Package org.orekit.geometry.fov

Class PolygonalFieldOfView

  • All Implemented Interfaces:
    FieldOfView
    Direct Known Subclasses:
    DoubleDihedraFieldOfView
    public class PolygonalFieldOfView
extends AbstractFieldOfView
    Class representing a spacecraft sensor Field Of View with polygonal shape.

    Fields Of View are zones defined on the unit sphere centered on the spacecraft. They can have any shape, they can be split in several non-connected patches and can have holes.

    Since:
    10.1
    Author:
    Luc Maisonobe

    • Constructor Detail

      • PolygonalFieldOfView

        public PolygonalFieldOfView​(SphericalPolygonsSet zone,
                            double margin)
        Build a new instance.
        Parameters:
        zone - interior of the Field Of View, in spacecraft frame
        margin - angular margin to apply to the zone (if positive, points outside of the raw FoV but close enough to the boundary are considered visible; if negative, points inside of the raw FoV but close enough to the boundary are considered not visible)

      • PolygonalFieldOfView

        public PolygonalFieldOfView​(Vector3D center,
                            PolygonalFieldOfView.DefiningConeType coneType,
                            Vector3D meridian,
                            double radius,
                            int n,
                            double margin)
        Build Field Of View with a regular polygon shape.
        Parameters:
        center - center of the polygon (the center is in the inside part)
        coneType - type of defining cone
        meridian - point defining the reference meridian for one contact point between defining cone and polygon (i.e. either a polygon edge middle point or a polygon vertex)
        radius - defining cone angular radius
        n - number of sides of the polygon
        margin - angular margin to apply to the zone (if positive, points outside of the raw FoV but close enough to the boundary are considered visible; if negative, points inside of the raw FoV but close enough to the boundary are considered not visible)
        Since:
        10.1

    • Method Detail

      • offsetFromBoundary

        public double offsetFromBoundary​(Vector3D lineOfSight,
                                 double angularRadius,
                                 VisibilityTrigger trigger)
        Get the offset of target body with respect to the Field Of View Boundary.

        The offset is the signed angular distance between target body and closest boundary point, taking into account VisibilityTrigger and margin.

        As Field Of View can have complex shapes that may require long computation, when the target point can be proven to be outside of the Field Of View, a faster but approximate computation can be used. This approximation is only performed about 0.01 radians outside of the Field Of View augmented by the deadband defined by target body radius and Field Of View margin and should be designed to still return a positive value if the full accurate computation would return a positive value. When target point is close to the zone (and furthermore when it is inside the zone), the full accurate computation is performed. This design allows this offset to be used as a reliable way to detect Field Of View boundary crossings (taking VisibilityTrigger and margin into account), which correspond to sign changes of the offset.

        Parameters:
        lineOfSight - line of sight from the center of the Field Of View support unit sphere to the target in spacecraft frame
        angularRadius - target body angular radius
        trigger - visibility trigger for spherical bodies
        Returns:
        an offset negative if the target is visible within the Field Of View and positive if it is outside of the Field Of View (note that this cannot take into account interposing bodies)
        See Also:
        FieldOfView.offsetFromBoundary(Vector3D, double, VisibilityTrigger)

      • projectToBoundary

        public Vector3D projectToBoundary​(Vector3D lineOfSight)
        Find the direction on Field Of View Boundary closest to a line of sight.
        Parameters:
        lineOfSight - line of sight from the center of the Field Of View support unit sphere to the target in spacecraft frame
        Returns:
        direction on Field Of View Boundary closest to a line of sight

      • getFootprint

        public List<List<GeodeticPoint>> getFootprint​(Transform fovToBody,
                                              OneAxisEllipsoid body,
                                              double angularStep)
        Get the footprint of the Field Of View on ground.

        This method assumes the Field Of View is centered on some carrier, which will typically be a spacecraft or a ground station antenna. The points in the footprint boundary loops are all at altitude zero with respect to the ellipsoid, they correspond either to projection on ground of the edges of the Field Of View, or to points on the body limb if the Field Of View goes past horizon. The points on the limb see the carrier origin at zero elevation. If the Field Of View is so large it contains entirely the body, all points will correspond to points at limb. If the Field Of View looks away from body, the boundary loops will be an empty list. The points within footprint loops are sorted in trigonometric order as seen from the carrier. This implies that someone traveling on ground from one point to the next one will have the points visible from the carrier on his left hand side, and the points not visible from the carrier on his right hand side.

        The truncation of Field Of View at limb can induce strange results for complex Fields Of View. If for example a Field Of View is a ring with a hole and part of the ring goes past horizon, then instead of having a single loop with a C-shaped boundary, the method will still return two loops truncated at the limb, one clockwise and one counterclockwise, hence "closing" the C-shape twice. This behavior is considered acceptable.

        If the carrier is a spacecraft, then the fovToBody transform can be computed from a SpacecraftState as follows: 

         Transform inertToBody = state.getFrame().getTransformTo(body.getBodyFrame(), state.getDate());
 Transform fovToBody   = new Transform(state.getDate(),
                                       state.toTransform().getInverse(),
                                       inertToBody);

        If the carrier is a ground station, located using a topocentric frame and managing its pointing direction using a transform between the dish frame and the topocentric frame, then the fovToBody transform can be computed as follows: 

         Transform topoToBody = topocentricFrame.getTransformTo(body.getBodyFrame(), date);
 Transform topoToDish = ...
 Transform fovToBody  = new Transform(date,
                                      topoToDish.getInverse(),
                                      topoToBody);

        Only the raw zone is used, the angular margin is ignored here.

        Parameters:
        fovToBody - transform between the frame in which the Field Of View is defined and body frame.
        body - body surface the Field Of View will be projected on
        angularStep - step used for boundary loops sampling (radians), beware this is generally not an angle on the unit sphere, but rather a phase angle used by the underlying Field Of View boundary model
        Returns:
        list footprint boundary loops (there may be several independent loops if the Field Of View shape is complex)