This tutorial emphasizes a specific usage of the attitude package described in the attitudes section of the library architecture documentation.
AttitudesSequence enables easy switching between attitude laws on event occurrences when propagating some SpacecraftState.
Let’s set up an initial state as:
The initial orbit is here defined as a KeplerianOrbit.
AbsoluteDate initialDate = new AbsoluteDate(2004, 01, 01, 23, 30, 00.000, TimeScalesFactory.getUTC());
Vector3D position = new Vector3D(-6142438.668, 3492467.560, -25767.25680);
Vector3D velocity = new Vector3D(505.8479685, 942.7809215, 7435.922231);
Orbit initialOrbit = new KeplerianOrbit(new PVCoordinates(position, velocity),
FramesFactory.getEME2000(), initialDate,
Constants.EIGEN5C_EARTH_MU);
More details on the orbit representation can be found in the orbits section of the library architecture documentation.
We will put all switching events in a set.
final SortedSet<String> output = new TreeSet<String>();
Let’s define a couple of AttitudeProvider, built upon LofOffset laws for instance.
final AttitudeProvider dayObservationLaw = new LofOffset(initialOrbit.getFrame(), LOFType.VVLH,
RotationOrder.XYZ, FastMath.toRadians(20), FastMath.toRadians(40), 0);
final AttitudeProvider nightRestingLaw = new LofOffset(initialOrbit.getFrame(), LOFType.VVLH);
Let’s also define some EventDetector. For this tutorial’s requirements, two EclipseDetector, each one using a customized implementation of EventHandler with a dedicated eventOccurred method: dayNightEvent, to detect the day to night transition, nightDayEvent, to detect the night to day transition:
PVCoordinatesProvider sun = CelestialBodyFactory.getSun();
PVCoordinatesProvider earth = CelestialBodyFactory.getEarth();
EventDetector dayNightEvent = new EclipseDetector(sun, 696000000., earth, Constants.WGS84_EARTH_EQUATORIAL_RADIUS).
withHandler(new ContinueOnEvent<EclipseDetector>());
EventDetector nightDayEvent = new EclipseDetector(sun, 696000000., earth, Constants.WGS84_EARTH_EQUATORIAL_RADIUS).
withHandler(new ContinueOnEvent<EclipseDetector>());
More details on event detectors and event handlers can be found in the propagation section of the library architecture documentation.
An AttitudesSequence is then defined, for the sake of this tutorial, by adding two switching conditions acting as a simple loop:
As the two conditions reverse each other effect, the combined AttitudesSequence acts as a loop. We also define a handler to monitor attitude switches:
AttitudesSequence attitudesSequence = new AttitudesSequence();
AttitudesSequence.SwitchHandler switchHandler =
new AttitudesSequence.SwitchHandler() {
public void switchOccurred(AttitudeProvider preceding, AttitudeProvider following,
SpacecraftState s) {
if (preceding == dayObservationLaw) {
output.add(s.getDate() + ": switching to night law");
} else {
output.add(s.getDate() + ": switching to day law");
}
}
};
attitudesSequence.addSwitchingCondition(dayObservationLaw, dayNightEvent,
false, true, 10.0,
AngularDerivativesFilter.USE_R, nightRestingLaw, switchHandler);
attitudesSequence.addSwitchingCondition(nightRestingLaw, nightDayEvent,
true, false, 10.0,
AngularDerivativesFilter.USE_R, dayObservationLaw, switchHandler);
An AttitudesSequence needs at least one switching condition to be meaningful, but there is no upper limit.
An active AttitudeProvider may have several switch events and next law settings, leading to different activation patterns depending on which event is triggered first.
Don’t forget to set the current active law according to the current state:
if (dayNightEvent.g(new SpacecraftState(initialOrbit)) >= 0) {
// initial position is in daytime
attitudesSequence.resetActiveProvider(dayObservationLaw);
} else {
// initial position is in nighttime
attitudesSequence.resetActiveProvider(nightRestingLaw);
}
Now, let’s choose some propagator to compute the spacecraft motion. We will use an EcksteinHechlerPropagator based on the analytical Eckstein-Hechler model. The propagator is built upon the initialOrbit, the attitudeSequence and physical constants for the potential.
Propagator propagator = new EcksteinHechlerPropagator(initialOrbit, attitudesSequence,
Constants.EIGEN5C_EARTH_EQUATORIAL_RADIUS,
Constants.EIGEN5C_EARTH_MU, Constants.EIGEN5C_EARTH_C20,
Constants.EIGEN5C_EARTH_C30, Constants.EIGEN5C_EARTH_C40,
Constants.EIGEN5C_EARTH_C50, Constants.EIGEN5C_EARTH_C60);
The attitudeSequence must register all the switching events before propagation.
attitudesSequence.registerSwitchEvents(propagator);
The propagator operating mode is set to master mode with fixed step. The implementation of the interface OrekitFixedStepHandler aims to define the handleStep method called within the loop. For the purpose of this tutorial, the handleStep method will print at the current date two angles, the first one indicates if the spacecraft is eclipsed while the second informs about the current attitude law.
propagator.setMasterMode(180., new OrekitFixedStepHandler() {
public void handleStep(SpacecraftState currentState, boolean isLast) throws OrekitException {
DecimalFormatSymbols angleDegree = new DecimalFormatSymbols(Locale.US);
angleDegree.setDecimalSeparator('\u00b0');
DecimalFormat ad = new DecimalFormat(" 00.000;-00.000", angleDegree);
// the g function is the eclipse indicator, its an angle between Sun and Earth limb,
// positive when Sun is outside of Earth limb, negative when Sun is hidden by Earth limb
final double eclipseAngle = dayNightEvent.g(currentState);
// the Earth position in spacecraft frame should be along spacecraft Z axis
// during nigthtime and away from it during daytime due to roll and pitch offsets
final Vector3D earth = currentState.toTransform().transformPosition(Vector3D.ZERO);
final double pointingOffset = Vector3D.angle(earth, Vector3D.PLUS_K);
output.add(currentState.getDate() +
" " + ad.format(FastMath.toDegrees(eclipseAngle) +
" " + ad.format(FastMath.toDegrees(pointingOffset)));
}
});
More details on propagation modes can be found in the propagation section of the library architecture documentation.
Finally, the propagator is just asked to propagate for a given duration, and we print the results:
SpacecraftState finalState = propagator.propagate(initialDate.shiftedBy(12600.));
for (final String line : output) {
System.out.println(line);
}
Note that we use an intermediate SortedSet to first gather both the switching events and the step outputs instead of just letting the event handler and step handler directly print their results. The rationale is that as events handlers could truncate a step (if their eventOccurred method returned Action.STOP), the library design is to always call eventOccurred on the event handler first, and then to call handleStep on the step handler afterwards, with the isLast boolean set up correctly if the event handler decided to stop propagation. A side effect is that if both methods print something, then the switch from the end of the step would be printed first and the step itself printed afterwards, which would lead to out of order output. The SortedSet ensures the various lines will be sorted in lexicographic order, which is chronological order here, despite they will be generated slightly out of order near events occurrences.
As the propagation goes along, events occur switching from one attitude law to another.
The printed results are shown below:
2004-01-01T23:30:00.000 -11°649 00°000 2004-01-01T23:33:00.000 -17°804 00°000 2004-01-01T23:36:00.000 -22°458 00°000 2004-01-01T23:39:00.000 -25°045 00°000 2004-01-01T23:42:00.000 -25°140 00°000 2004-01-01T23:45:00.000 -22°726 00°000 2004-01-01T23:48:00.000 -18°207 00°000 2004-01-01T23:51:00.000 -12°146 00°000 2004-01-01T23:54:00.000 -05°042 00°000 2004-01-01T23:55:57.968: switching to day law 2004-01-01T23:57:00.000 02°741 43°958 2004-01-02T00:00:00.000 10°946 43°958 2004-01-02T00:03:00.000 19°390 43°958 2004-01-02T00:06:00.000 27°931 43°958 2004-01-02T00:09:00.000 36°441 43°958 2004-01-02T00:12:00.000 44°787 43°958 2004-01-02T00:15:00.000 52°808 43°958 2004-01-02T00:18:00.000 60°286 43°958 2004-01-02T00:21:00.000 66°913 43°958 2004-01-02T00:24:00.000 72°251 43°958 2004-01-02T00:27:00.000 75°751 43°958 2004-01-02T00:30:00.000 76°896 43°958 2004-01-02T00:33:00.000 75°480 43°958 2004-01-02T00:36:00.000 71°756 43°958 2004-01-02T00:39:00.000 66°259 43°958 2004-01-02T00:42:00.000 59°533 43°958 2004-01-02T00:45:00.000 51°999 43°958 2004-01-02T00:48:00.000 43°955 43°958 2004-01-02T00:51:00.000 35°608 43°958 2004-01-02T00:54:00.000 27°112 43°958 2004-01-02T00:57:00.000 18°596 43°958 2004-01-02T01:00:00.000 10°184 43°958 2004-01-02T01:03:00.000 02°022 43°958 2004-01-02T01:03:45.919: switching to night law 2004-01-02T01:06:00.000 -05°706 00°000 2004-01-02T01:09:00.000 -12°733 00°000 2004-01-02T01:12:00.000 -18°680 00°000 2004-01-02T01:15:00.000 -23°037 00°000 2004-01-02T01:18:00.000 -25°240 00°000 2004-01-02T01:21:00.000 -24°914 00°000 2004-01-02T01:24:00.000 -22°122 00°000 2004-01-02T01:27:00.000 -17°313 00°000 2004-01-02T01:30:00.000 -11°051 00°000 2004-01-02T01:33:00.000 -03°814 00°000 2004-01-02T01:34:28.690: switching to day law 2004-01-02T01:36:00.000 04°052 43°958 2004-01-02T01:39:00.000 12°308 43°958 2004-01-02T01:42:00.000 20°777 43°958 2004-01-02T01:45:00.000 29°322 43°958 2004-01-02T01:48:00.000 37°815 43°958 2004-01-02T01:51:00.000 46°121 43°958 2004-01-02T01:54:00.000 54°070 43°958 2004-01-02T01:57:00.000 61°434 43°958 2004-01-02T02:00:00.000 67°885 43°958 2004-01-02T02:03:00.000 72°963 43°958 2004-01-02T02:06:00.000 76°111 43°958 2004-01-02T02:09:00.000 76°841 43°958 2004-01-02T02:12:00.000 75°020 43°958 2004-01-02T02:15:00.000 70°968 43°958 2004-01-02T02:18:00.000 65°236 43°958 2004-01-02T02:21:00.000 58°353 43°958 2004-01-02T02:24:00.000 50°719 43°958 2004-01-02T02:27:00.000 42°613 43°958 2004-01-02T02:30:00.000 34°231 43°958 2004-01-02T02:33:00.000 25°723 43°958 2004-01-02T02:36:00.000 17°215 43°958 2004-01-02T02:39:00.000 08°834 43°958 2004-01-02T02:42:00.000 00°727 43°958 2004-01-02T02:42:16.591: switching to night law 2004-01-02T02:45:00.000 -06°907 00°000 2004-01-02T02:48:00.000 -13°788 00°000 2004-01-02T02:51:00.000 -19°515 00°000 2004-01-02T02:54:00.000 -23°558 00°000 2004-01-02T02:57:00.000 -25°366 00°000 2004-01-02T03:00:00.000 -24°622 00°000 Propagation ended at 2004-01-02T03:00:00.000
The complete code for this example can be found in the source tree of the library, in file src/tutorials/fr/cs/examples/attitude/EarthObservation.java.