Procedure

   void surfnm_c ( SpiceDouble        a, 
                   SpiceDouble        b, 
                   SpiceDouble        c, 
                   ConstSpiceDouble   point[3], 
                   SpiceDouble        normal[3] ) 

Abstract

 
   This routine computes the outward-pointing, unit normal vector 
   from a point on the surface of an ellipsoid. 
 

Required_Reading

 
   None. 
 

Keywords

 
   ELLIPSOID,  GEOMETRY 
 

Brief_I/O

 
   VARIABLE  I/O  DESCRIPTION 
   --------  ---  -------------------------------------------------- 
   a          I   Length of the ellisoid semi-axis along the x-axis. 
   b          I   Length of the ellisoid semi-axis along the y-axis. 
   c          I   Length of the ellisoid semi-axis along the z-axis. 
   point      I   Body-fixed coordinates of a point on the ellipsoid 
   normal     O   Outward pointing unit normal to ellipsoid at point 
 

Detailed_Input

   a          This is the length of the semi-axis of the ellipsoid 
              that is parallel to the x-axis of the body-fixed 
              coordinate system. 

   b          This is the length of the semi-axis of the ellipsoid 
              that is parallel to the y-axis of the body-fixed 
              coordinate system. 

   c          This is the length of the semi-axis of the ellipsoid 
              that is parallel to the z-axis of the body-fixed 
              coordinate system. 

   point      This is a 3-vector giving the bodyfixed coordinates 
              of a point on the ellipsoid. In bodyfixed coordinates, 
              the semi-axes of the ellipsoid are aligned with the 
              x, y, and z-axes of the coordinate system. 
 

Detailed_Output

 
   normal    A unit vector pointing away from the ellipsoid and 
             normal to the ellipsoid at point. 
 

Parameters

 
   None. 
 

Exceptions

 
   1) If any of the axes are non-positive, the error 
      SPICE(BADAXISLENGTH) will be signalled. 
 

Files

 
   None. 
 

Particulars

 
   This routine computes the outward pointing unit normal vector to 
   the ellipsoid having semi-axes of length a, b, and c from the 
   point point. 
 

Examples

 
   A typical use of surfnm_c would be to find the angle of incidence 
   of the light from the sun at a point on the surface of an 
   ellipsoid. 

   Let q be a 3-vector representing the rectangular body-fixed 
   coordinates of a point on the ellipsoid (we are assuming that 
   the axes of the ellipsoid are aligned with the axes of the 
   body fixed frame.)  Let v be the vector from q to the sun in 
   bodyfixed coordinates.  Then the following code fragment could 
   be used to compute angle of incidence of sunlight at q. 

      surfnm_c   ( a, b, c, q, nrml );

      incidn = vsep_c ( v, nrml );
 
 

Restrictions

 
   It is assumed that the input point is indeed on the ellipsoid. 
   No checking for this is done. 
 

Literature_References

 
   None. 
 

Author_and_Institution

 
   W.L. Taber      (JPL) 
   N.J. Bachman    (JPL)
   B.V. Semenov    (JPL)
   

Version

 
   -CSPICE Version 1.3.1, 31-JAN-2008 (BVS)

      Removed '-Revisions' from the header.

   -CSPICE Version 1.3.0, 22-OCT-1998 (NJB)

      Made input vector 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 1.2.0, 07-AUG-1996 (WLT)

Index_Entries

 
   surface normal vector on an ellipsoid 
 

Link to routine surfnm_c source file surfnm_c.c