Correct the apparent position of an object for stellar 
    aberration. 

   None. 

    VARIABLE  I/O  DESCRIPTION 
    --------  ---  -------------------------------------------------- 
    pobj       I   Position of an object with respect to the 
                   observer. 
    vobs       I   Velocity of the observer with respect to the 
                   Solar System barycenter. 
    appobj     O   Apparent position of the object with respect to 
                   the observer, corrected for stellar aberration. 

    pobj        is the position (x, y, z, km) of an object with 
                respect to the observer, possibly corrected for 
                light time. 

    vobs        is the velocity (dx/dt, dy/dt, dz/dt, km/sec) 
                of the observer with respect to the Solar System 
                barycenter. 

    appobj      is the apparent position of the object relative 
                to the observer, corrected for stellar aberration. 

                appobj may overwrite pobj. 

   None. 

    Let r be the vector from the observer to the object, and v be 
        -                                                    - 
    the velocity of the observer with respect to the Solar System 
    barycenter. Let w be the angle between them. The aberration 
    angle phi is given by 

         sin(phi) = v sin(w) / c 

    Let h be the vector given by the cross product 
        - 

          h = r X v 
          -   -   - 

    Rotate r by phi radians about h to obtain the apparent position 
           -                      - 
    of the object. 

    In the following example, stelab_c is used to correct the position 
    of a target body for stellar aberration. 

       /.
       (Previous subroutine calls have loaded the SPK file and 
       the leapseconds kernel file.) 
       ./

       /.
       Get the state of the observer with respect to the solar 
       system barycenter. 
       ./
       spkssb_c ( idobs,  et, "J2000", sobs ); 

       /.
       Get the light-time corrected state of the target body as 
       seen by the observer. 
       ./
       spkapp_c ( idtarg, et, "J2000",  sobs,  "LT", starg, &lt ); 

       /.
       Apply the correction for stellar aberration to the 
       light-time corrected state of the target body. 
       ./
       stelab_c ( starg, starg+3, starg ) 


    Note that this example is somewhat contrived. The correction 
    flag "LT+S" could have been used (in place of "LT") to do the 
    correction automatically in spkapp_c. 

    None. 

   1) If the velocity of the observer is greater than or equal 
      to the speed of light, the error SPICE(VALUEOUTOFRANGE) 
      is signalled. 

    None. 

    N.J. Bachman    (JPL)
    H.A. Neilan     (JPL) 
    W.L. Taber      (JPL) 
    I.M. Underwood  (JPL) 

    1) W.M. Owen, Jr., JPL IOM #314.8-524, "The Treatment of 
       Aberration in Optical Navigation", 8 February 1985. 

   -CSPICE Version 1.0.0, 22-OCT-1998 (NJB)

      Based on SPICELIB Version 1.0.2, 10-MAR-1992 (WLT)

   stellar aberration