FootprintOverlapDetector.java
/* Copyright 2002-2019 CS Systèmes d'Information
* Licensed to CS Systèmes d'Information (CS) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* CS licenses this file to You under the Apache License, Version 2.0
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*
* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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package org.orekit.propagation.events;
import java.util.ArrayList;
import java.util.List;
import org.hipparchus.geometry.enclosing.EnclosingBall;
import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.hipparchus.geometry.spherical.twod.Edge;
import org.hipparchus.geometry.spherical.twod.S2Point;
import org.hipparchus.geometry.spherical.twod.Sphere2D;
import org.hipparchus.geometry.spherical.twod.SphericalPolygonsSet;
import org.hipparchus.geometry.spherical.twod.Vertex;
import org.hipparchus.ode.events.Action;
import org.hipparchus.util.FastMath;
import org.orekit.bodies.BodyShape;
import org.orekit.bodies.GeodeticPoint;
import org.orekit.bodies.OneAxisEllipsoid;
import org.orekit.frames.Transform;
import org.orekit.models.earth.tessellation.DivertedSingularityAiming;
import org.orekit.models.earth.tessellation.EllipsoidTessellator;
import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.propagation.events.handlers.StopOnIncreasing;
/** Detector triggered by geographical region entering/leaving a spacecraft sensor
* {@link FieldOfView Field Of View}.
* <p>
* This detector is a mix between to {@link FieldOfViewDetector} and {@link
* GeographicZoneDetector}. Similar to the first detector above, it triggers events
* related to entry/exit of targets in a Field Of View, taking attitude into account.
* Similar to the second detector above, its target is an entire geographic region
* (which can even be split in several non-connected patches and can have holes).
* </p>
* <p>
* This detector is typically used for ground observation missions with agile
* satellites than can look away from nadir.
* </p>
* <p>The default implementation behavior is to {@link Action#CONTINUE continue}
* propagation at FOV entry and to {@link Action#STOP stop} propagation
* at FOV exit. This can be changed by calling
* {@link #withHandler(EventHandler)} after construction.</p>
* @see org.orekit.propagation.Propagator#addEventDetector(EventDetector)
* @see FieldOfViewDetector
* @see GeographicZoneDetector
* @author Luc Maisonobe
* @since 7.1
*/
public class FootprintOverlapDetector extends AbstractDetector<FootprintOverlapDetector> {
/** Field of view. */
private final FieldOfView fov;
/** Body on which the geographic zone is defined. */
private final OneAxisEllipsoid body;
/** Geographic zone to consider. */
private final SphericalPolygonsSet zone;
/** Linear step used for sampling the geographic zone. */
private final double samplingStep;
/** Sampling of the geographic zone. */
private final List<SamplingPoint> sampledZone;
/** Center of the spherical cap surrounding the zone. */
private final Vector3D capCenter;
/** Cosine of the radius of the spherical cap surrounding the zone. */
private final double capCos;
/** Sine of the radius of the spherical cap surrounding the zone. */
private final double capSin;
/** Build a new instance.
* <p>The maximal interval between distance to FOV boundary checks should
* be smaller than the half duration of the minimal pass to handle,
* otherwise some short passes could be missed.</p>
* @param fov sensor field of view
* @param body body on which the geographic zone is defined
* @param zone geographic zone to consider
* @param samplingStep linear step used for sampling the geographic zone (in meters)
*/
public FootprintOverlapDetector(final FieldOfView fov,
final OneAxisEllipsoid body,
final SphericalPolygonsSet zone,
final double samplingStep) {
this(DEFAULT_MAXCHECK, DEFAULT_THRESHOLD, DEFAULT_MAX_ITER,
new StopOnIncreasing<FootprintOverlapDetector>(),
fov, body, zone, samplingStep, sample(body, zone, samplingStep));
}
/** Private constructor with full parameters.
* <p>
* This constructor is private as users are expected to use the builder
* API with the various {@code withXxx()} methods to set up the instance
* in a readable manner without using a huge amount of parameters.
* </p>
* @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
* @param body body on which the geographic zone is defined
* @param zone geographic zone to consider
* @param fov sensor field of view
* @param sampledZone sampling of the geographic zone
* @param samplingStep linear step used for sampling the geographic zone (in meters)
*/
private FootprintOverlapDetector(final double maxCheck, final double threshold,
final int maxIter, final EventHandler<? super FootprintOverlapDetector> handler,
final FieldOfView fov,
final OneAxisEllipsoid body,
final SphericalPolygonsSet zone,
final double samplingStep,
final List<SamplingPoint> sampledZone) {
super(maxCheck, threshold, maxIter, handler);
this.fov = fov;
this.body = body;
this.samplingStep = samplingStep;
this.zone = zone;
this.sampledZone = sampledZone;
final EnclosingBall<Sphere2D, S2Point> cap = zone.getEnclosingCap();
this.capCenter = cap.getCenter().getVector();
this.capCos = FastMath.cos(cap.getRadius());
this.capSin = FastMath.sin(cap.getRadius());
}
/** Sample the region.
* @param body body on which the geographic zone is defined
* @param zone geographic zone to consider
* @param samplingStep linear step used for sampling the geographic zone (in meters)
* @return sampling points
*/
private static List<SamplingPoint> sample(final OneAxisEllipsoid body,
final SphericalPolygonsSet zone,
final double samplingStep) {
final List<SamplingPoint> sampledZone = new ArrayList<SamplingPoint>();
// sample the zone boundary
final List<Vertex> boundary = zone.getBoundaryLoops();
for (final Vertex loopStart : boundary) {
int count = 0;
for (Vertex v = loopStart; count == 0 || v != loopStart; v = v.getOutgoing().getEnd()) {
++count;
final Edge edge = v.getOutgoing();
final int n = (int) FastMath.ceil(edge.getLength() * body.getEquatorialRadius() / samplingStep);
for (int i = 0; i < n; ++i) {
final S2Point intermediate = new S2Point(edge.getPointAt(i * edge.getLength() / n));
final GeodeticPoint gp = new GeodeticPoint(0.5 * FastMath.PI - intermediate.getPhi(),
intermediate.getTheta(), 0.0);
sampledZone.add(new SamplingPoint(body.transform(gp), gp.getZenith()));
}
}
}
// sample the zone interior
final EllipsoidTessellator tessellator =
new EllipsoidTessellator(body, new DivertedSingularityAiming(zone), 1);
final List<List<GeodeticPoint>> gpSample = tessellator.sample(zone, samplingStep, samplingStep);
for (final List<GeodeticPoint> list : gpSample) {
for (final GeodeticPoint gp : list) {
sampledZone.add(new SamplingPoint(body.transform(gp), gp.getZenith()));
}
}
return sampledZone;
}
/** {@inheritDoc} */
@Override
protected FootprintOverlapDetector create(final double newMaxCheck, final double newThreshold,
final int newMaxIter,
final EventHandler<? super FootprintOverlapDetector> newHandler) {
return new FootprintOverlapDetector(newMaxCheck, newThreshold, newMaxIter, newHandler,
fov, body, zone, samplingStep, sampledZone);
}
/** Get the geographic zone triggering the events.
* <p>
* The zone is mapped on the unit sphere
* </p>
* @return geographic zone triggering the events
*/
public SphericalPolygonsSet getZone() {
return zone;
}
/** Get the Field Of View.
* @return Field Of View
*/
public FieldOfView getFieldOfView() {
return fov;
}
/** Get the body on which the geographic zone is defined.
* @return body on which the geographic zone is defined
*/
public BodyShape getBody() {
return body;
}
/** {@inheritDoc}
* <p>
* The g function value is the minimum offset among the region points
* with respect to the Field Of View boundary. It is positive if all region
* points are outside of the Field Of View, and negative if at least some
* of the region points are inside of the Field Of View. The minimum is
* computed by sampling the region, considering only the points for which
* the spacecraft is above the horizon. The accuracy of the detection
* depends on the linear sampling step set at detector construction. If
* the spacecraft is below horizon for all region points, an arbitrary
* positive value is returned.
* </p>
* <p>
* As per the previous definition, when the region enters the Field Of
* View, a decreasing event is generated, and when the region leaves
* the Field Of View, an increasing event is generated.
* </p>
*/
public double g(final SpacecraftState s) {
// initial arbitrary positive value
double value = FastMath.PI;
// get spacecraft position in body frame
final Vector3D scBody = s.getPVCoordinates(body.getBodyFrame()).getPosition();
// map the point to a sphere
final GeodeticPoint gp = body.transform(scBody, body.getBodyFrame(), s.getDate());
final S2Point s2p = new S2Point(gp.getLongitude(), 0.5 * FastMath.PI - gp.getLatitude());
// for faster computation, we start using only the surrounding cap, to filter out
// far away points (which correspond to most of the points if the zone is small)
final Vector3D p = s2p.getVector();
final double dot = Vector3D.dotProduct(p, capCenter);
if (dot < capCos) {
// the spacecraft is outside of the cap, look for the closest cap point
final Vector3D t = p.subtract(dot, capCenter).normalize();
final Vector3D close = new Vector3D(capCos, capCenter, capSin, t);
if (Vector3D.dotProduct(p, close) < -0.01) {
// the spacecraft is not visible from the cap edge,
// even taking some margin into account for sphere/ellipsoid different shapes
// this induces no points in the zone can see the spacecraft,
// we can return the arbitrary initial positive value without performing further computation
return value;
}
}
// the spacecraft may be visible from some points in the zone, check them all
final Transform bodyToSc = new Transform(s.getDate(),
body.getBodyFrame().getTransformTo(s.getFrame(), s.getDate()),
s.toTransform());
for (final SamplingPoint point : sampledZone) {
final Vector3D lineOfSightBody = point.getPosition().subtract(scBody);
if (Vector3D.dotProduct(lineOfSightBody, point.getZenith()) <= 0) {
// spacecraft is above this sample point local horizon
// get line of sight in spacecraft frame
final double offset = fov.offsetFromBoundary(bodyToSc.transformVector(lineOfSightBody));
value = FastMath.min(value, offset);
}
}
return value;
}
/** Container for sampling points. */
private static class SamplingPoint {
/** Position of the point. */
private final Vector3D position;
/** Zenith vector of the point. */
private final Vector3D zenith;
/** Simple constructor.
* @param position position of the point
* @param zenith zenith vector of the point
*/
SamplingPoint(final Vector3D position, final Vector3D zenith) {
this.position = position;
this.zenith = zenith;
}
/** Get the point position.
* @return point position
*/
public Vector3D getPosition() {
return position;
}
/** Get the point zenith vector.
* @return point zenith vector
*/
public Vector3D getZenith() {
return zenith;
}
}
}