void surfpt_c ( ConstSpiceDouble positn[3],
ConstSpiceDouble u[3],
SpiceDouble a,
SpiceDouble b,
SpiceDouble c,
SpiceDouble point[3],
SpiceBoolean * found )
Determine the intersection of a line-of-sight vector with the
surface of an ellipsoid.
None.
ELLIPSOID
GEOMETRY
INTERSECTION
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
positn I Position of the observer in body-fixed frame.
u I Vector from the observer in some direction.
a I Length of the ellipsoid semi-axis along the x-axis.
b I Length of the ellipsoid semi-axis along the y-axis.
c I Length of the ellipsoid semi-axis along the z-axis.
point O Point on the ellipsoid pointed to by u.
found O Flag indicating if u points at the ellipsoid.
positn 3-vector giving the position of an observer with respect
to the center of an ellipsoid. The vector is expressed
in a body-fixed reference frame. The semi-axes of the
ellipsoid are aligned with the x, y, and z-axes of the
body-fixed frame.
u Direction vector emanating from the observer.
a Length of the semi-axis of the ellipsoid that is parallel
to the x-axis of the body-fixed reference frame.
b Length of the semi-axis of the ellipsoid that is parallel
to the y-axis of the body-fixed reference frame.
c Length of the semi-axis of the ellipsoid that is parallel
to the z-axis of the body-fixed reference frame.
point If the ray with direction vector u emanating from
positn intersects the ellipsoid, point will be
returned with the body-fixed coordinates of the point
where the ray first meets the ellipsoid. Otherwise,
point will be returned as (0, 0, 0).
found A logical flag indicating whether or not the ray from
positn with direction u actually intersects the
ellipsoid. If the ray does intersect the ellipsoid,
found will be returned as SPICETRUE. If the ray misses
the ellipsoid, found will be returned as SPICEFALSE.
None.
1) If the input vector is the zero vector, the error
SPICE(ZEROVECTOR) is signaled.
2) If any of the body's axes is zero, the error
SPICE(BADAXISLENGTH) is signaled.
None.
This routine assumes that an ellipsoid having semi-axes of length a,
b and c is given. Moreover, it is assumed that these axes are
parallel to the x-, y-, and z-axes of a coordinate system whose
origin is the geometric center of the ellipsoid---this is called the
body-fixed coordinate frame.
A typical use of surfpt_c would be to obtain the planetocentric
coordinates of the point at which the optic axis of a
spacecraft-mounted instrument intersects the surface of a target
body, given the following items.
1) The epoch (et) of observation, and the inertial
pointing (vpnt) of the instrument at this epoch.
2) The apparent position (vtarg) of the center of the
target body as seen from the spacecraft at the epoch
of observation, and the one-way light time (tau)
from the target to the spacecraft.
In order to find the point of intersection, the following items are
also needed.
3) The transformation (tibf) from inertial
to body-fixed coordinates at epoch et-tau.
4) The radii (r) of the tri-axial ellipsoid
used to model the target body.
These may be obtained from the kernel pool via calls to pxform_c and
bodvrd_c or bodvcd_c respectively.
The position of the observer is just the negative of the
spacecraft-target vector, vtarg, computed using the vminus_c module.
(Note that this is NOT the same as the apparent position of the
spacecraft as seen from the target!) Both vectors must be in
body-fixed coordinates. The point of intersection is found as
follows.
vminus_c ( vtarg, vpos );
mxv_c ( tibf, vpos, vpos );
mxv_c ( tibf, vpnt, vpnt );
surfpt_c ( vpos, vpnt, r[0], r[1], r[2], vsurf, &found );
Note that vsurf may or may not be a point of intersection, depending
on whether found is SPICETRUE or SPICEFALSE. Note also that vsurf is
a vector from the center to the surface of the target, in body-fixed
coordinates, which may be converted directly to planetocentric
latitude, longitude, and radius:
reclat_c ( vsurf, &radius, &long, &lat );
To get the inertial vector from the spacecraft to the surface point,
you must subtract vpos from vsurf, and rotate the resulting vector
back to inertial coordinates:
vsub_c ( vsurf, vpos, vsurf );
mtxv_c ( tibf, vsurf, vsurf );
None.
None.
C.H. Acton (JPL)
N.J. Bachman (JPL)
W.L. Taber (JPL)
-CSPICE Version 1.4.3, 24-OCT-2005 (NJB)
Header update: reference to bodvar_c was replaced with
references to bodvrd_c and bodvcd_c.
-CSPICE Version 1.4.2, 27-JUL-2003 (NJB) (CHA)
Various header corrections were made.
-CSPICE Version 1.4.1, 28-NOV-2002 (NJB)
Made miscellaneous updates to header comments.
-CSPICE Version 1.4.0, 27-AUG-1999 (NJB)
Now uses local "found" flag of type logical.
-CSPICE Version 1.3.0, 22-OCT-1998 (NJB)
Made input vectors const.
-CSPICE Version 1.2.0, 08-FEB-1998 (NJB)
Removed local variables used for temporary capture of outputs.
-CSPICE Version 1.0.0, 25-OCT-1997 (NJB)
Based on SPICELIB Version 1.1.0, 07-AUG-1996 (WLT)
line of sight intercept with body
point of intersection between ray and ellipsoid
surface point of intersection of ray and ellipsoid
Link to routine surfpt_c source file surfpt_c.c
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