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

Table of contents

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

       stelab_c ( Stellar Aberration )

       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 by a routine in the call tree of this routine.

#### 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

       The numerical results shown for this example may differ across
       platforms. The results depend on the SPICE kernels used as
       input, the compiler and supporting libraries, and the machine
       specific arithmetic implementation.

       1) Compute the apparent position of the Moon relative to the
          Earth, corrected for one light-time and stellar aberration,
          given the geometric state of the Earth relative to the Solar
          System Barycenter, and the difference between the stellar
          aberration corrected and uncorrected position vectors, taking
          several steps.

          First, compute the light-time corrected state of the Moon body
          as seen by the Earth, using its geometric state. Then apply
          the correction for stellar aberration to the light-time
          corrected state of the target body.

          The code in this example could be replaced by a single call
          to spkpos_c:

             spkpos_c ( "MOON", et, "J2000", "LT+S", "EARTH", pos, &lt );

          Use the meta-kernel shown below to load the required SPICE
          kernels.

             KPL/MK

             File name: stelab_ex1.tm

             This meta-kernel is intended to support operation of SPICE
             example programs. The kernels shown here should not be
             assumed to contain adequate or correct versions of data
             required by SPICE-based user applications.

             In order for an application to use this meta-kernel, the
             kernels referenced here must be present in the user's
             current working directory.

             The names and contents of the kernels referenced
             by this meta-kernel are as follows:

                File name                     Contents
                ---------                     --------
                de418.bsp                     Planetary ephemeris
                naif0009.tls                  Leapseconds

             \begindata

                KERNELS_TO_LOAD = ( 'de418.bsp',
                                    'naif0009.tls'  )

             \begintext

             End of meta-kernel

          Example code begins here.

          /.
             Program stelab_ex1
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main ()
          {

             /.
             Local variables.
             ./
             SpiceChar             * reffrm;
             SpiceChar             * utcstr;

             SpiceDouble             appdif [ 3 ];
             SpiceDouble             et;
             SpiceDouble             lt;
             SpiceDouble             pcorr  [ 3 ];
             SpiceDouble             pos    [ 3 ];
             SpiceDouble             sobs   [ 6 ];

             SpiceInt                idobs;
             SpiceInt                idtarg;

             /.
             Assign an observer, Earth, target, Moon, time of interest and
             reference frame for returned vectors.
             ./
             idobs  = 399;
             idtarg = 301;
             utcstr = "July 4 2004";
             reffrm = "J2000";

             /.
             Load the needed kernels.
             ./
             furnsh_c ( "stelab_ex1.tm" );

             /.
             Convert the time string to ephemeris time, J2000.
             ./
             str2et_c ( utcstr, &et );

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

             /.
             Get the light-time corrected position `pos' of the target
             body `idtarg' as seen by the observer.
             ./
             spkapo_c ( idtarg, et, reffrm, sobs, "LT", pos, &lt );

             /.
             Output the uncorrected vector.
             ./
             printf ( "Uncorrected position vector\n" );
             printf ( "   %18.6f %18.6f %18.6f\n", pos[0], pos[1], pos[2] );

             /.
             Apply the correction for stellar aberration to the
             light-time corrected position of the target body.
             ./
             stelab_c ( pos, sobs+3, pcorr );

             /.
             Output the corrected position vector and the apparent
             difference from the uncorrected vector.
             ./
             printf ( "\n" );
             printf ( "Corrected position vector\n" );
             printf ( "   %18.6f %18.6f %18.6f\n",
                       pcorr[0], pcorr[1], pcorr[2] );

             /.
             Apparent difference.
             ./
             vsub_c ( pos, pcorr, appdif );
             printf ( "\n" );
             printf ( "Apparent difference\n" );
             printf ( "   %18.6f %18.6f %18.6f\n",
                       appdif[0], appdif[1], appdif[2] );

             return ( 0 );
          }

          When this program was executed on a Mac/Intel/cc/64-bit
          platform, the output was:

          Uncorrected position vector
                  201738.725087     -260893.141602     -147722.589056

          Corrected position vector
                  201765.929516     -260876.818077     -147714.262441

          Apparent difference
                     -27.204429         -16.323525          -8.326615

#### Restrictions

       None.

#### Literature_References

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

#### Author_and_Institution

       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
       W.L. Taber          (JPL)
       I.M. Underwood      (JPL)

#### Version

       -CSPICE Version 1.0.2, 05-JUL-2021 (JDR)

           Edited the header to comply with NAIF standard.

           Added example's meta-kernel and problem statement. Created complete
           code example from existing code fragments.

       -CSPICE Version 1.0.1, 08-JAN-2008 (NJB)

           The header example was updated to remove references
           to spkapp_c.

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

           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

    Fri Dec 31 18:41:13 2021