:

field - field to which the elements belong

propagator - propagator that will handle the events

addSwitchingCondition

public <T extends EventDetector> void addSwitchingCondition(AttitudeProvider past,
                                                            AttitudeProvider future,
                                                            T switchEvent,
                                                            boolean switchOnIncrease,
                                                            boolean switchOnDecrease,
                                                            double transitionTime,
                                                            AngularDerivativesFilter transitionFilter,
                                                            AttitudesSequence.SwitchHandler handler)
                                                     throws OrekitException

Add a switching condition between two attitude providers.

The past and future attitude providers are defined with regard to the natural flow of time. This means that if the propagation is forward, the propagator will switch from past provider to future provider at event occurrence, but if the propagation is backward, the propagator will switch from future provider to past provider at event occurrence. The transition between the two attitude laws is not instantaneous, the switch event defines the start of the transition (i.e. when leaving the past attitude law and entering the interpolated transition law). The end of the transition (i.e. when leaving the interpolating transition law and entering the future attitude law) occurs at switch time plus transitionTime.

An attitude provider may have several different switch events associated to it. Depending on which event is triggered, the appropriate provider is switched to.

The switch events specified here must not be registered to the propagator directly. The proper way to register these events is to call registerSwitchEvents(Propagator) once after all switching conditions have been set up. The reason for this is that the events will be wrapped before being registered.

If the underlying detector has an event handler associated to it, this handler will be triggered (i.e. its eventOccurred method will be called), regardless of the event really triggering an attitude switch or not. As an example, if an eclipse detector is used to switch from day to night attitude mode when entering eclipse, with switchOnIncrease set to false and switchOnDecrease set to true. Then a handler set directly at eclipse detector level would be triggered at both eclipse entry and eclipse exit, but attitude switch would occur only at eclipse entry. Note that for the sake of symmetry, the transition start and end dates should match for both forward and backward propagation. This implies that for backward propagation, we have to compensate for the transitionTime when looking for the event. An unfortunate consequence is that the eventOccurred method may appear to be called out of sync with respect to the propagation (it will be called when propagator reaches transition end, despite it refers to transition start, as per transitionTime compensation), and if the method returns EventHandler.Action.STOP, it will stop at the end of the transition instead of at the start. For these reasons, it is not recommended to set up an event handler for events that are used to switch attitude. If an event handler is needed for other purposes, a second handler should be registered to the propagator rather than relying on the side effects of attitude switches.

The smoothness of the transition between past and future attitude laws can be tuned using the transitionTime and transitionFilter parameters. The transitionTime parameter specifies how much time is spent to switch from one law to the other law. It should be larger than the event convergence threshold in order to ensure attitude continuity. The transitionFilter parameter specifies the attitude time derivatives that should match at the boundaries between past attitude law and transition law on one side, and between transition law and future law on the other side. AngularDerivativesFilter.USE_R means only the rotation should be identical, AngularDerivativesFilter.USE_RR means both rotation and rotation rate should be identical, AngularDerivativesFilter.USE_RRA means both rotation, rotation rate and rotation acceleration should be identical. During the transition, the attitude law is computed by interpolating between past attitude law at switch time and future attitude law at current intermediate time.

Type Parameters:

T - class type for the switch event

Parameters:

past - attitude provider applicable for times in the switch event occurrence past

future - attitude provider applicable for times in the switch event occurrence future

switchEvent - event triggering the attitude providers switch

switchOnIncrease - if true, switch is triggered on increasing event

switchOnDecrease - if true, switch is triggered on decreasing event

transitionTime - duration of the transition between the past and future attitude laws

transitionFilter - specification of transition law time derivatives that should match past and future attitude laws

handler - handler to call for notifying when switch occurs (may be null)

Throws:

OrekitException - if transition time is shorter than event convergence threshold

Since:

7.1

getAttitude

public Attitude getAttitude(PVCoordinatesProvider pvProv,
                            AbsoluteDate date,
                            Frame frame)
                     throws OrekitException

Compute the attitude corresponding to an orbital state.

Specified by:

getAttitude in interface AttitudeProvider

Parameters:

pvProv - local position-velocity provider around current date

date - current date

frame - reference frame from which attitude is computed

Returns:

attitude attitude on the specified date and position-velocity state

Throws:

OrekitException - if attitude cannot be computed

getAttitude

public <T extends RealFieldElement<T>> FieldAttitude<T> getAttitude(FieldPVCoordinatesProvider<T> pvProv,
                                                                    FieldAbsoluteDate<T> date,
                                                                    Frame frame)
                                                             throws OrekitException

Compute the attitude corresponding to an orbital state.

Specified by:

getAttitude in interface AttitudeProvider

Type Parameters:

T - type of the field elements

Parameters:

pvProv - local position-velocity provider around current date

date - current date

frame - reference frame from which attitude is computed

Returns:

attitude attitude on the specified date and position-velocity state

Throws:

OrekitException - if attitude cannot be computed