Procedure

   void stelab_c ( ConstSpiceDouble   pobj[3],
                   ConstSpiceDouble   vobs[3],
                   SpiceDouble        appobj[3] ) 

Abstract

 
    Correct the apparent position of an object for stellar 
    aberration. 
 

Required_Reading

 
   None. 
 

Keywords

 
    EPHEMERIS 
 

Brief_I/O

 
    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. 
 

Detailed_Input

 
    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. 
 

Detailed_Output

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

Parameters

 
   None. 
 

Exceptions

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

Files

 
    None. 
 

Particulars

 
    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. 
 

Examples

 
    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 position `tpos' of the target
       body `targ' as seen by the observer. Normally we would
       call spkpos_c to obtain this vector, but we already have
       the state of the observer relative to the solar system
       barycenter, so we can avoid looking up that state twice
       by calling spkapo_c.
       ./
       spkapo_c ( targ, et, "j2000", sobs, "lt", tpos, &lt );

       /.
       Apply the correction for stellar aberration to the
       light-time corrected position of the target body.
       The corrected position is returned in the argument
       `pcorr'.
       ./
       stelab_c ( tpos, sobs+3, pcorr );
 

    Note that this example is somewhat contrived. The sequence
    of calls above could be replaced by a single call to spkezp_c,
    using the aberration correction flag "lt+s".

    For more information on aberration-corrected states or
    positions, see the headers of any of the routines

       spkezr_c
       spkez_c
       spkpos_c
       spkezp_c

 

Restrictions

 
    None. 
 

Literature_References

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

Author_and_Institution

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

Version

 
  -CSPICE Version 1.0.1, 8-JAN-2008 (NJB)

      The header example was updated to remove references
      to spkapp_c. 

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

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

Index_Entries

 
   stellar aberration 
 

Link to routine stelab_c source file stelab_c.c