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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

       gfocce_c ( GF, occultation event )

       void gfocce_c ( ConstSpiceChar     * occtyp,
                       ConstSpiceChar     * front,
                       ConstSpiceChar     * fshape,
                       ConstSpiceChar     * fframe,
                       ConstSpiceChar     * back,
                       ConstSpiceChar     * bshape,
                       ConstSpiceChar     * bframe,
                       ConstSpiceChar     * abcorr,
                       ConstSpiceChar     * obsrvr,
                       SpiceDouble          tol,

                       void             ( * udstep ) ( SpiceDouble       et,
                                                       SpiceDouble     * step ),

                       void             ( * udrefn ) ( SpiceDouble       t1,
                                                       SpiceDouble       t2,
                                                       SpiceBoolean      s1,
                                                       SpiceBoolean      s2,
                                                       SpiceDouble     * t    ),
                       SpiceBoolean         rpt,

                       void             ( * udrepi ) ( SpiceCell       * cnfine,
                                                       ConstSpiceChar  * srcpre,
                                                       ConstSpiceChar  * srcsuf ),

                       void             ( * udrepu ) ( SpiceDouble       ivbeg,
                                                       SpiceDouble       ivend,
                                                       SpiceDouble       et      ),

                       void             ( * udrepf ) ( void ),
                       SpiceBoolean         bail,
                       SpiceBoolean     ( * udbail ) ( void ),
                       SpiceCell          * cnfine,
                       SpiceCell          * result                                )

#### Abstract

       Determine time intervals when an observer sees one target
       occulted by another. Report progress and handle interrupts
       if so commanded.

       The surfaces of the target bodies may be represented by triaxial
       ellipsoids or by topographic data provided by DSK files.

#### Required_Reading

       FRAMES
       GF
       KERNEL
       NAIF_IDS
       SPK
       TIME
       WINDOWS

#### Keywords

       EVENT
       GEOMETRY
       SEARCH
       WINDOW

#### Brief_I/O

       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       occtyp     I   Type of occultation.
       front      I   Name of body occulting the other.
       fshape     I   Type of shape model used for front body.
       fframe     I   Body-fixed, body-centered frame for front body.
       back       I   Name of body occulted by the other.
       bshape     I   Type of shape model used for back body.
       bframe     I   Body-fixed, body-centered frame for back body.
       abcorr     I   Aberration correction flag.
       obsrvr     I   Name of the observing body.
       tol        I   Convergence tolerance in seconds.
       udstep     I   Name of the routine that returns a time step.
       udrefn     I   Name of the routine that computes a refined time.
       rpt        I   Progress report flag.
       udrepi     I   Function that initializes progress reporting.
       udrepu     I   Function that updates the progress report.
       udrepf     I   Function that finalizes progress reporting.
       bail       I   Logical indicating program interrupt monitoring.
       udbail     I   Name of a routine that signals a program interrupt.
       cnfine    I-O  SPICE window to which the search is restricted.
       result     O   SPICE window containing results.

#### Detailed_Input

       occtyp      indicates the type of occultation that is to be found.
                   Supported values and corresponding definitions are:

                      "FULL"      denotes the full occultation of the body
                                  designated by `back' by the body designated
                                  by `front', as seen from the location of the
                                  observer. In other words, the occulted
                                  body is completely invisible as seen from
                                  the observer's location.

                      "ANNULAR"   denotes an annular occultation: the body
                                  designated by `front' blocks part of, but
                                  not the limb of, the body designated by
                                  `back', as seen from the location of the
                                  observer.

                      "PARTIAL"   denotes a partial, non-annular
                                  occultation: the body designated by `front'
                                  blocks part, but not all, of the limb of
                                  the body designated by `back', as seen from
                                  the location of the observer.

                      "ANY"       denotes any of the above three types of
                                  occultations: "PARTIAL", "ANNULAR", or
                                  "FULL".

                                  "ANY" should be used to search for times
                                  when the body designated by `front' blocks
                                  any part of the body designated by `back'.

                                  The option "ANY" must be used if either
                                  the front or back target body is modeled
                                  as a point.

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

       front       is the name of the target body that occults --- that is,
                   passes in front of --- the other. Optionally, you may
                   supply the integer NAIF ID code for the body as a string.
                   For example both "MOON" and "301" are legitimate strings
                   that designate the Moon.

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

       fshape      is a string indicating the geometric model used to
                   represent the shape of the front target body. The
                   supported options are:

                      "ELLIPSOID"

                          Use a triaxial ellipsoid model with radius values
                          provided via the kernel pool. A kernel variable
                          having a name of the form

                             BODYnnn_RADII

                          where nnn represents the NAIF integer code
                          associated with the body, must be present in the
                          kernel pool. This variable must be associated with
                          three numeric values giving the lengths of the
                          ellipsoid's X, Y, and Z semi-axes.

                      "POINT"

                          Treat the body as a single point. When a point
                          target is specified, the occultation type must be
                          set to "ANY".

                      "DSK/UNPRIORITIZED[/SURFACES = <surface list>]"

                          Use topographic data provided by DSK files to
                          model the body's shape. These data must be
                          provided by loaded DSK files.

                          The surface list specification is optional. The
                          syntax of the list is

                             <surface 1> [, <surface 2>...]

                          If present, it indicates that data only for the
                          listed surfaces are to be used; however, data need
                          not be available for all surfaces in the list. If
                          absent, loaded DSK data for any surface associated
                          with the target body are used.

                          The surface list may contain surface names or
                          surface ID codes. Names containing blanks must be
                          delimited by double quotes, for example

                             SURFACES = "Mars MEGDR 128 PIXEL/DEG"

                          If multiple surfaces are specified, their names or
                          IDs must be separated by commas.

                          See the -Particulars section below for details
                          concerning use of DSK data.

                   The combinations of the shapes of the target bodies
                   `front' and `back' must be one of:

                      One ELLIPSOID, one POINT
                      Two ELLIPSOIDs
                      One DSK, one POINT

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

       fframe      is the name of the body-fixed, body-centered reference
                   frame associated with the front target body. Examples
                   of such names are "IAU_SATURN" (for Saturn) and
                   "ITRF93" (for the Earth).

                   If the front target body is modeled as a point, `fframe'
                   should be left empty or blank.

                   Case and leading or trailing blanks bracketing a
                   non-blank frame name are not significant in the string
                   `fframe'.

       back        is the name of the target body that is occulted by ---
                   that is, passes in back of --- the other. Optionally, you
                   may supply the integer NAIF ID code for the body as a
                   string. For example both "MOON" and "301" are legitimate
                   strings that designate the Moon.

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

       bshape      is the shape specification for the body designated by
                   `back'. The supported options are those for `fshape'. See the
                   description of `fshape' above for details.

       bframe      is the name of the body-fixed, body-centered reference
                   frame associated with the "back" target body. See the
                   description of `fframe' above for details. Examples of such
                   names are "IAU_SATURN" (for Saturn) and "ITRF93" (for the
                   Earth).

                   If the back target body is modeled as a point, `bframe'
                   should be left empty or blank.

                   Case and leading or trailing blanks bracketing a
                   non-blank frame name are not significant in the string
                   `bframe'.

       abcorr      indicates the aberration corrections to be applied to the
                   state of the target body to account for one-way light
                   time. Stellar aberration corrections are ignored if
                   specified, since these corrections don't improve the
                   accuracy of the occultation determination.

                   See the header of the SPICE routine spkezr_c for a detailed
                   description of the aberration correction options. For
                   convenience, the options supported by this routine are
                   listed below:

                      "NONE"     Apply no correction.

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

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

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

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

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

       obsrvr      is the name of the body from which the occultation is
                   observed. Optionally, you may supply the integer NAIF
                   ID code for the body as a string.

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

       tol         is a tolerance value used to determine convergence of
                   root-finding operations. `tol' is measured in TDB seconds
                   and must be greater than zero.

       udstep      is an externally specified routine that computes a
                   time step used to find transitions of the state being
                   considered. A state transition occurs where the state
                   changes from being "in occultation" to being "not in
                   occultation" or vice versa.

                   This routine relies on `udstep' returning step sizes small
                   enough so that state transitions within the confinement
                   window are not overlooked. There must never be two roots
                   A and B separated by less than `step', where `step' is the
                   minimum step size returned by `udstep' for any value of `et';
                   in the interval [A, B].

                   The prototype for `udstep' is

                      void   ( * udstep ) ( SpiceDouble       et,
                                            SpiceDouble     * step )

                   where:

                      et      is the input start time from which the
                              algorithm is to search forward for a state
                              transition. `et' is expressed as seconds past
                              J2000 TDB.

                      step    is the output step size. `step' indicates
                              how far to advance `et' so that `et' and
                              et+step may bracket a state transition and
                              definitely do not bracket more than one
                              state transition. Units are TDB seconds.

                   If a constant step size is desired, the CSPICE routine

                      gfstep_c

                   may be used as the step size function. If gfstep_c is used,
                   the step size must be set by calling gfsstp_c prior to
                   calling this routine.

       udrefn      is the name of the externally specified routine that
                   refines the times that bracket a transition point. In
                   other words, once a pair of times, `t1' and `t2', that
                   bracket a state transition have been found, `udrefn'
                   computes an intermediate time `t' such that either [t1, t]
                   or [t, t2] contains the time of the state transition. The
                   prototype for `udrefn' is:

                      void   ( * udrefn ) ( SpiceDouble       t1,
                                            SpiceDouble       t2,
                                            SpiceBoolean      s1,
                                            SpiceBoolean      s2,
                                            SpiceDouble     * t   )

                   where the inputs are:

                      t1    is a time when the visibility state is `s1'. `t1'
                            is expressed as seconds past J2000 TDB.

                      t2    is a time when the visibility state is `s2'. `t2' is
                            expressed as seconds past J2000 TDB. `t2' is
                            assumed to be larger than `t1'.

                      s1    is the visibility state at time `t1'.

                      s2    is the visibility state at time `t2'.

                   The output is:

                      t     is the next time to check for a state
                            transition. `t' is expressed as seconds past
                            J2000 TDB and is between `t1' and `t2'.

                   If a simple bisection method is desired, the CSPICE
                   routine gfrefn_c may be used.

       rpt         is a logical variable which controls whether progress
                   reporting is enabled. When `rpt' is SPICETRUE, progress
                   reporting is enabled and the routines `udrepi', `udrepu', and
                   `udrepf' (see descriptions below) are used to report
                   progress.

       udrepi      is a user-defined routine that initializes a progress
                   report. When progress reporting is enabled, `udrepi' is
                   called at the start of a search. The prototype for
                   `udrepi' is

                      void   ( * udrepi ) ( SpiceCell       * cnfine,
                                            ConstSpiceChar  * srcpre,
                                            ConstSpiceChar  * srcsuf )

                   where

                      cnfine

                   is the confinement window specifying the time period over
                   which a search is conducted, and

                      srcpre
                      srcsuf

                   are prefix and suffix strings used in the progress
                   report: these strings are intended to bracket a
                   representation of the fraction of work done. For example,
                   when the CSPICE progress reporting functions are used, if
                   `srcpre' and `srcsuf' are, respectively,

                      "Occultation/transit search"
                      "done."

                   the progress report display at the end of the
                   search will be:

                      Occultation/transit search 100.00% done.

                   The CSPICE routine gfrepi_c may be used as the actual
                   argument corresponding to `udrepi'. If so, the CSPICE
                   routines gfrepu_c and gfrepf_c must be the actual arguments
                   corresponding to `udrepu' and `udrepf'.

       udrepu      is a user-defined routine that updates the progress
                   report for a search. The prototype of `udrepu' is

                      void   ( * udrepu ) ( SpiceDouble       ivbeg,
                                            SpiceDouble       ivend,
                                            SpiceDouble       et      )

                   Here `ivbeg', `ivend' are the bounds of an interval that is
                   contained in some interval belonging to the confinement
                   window. The confinement window is associated with some
                   root finding activity. It is used to determine how much
                   total time is being searched in order to find the events
                   of interest.

                   `et' is an epoch belonging to the interval
                   [`ivbeg', `ivend'].

                   In order for a meaningful progress report to be
                   displayed, `ivbeg' and `ivend' must satisfy the following
                   constraints:

                    - `ivbeg' must be less than or equal to `ivend'.

                    - The interval [ `ivbeg', `ivend' ] must be contained in
                      some interval of the confinement window. It can be
                      a proper subset of the containing interval; that
                      is, it can be smaller than the interval of the
                      confinement window that contains it.

                    - Over a search, the sum of the differences

                         ivend - ivbeg

                      for all calls to this routine made during the search
                      must equal the measure (that is, the sum of the
                      lengths of the intervals) of the confinement window
                      `cnfine'.

                   `et' is the current time reached in the search for an event.
                   `et' must lie in the interval

                      ivbeg : ivend

                   inclusive. The input values of `et' for a given interval
                   need not form an increasing sequence.

                   The CSPICE routine gfrepu_c may be used as the actual
                   argument corresponding to `udrepu'. If so, the CSPICE
                   routines gfrepi_c and gfrepf_c must be the actual arguments
                   corresponding to `udrepi' and `udrepf'.

       udrepf      is a user-defined routine that finalizes a
                   progress report. `udrepf' has no arguments.

                   The CSPICE routine gfrepf_c may be used as the actual
                   argument corresponding to `udrepf'. If so, the CSPICE
                   routines gfrepi_c and gfrepu_c must be the actual arguments
                   corresponding to `udrepi' and `udrepu'.

       bail        is a logical variable indicating whether or not interrupt
                   handling is enabled. When `bail' is set to SPICETRUE, the
                   input function `udbail' (see description below) is used to
                   determine whether an interrupt has been issued.

       udbail      is the name of a user defined logical function that
                   indicates whether an interrupt signal has been issued
                   (for example, from the keyboard). The prototype of `udbail'
                   is

                      SpiceBoolean   ( * udbail ) ( void )

                   The return value is SPICETRUE if an interrupt has
                   been issued; otherwise the value is SPICEFALSE.

                   gfocce_c uses `udbail' only when `bail' (see above) is set to
                   SPICETRUE, indicating that interrupt handling is enabled.
                   When interrupt handling is enabled, gfocce_c and routines
                   in its call tree will call `udbail' to determine whether to
                   terminate processing and return immediately.

                   If interrupt handing is not enabled, a logical function
                   must still be passed to gfocce_c as an input argument. The
                   CSPICE function

                      gfbail_c

                   may be used for this purpose.

                   The function `udbail' will usually be tested multiple times by
                   the GF system between the time an interrupt is issued and the
                   time when control is returned to the calling program, so
                   `udbail' must continue to return SPICETRUE until explicitly
                   reset by the calling application. Therefore `udbail' must
                   provide a "reset" mechanism." In the case of gfbail_c, the reset
                   function is

                      gfclrh_c

                   See the -Examples header section below for a complete code
                   example demonstrating use of the CSPICE interrupt handling
                   capability.

       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.

                   `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 a SPICE window representing the set of time intervals,
                   within the confinement period, when the specified
                   occultation occurs.

                   `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 gfocce_c conducts its search.

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

                   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

       None.

#### 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 name of either target or the observer cannot be translated
           to a NAIF ID code, an error is signaled by a routine in the
           call tree of this routine.

       4)  If the radii of a target body modeled as an ellipsoid cannot
           be determined by searching the kernel pool for a kernel
           variable having a name of the form

              "BODYnnn_RADII"

           where nnn represents the NAIF integer code associated with
           the body, an error is signaled by a routine in the call tree
           of this routine.

       5)  If either of the target bodies `front' or `back' coincides with
           the observer body `obsrvr', an error is signaled by a routine in
           the call tree of this routine.

       6)  If the body designated by `front' coincides with that designated
           by `back', an error is signaled by a routine in the call tree of
           this routine.

       7)  If either of the body model specifiers `fshape' or `bshape' is not
           recognized, an error is signaled by a routine in the call tree
           of this routine.

       8)  If both of the body model specifiers `fshape' and `bshape'
           specify point targets, the error SPICE(INVALIDSHAPECOMBO)
           is signaled by a routine in the call tree of this routine.

       9)  If a target body-fixed reference frame associated with a
           non-point target is not recognized, an error is signaled by a
           routine in the call tree of this routine.

       10) If a target body-fixed reference frame is not centered at the
           corresponding target body, an error is signaled by a routine
           in the call tree of this routine.

       11) If the loaded kernels provide insufficient data to compute the
           requested state vector, an error is signaled by a routine in
           the call tree of this routine.

       12) If an error occurs while reading an SPK or other kernel file,
           the error is signaled by a routine in the call tree
           of this routine.

       13) If a point target is specified and the occultation type is set
           to a valid value other than "ANY", an error is signaled by a
           routine in the call tree of this routine.

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

       15) If the result window has size less than 2, the error
           SPICE(WINDOWTOOSMALL) is signaled by a routine in the call
           tree of this routine.

       16) If the occultation type `occtyp' is invalid, an error is
           signaled by a routine in the call tree of this routine.

       17) If the aberration correction specification `abcorr' is invalid,
           an error is signaled by a routine in the call tree of this
           routine.

       18) If the convergence tolerance size is non-positive, the error
           SPICE(INVALIDTOLERANCE) is signaled by a routine in the call
           tree of this routine.

       19) If either `fshape' or `bshape' specifies that the target surface
           is represented by DSK data, and no DSK files are loaded for
           the specified target, an error is signaled by a routine in
           the call tree of this routine.

       20) If either `fshape' or `bshape' specifies that the target surface
           is represented by DSK data, but the shape specification is
           invalid, an error is signaled by a routine in the call tree
           of this routine.

       21) If operation of this routine is interrupted, the output result
           window will be invalid.

       22) If any of the `occtyp', `front', `fshape', `back', `bshape',
           `abcorr', `obsrvr', `bframe' or `fframe' input string pointers
           is null, the error SPICE(NULLPOINTER) is signaled.

       23) If any of the `occtyp', `front', `fshape', `back', `bshape',
           `abcorr' or `obsrvr' input strings has zero length, the error
           SPICE(EMPTYSTRING) is signaled.

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

       25) If any attempt to change the handler for the interrupt signal
           SIGINT fails, the error SPICE(SIGNALFAILED) 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: the calling application must load ephemeris data
          for the targets, source and observer that cover the time
          period specified by the window `cnfine'. If aberration
          corrections are used, the states of the target bodies and of
          the 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.

       -  PCK data: bodies modeled as triaxial ellipsoids must have
          semi-axis lengths provided by variables in the kernel pool.
          Typically these data are made available by loading a text
          PCK file via furnsh_c.

       -  FK data: if either of the reference frames designated by
          `bframe' or `fframe' are not built in to the SPICE system,
          one or more FKs specifying these frames must be loaded.

       The following data may be required:

       -  DSK data: if either `fshape' or `bshape' indicates that DSK
          data are to be used, DSK files containing topographic data
          for the target body must be loaded. If a surface list is
          specified, data for at least one of the listed surfaces must
          be loaded.

       -  Surface name-ID associations: if surface names are specified
          in `fshape' or `bshape', the association of these names with
          their corresponding surface ID codes must be established by
          assignments of the kernel variables

             NAIF_SURFACE_NAME
             NAIF_SURFACE_CODE
             NAIF_SURFACE_BODY

          Normally these associations are made by loading a text
          kernel containing the necessary assignments. An example
          of such a set of assignments is

             NAIF_SURFACE_NAME += 'Mars MEGDR 128 PIXEL/DEG'
             NAIF_SURFACE_CODE += 1
             NAIF_SURFACE_BODY += 499

       -  CK data: either of the body-fixed frames to which `fframe' or
          `bframe' refer might be a CK frame. If so, at least one CK
          file will be needed to permit transformation of vectors
          between that frame and the J2000 frame.

       -  SCLK data: if a CK file is needed, an associated SCLK
          kernel is required to enable conversion between encoded SCLK
          (used to time-tag CK data) and barycentric dynamical time
          (TDB).

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

#### Particulars

       This routine provides the SPICE GF system's most flexible
       interface for searching for occultation events.

       Applications that require do not require support for progress
       reporting, interrupt handling, non-default step or refinement
       functions, or non-default convergence tolerance normally should
       call gfoclt_c rather than this routine.

       This routine determines a set of one or more time intervals
       within the confinement window when a specified type of
       occultation occurs. 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
       ==================

       The search for occultations is treated as a search for state
       transitions: times are sought when the state of the BACK body
       changes from "not occulted" to "occulted" or vice versa.

       Step Size
       =========

       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 occultation state will be sampled.
       Starting at the left endpoint of an interval, samples will be
       taken at each step. If a state change is detected, a root has
       been bracketed; at that point, the "root"--the time at which the
       state change occurs---is 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 occultation state is constant:
       the step size should be shorter than the shortest occultation
       duration and the shortest period between occultations, within
       the confinement window.

       Having some knowledge of the relative geometry of the targets 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
       =====================

       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 high-level GF routines
       that call this routine is set via the parameter SPICE_GF_CNVTOL,
       which is declared in the header file 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.

       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. For an example, see the program CASCADE in the GF
       Example Programs chapter of the GF Required Reading, gf.req.

       Using DSK data
       ==============

          DSK loading and unloading
          -------------------------

          DSK files providing data used by this routine are loaded by
          calling furnsh_c and can be unloaded by calling unload_c or
          kclear_c. See the documentation of furnsh_c for limits on numbers
          of loaded DSK files.

          For run-time efficiency, it's desirable to avoid frequent
          loading and unloading of DSK files. When there is a reason to
          use multiple versions of data for a given target body---for
          example, if topographic data at varying resolutions are to be
          used---the surface list can be used to select DSK data to be
          used for a given computation. It is not necessary to unload
          the data that are not to be used. This recommendation presumes
          that DSKs containing different versions of surface data for a
          given body have different surface ID codes.

          DSK data priority
          -----------------

          A DSK coverage overlap occurs when two segments in loaded DSK
          files cover part or all of the same domain---for example, a
          given longitude-latitude rectangle---and when the time
          intervals of the segments overlap as well.

          When DSK data selection is prioritized, in case of a coverage
          overlap, if the two competing segments are in different DSK
          files, the segment in the DSK file loaded last takes
          precedence. If the two segments are in the same file, the
          segment located closer to the end of the file takes
          precedence.

          When DSK data selection is unprioritized, data from competing
          segments are combined. For example, if two competing segments
          both represent a surface as a set of triangular plates, the
          union of those sets of plates is considered to represent the
          surface.

          Currently only unprioritized data selection is supported.
          Because prioritized data selection may be the default behavior
          in a later version of the routine, the UNPRIORITIZED keyword is
          required in the `fshape' and `bshape' arguments.

          Syntax of the shape input arguments for the DSK case
          ----------------------------------------------------

          The keywords and surface list in the target shape arguments
          `bshape' and `fshape' are called "clauses." The clauses may
          appear in any order, for example

             "DSK/<surface list>/UNPRIORITIZED"
             "DSK/UNPRIORITIZED/<surface list>"
             "UNPRIORITIZED/<surface list>/DSK"

          The simplest form of the `method' argument specifying use of
          DSK data is one that lacks a surface list, for example:

             "DSK/UNPRIORITIZED"

          For applications in which all loaded DSK data for the target
          body are for a single surface, and there are no competing
          segments, the above string suffices. This is expected to be
          the usual case.

          When, for the specified target body, there are loaded DSK
          files providing data for multiple surfaces for that body, the
          surfaces to be used by this routine for a given call must be
          specified in a surface list, unless data from all of the
          surfaces are to be used together.

          The surface list consists of the string

             "SURFACES = "

          followed by a comma-separated list of one or more surface
          identifiers. The identifiers may be names or integer codes in
          string format. For example, suppose we have the surface
          names and corresponding ID codes shown below:

             Surface Name                              ID code
             ------------                              -------
             "Mars MEGDR 128 PIXEL/DEG"                1
             "Mars MEGDR 64 PIXEL/DEG"                 2
             "Mars_MRO_HIRISE"                         3

          If data for all of the above surfaces are loaded, then
          data for surface 1 can be specified by either

             "SURFACES = 1"

          or

             "SURFACES = \"Mars MEGDR 128 PIXEL/DEG\""

          Escaped double quotes are used to delimit the surface name because
          it contains blank characters.

          To use data for surfaces 2 and 3 together, any
          of the following surface lists could be used:

             "SURFACES = 2, 3"

             "SURFACES = \"Mars MEGDR  64 PIXEL/DEG\", 3"

             "SURFACES = 2, Mars_MRO_HIRISE"

             "SURFACES = \"Mars MEGDR 64 PIXEL/DEG\", Mars_MRO_HIRISE"

          An example of a shape argument that could be constructed
          using one of the surface lists above is

             "DSK/UNPRIORITIZED/SURFACES = \"Mars MEGDR 64 PIXEL/DEG\", 3"

#### 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) Conduct a search using default GF progress reporting
          and interrupt handling capabilities.

          The program will use console I/O to display a simple
          ASCII-based progress report.

          The program will trap keyboard interrupts (on most systems,
          generated by typing the "control C" key combination). This
          feature can be used in non-trivial applications to allow
          the application to continue after a search as been interrupted.

          The program will find occultations of the Sun by the Moon as seen
          from the center of the Earth over the month December, 2001.

          Use light time corrections to model apparent positions of Sun
          and Moon. Stellar aberration corrections are not specified
          because they don't affect occultation computations.

          We select a step size of 20 seconds, which implies we ignore
          occultation events lasting less than 20 seconds, if any exist.
          Given this step size and the length of the search interval, the
          user has time to interrupt the computation. In an interactive
          setting, the user might speed up the search by lengthening the
          step size or shortening the search interval, as long as these
          adjustments don't prevent the search from finding the correct
          solution.

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

             KPL/MK

             File name: gfocce_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
                ---------                     --------
                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.

          /.
             Program gfocce_ex1
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main()
          {
             /.
             Constants
             ./
             #define  TIMFMT  "YYYY MON DD HR:MN:SC.###### ::TDB (TDB)"
             #define  CNVTOL  1.e-6
             #define  MAXWIN  200
             #define  TIMLEN  41

             /.
             Local variables
             ./
             SpiceBoolean            bail;
             SpiceBoolean            rpt;

             SpiceChar             * win0;
             SpiceChar             * win1;
             SpiceChar               begstr [ TIMLEN ];
             SpiceChar               endstr [ TIMLEN ];

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

             SpiceDouble             et0;
             SpiceDouble             et1;
             SpiceDouble             left;
             SpiceDouble             right;

             SpiceInt                i;

             /.
             Load kernels.
             ./
             furnsh_c ( "gfocce_ex1.tm" );

             /.
             Obtain the TDB time bounds of the confinement
             window, which is a single interval in this case.
             ./
             win0 = "2001 DEC 10 00:00:00 TDB";
             win1 = "2002 JAN 01 00:00:00 TDB";

             str2et_c ( win0, &et0 );
             str2et_c ( win1, &et1 );

             /.
             Insert the time bounds into the confinement
             window.
             ./
             wninsd_c ( et0, et1, &cnfine );

             /.
             Select a twenty-second step. We'll ignore any occultations
             lasting less than 20 seconds.
             ./
             gfsstp_c ( 20.0 );

             /.
             Turn on interrupt handling and progress reporting.
             ./
             bail = SPICETRUE;
             rpt  = SPICETRUE;

             /.
             Perform the search.
             ./
             gfocce_c ( "ANY",
                        "MOON",     "ellipsoid",  "IAU_MOON",
                        "SUN",      "ellipsoid",  "IAU_SUN",
                        "LT",       "EARTH",      CNVTOL,
                        gfstep_c,   gfrefn_c,     rpt,
                        gfrepi_c,   gfrepu_c,     gfrepf_c,
                        bail,       gfbail_c,     &cnfine,
                        &result                              );

             if ( gfbail_c() )
             {
                /.
                Clear the CSPICE interrupt indication. This is
                an essential step for programs that continue
                running after an interrupt; gfbail_c will
                continue to return SPICETRUE until this step
                has been performed.
                ./
                gfclrh_c();

                /.
                We've trapped an interrupt signal. In a realistic
                application, the program would continue operation
                from this point. In this simple example, we simply
                display a message and quit.
                ./
                printf ( "\nSearch was interrupted.\n\nThis message "
                         "was written after an interrupt signal\n"
                         "was trapped. By default, the program "
                         "would have terminated \nbefore this message "
                         "could be written.\n\n"                       );
             }
             else
             {

                if ( wncard_c(&result) == 0 )
                {
                   printf ( "No occultation was found.\n" );
                }
                else
                {
                   for ( i = 0;  i < wncard_c(&result);  i++ )
                   {
                      /.
                      fetch and display each occultation interval.
                      ./
                      wnfetd_c ( &result, i, &left, &right );

                      timout_c ( left,  TIMFMT, TIMLEN, begstr );
                      timout_c ( right, TIMFMT, TIMLEN, endstr );

                      printf ( "Interval %d\n", (int)i );
                      printf ( "   Start time: %s\n", begstr );
                      printf ( "   Stop time:  %s\n", endstr );
                   }
                }

             }

             return ( 0 );
          }

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

          Occultation/transit search 100.00% done.

          Interval 0
             Start time: 2001 DEC 14 20:10:14.195952  (TDB)
             Stop time:  2001 DEC 14 21:35:50.317994  (TDB)

          Note that the progress report has the format shown below:

             Occultation/transit search   6.02% done.

          The completion percentage was updated approximately once per
          second.

          When the program was interrupted at an arbitrary time,
          the output was:

             Occultation/transit search  13.63% done.
             Search was interrupted.

          This message was written after an interrupt signal
          was trapped. By default, the program would have terminated
          before this message could be written.

#### Restrictions

       1)  If the caller passes in the default, constant step
           size routine, gfstep_c, the caller must set the step
           size by calling the entry point gfsstp_c before
           calling gfocce_c. The call syntax for gfsstp_c is

              gfsstp_c ( step );

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
       L.S. Elson          (JPL)
       W.L. Taber          (JPL)
       I.M. Underwood      (JPL)
       E.D. Wright         (JPL)

#### Version

       -CSPICE Version 2.0.1, 06-AUG-2021 (JDR)

           Edited the header to comply with NAIF standard.

           Renamed example's meta-kernel.

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

           Added entries #15 and #24 in -Exceptions section.

       -CSPICE Version 2.0.0, 29-FEB-2016 (NJB) (EDW)

           Edit to example program to use "%d" with explicit casts
           to int for printing SpiceInts with printf.

           Updated to support use of DSKs.

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

#### Index_Entries

       GF mid-level occultation search

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

    Fri Dec 31 18:41:07 2021