stelab_c

Index Page stelab_c
A B C D E F G H I J K L M N O P Q R S T U V W X
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
       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

    Thu Apr 16 08:59:18 2009