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 stlabx_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

       stlabx_c ( Stellar aberration, transmission case )

       void stlabx_c ( ConstSpiceDouble    pobj   [3],
                       ConstSpiceDouble    vobs   [3],
                       SpiceDouble         corpos [3] )

#### Abstract

       Correct the position of a target for the stellar aberration
       effect on radiation transmitted from a specified observer to
       the target.

#### 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.
       corpos     O   Corrected position of the object.

#### Detailed_Input

       pobj        is the cartesian position vector of an object with
                   respect to the observer, possibly corrected for
                   light time. Units are km.

       vobs        is the cartesian velocity vector of the observer
                   with respect to the Solar System barycenter. Units
                   are km/s.

#### Detailed_Output

       corpos      is the  position of the object relative to the
                   observer, corrected for the stellar aberration
                   effect on radiation directed toward the target. This
                   correction is the inverse of the usual stellar
                   aberration correction: the corrected vector
                   indicates the direction in which radiation must be
                   emitted from the observer, as seen in an inertial
                   reference frame having velocity equal to that of the
                   observer, in order to reach the position indicated by
                   the input vector `pobj'.

#### Parameters

       None.

#### Exceptions

       1)  If the velocity of the observer is greater than or equal to
           the speed of light, an error is signaled by a routine in the
           call tree of this routine. The outputs are undefined.

#### Files

       None.

#### Particulars

       In order to transmit radiation from an observer to a specified
       target, the emission direction must be corrected for one way
       light time and for the motion of the observer relative to the
       solar system barycenter. The correction for the observer's
       motion when transmitting to a target is the inverse of the
       usual stellar aberration correction applied to the light-time
       corrected position of the target as seen by the observer.

       Below is the description of the stellar aberration correction
       used in the CSPICE routine stelab_c (with the notation changed
       slightly):

          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.

       This routine applies the inverse correction, so here the rotation
       about `h' is by -phi radians.

#### 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 way light-time and stellar aberration
          effect on radiation transmitted from the Earth to the Moon,
          given the geometric state of the Earth relative to the Solar
          System Barycenter, and the difference between the stelar
          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, both for the transmission
          case.

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

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

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

             KPL/MK

             File name: stlabx_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 stlabx_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 ( "stlabx_ex1.tm" );

             /.
             Convert the time string to ephemeris time.
             ./
             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. 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 ( idtarg, et, reffrm, sobs, "XLT", 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.
             ./
             stlabx_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
                   201809.933536     -260878.049826     -147716.077987

          Corrected position vector
                   201782.730972     -260894.375627     -147724.405897

          Apparent difference
                       27.202563          16.325802           8.327911

#### 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

       J. Diaz del Rio     (ODC Space)

#### Version

       -CSPICE Version 1.0.0, 13-AUG-2021 (JDR)

#### Index_Entries

       stellar aberration for transmission case

#### Link to routine stlabx_c source file stlabx_c.c

    Fri Dec 31 18:41:13 2021