Index Page spkgeo_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 spkgeo_c ( SpiceInt          targ,
                       SpiceDouble       et,
                       ConstSpiceChar  * ref,
                       SpiceInt          obs,
                       SpiceDouble       state[6],
                       SpiceDouble     * lt       )

#### Abstract

       Compute the geometric state (position and velocity) 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.
       state      O   State 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 state
                   of the target body is to be computed.

       ref         is the name of the reference frame to which the state
                   vector returned by the routine should be rotated. This
                   may be any frame supported by the SPICELIB subroutine
                   FRMCHG.  See also the Frames Required Reading for a list
                   of supported frames.

       obs         is the standard NAIF ID code for an observing body.

#### Detailed_Output

       state       contains the geometric position and velocity of the
                   target body, relative to the observing body, at epoch
                   'et'. 'state' has six elements: the first three contain
                   the target's position; the last three contain the
                   target's velocity. These vectors are transformed into
                   the specified reference frame.

                   Units are always km and km/sec.

       lt          is the one-way light time from the observing body
                   to the geometric position of the target body
                   in seconds at the specified epoch.

#### Parameters

       None.

#### Exceptions

       1) If insufficient ephemeris data have been loaded to compute
          the requested state, the error SPICE(SPKINSUFFDATA) is
          signalled.

#### Files

       See Restrictions.

#### Particulars

       spkgeo_c computes the geometric state, T(t), of the target
       body and the geometric state, O(t), of the observing body
       relative to the first common center of motion.  Subtracting
       O(t) from T(t) gives the geometric state 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 state of -94 relative to 4 and T(t) is the
       state 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 state of 399 relative
       to 0 and T(t) would be the state of 299 relative to 0.

       Ephemeris data from more than one segment may be required
       to determine the states of the target body and observer
       relative to a common center.  spkgeo_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).

       spkgeo_c is similar to spkez_c but returns geometric states
       only, with no option to make planetary (light-time) nor
       stellar aberration corrections.  The geometric states
       returned by spkez_c and spkgeo_c are the same.

#### Examples

       The following code example computes the geometric
       state 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
       from BEGIN to END.

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

          #define   EARTH         399
          #define   MOON          301
          #define   N             100
          #define   UTCLEN        35

          SpiceChar               utc [ 20 ];

          SpiceInt                handle;
          SpiceInt                i;

          SpiceDouble             begin;
          SpiceDouble             delta;
          SpiceDouble             end;
          SpiceDouble             et;
          SpiceDouble             lt;
          SpiceDouble             state [6];

          /.
          Load the binary SPK ephemeris file.
          ./
          furnsh_c ( "sample.bsp" );

              .
              .
              .

          /.
          Divide the interval of coverage [BEGIN,END] into
          N steps.  At each step, compute the state, 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;

             spkgeo_c ( MOON, et, "J2000", EARTH, state, &lt );

             et2utc_c ( et, "c", 0, UTCLEN, utc );

             printf ( "UTC = %s; ||pos|| = %f\n", utc, vnorm_c(state) );
          }

#### Restrictions

       1) The ephemeris files to be used by spkgeo_c must be loaded
          by SPKLEF before spkgeo_c is called.

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman  (JPL)
       J.E. McLean   (JPL)
       W.L. Taber    (JPL)

#### Version

       -CSPICE Version 1.1.2, 08-JAN-2014 (BVS)

           Fixed description of 'state' in Detailed Output. Replaced
           spklef_c with furnsh_c and fixed errors in Examples.

       -CSPICE Version 1.1.1, 13-OCT-2003 (EDW)

           Various minor header changes were made to improve clarity.
           Added mention that 'lt' returns a value in seconds.

       -CSPICE Version 1.1.0, 08-FEB-1998 (NJB)

           References to C2F_CreateStr_Sig were removed; code was
           cleaned up accordingly.  String checks are now done using
           the macro CHKFSTR.

       -CSPICE Version 1.0.0, 25-OCT-1997 (NJB)

          Based on SPICELIB Version 2.2.0, 11-APR-1997 (WLT)

#### Index_Entries

       geometric state of one body relative to another

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

    Wed Apr  5 17:54:43 2017