InterSatDirectViewDetector.java
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package org.orekit.propagation.events;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.hipparchus.util.FastMath;
import org.orekit.bodies.OneAxisEllipsoid;
import org.orekit.frames.Frame;
import org.orekit.propagation.PropagatorsParallelizer;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.ContinueOnEvent;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.time.AbsoluteDate;
import org.orekit.utils.PVCoordinatesProvider;
/** Detector for inter-satellites direct view (i.e. no masking by central body limb).
* <p>
* As this detector needs two satellites, it embeds one {@link
* PVCoordinatesProvider coordinates provider} for the secondary satellite
* and is registered as an event detector in the propagator of the primary
* satellite. The secondary satellite provider will therefore be driven by this
* detector (and hence by the propagator in which this detector is registered).
* </p>
* <p>
* In order to avoid infinite recursion, care must be taken to have the secondary
* satellite provider being <em>completely independent</em> from anything else.
* In particular, if the provider is a propagator, it should <em>not</em> be run
* together in a {@link PropagatorsParallelizer propagators parallelizer} with
* the propagator this detector is registered in. It is fine however to configure
* two separate propagators PsA and PsB with similar settings for the secondary satellite
* and one propagator Pm for the primary satellite and then use Psa in this detector
* registered within Pm while Pm and Psb are run in the context of a {@link
* PropagatorsParallelizer propagators parallelizer}.
* </p>
* <p>
* For efficiency reason during the event search loop, it is recommended to have
* the secondary provider be an analytical propagator or an ephemeris. A numerical propagator
* as a secondary propagator works but is expected to be computationally costly.
* </p>
* <p>
* The {@code g} function of this detector is positive when satellites can see
* each other directly and negative when the central body limb is in between and
* blocks the direct view.
* </p>
* <p>
* This detector only checks masking by central body limb, it does not take into
* account satellites antenna patterns. If these patterns must be considered, then
* this detector can be {@link BooleanDetector#andCombine(EventDetector...) and combined}
* with the {@link BooleanDetector#notCombine(EventDetector) logical not} of
* {@link FieldOfViewDetector field of view detectors}.
* </p>
* @author Luc Maisonobe
* @since 9.3
*/
public class InterSatDirectViewDetector extends AbstractDetector<InterSatDirectViewDetector> {
/** Central body. */
private final OneAxisEllipsoid body;
/** Equatorial radius squared. */
private final double ae2;
/** 1 minus flatness squared. */
private final double g2;
/** Coordinates provider for the secondary satellite. */
private final PVCoordinatesProvider secondary;
/** simple constructor.
*
* @param body central body
* @param secondary provider for the secondary satellite
*/
public InterSatDirectViewDetector(final OneAxisEllipsoid body, final PVCoordinatesProvider secondary) {
this(body, secondary, DEFAULT_MAXCHECK, DEFAULT_THRESHOLD, DEFAULT_MAX_ITER,
new ContinueOnEvent<>());
}
/** Private constructor.
* @param body central body
* @param secondary provider for the secondary satellite
* @param maxCheck maximum checking interval (s)
* @param threshold convergence threshold (s)
* @param maxIter maximum number of iterations in the event time search
* @param handler event handler to call at event occurrences
*/
private InterSatDirectViewDetector(final OneAxisEllipsoid body,
final PVCoordinatesProvider secondary,
final double maxCheck,
final double threshold,
final int maxIter,
final EventHandler<? super InterSatDirectViewDetector> handler) {
super(maxCheck, threshold, maxIter, handler);
this.body = body;
this.ae2 = body.getEquatorialRadius() * body.getEquatorialRadius();
this.g2 = (1.0 - body.getFlattening()) * (1.0 - body.getFlattening());
this.secondary = secondary;
}
/** Get the central body.
* @return central body
*/
public OneAxisEllipsoid getCentralBody() {
return body;
}
/** Get the provider for the secondary satellite.
* @return provider for the secondary satellite
*/
public PVCoordinatesProvider getSecondary() {
return secondary;
}
/** {@inheritDoc} */
@Override
protected InterSatDirectViewDetector create(final double newMaxCheck,
final double newThreshold,
final int newMaxIter,
final EventHandler<? super InterSatDirectViewDetector> newHandler) {
return new InterSatDirectViewDetector(body, secondary, newMaxCheck, newThreshold, newMaxIter, newHandler);
}
/** {@inheritDoc}
* <p>
* The {@code g} function of this detector is positive when satellites can see
* each other directly and negative when the central body limb is in between and
* blocks the direct view.
* </p>
*/
@Override
public double g(final SpacecraftState state) {
// get the line between primary and secondary in body frame
final AbsoluteDate date = state.getDate();
final Frame frame = body.getBodyFrame();
final Vector3D pPrimary = state.getPVCoordinates(frame).getPosition();
final Vector3D pSecondary = secondary.getPVCoordinates(date, frame).getPosition();
// points along the primary/secondary lines are defined as
// xk = x + k * dx, yk = y + k * dy, zk = z + k * dz
// so k is 0 at primary and 1 at secondary
final double x = pPrimary.getX();
final double y = pPrimary.getY();
final double z = pPrimary.getZ();
final double dx = pSecondary.getX() - x;
final double dy = pSecondary.getY() - y;
final double dz = pSecondary.getZ() - z;
// intersection between line and central body surface
// is a root of a 2nd degree polynomial :
// a k^2 - 2 b k + c = 0
final double a = g2 * (dx * dx + dy * dy) + dz * dz;
final double b = -(g2 * (x * dx + y * dy) + z * dz);
final double c = g2 * (x * x + y * y - ae2) + z * z;
final double s = b * b - a * c;
if (s < 0) {
// the quadratic has no solution, the line between primary and secondary
// doesn't crosses central body limb, direct view is possible
// return a positive value, preserving continuity across zero crossing
return -s;
}
// the quadratic has two solutions (degenerated to one if s = 0)
// direct view is blocked when one of these solutions is between 0 and 1
final double k1 = (b < 0) ? (b - FastMath.sqrt(s)) / a : c / (b + FastMath.sqrt(s));
final double k2 = c / (a * k1);
if (FastMath.max(k1, k2) < 0.0 || FastMath.min(k1, k2) > 1.0) {
// the intersections are either behind primary or farther away than secondary
// along the line, direct view is possible
// return a positive value, preserving continuity across zero crossing
return s;
} else {
// part of the central body is between primary and secondary
// this includes unrealistic cases where primary, secondary or both are inside the central body ;-)
// in all these cases, direct view is blocked
// return a negative value, preserving continuity across zero crossing
return -s;
}
}
}