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Index Page surfpt_c

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Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries

#### Procedure

       surfpt_c ( Surface point on an ellipsoid )

       void surfpt_c ( ConstSpiceDouble   positn[3],
                       ConstSpiceDouble   u[3],
                       SpiceDouble        a,
                       SpiceDouble        b,
                       SpiceDouble        c,
                       SpiceDouble        point[3],
                       SpiceBoolean     * found     )

#### Abstract

       Determine the intersection of a line-of-sight vector with the
       surface of an ellipsoid.

#### Required_Reading

       None.

#### Keywords

       ELLIPSOID
       GEOMETRY
       INTERSECTION

#### Brief_I/O

       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.

#### Detailed_Input

       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           Pointing 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.

#### Detailed_Output

       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.

#### Parameters

       None.

#### Exceptions

       1)  If the input vector is the zero vector, the error
           SPICE(ZEROVECTOR) is signaled by a routine in the call tree of
           this routine.

       2)  If any of the body's axes is zero, the error
           SPICE(BADAXISLENGTH) is signaled by a routine in the call tree
           of this routine.

#### Files

       None.

#### Particulars

       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.

#### Examples

       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, &lon, &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 );

#### Restrictions

       None.

#### Literature_References

       None.

#### Author_and_Institution

       C.H. Acton          (JPL)
       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
       W.L. Taber          (JPL)

#### Version

       -CSPICE Version 1.4.4, 05-AUG-2021 (NJB) (JDR)

           Edited the header to comply with NAIF standard.

           Changed variable name `long' to `lon' in header code fragment.

       -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) (WLT)

           Based on SPICELIB Version 1.1.0, 07-AUG-1996 (WLT)

#### Index_Entries

       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

    Fri Dec 31 18:41:13 2021