Index Page gfdist_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 gfdist_c ( ConstSpiceChar     * target,
                       ConstSpiceChar     * abcorr,
                       ConstSpiceChar     * obsrvr,
                       ConstSpiceChar     * relate,
                       SpiceDouble          refval,
                       SpiceDouble          adjust,
                       SpiceDouble          step,
                       SpiceInt             nintvls,
                       SpiceCell          * cnfine,
                       SpiceCell          * result     )

#### Abstract

       Return the time window over which a specified constraint on
       observer-target distance is met.

#### Required_Reading

       GF
       NAIF_IDS
       SPK
       TIME
       WINDOWS

#### Keywords

       EPHEMERIS
       EVENT
       GEOMETRY
       SEARCH
       WINDOW

#### Brief_I/O

       Variable         I/O  Description
       ---------------  ---  ------------------------------------------------
       SPICE_GF_CNVTOL   P   Convergence tolerance
       target            I   Name of the target body.
       abcorr            I   Aberration correction flag.
       obsrvr            I   Name of the observing body.
       relate            I   Relational operator.
       refval            I   Reference value.
       adjust            I   Adjustment value for absolute extrema searches.
       step              I   Step size used for locating extrema and roots.
       nintvls           I   Workspace window interval count.
       cnfine           I-O  SPICE window to which the search is confined.
       result            O   SPICE window containing results.

#### Detailed_Input

       target      is the name of a target body. Optionally, you may supply
                   a string containing the integer ID code for the object.
                   For example both "MOON" and "301" are legitimate strings
                   that indicate the Moon is the target body.

                   The target and observer define a position vector which
                   points from the observer to the target; the length of
                   this vector is the "distance" that serves as the subject
                   of the search performed by this routine.

                   Case and leading or trailing blanks are not significant
                   in the string `target'.

       abcorr      indicates the aberration corrections to be applied to
                   the observer-target position vector to account for
                   one-way light time and stellar aberration.

                   Any aberration correction accepted by the SPICE
                   routine spkezr_c is accepted here. See the header
                   of spkezr_c for a detailed description of the
                   aberration correction options. For convenience,
                   the options are listed below:

                      "NONE"     Apply no correction.

                      "LT"       "Reception" case:  correct for
                                 one-way light time using a Newtonian
                                 formulation.

                      "LT+S"     "Reception" case:  correct for
                                 one-way light time and stellar
                                 aberration using a Newtonian
                                 formulation.

                      "CN"       "Reception" case:  converged
                                 Newtonian light time correction.

                      "CN+S"     "Reception" case:  converged
                                 Newtonian light time and stellar
                                 aberration corrections.

                      "XLT"      "Transmission" case:  correct for
                                 one-way light time using a Newtonian
                                 formulation.

                      "XLT+S"    "Transmission" case:  correct for
                                 one-way light time and stellar
                                 aberration using a Newtonian
                                 formulation.

                      "XCN"      "Transmission" case:  converged
                                 Newtonian light time correction.

                      "XCN+S"    "Transmission" case:  converged
                                 Newtonian light time and stellar
                                 aberration corrections.

                   Case and leading or trailing blanks are not significant
                   in the string `abcorr'.

       obsrvr      is the name of the observing body. Optionally, you may
                   supply a string containing the integer ID code for the
                   object. For example both "MOON" and "301" are legitimate
                   strings that indicate the Moon is the observer.

                   Case and leading or trailing blanks are not significant
                   in the string `obsrvr'.

       relate      is a relational operator used to define a constraint on
                   the observer-target distance. The result window found by
                   this routine indicates the time intervals where the
                   constraint is satisfied. Supported values of `relate'
                   and corresponding meanings are shown below:

                      ">"      Distance is greater than the reference
                               value `refval'.

                      "="      Distance is equal to the reference
                               value `refval'.

                      "<"      Distance is less than the reference
                               value `refval'.

                      "ABSMAX"  Distance is at an absolute maximum.

                      "ABSMIN"  Distance is at an absolute  minimum.

                      "LOCMAX"  Distance is at a local maximum.

                      "LOCMIN"  Distance is at a local minimum.

                   `relate' may be used to specify an "adjusted" absolute
                   extremum constraint: this requires the distance
                   to be within a specified offset relative to an
                   absolute extremum. The argument `adjust' (described
                   below) is used to specify this offset.

                   Local extrema are considered to exist only in the
                   interiors of the intervals comprising the confinement
                   window:  a local extremum cannot exist at a boundary
                   point of the confinement window.

                   Case and leading or trailing blanks are not significant
                   in the string `relate'.

       `refval'    is the reference value used together with the argument
                   `relate' to define an equality or inequality to be
                   satisfied by the distance between the specified target
                   and observer. See the discussion of `relate' above for
                   further information.

                   The units of `refval' are km.

       adjust      is a parameter used to modify searches for absolute
                   extrema: when `relate' is set to "ABSMAX" or "ABSMIN"
                   and `adjust' is set to a positive value, gfdist_c will
                   find times when the observer-target distance is within
                   `adjust' km of the specified extreme value.

                   If `adjust' is non-zero and a search for an absolute
                   minimum `min' is performed, the result window contains
                   time intervals when the observer-target distance has
                   values between `min' and min+adjust.

                   If the search is for an absolute maximum `max', the
                   corresponding range is from max-adjust to `max'.

                   `adjust' is not used for searches for local extrema,
                   equality or inequality conditions.

       step        is the step size to be used in the search. `step' must
                   be shorter than any maximal time interval on which the
                   specified distance function is monotone increasing or
                   decreasing. That is, if the confinement window is
                   partitioned into alternating intervals on which the
                   distance function is either monotone increasing or
                   decreasing, `step' must be shorter than any of these
                   intervals.

                   However, `step' must not be *too* short, or the search
                   will take an unreasonable amount of time.

                   The choice of `step' affects the completeness but not
                   the precision of solutions found by this routine; the
                   precision is controlled by the convergence tolerance.
                   See the discussion of the parameter SPICE_GF_CNVTOL for
                   details.

                   STEP has units of TDB seconds.

       nintvls     is a parameter specifying the number of intervals that
                   can be accommodated by each of the dynamically allocated
                   workspace windows used internally by this routine.

                   In many cases, it's not necessary to compute an accurate
                   estimate of how many intervals are needed; rather, the
                   user can pick a size considerably larger than what's
                   really required.

                   However, since excessively large arrays can prevent
                   applications from compiling, linking, or running
                   properly, sometimes `nintvls' must be set according to
                   the actual workspace requirement. A rule of thumb for
                   the number of intervals needed is

                      nintvls  =  2*n  +  ( m / step )

                   where

                      n     is the number of intervals in the confinement
                            window

                      m     is the measure of the confinement window, in
                            units of seconds

                      step  is the search step size in seconds

       cnfine      is a SPICE window that confines the time period over
                   which the specified search is conducted. `cnfine' may
                   consist of a single interval or a collection of
                   intervals.

                   The endpoints of the time intervals comprising `cnfine'
                   are interpreted as seconds past J2000 TDB.

                   See the Examples section below for a code example that
                   shows how to create a confinement window.

#### Detailed_Output

       cnfine      is the input confinement window, updated if necessary so
                   the control area of its data array indicates the
                   window's size and cardinality. The window data are
                   unchanged.

       result      is the window of intervals, contained within the
                   confinement window `cnfine', on which the specified
                   distance constraint is satisfied.

                   The endpoints of the time intervals comprising `result'
                   are interpreted as seconds past J2000 TDB.

                   If `result' is non-empty on input, its contents will be
                   discarded before gfdist_c conducts its search.

#### Parameters

       SPICE_GF_CNVTOL

                   is the convergence tolerance used for finding endpoints
                   of the intervals comprising the result window.
                   SPICE_GF_CNVTOL is used to determine when binary
                   searches for roots should terminate: when a root is
                   bracketed within an interval of length SPICE_GF_CNVTOL,
                   the root is considered to have been found.

                   The accuracy, as opposed to precision, of roots found by
                   this routine depends on the accuracy of the input data.
                   In most cases, the accuracy of solutions will be
                   inferior to their precision.

                   SPICE_GF_CNVTOL is declared in the header file
                   SpiceGF.h.

#### Exceptions

       1)  In order for this routine to produce correct results,
           the step size must be appropriate for the problem at hand.
           Step sizes that are too large may cause this routine to miss
           roots; step sizes that are too small may cause this routine
           to run unacceptably slowly and in some cases, find spurious
           roots.

           This routine does not diagnose invalid step sizes, except
           that if the step size is non-positive, an error is signaled
           by a routine in the call tree of this routine.

       2)  Due to numerical errors, in particular,

              - Truncation error in time values
              - Finite tolerance value
              - Errors in computed geometric quantities

           it is *normal* for the condition of interest to not always be
           satisfied near the endpoints of the intervals comprising the
           result window.

           The result window may need to be contracted slightly by the
           caller to achieve desired results. The SPICE window routine
           wncond_c can be used to contract the result window.

       3)  If an error (typically cell overflow) occurs while performing
           window arithmetic, the error will be diagnosed by a routine
           in the call tree of this routine.

       4)  If the relational operator `relate' is not recognized, an
           error is signaled by a routine in the call tree of this
           routine.

       5)  If the aberration correction specifier contains an
           unrecognized value, an error is signaled by a routine in the
           call tree of this routine.

       6)  If `adjust' is negative, an error is signaled by a routine in
           the call tree of this routine.

       7)  If either of the input body names do not map to NAIF ID
           codes, an error is signaled by a routine in the call tree of
           this routine.

       8)  If required ephemerides or other kernel data are not
           available, an error is signaled by a routine in the call tree
           of this routine.

       9)  If the workspace interval count is less than 1, the error
           SPICE(VALUEOUTOFRANGE) will be signaled.

       10) If the required amount of workspace memory cannot be
           allocated, the error SPICE(MALLOCFAILURE) will be
           signaled.

       11) If the output SPICE window `result' has insufficient capacity to
           contain the number of intervals on which the specified distance
           condition is met, the error will be diagnosed by a routine in
           the call tree of this routine. If the result window has size
           less than 2, the error SPICE(INVALIDDIMENSION) will be signaled
           by this routine.

       12) If any input string argument pointer is null, the error
           SPICE(NULLPOINTER) will be signaled.

       13) If any input string argument is empty, the error
           SPICE(EMPTYSTRING) will be signaled.

       14) If either input cell has type other than SpiceDouble,
           the error SPICE(TYPEMISMATCH) is signaled.

#### Files

       Appropriate SPICE kernels must be loaded by the calling program before
       this routine is called.

       The following data are required:

          - SPK data: ephemeris data for target and observer for the
            time period defined by the confinement window must be
            loaded. If aberration corrections are used, the states of
            target and observer relative to the solar system barycenter
            must be calculable from the available ephemeris data.
            Typically ephemeris data are made available by loading one
            or more SPK files via furnsh_c.

          - If non-inertial reference frames are used by the SPK files,
            then PCK files, frame kernels, C-kernels, and SCLK kernels may
            be needed.

       Kernel data are normally loaded once per program run, NOT every time
       this routine is called.

#### Particulars

       This routine determines a set of one or more time intervals
       within the confinement window when the distance between the
       specified target and observer satisfies a caller-specified
       constraint. The resulting set of intervals is returned as a SPICE
       window.

       Below we discuss in greater detail aspects of this routine's
       solution process that are relevant to correct and efficient
       use of this routine in user applications.

       The Search Process
       ==================

       Regardless of the type of constraint selected by the caller, this
       routine starts the search for solutions by determining the time
       periods, within the confinement window, over which the specified
       distance function is monotone increasing and monotone decreasing.
       Each of these time periods is represented by a SPICE window. Having
       found these windows, all of the distance function's local extrema
       within the confinement window are known. Absolute extrema then can
       be found very easily.

       Within any interval of these "monotone" windows, there will be at
       most one solution of any equality constraint. With these solutions
       in hand, solutions of inequalities are easily found as well.

       Step Size
       =========

       The monotone windows (described above) are found via a two-step
       search process. Each interval of the confinement window is searched
       as follows: first, the input step size is the time separation at
       which the sign of the rate of change of distance ("range rate") is
       sampled. Starting at the left endpoint of the interval, samples will
       be taken at each step. If a change of sign is found, a root has been
       bracketed; at that point, the time at which the range rate is zero
       can be found by a refinement process, for example, via binary
       search.

       Note that the optimal choice of step size depends on the lengths
       of the intervals over which the distance function is monotone:
       the step size should be shorter than the shortest of these
       intervals (within the confinement window).

       The optimal step size is *not* necessarily related to the lengths
       of the intervals comprising the result window. For example, if
       the shortest monotone interval has length 10 days, and if the
       shortest result window interval has length 5 minutes, a step size
       of 9.9 days is still adequate to find all of the intervals in the
       result window. In situations like this, the technique of using
       monotone windows yields a dramatic efficiency improvement over a
       state-based search that simply tests at each step whether the
       specified constraint is satisfied. The latter type of search can
       miss solution intervals if the step size is longer than the
       shortest solution interval.

       Having some knowledge of the relative geometry of the target and
       observer can be a valuable aid in picking a reasonable step size.
       In general, the user can compensate for lack of such knowledge by
       picking a very short step size; the cost is increased computation
       time.

       Note that the step size is not related to the precision with which
       the endpoints of the intervals of the result window are computed.
       That precision level is controlled by the convergence tolerance.

       Convergence Tolerance
       =====================

       As described above, the root-finding process used by this routine
       involves first bracketing roots and then using a search process to
       locate them.  "Roots" include times when extrema are attained and
       times when the distance function is equal to a reference value or
       adjusted extremum. All endpoints of the intervals comprising the
       result window are either endpoints of intervals of the confinement
       window or roots.

       Once a root has been bracketed, a refinement process is used to
       narrow down the time interval within which the root must lie.
       This refinement process terminates when the location of the root
       has been determined to within an error margin called the
       "convergence tolerance." The convergence tolerance used by this
       routine is set via the parameter SPICE_GF_CNVTOL.

       The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
       tolerance doesn't limit the accuracy of solutions found by this
       routine. In general the accuracy of input data will be the limiting
       factor.

       The user may change the convergence tolerance from the default
       SPICE_GF_CNVTOL value by calling the routine gfstol_c, e.g.

          gfstol_c( tolerance value )

       Call gfstol_c prior to calling this routine. All subsequent
       searches will use the updated tolerance value.

       To use a different tolerance value, a lower-level GF routine such
       as gfevnt_c  must be called. Making the tolerance tighter than
       SPICE_GF_CNVTOL is unlikely to be useful, since the results are unlikely
       to be more accurate. Making the tolerance looser will speed up
       searches somewhat, since a few convergence steps will be omitted.
       However, in most cases, the step size is likely to have a much
       greater affect on processing time than would the convergence
       tolerance.

       The Confinement Window
       ======================

       The simplest use of the confinement window is to specify a time
       interval within which a solution is sought. However, the
       confinement window can, in some cases, be used to make searches
       more efficient. Sometimes it's possible to do an efficient search
       to reduce the size of the time period over which a relatively
       slow search of interest must be performed. See the "CASCADE"
       example program in gf.req for a demonstration.

#### Examples

       The numerical results shown for these examples 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) Find times during the first three months of the year 2007
          when the Earth-Moon distance is greater than 400000 km.
          Display the start and stop times of the time intervals
          over which this constraint is met, along with the Earth-Moon
          distance at each interval endpoint.

          We expect the Earth-Moon distance to be an oscillatory function
          with extrema roughly two weeks apart. Using a step size of one
          day will guarantee that the GF system will find all distance
          extrema. (Recall that a search for distance extrema is an
          intermediate step in the GF search process.)

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

             KPL/MK

             File name: standard.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
                ---------                     --------
                de421.bsp                     Planetary ephemeris
                pck00008.tpc                  Planet orientation and
                                              radii
                naif0009.tls                  Leapseconds

             \begindata

                KERNELS_TO_LOAD = ( 'de421.bsp',
                                    'pck00008.tpc',
                                    'naif0009.tls'  )

             \begintext

             End of meta-kernel

          Example code begins here.

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

             int main()
             {
                /.
                Constants
                ./
                #define  TIMFMT  "YYYY MON DD HR:MN:SC.###"
                #define  MAXWIN  200
                #define  NINTVL  100
                #define  TIMLEN  41

                /.
                Local variables
                ./
                SpiceChar               begstr [ TIMLEN ];
                SpiceChar               endstr [ TIMLEN ];

                SPICEDOUBLE_CELL      ( cnfine, MAXWIN );
                SPICEDOUBLE_CELL      ( result, MAXWIN );

                SpiceDouble             adjust;
                SpiceDouble             dist;
                SpiceDouble             et0;
                SpiceDouble             et1;
                SpiceDouble             lt;
                SpiceDouble             pos    [3];
                SpiceDouble             refval;
                SpiceDouble             start;
                SpiceDouble             step;
                SpiceDouble             stop;

                SpiceInt                i;

                /.
                Load kernels.
                ./
                furnsh_c ( "standard.tm" );

                /.
                Store the time bounds of our search interval in
                the confinement window.
                ./
                str2et_c ( "2007 JAN 1", &et0 );
                str2et_c ( "2007 APR 1", &et1 );

                wninsd_c ( et0, et1, &cnfine );

                /.
                Search using a step size of 1 day (in units of
                seconds).  The reference value is 400000 km.
                We're not using the adjustment feature, so
                we set `adjust' to zero.
                ./
                step   = spd_c();
                refval = 4.e5;
                adjust = 0.0;

                /.
                Perform the search. The set of times when the
                constraint is met will be stored in the SPICE
                window `result'.
                ./
                gfdist_c ( "MOON", "NONE", "EARTH", ">",     refval,
                           adjust, step,   NINTVL,  &cnfine, &result );

                /.
                Display the results.
                ./
                if ( wncard_c(&result) == 0 )
                {
                   printf ( "Result window is empty.\n\n" );
                }
                else
                {
                   for ( i = 0;  i < wncard_c(&result);  i++ )
                   {
                      /.
                      Fetch the endpoints of the Ith interval
                      of the result window.
                      ./
                      wnfetd_c ( &result, i, &start, &stop );

                      /.
                      Check the distance at the interval's
                      start and stop times.
                      ./
                      spkpos_c ( "MOON",  start, "J2000", "NONE",
                                 "EARTH", pos,   &lt              );

                      dist = vnorm_c(pos);

                      timout_c ( start, TIMFMT, TIMLEN, begstr );

                      printf ( "Start time, distance = %s %17.9f\n",
                               begstr, dist                          );

                      spkpos_c ( "MOON",  stop, "J2000", "NONE",
                                 "EARTH", pos,  &lt              );

                      dist = vnorm_c(pos);

                      timout_c ( stop, TIMFMT, TIMLEN, endstr );

                      printf ( "Stop time,  distance = %s %17.9f\n",
                               endstr, dist                          );
                   }
                }

                return ( 0 );
             }

          When this program was executed on a PC/Linux/gcc platform, the
          output was:

             Start time, distance = 2007 JAN 08 00:10:02.439  399999.999999989
             Stop time,  distance = 2007 JAN 13 06:36:42.770  400000.000000010
             Start time, distance = 2007 FEB 04 07:01:30.094  399999.999999990
             Stop time,  distance = 2007 FEB 10 09:29:56.659  399999.999999998
             Start time, distance = 2007 MAR 03 00:19:19.998  400000.000000006
             Stop time,  distance = 2007 MAR 10 14:03:33.312  400000.000000007
             Start time, distance = 2007 MAR 29 22:52:52.961  399999.999999995
             Stop time,  distance = 2007 APR 01 00:00:00.000  404531.955232216

          Note that the distance at the final solutions interval's stop
          time is not close to the reference value of 400000 km. This is
          because the interval's stop time was determined by the stop time
          of the confinement window.

       2) Extend the first example to demonstrate use of all supported
          relational operators. Find times when

             Earth-Moon distance is = 400000 km
             Earth-Moon distance is < 400000 km
             Earth-Moon distance is > 400000 km
             Earth-Moon distance is at a local minimum
             Earth-Moon distance is at a absolute minimum
             Earth-Moon distance is > the absolute minimum + 100 km
             Earth-Moon distance is at a local maximum
             Earth-Moon distance is at a absolute maximum
             Earth-Moon distance is > the absolute maximum - 100 km

          To shorten the search time and output, use the
          shorter search interval

             2007 JAN 15 00:00:00 UTC  to
             2007 MAR 15 00:00:00 UTC

          As before, use geometric (uncorrected) positions, so
          set the aberration correction flag to 'NONE'.

          Use the meta-kernel from the first example.

          Example code begins here.

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

             int main()
             {
                /.
                Constants
                ./
                #define  TIMFMT  "YYYY MON DD HR:MN:SC.###"
                #define  LNSIZE  81
                #define  MAXWIN  200
                #define  NINTVL  100
                #define  TIMLEN  41
                #define  NRELOP  9

                /.
                Local variables
                ./
                SpiceChar               begstr [ TIMLEN ];
                SpiceChar               endstr [ TIMLEN ];

                static ConstSpiceChar * relate [NRELOP] =
                                        {
                                           "=",
                                           "<",
                                           ">",
                                           "LOCMIN",
                                           "ABSMIN",
                                           "ABSMIN",
                                           "LOCMAX",
                                           "ABSMAX",
                                           "ABSMAX"
                                        };

                static ConstSpiceChar * templt [NRELOP] =
                   {
                      "Condition: distance = # km",
                      "Condition: distance < # km",
                      "Condition: distance > # km",
                      "Condition: distance is a local minimum",
                      "Condition: distance is the absolute minimum",
                      "Condition: distance < the absolute minimum + * km",
                      "Condition: distance is a local maximum",
                      "Condition: distance is the absolute maximum",
                      "Condition: distance > the absolute maximum - * km"
                   };

                SpiceChar               title [ LNSIZE ];

                SPICEDOUBLE_CELL      ( cnfine, MAXWIN );
                SPICEDOUBLE_CELL      ( result, MAXWIN );

                static SpiceDouble      adjust [NRELOP] =
                                        {
                                           0.0,
                                           0.0,
                                           0.0,
                                           0.0,
                                           0.0,
                                           100.0,
                                           0.0,
                                           0.0,
                                           100.0
                                        };

                SpiceDouble             dist;
                SpiceDouble             et0;
                SpiceDouble             et1;
                SpiceDouble             lt;
                SpiceDouble             pos    [3];
                SpiceDouble             refval;
                SpiceDouble             start;
                SpiceDouble             step;
                SpiceDouble             stop;

                SpiceInt                i;
                SpiceInt                j;

                /.
                Load kernels.
                ./
                furnsh_c ( "standard.tm" );

                /.
                Store the time bounds of our search interval in
                the confinement window.
                ./
                str2et_c ( "2007 JAN 15", &et0 );
                str2et_c ( "2007 MAR 15", &et1 );

                wninsd_c ( et0, et1, &cnfine );

                /.
                Search using a step size of 1 day (in units of
                seconds). Use a reference value of 400000 km.
                ./
                refval = 400000.0;
                step   = spd_c();

                for ( i = 0;  i < NRELOP;  i++ )
                {
                   gfdist_c ( "MOON",    "NONE", "EARTH", relate[i], refval,
                              adjust[i], step,   NINTVL,  &cnfine,   &result );

                   /.
                   Display the results.
                   ./
                   printf ( "\n" );

                   /.
                   Substitute the reference and adjustment values,
                   where applicable, into the title string:
                   ./
                   repmd_c ( templt[i], "#", refval,    6, LNSIZE, title );
                   repmd_c ( title,     "*", adjust[i], 6, LNSIZE, title );

                   printf ( "%s\n", title );

                   if ( wncard_c(&result) == 0 )
                   {
                      printf ( " Result window is empty.\n" );
                   }
                   else
                   {
                      printf ( " Result window:\n" );

                      for ( j = 0;  j < wncard_c(&result);  j++ )
                      {
                         /.
                         Fetch the endpoints of the jth interval
                         of the result window.
                         ./
                         wnfetd_c ( &result, j, &start, &stop );

                         /.
                         Check the distance at the interval's
                         start and stop times.
                         ./
                         spkpos_c ( "MOON",  start, "J2000", "NONE",
                                    "EARTH", pos,   &lt              );

                         dist = vnorm_c(pos);

                         timout_c ( start, TIMFMT, TIMLEN, begstr );

                         printf ( "  Start time, distance = %s %17.9f\n",
                                  begstr, dist                          );

                         spkpos_c ( "MOON",  stop, "J2000", "NONE",
                                    "EARTH", pos,  &lt              );

                         dist = vnorm_c(pos);

                         timout_c ( stop, TIMFMT, TIMLEN, endstr );

                         printf ( "  Stop time,  distance = %s %17.9f\n",
                                  endstr, dist                          );
                      }
                   }
                }
                printf ( "\n" );

                return ( 0 );
             }

          When this program was executed on a PC/Linux/gcc platform, the
          output was:

             Condition: distance = 4.00000E+05 km
              Result window:
               Start time, distance = 2007 FEB 04 07:01:30.094  399999.999999998
               Stop time,  distance = 2007 FEB 04 07:01:30.094  399999.999999998
               Start time, distance = 2007 FEB 10 09:29:56.659  399999.999999989
               Stop time,  distance = 2007 FEB 10 09:29:56.659  399999.999999989
               Start time, distance = 2007 MAR 03 00:19:19.998  399999.999999994
               Stop time,  distance = 2007 MAR 03 00:19:19.998  399999.999999994
               Start time, distance = 2007 MAR 10 14:03:33.312  400000.000000000
               Stop time,  distance = 2007 MAR 10 14:03:33.312  400000.000000000

             Condition: distance < 4.00000E+05 km
              Result window:
               Start time, distance = 2007 JAN 15 00:00:00.000  393018.609906208
               Stop time,  distance = 2007 FEB 04 07:01:30.094  399999.999999990
               Start time, distance = 2007 FEB 10 09:29:56.659  399999.999999998
               Stop time,  distance = 2007 MAR 03 00:19:19.998  400000.000000006
               Start time, distance = 2007 MAR 10 14:03:33.312  400000.000000010
               Stop time,  distance = 2007 MAR 15 00:00:00.000  376255.453934464

             Condition: distance > 4.00000E+05 km
              Result window:
               Start time, distance = 2007 FEB 04 07:01:30.094  399999.999999990
               Stop time,  distance = 2007 FEB 10 09:29:56.659  399999.999999998
               Start time, distance = 2007 MAR 03 00:19:19.998  400000.000000006
               Stop time,  distance = 2007 MAR 10 14:03:33.312  400000.000000010

             Condition: distance is a local minimum
              Result window:
               Start time, distance = 2007 JAN 22 12:30:49.458  366925.804109350
               Stop time,  distance = 2007 JAN 22 12:30:49.458  366925.804109350
               Start time, distance = 2007 FEB 19 09:36:29.968  361435.646812061
               Stop time,  distance = 2007 FEB 19 09:36:29.968  361435.646812061

             Condition: distance is the absolute minimum
              Result window:
               Start time, distance = 2007 FEB 19 09:36:29.968  361435.646812061
               Stop time,  distance = 2007 FEB 19 09:36:29.968  361435.646812061

             Condition: distance < the absolute minimum + 1.00000E+02 km
              Result window:
               Start time, distance = 2007 FEB 19 01:09:52.706  361535.646812062
               Stop time,  distance = 2007 FEB 19 18:07:45.136  361535.646812061

             Condition: distance is a local maximum
              Result window:
               Start time, distance = 2007 FEB 07 12:38:29.870  404992.424288620
               Stop time,  distance = 2007 FEB 07 12:38:29.870  404992.424288620
               Start time, distance = 2007 MAR 07 03:37:02.122  405853.452130754
               Stop time,  distance = 2007 MAR 07 03:37:02.122  405853.452130754

             Condition: distance is the absolute maximum
              Result window:
               Start time, distance = 2007 MAR 07 03:37:02.122  405853.452130754
               Stop time,  distance = 2007 MAR 07 03:37:02.122  405853.452130754

             Condition: distance > the absolute maximum - 1.00000E+02 km
              Result window:
               Start time, distance = 2007 MAR 06 15:56:00.957  405753.452130753
               Stop time,  distance = 2007 MAR 07 15:00:38.674  405753.452130753

#### Restrictions

       1) The kernel files to be used by this routine must be loaded
          (normally via the CSPICE routine furnsh_c) before this routine
          is called.

       2) This routine has the side effect of re-initializing the
          distance quantity utility package.

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman   (JPL)
       E.D. Wright    (JPL)

#### Version

       -CSPICE Version 1.0.1, 28-FEB-2013 (NJB) (EDW)

           Header was updated to discuss use of gfstol_c. A
           header typo was corrected.

       -CSPICE Version 1.0.0, 15-APR-2009 (NJB) (EDW)

#### Index_Entries

       GF distance search

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

    Wed Apr  5 17:54:35 2017