Find the unit vector corresponding to a state vector and the
   derivative of the unit vector.

   None.

   VARIABLE  I/O  DESCRIPTION
   --------  ---  --------------------------------------------------
   s1        I     State to be normalized.
   sout      O     Unit vector s1 / |s1|, and its time derivative.

   s1       This is any double precision state. If the position
            component of the state is the zero vector, this routine
            will detect it and will not attempt to divide by zero.

   sout     sout is a state containing the unit vector pointing in
            the direction of position component of s1 and the
            derivative of the unit vector with respect to time.

            sout may overwrite s1.

   None.

   Let s1 be a state vector with position and velocity components p
   and v respectively.  From these components one can compute the
   unit vector parallel to p, call it u and the derivative of u
   with respect to time, du.  This pair (u,du) is the state returned
   by this routine in sout.

   Suppose that STATE gives the apparent state of a body with
   respect to an observer.  This routine can be used to compute the
   instantaneous angular rate of the object across the sky as seen
   from the observers vantage.

      dvhat_c ( state, ustate );

      vel[0] = ustate[3];
      vel[1] = ustate[4];
      vel[2] = ustate[5];

      angular_rate = vnorm_c ( vel );

   None.

   Error free.

   1) If s1 represents the zero vector, then the position
      component of sout will also be the zero vector.  The
      velocity component will be the velocity component
      of s1.

   None.

     W.L. Taber      (JPL)
     E.D. Wright     (JPL)

     None.

   -CSPICE Version 1.0.0, 7-JUL-1999

   State of a unit vector parallel to a state vector