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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

       gfsubc_c (GF, subpoint vector coordinate search)

       void gfsubc_c ( ConstSpiceChar     * target,
                       ConstSpiceChar     * fixref,
                       ConstSpiceChar     * method,
                       ConstSpiceChar     * abcorr,
                       ConstSpiceChar     * obsrvr,
                       ConstSpiceChar     * crdsys,
                       ConstSpiceChar     * coord,
                       ConstSpiceChar     * relate,
                       SpiceDouble          refval,
                       SpiceDouble          adjust,
                       SpiceDouble          step,
                       SpiceInt             nintvls,
                       SpiceCell          * cnfine,
                       SpiceCell          * result  )

#### Abstract

       Determine time intervals for which a coordinate of an
       subpoint position vector satisfies a numerical constraint.

#### Required_Reading

       GF
       SPK
       CK
       TIME
       WINDOWS

#### Keywords

       COORDINATE
       EVENT
       GEOMETRY
       SEARCH

#### Brief_I/O

       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       SPICE_GF_CNVTOL
                  P   Convergence tolerance.
       target     I   Name of the target body.
       fixref     I   Body fixed frame associated with `target'.
       method     I   Name of method type for subpoint calculation.
       abcorr     I   Aberration correction flag.
       obsrvr     I   Name of the observing body.
       crdsys     I   Name of the coordinate system containing `coord'.
       coord      I   Name of the coordinate of interest.
       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 string name of a target body. Optionally, you may
                   supply the integer ID code for the object as an
                   integer string. 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
                   that points from the observer to the target.

       fixref      is the string name of the body-fixed, body-centered
                   reference frame associated with the target body `target'.

                   The SPICE frame subsystem must recognize the "fixref"
                   name.

       method      is the string name of the method to use for the subpoint
                   calculation. The accepted values for `method':

                      "Near point: ellipsoid"   The sub-observer point
                                                computation uses a
                                                triaxial ellipsoid to
                                                model the surface of the
                                                target body. The
                                                sub-observer point is
                                                defined as the nearest
                                                point on the target
                                                relative to the
                                                observer.

                      "Intercept: ellipsoid"    The sub-observer point
                                                computation uses a
                                                triaxial ellipsoid to
                                                model the surface of the
                                                target body. The
                                                sub-observer point is
                                                defined as the target
                                                surface intercept of the
                                                line containing the
                                                observer and the
                                                target's center.

                   The `method' string lacks sensitivity to case, embedded,
                   leading and trailing blanks.

       abcorr      is the string description of the aberration corrections
                   to apply to the state evaluations to account for one-way
                   light time and stellar aberration.

                   This routine accepts the same aberration corrections
                   as does the SPICE routine spkezr_c. 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. Returns the "true"
                                 geometric state.

                      "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.

                   The `abcorr' string lacks sensitivity to case, leading
                   and trailing blanks.

       obsrvr      is the string name of an observing body. Optionally, you
                   may supply the ID code of the object as an integer
                   string. For example, both "EARTH" and "399" are
                   legitimate strings to indicate that the observer is the
                   Earth.

       crdsys      is the string name of the coordinate system for which the
                   coordinate of interest is a member.

       coord       is the string name of the coordinate of interest in
                   `crdsys'.

                   The supported coordinate systems and coordinate names:

                      crdsys             coord               Range
                      ----------------   -----------------   ------------

                      "RECTANGULAR"      "X"
                                         "Y"
                                         "Z"

                      "LATITUDINAL"      "RADIUS"
                                         "LONGITUDE"         (-Pi,Pi]
                                         "LATITUDE"          [-Pi/2,Pi/2]

                      "RA/DEC"           "RANGE"
                                         "RIGHT ASCENSION"   [0,2Pi)
                                         "DECLINATION"       [-Pi/2,Pi/2]

                      "SPHERICAL"        "RADIUS"
                                         "COLATITUDE"        [0,Pi]
                                         "LONGITUDE"         (-Pi,Pi]

                      "CYLINDRICAL"      "RADIUS"
                                         "LONGITUDE"         [0,2Pi)
                                         "Z"

                      "GEODETIC"         "LONGITUDE"         (-Pi,Pi]
                                         "LATITUDE"          [-Pi/2,Pi/2]
                                         "ALTITUDE"

                      "PLANETOGRAPHIC"   "LONGITUDE"         [0,2Pi)
                                         "LATITUDE"          [-Pi/2,Pi/2]
                                         "ALTITUDE"

                   The "ALTITUDE" coordinates have a constant value of
                   zero +/- roundoff for ellipsoid targets.

                   Limit searches for coordinate events in the "GEODETIC"
                   and "PLANETOGRAPHIC" coordinate systems to `target' bodies
                   with axial symmetry in the equatorial plane, i.e.
                   equality of the body X and Y radii (oblate or prolate
                   spheroids).

                   Searches on "GEODETIC" or "PLANETOGRAPHIC" coordinates
                   requires body shape data, and in the case of
                   "PLANETOGRAPHIC" coordinates, body rotation data.

                   The body associated with "GEODETIC" or "PLANETOGRAPHIC"
                   coordinates is the center of the frame `fixref'.

       relate      is the string or character describing the relational
                   operator used to define a constraint on the selected
                   coordinate of the subpoint vector. 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:

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

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

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

                      "ABSMAX"   The coordinate value is at an absolute
                                 maximum.

                      "ABSMIN"   The coordinate value is at an absolute
                                 minimum.

                      "LOCMAX"   The coordinate value is at a local
                                 maximum.

                      "LOCMIN"   The coordinate value is at a local
                                 minimum.

                   The caller may indicate that the region of interest
                   is the set of time intervals where the quantity is
                   within a specified measure of an absolute extremum.
                   The argument `adjust' (described below) is used to
                   specify this measure.

                   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.

                   The `relate' string lacks sensitivity to case, leading
                   and trailing blanks.

       refval      is the double precision reference value used together
                   with the argument `relate' to define an equality or
                   inequality to satisfy by the selected coordinate of the
                   subpoint vector. See the discussion of `relate' above for
                   further information.

                   The units of `refval' correspond to the type as defined
                   by `coord', radians for angular measures, kilometers for
                   distance measures.

       adjust      is a double precision value used to modify searches for
                   absolute extrema: when `relate' is set to "ABSMAX" or
                   "ABSMIN" and `adjust' is set to a positive value, gfsubc_c
                   finds times when the subpoint position vector coordinate
                   is within `adjust' radians/kilometers of the specified
                   extreme value.

                   For `relate' set to "ABSMAX", the `result' window contains
                   time intervals when the position vector coordinate has
                   values between absmax - adjust and `absmax'.

                   For `relate' set to "ABSMIN", the `result' window contains
                   time intervals when the position vector coordinate has
                   values between `absmin' and absmin + adjust.

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

       step        is the double precision time step size to use in the
                   search.

                   `step' must be short enough to for a search using this step
                   size to locate the time intervals where coordinate
                   function of the subpoint vector is monotone increasing or
                   decreasing. However, `step' must not be *too* short, or
                   the search will take an unreasonable amount of time.

                   For coordinates other than "LONGITUDE" and 'RIGHT
                   ASCENSION', the step size must be shorter than the
                   shortest interval, within the confinement window, over
                   which the coordinate is monotone increasing or
                   decreasing.

                   For "LONGITUDE" and "RIGHT ASCENSION", the step size must
                   be shorter than the shortest interval, within the
                   confinement window, over which either the sin or cos
                   of the coordinate is monotone increasing or decreasing.

                   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 an integer 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 double precision 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.

                   In some cases the confinement window can be used to
                   greatly reduce the time period that must be searched
                   for the desired solution. See the -Particulars section
                   below for further discussion.

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

                   In some cases the observer's state may be computed at
                   times outside of `cnfine' by as much as 2 seconds. See
                   -Particulars for details.

                   `cnfine' must be declared as a double precision SpiceCell.

                   CSPICE provides the following macro, which declares and
                   initializes the cell

                      SPICEDOUBLE_CELL        ( cnfine, CNFINESZ );

                   where CNFINESZ is the maximum capacity of `cnfine'.

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

                   `result' must be declared and initialized with sufficient
                   size to capture the full set of time intervals within the
                   search region on which the specified condition is satisfied.

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

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

                   If the search is for local extrema, or for absolute
                   extrema with `adjust' set to zero, then normally each
                   interval of `result' will be a singleton: the left and
                   right endpoints of each interval will be identical.

                   If no times within the confinement window satisfy the
                   search criteria, `result' will be returned with a
                   cardinality of zero.

                   `result' must be declared as a double precision SpiceCell.

                   CSPICE provides the following macro, which declares and
                   initializes the cell

                      SPICEDOUBLE_CELL        ( result, RESULTSZ );

                   where RESULTSZ is the maximum capacity of `result'.

#### 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 has the value 1.0e-6. Units are TDB
                  seconds.

#### 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. One technique to handle such a situation,
           slightly contract `result' using the window routine wncond_c.

       3)  If the number of intervals `nintvls' is less than 1, the error
           SPICE(VALUEOUTOFRANGE) is signaled.

       4)  If the window size of `result' is less than 2, the error
           SPICE(INVALIDDIMENSION) is signaled by a routine in the call
           tree of this routine.

       5)  If the output SPICE window `result' has insufficient capacity
           to contain the number of intervals on which the specified
           distance condition is met, an error is signaled
           by a routine in the call tree of this routine.

       6)  If an error (typically cell overflow) occurs during
           window arithmetic, the error is signaled by a routine
           in the call tree of this routine.

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

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

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

       10) 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.

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

       12) If the search uses GEODETIC or PLANETOGRAPHIC coordinates, and
           the center body of the reference frame has unequal equatorial
           radii, an error is signaled by a routine in the call tree of
           this routine.

       13) If any of the `target', `fixref', `method', `abcorr',
           `obsrvr', `crdsys', `coord' or `relate' input string pointers
           is null, the error SPICE(NULLPOINTER) is signaled.

       14) If any of the `target', `fixref', `method', `abcorr',
           `obsrvr', `crdsys', `coord' or `relate' input strings has zero
           length, the error SPICE(EMPTYSTRING) is signaled.

       15) If any the `cnfine' or `result' cell arguments has a type
           other than SpiceDouble, the error SPICE(TYPEMISMATCH) is
           signaled.

       16) If memory cannot be allocated to create the temporary variable
           required for the execution of the underlying Fortran routine,
           the error SPICE(MALLOCFAILED) is signaled.

#### Files

       Appropriate SPK and PCK kernels must be loaded by the calling
       program before this routine is called.

       The following data are required:

       -  SPK data: the calling application must load ephemeris data
          for the targets, observer, and any intermediate objects in
          a chain connecting the targets and observer that cover the
          time period specified by the window `cnfine'. 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 using
          furnsh_c.

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

       -  In some cases the observer's state may be computed at times
          outside of `cnfine' by as much as 2 seconds; data required to
          compute this state must be provided by loaded kernels. See
          -Particulars for details.

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

#### Particulars

       This routine provides a simpler, but less flexible interface
       than does the routine gfevnt_c for conducting searches for
       subpoint position vector coordinate value events.
       Applications that require support for progress reporting,
       interrupt handling, non-default step or refinement functions, or
       non-default convergence tolerance should call gfevnt_c rather than
       this routine.

       This routine determines a set of one or more time intervals
       within the confinement window when the selected coordinate of
       the subpoint position vector 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
       coordinate 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 coordinate
       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. Since the boundary
       of the solution set for any inequality constraint is contained in
       the union of

       -  the set of points where an equality constraint is met

       -  the boundary points of the confinement window

       the solutions of both equality and inequality constraints can be
       found easily once the monotone windows have been found.

       Step Size
       =========

       The monotone windows (described above) are found using a two-step
       search process. Each interval of the confinement window is
       searched as follows: first, the input step size is used to
       determine the time separation at which the sign of the rate of
       change of coordinate will be sampled. Starting at
       the left endpoint of an 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 time derivative of the
       coordinate is zero can be found by a refinement process, for
       example, using a binary search.

       Note that the optimal choice of step size depends on the lengths
       of the intervals over which the coordinate 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" are both times when local extrema are
       attained and times when the coordinate function is equal to a
       reference value. 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 default convergence tolerance
       used by this routine is set by the parameter SPICE_GF_CNVTOL (defined
       in SpiceGF.h).

       The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
       tolerance doesn't become the limiting factor in 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.

       Setting 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 effect
       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.

       Practical use of the coordinate search capability would likely
       consist of searches over multiple coordinate constraints to find
       time intervals that satisfies the constraints. An
       effective technique to accomplish such a search is
       to use the result window from one search as the confinement window
       of the next.

       Certain types of searches require the state of the observer,
       relative to the solar system barycenter, to be computed at times
       slightly outside the confinement window `cnfine'. The time window
       that is actually used is the result of "expanding" `cnfine' by a
       specified amount "T": each time interval of `cnfine' is expanded by
       shifting the interval's left endpoint to the left and the right
       endpoint to the right by T seconds. Any overlapping intervals are
       merged. (The input argument `cnfine' is not modified.)

       The window expansions listed below are additive: if both
       conditions apply, the window expansion amount is the sum of the
       individual amounts.

       -  If a search uses an equality constraint, the time window
          over which the state of the observer is computed is expanded
          by 1 second at both ends of all of the time intervals
          comprising the window over which the search is conducted.

       -  If a search uses stellar aberration corrections, the time
          window over which the state of the observer is computed is
          expanded as described above.

       When light time corrections are used, expansion of the search
       window also affects the set of times at which the light time-
       corrected state of the target is computed.

       In addition to the possible 2 second expansion of the search
       window that occurs when both an equality constraint and stellar
       aberration corrections are used, round-off error should be taken
       into account when the need for data availability is analyzed.

       Longitude and Right Ascension
       =============================

       The cyclic nature of the longitude and right ascension coordinates
       produces branch cuts at +/- 180 degrees longitude and 0-360
       longitude. Round-off error may cause solutions near these branches
       to cross the branch. Use of the SPICE routine wncond_c will contract
       solution windows by some epsilon, reducing the measure of the
       windows and eliminating the branch crossing. A one millisecond
       contraction will in most cases eliminate numerical round-off
       caused branch crossings.

#### 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) Find the time during 2007 for which the subpoint position vector
          of the sun on earth in the IAU_EARTH frame lies within a geodetic
          latitude-longitude "box" defined as

             16 degrees <= latitude  <= 17 degrees
             85 degrees <= longitude <= 86 degrees

          This problem requires four searches, each search on one of the
          box restrictions. The user needs also realize the temporal
          behavior of latitude greatly differs from that of the longitude. The
          sub-observer point latitude varies between approximately 23.44 degrees
          and -23.44 degrees during the year. The sub-observer point longitude
          varies between -180 degrees and 180 degrees in one day.

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

             KPL/MK

             File name: gfsubc_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
                ---------                     --------
                de414.bsp                     Planetary ephemeris
                pck00008.tpc                  Planet orientation and
                                              radii
                naif0008.tls                  Leapseconds

             \begindata

                KERNELS_TO_LOAD = ( 'de414.bsp',
                                    'pck00008.tpc',
                                    'naif0008.tls'  )

             \begintext

             End of meta-kernel

          Example code begins here.

          /.
             Program gfsubc_ex1
          ./
          #include <stdio.h>
          #include <stdlib.h>
          #include <string.h>
          #include "SpiceUsr.h"

          #define   MAXWIN   100
          #define   TIMFMT   "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND"
          #define   STRLEN   64

          int main( )
             {

             /.
             Create the needed windows. Note, one window
             consists of two values, so the total number
             of cell values to allocate equals twice
             the number of intervals.
             ./
             SPICEDOUBLE_CELL ( result1, 2*MAXWIN );
             SPICEDOUBLE_CELL ( result2, 2*MAXWIN );
             SPICEDOUBLE_CELL ( result3, 2*MAXWIN );
             SPICEDOUBLE_CELL ( result4, 2*MAXWIN );
             SPICEDOUBLE_CELL ( cnfine, 2       );

             SpiceDouble       begtim;
             SpiceDouble       endtim;
             SpiceDouble       step;
             SpiceDouble       adjust;
             SpiceDouble       refval;
             SpiceDouble       beg;
             SpiceDouble       end;

             SpiceChar         begstr [ STRLEN ];
             SpiceChar         endstr [ STRLEN ];
             SpiceChar       * target = "EARTH";
             SpiceChar       * obsrvr = "SUN";
             SpiceChar       * fixref = "IAU_EARTH";
             SpiceChar       * method = "Near point: ellipsoid";
             SpiceChar       * crdsys = "GEODETIC";
             SpiceChar       * abcorr = "NONE";

             SpiceInt          count;
             SpiceInt          i;

             /.
             Load kernels.
             ./
             furnsh_c( "gfsubc_ex1.tm" );

             /.
             Store the time bounds of our search interval in
             the cnfine confinement window.
             ./
             str2et_c( "2007 JAN 01", &begtim );
             str2et_c( "2008 JAN 01", &endtim );

             wninsd_c ( begtim, endtim, &cnfine );

             /.
             Perform four searches to determine the times when the
             latitude-longitude box restriction conditions apply to
             the subpoint vector.

             Perform the searches such that the result window of a search
             serves as the confinement window of the subsequent search.

             Since the latitude coordinate varies slowly and is well behaved
             over the time of the confinement window, search first for the
             windows satisfying the latitude requirements, then use that result
             as confinement for the longitude search.
             ./

             /.
             The latitude varies relatively slowly, ~46 degrees during the
             year. The extrema occur approximately every six months.
             Search using a step size less than half that value (180 days).
             For this example use ninety days (in units of seconds).
             ./

             step   = (90.)*spd_c();
             adjust = 0.;

             {
             SpiceChar       * coord  = "LATITUDE";
             SpiceChar       * relate = ">";

             refval = 16. *rpd_c();

             gfsubc_c (  target,  fixref,
                         method,  abcorr, obsrvr,
                         crdsys,  coord,
                         relate,  refval,
                         adjust,  step,
                         MAXWIN,
                         &cnfine, &result1 );
             }

             {
             SpiceChar       * coord  = "LATITUDE";
             SpiceChar       * relate = "<";

             refval = 17. *rpd_c();

             gfsubc_c (  target,  fixref,
                         method,  abcorr, obsrvr,
                         crdsys,  coord,
                         relate,  refval,
                         adjust,  step,
                         MAXWIN,
                         &result1, &result2 );
             }

             /.
             Now the longitude search.
             ./

             /.
             Reset the stepsize to something appropriate for the 360
             degrees in 24 hours domain. The longitude shows near
             linear behavior so use a stepsize less than half the period
             of twelve hours. Ten hours will suffice in this case.
             ./
             step   = (10./24.)*spd_c();

             {
             SpiceChar       * coord  = "LONGITUDE";
             SpiceChar       * relate = ">";

             refval = 85. *rpd_c();

             gfsubc_c (  target,  fixref,
                         method,  abcorr, obsrvr,
                         crdsys,  coord,
                         relate,  refval,
                         adjust,  step,
                         MAXWIN,
                         &result2, &result3 );

             /.
             Contract the endpoints of each window to account
             for possible round-off error at the -180/180 degree branch.

             A contraction value of a millisecond should eliminate
             any round-off caused branch crossing.
             ./

             wncond_c( 1e-3, 1e-3, &result3 );
             }

             {
             SpiceChar       * coord  = "LONGITUDE";
             SpiceChar       * relate = "<";

             refval = 86. *rpd_c();

             gfsubc_c (  target,  fixref,
                         method,  abcorr, obsrvr,
                         crdsys,  coord,
                         relate,  refval,
                         adjust,  step,
                         MAXWIN,
                         &result3, &result4 );
             }

             /.
             List the beginning and ending points in each interval
             if result contains data.
             ./
             count = wncard_c( &result4 );

             /.
             Display the results.
             ./
             if (count == 0 )
                {
                printf ( "Result window is empty.\n\n" );
                }
             else
                {
                for ( i = 0;  i < count;  i++ )
                   {

                   /.
                   Fetch the endpoints of the Ith interval
                   of the result window.
                   ./
                   wnfetd_c ( &result4, i, &beg, &end );

                   timout_c ( beg, TIMFMT, STRLEN, begstr );
                   timout_c ( end, TIMFMT, STRLEN, endstr );

                   printf ( "Interval %d\n", i + 1);
                   printf ( "Beginning TDB %s \n",   begstr );
                   printf ( "Ending TDB    %s \n\n", endstr );

                   }
                }

             kclear_c();
             return( 0 );
             }

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

          Interval 1
          Beginning TDB 2007-MAY-05 06:14:04.637735 (TDB)
          Ending TDB    2007-MAY-05 06:18:04.621907 (TDB)

          Interval 2
          Beginning TDB 2007-MAY-06 06:13:59.583483 (TDB)
          Ending TDB    2007-MAY-06 06:17:59.569240 (TDB)

          Interval 3
          Beginning TDB 2007-MAY-07 06:13:55.102940 (TDB)
          Ending TDB    2007-MAY-07 06:17:55.090299 (TDB)

          Interval 4
          Beginning TDB 2007-MAY-08 06:13:51.202604 (TDB)
          Ending TDB    2007-MAY-08 06:17:51.191583 (TDB)

          Interval 5
          Beginning TDB 2007-AUG-06 06:23:17.282927 (TDB)
          Ending TDB    2007-AUG-06 06:27:17.264010 (TDB)

          Interval 6
          Beginning TDB 2007-AUG-07 06:23:10.545441 (TDB)
          Ending TDB    2007-AUG-07 06:27:10.524925 (TDB)

          Interval 7
          Beginning TDB 2007-AUG-08 06:23:03.233996 (TDB)
          Ending TDB    2007-AUG-08 06:27:03.211889 (TDB)

#### 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
           coordinate quantity utility package. Callers may
           need to re-initialize the package after calling this routine.

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
       E.D. Wright         (JPL)

#### Version

       -CSPICE Version 1.1.0, 01-NOV-2021 (JDR) (EDW)

           Added use of ALLOC_CHECK_INTRA to check net null effect on
           alloc count.

           Updated header to describe use of expanded confinement window.

           Edited the header to comply with NAIF standard.

           Added entries #3, #4, #12 and #15 in -Exceptions section.

           Updated the description of "nintvls", "cnfine" and "result" arguments.

       -CSPICE Version 1.0.2, 29-AUG-2014 (EDW)

           Edit to header, replaced ' character with character " to indicate
           C strings. Removed extraneous line from header.

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

           Header was updated to discuss use of gfstol_c.

           Edit to comments to correct search description.

           Edit to Example description, replaced "intercept" with
           "sub-observer point."

           Correction of several typos.

       -CSPICE Version 1.0.0, 10-FEB-2009 (NJB) (EDW)

#### Index_Entries

       GF subpoint coordinate search

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

    Fri Dec 31 18:41:07 2021