spkgps_c

Index Page spkgps_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 spkgps_c ( SpiceInt           targ,
                       SpiceDouble        et,
                       ConstSpiceChar   * ref,
                       SpiceInt           obs,
                       SpiceDouble        pos[3],
                       SpiceDouble      * lt     )
#### Abstract
       Compute the geometric position of a target body relative to an
       observing body.
#### Required_Reading
       SPK
#### Keywords
       EPHEMERIS
#### Brief_I/O
       Variable  I/O  Description
       --------  ---  --------------------------------------------------
       targ       I   Target body.
       et         I   Target epoch.
       ref        I   Target reference frame.
       obs        I   Observing body.
       pos        O   Position of target.
       lt         O   Light time.
#### Detailed_Input
       targ        is the standard NAIF ID code for a target body.

       et          is the epoch (ephemeris time) at which the position
                   of the target body is to be computed.

       ref         is the name of the reference frame to
                   which the vectors returned by the routine should
                   be rotated. This may be any frame supported by
                   the CSPICE subroutine sxform_c.

       obs         is the standard NAIF ID code for an observing body.
#### Detailed_Output
       pos         contains the position of the target
                   body, relative to the observing body. This vector is
                   rotated into the specified reference frame. Units
                   are always km.

       lt          is the one-way light time from the observing body
                   to the geometric position of the target body at the
                   specified epoch.
#### Parameters
       None.
#### Exceptions
       1) If insufficient ephemeris data has been loaded to compute
          the necessary positions, the error SPICE(SPKINSUFFDATA) is
          signalled.
#### Files
       See: $Restrictions.
#### Particulars
       spkgps_c computes the geometric position, T(t), of the target
       body and the geometric position, O(t), of the observing body
       relative to the first common center of motion.  Subtracting
       O(t) from T(t) gives the geometric position of the target
       body relative to the observer.

          CENTER ----- O(t)
              |      /
              |     /
              |    /
              |   /  T(t) - O(t)
              |  /
             T(t)

       The one-way light time, tau, is given by

                 | T(t) - O(t) |
          tau = -----------------
                        c

       For example, if the observing body is -94, the Mars Observer
       spacecraft, and the target body is 401, Phobos, then the
       first common center is probably 4, the Mars Barycenter.
       O(t) is the position of -94 relative to 4 and T(t) is the
       position of 401 relative to 4.

       The center could also be the Solar System Barycenter, body 0.
       For example, if the observer is 399, Earth, and the target
       is 299, Venus, then O(t) would be the position of 399 relative
       to 0 and T(t) would be the position of 299 relative to 0.

       Ephemeris data from more than one segment may be required
       to determine the positions of the target body and observer
       relative to a common center.  spkgps_c reads as many segments
       as necessary, from as many files as necessary, using files
       that have been loaded by previous calls to spklef_c (load
       ephemeris file).

       spkgps_c is similar to spkgeo_c but returns geometric positions
       only.
#### Examples
       The following code example computes the geometric
       position of the moon with respect to the earth and
       then prints the distance of the moon from the
       the earth at a number of epochs.

       Assume the SPK file SAMPLE.BSP contains ephemeris data
       for the moon relative to earth over the time interval
       whose endpoints are represented by the variables begin and
       end.

          #include <stdio.h>
          #include "SpiceUsr.h"
              .
              .
              .

          int main()
          {

             #define EARTH           399
             #define MOON            301
             #define N               100
             #define TIMLEN          30

             SpiceChar               utc    [TIMLEN];

             SpiceDouble             begin;
             SpiceDouble             delta;
             SpiceDouble             end;
             SpiceDouble             et;
             SpiceDouble             pos    [3];

             SpiceInt                handle;

             /.
             Load the binary SPK ephemeris file.
             ./

             spklef_c ( "SAMPLE.BSP", &handle );

                 .
                 .
                 .

             /.
             Divide the interval of coverage [begin,end] into
             n steps.  At each step, compute the position, and
             print out the epoch in UTC time and position norm.
             ./

             delta = ( end - begin ) / n

             for ( i = 0;  i < N;  i++ )
             {
                et  =  begin + i * delta;

                spkgps_c ( MOON, et, "J2000", EARTH, pos, &lt );

                et2utc_c ( et, "C", 0, utc );

                printf   ( "%s  %25.15e\n", utc, vnorm_c(pos) );
             }

             return ( 0 );
          }
#### Restrictions
       1) The ephemeris files to be used by spkgps_c must be loaded
          by spklef_c before spkgps_c is called.
#### Literature_References
       None.
#### Author_and_Institution
       N.J. Bachman (JPL)
       J.E. McLean  (JPL)
       W.L. Taber   (JPL)
#### Version
       -CSPICE Version 1.0.0, 30-MAY-1999 (NJB) (JEM) (WLT)
#### Index_Entries
       geometric position of one body relative to another
#### Link to routine spkgps_c source file spkgps_c.c

    Thu Apr 16 08:59:13 2009