Index Page gfoclt_c

A B C D E F G H I J K L M N O P Q R S T U V W X

Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries

#### Procedure

       void gfoclt_c ( ConstSpiceChar   * occtyp,
                       ConstSpiceChar   * front,
                       ConstSpiceChar   * fshape,
                       ConstSpiceChar   * fframe,
                       ConstSpiceChar   * back,
                       ConstSpiceChar   * bshape,
                       ConstSpiceChar   * bframe,
                       ConstSpiceChar   * abcorr,
                       ConstSpiceChar   * obsrvr,
                       SpiceDouble        step,
                       SpiceCell        * cnfine,
                       SpiceCell        * result )

#### Abstract

       Determine time intervals when an observer sees one target occulted
       by, or in transit across, another.

       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
       --------------- ---  -------------------------------------------------
       SPICE_GF_CNVTOL  P   Convergence tolerance.
       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.
       step             I   Step size in seconds for finding occultation
                            events.
       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.
                  Note that transits are considered to be a type of
                  occultation.

                  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.
                  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 each 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'.

       step       is the step size to be used in the search. `step' must
                  be shorter than any interval, within the confinement
                  window, over which the specified condition is met. In
                  other words, `step' must be shorter than the shortest
                  occultation event that the user wishes to detect; `step'
                  must also be shorter than the shortest time interval
                  between two occultation events that occur within the
                  confinement window (see below). However, `step' must not
                  be *too* short, or the search will take an unreasonable
                  amount of time.

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

                  `step' has units of TDB seconds.

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

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

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

#### Detailed_Output

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

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

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

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

#### Parameters

       SPICE_GF_CNVTOL

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

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

                  SPICE_GF_CNVTOL is declared in the header file

                     SpiceGF.h

#### Exceptions

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

           This routine does not diagnose invalid step sizes, except
           that if the step size is non-positive, the error
           SPICE(INVALIDSTEPSIZE) will be signaled.

       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, the error will be diagnosed 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, the error will be diagnosed 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', the error will be diagnosed by a
           routine in the call tree of this routine.

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

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

       11) If the loaded kernels provide insufficient data to
           compute any required state vector, the deficiency will
           be diagnosed 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 will be diagnosed by a routine in the call tree
           of this routine.

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

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

       15) Invalid occultation types will be diagnosed by a routine in
           the call tree of this routine.

       16) Invalid aberration correction specifications will be
           diagnosed by a routine in the call tree of this routine.

       17) 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, the error is signaled by a routine in
           the call tree of this routine.

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

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

       20) If any input string argument, other than `fframe' or `bframe',
           is empty, the error SPICE(EMPTYSTRING) will be 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 a simpler, but less flexible, interface
       than does the CSPICE routine gfocce_c for conducting searches for
       occultation events. Applications that require support for
       progress reporting, interrupt handling, non-default step or
       refinement functions, or non-default convergence tolerance should
       call gfocce_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 the interval, samples of the occultation state 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 this routine is set
       via the parameter SPICE_GF_CNVTOL.

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

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

       The confinement window also can be used to restrict a search to
       a time window over which required data (typically ephemeris
       data, in the case of occultation searches) are known to be
       available.

       In some cases, the confinement window be used to make searches
       more efficient. Sometimes it's possible to do an efficient search
       to reduce the size of the time period over which a relatively
       slow search of interest must be performed. See the "CASCADE"
       example program in gf.req for a demonstration.

       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 these examples may differ across
       platforms. The results depend on the SPICE kernels used as
       input, the compiler and supporting libraries, and the machine
       specific arithmetic implementation.

       1) Find occultations of the Sun by the Moon (that is, solar
          eclipses) 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 3 minutes, which means we
          ignore occultation events lasting less than 3 minutes,
          if any exist.

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

             KPL/MK

             File name: standard.tm

             This meta-kernel is intended to support operation of SPICE
             example programs. The kernels shown here should not be
             assumed to contain adequate or correct versions of data
             required by SPICE-based user applications.

             In order for an application to use this meta-kernel, the
             kernels referenced here must be present in the user's
             current working directory.

             \begindata

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

             \begintext

          Example code begins here.

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

             int main()
             {
                /.
                Local constants
                ./

                #define TIMFMT          "YYYY MON DD HR:MN:SC.###### (TDB)::TDB"
                #define MAXWIN          200
                #define TIMLEN          41

                /.
                Local variables
                ./
                SPICEDOUBLE_CELL      ( cnfine, MAXWIN );
                SPICEDOUBLE_CELL      ( result, MAXWIN );

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

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

                SpiceInt                i;

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

                /.
                Obtain the TDB time bounds of the confinement
                window, which is a single interval in this case.
                ./
                win0 = "2001 DEC 01 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 3-minute step. We'll ignore any occultations
                lasting less than 3 minutes.  Units are TDB seconds.
                ./
                step = 180.0;

                /.
                Perform the search.
                ./
                gfoclt_c ( "any",
                           "moon",    "ellipsoid",  "iau_moon",
                           "sun",     "ellipsoid",  "iau_sun",
                           "lt",      "earth",      step,
                           &cnfine,   &result                 );

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

                return ( 0 );
             }

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

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

       2) Find occultations of Titan by Saturn or of Saturn by
          Titan as seen from the center of the Earth over the
          last four months of 2008. Model both target bodies as
          ellipsoids. Search for every type of occultation.

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

          We select a step size of 15 minutes, which means we
          ignore occultation events lasting less than 15 minutes,
          if any exist.

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

             KPL/MK

             File name: gfoclt_ex2.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
                sat288.bsp                    Satellite ephemeris for
                                              Saturn
                pck00008.tpc                  Planet orientation and
                                              radii
                naif0009.tls                  Leapseconds

             \begindata

                KERNELS_TO_LOAD = ( 'de421.bsp',
                                    'sat288.bsp',
                                    'pck00008.tpc',
                                    'naif0009.tls'  )

             \begintext

             End of meta-kernel

         Example code begins here.

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

            int main()
            {
               /.
               Local constants
               ./
               #define TIMFMT          "YYYY MON DD HR:MN:SC.###### (TDB)::TDB"
               #define MAXWIN          200
               #define TIMLEN          41
               #define LNSIZE          81
               #define NTYPES          4

               /.
               Local variables
               ./
               SPICEDOUBLE_CELL      ( cnfine, MAXWIN );
               SPICEDOUBLE_CELL      ( result, MAXWIN );

               SpiceChar             * back;
               SpiceChar             * bframe;
               SpiceChar             * front;
               SpiceChar             * fframe;
               SpiceChar               line   [ LNSIZE ];
               SpiceChar             * obsrvr;

               SpiceChar             * occtyp [ NTYPES ] =
                                       {
                                          "FULL",
                                          "ANNULAR",
                                          "PARTIAL",
                                          "ANY"
                                       };

               SpiceChar             * templt [ NTYPES ] =
                                       {
                                          "Condition: # occultation of # by #",
                                          "Condition: # occultation of # by #",
                                          "Condition: # occultation of # by #",
                                          "Condition: # occultation of # by #"
                                       };

               SpiceChar               timstr [ TIMLEN ];
               SpiceChar               title  [ LNSIZE ];
               SpiceChar             * win0;
               SpiceChar             * win1;

               SpiceDouble             et0;
               SpiceDouble             et1;
               SpiceDouble             finish;
               SpiceDouble             start;
               SpiceDouble             step;

               SpiceInt                i;
               SpiceInt                j;
               SpiceInt                k;

               /.
               Load kernels.
               ./
               furnsh_c ( "gfoclt_ex2.tm" );

               /.
               Obtain the TDB time bounds of the confinement
               window, which is a single interval in this case.
               ./
               win0 = "2008 SEP 01 00:00:00 TDB";
               win1 = "2009 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 15-minute step. We'll ignore any occultations
               lasting less than 15 minutes. Units are TDB seconds.
               ./
               step = 900.0;

               /.
               The observation location is the Earth.
               ./
               obsrvr = "Earth";

               /.
               Loop over the occultation types.
               ./
               for ( i = 0;  i < NTYPES;  i++ )
               {
                  /.
                  For each type, do a search for both transits of
                  Titan across Saturn and occultations of Titan by
                  Saturn.
                  ./
                  for ( j = 0;  j < 2;  j++ )
                  {
                     if ( j == 0 )
                     {
                        front  = "TITAN";
                        fframe = "IAU_TITAN";
                        back   = "SATURN";
                        bframe = "IAU_SATURN";
                     }
                     else
                     {
                        front  = "SATURN";
                        fframe = "IAU_SATURN";
                        back   = "TITAN";
                        bframe = "IAU_TITAN";
                     }

                     /.
                     Perform the search. The target body shapes
                     are modeled as ellipsoids.
                     ./
                     gfoclt_c ( occtyp[i],
                                front,    "ellipsoid",  fframe,
                                back,     "ellipsoid",  bframe,
                                "lt",     obsrvr,       step,
                                &cnfine,  &result               );

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

                     /.
                     Substitute the occultation type and target
                     body names into the title string:
                     ./
                     repmc_c ( templt[i], "#", occtyp[i], LNSIZE, title );
                     repmc_c ( title,     "#", back,      LNSIZE, title );
                     repmc_c ( title,     "#", front,     LNSIZE, title );

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

                     if ( wncard_c(&result) == 0 )
                     {
                        printf ( " Result window is empty: "
                                 "no occultation was found.\n" );
                     }
                     else
                     {
                        printf ( " Result window start, stop times:\n" );

                        for ( k = 0;  k < wncard_c(&result);  k++ )
                        {
                           /.
                           Fetch the endpoints of the kth interval
                           of the result window.
                           ./
                           wnfetd_c ( &result, k, &start, &finish );

                           /.
                           Call strncpy with a length of 7 to include
                           a terminating null.
                           ./
                           strncpy ( line, "  #  #", 7 );

                           timout_c ( start,  TIMFMT, TIMLEN, timstr );

                           repmc_c  ( line, "#", timstr, LNSIZE, line );

                           timout_c ( finish, TIMFMT, TIMLEN, timstr );

                           repmc_c  ( line, "#", timstr, LNSIZE, line );

                           printf ( "%s\n", line );
                        }
                     }
                     /.
                     We've finished displaying the results of the
                     current search.
                     ./
                  }
                  /.
                  We've finished displaying the results of the
                  searches using the current occultation type.
                  ./
               }
               printf ( "\n" );

               return ( 0 );
            }

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

             Condition: FULL occultation of SATURN by TITAN
              Result window is empty: no occultation was found.

             Condition: FULL occultation of TITAN by SATURN
              Result window start, stop times:
               2008 OCT 27 22:08:01.627053 (TDB)  2008 OCT 28 01:05:03.375236 (TDB)
               2008 NOV 12 21:21:59.252262 (TDB)  2008 NOV 13 02:06:05.053051 (TDB)
               2008 NOV 28 20:49:02.402832 (TDB)  2008 NOV 29 02:13:58.986344 (TDB)
               2008 DEC 14 20:05:09.246177 (TDB)  2008 DEC 15 01:44:53.523002 (TDB)
               2008 DEC 30 19:00:56.577073 (TDB)  2008 DEC 31 00:42:43.222909 (TDB)

             Condition: ANNULAR occultation of SATURN by TITAN
              Result window start, stop times:
               2008 OCT 19 21:29:20.599087 (TDB)  2008 OCT 19 22:53:34.518737 (TDB)
               2008 NOV 04 20:15:38.620368 (TDB)  2008 NOV 05 00:18:59.139978 (TDB)
               2008 NOV 20 19:38:59.647712 (TDB)  2008 NOV 21 00:35:26.725908 (TDB)
               2008 DEC 06 18:58:34.073268 (TDB)  2008 DEC 07 00:16:17.647040 (TDB)
               2008 DEC 22 18:02:46.288289 (TDB)  2008 DEC 22 23:26:52.712459 (TDB)

             Condition: ANNULAR occultation of TITAN by SATURN
              Result window is empty: no occultation was found.

             Condition: PARTIAL occultation of SATURN by TITAN
              Result window start, stop times:
               2008 OCT 19 20:44:30.326771 (TDB)  2008 OCT 19 21:29:20.599087 (TDB)
               2008 OCT 19 22:53:34.518737 (TDB)  2008 OCT 19 23:38:26.250580 (TDB)
               2008 NOV 04 19:54:40.339331 (TDB)  2008 NOV 04 20:15:38.620368 (TDB)
               2008 NOV 05 00:18:59.139978 (TDB)  2008 NOV 05 00:39:58.612935 (TDB)
               2008 NOV 20 19:21:46.689523 (TDB)  2008 NOV 20 19:38:59.647712 (TDB)
               2008 NOV 21 00:35:26.725908 (TDB)  2008 NOV 21 00:52:40.604703 (TDB)
               2008 DEC 06 18:42:36.100544 (TDB)  2008 DEC 06 18:58:34.073268 (TDB)
               2008 DEC 07 00:16:17.647040 (TDB)  2008 DEC 07 00:32:16.324244 (TDB)
               2008 DEC 22 17:47:10.776722 (TDB)  2008 DEC 22 18:02:46.288289 (TDB)
               2008 DEC 22 23:26:52.712459 (TDB)  2008 DEC 22 23:42:28.850542 (TDB)

             Condition: PARTIAL occultation of TITAN by SATURN
              Result window start, stop times:
               2008 OCT 27 21:37:16.970175 (TDB)  2008 OCT 27 22:08:01.627053 (TDB)
               2008 OCT 28 01:05:03.375236 (TDB)  2008 OCT 28 01:35:49.266506 (TDB)
               2008 NOV 12 21:01:47.105498 (TDB)  2008 NOV 12 21:21:59.252262 (TDB)
               2008 NOV 13 02:06:05.053051 (TDB)  2008 NOV 13 02:26:18.227357 (TDB)
               2008 NOV 28 20:31:28.522707 (TDB)  2008 NOV 28 20:49:02.402832 (TDB)
               2008 NOV 29 02:13:58.986344 (TDB)  2008 NOV 29 02:31:33.691598 (TDB)
               2008 DEC 14 19:48:27.094229 (TDB)  2008 DEC 14 20:05:09.246177 (TDB)
               2008 DEC 15 01:44:53.523002 (TDB)  2008 DEC 15 02:01:36.360243 (TDB)
               2008 DEC 30 18:44:23.485898 (TDB)  2008 DEC 30 19:00:56.577073 (TDB)
               2008 DEC 31 00:42:43.222909 (TDB)  2008 DEC 31 00:59:17.030568 (TDB)

             Condition: ANY occultation of SATURN by TITAN
              Result window start, stop times:
               2008 OCT 19 20:44:30.326771 (TDB)  2008 OCT 19 23:38:26.250580 (TDB)
               2008 NOV 04 19:54:40.339331 (TDB)  2008 NOV 05 00:39:58.612935 (TDB)
               2008 NOV 20 19:21:46.689523 (TDB)  2008 NOV 21 00:52:40.604703 (TDB)
               2008 DEC 06 18:42:36.100544 (TDB)  2008 DEC 07 00:32:16.324244 (TDB)
               2008 DEC 22 17:47:10.776722 (TDB)  2008 DEC 22 23:42:28.850542 (TDB)

             Condition: ANY occultation of TITAN by SATURN
              Result window start, stop times:
               2008 OCT 27 21:37:16.970175 (TDB)  2008 OCT 28 01:35:49.266506 (TDB)
               2008 NOV 12 21:01:47.105498 (TDB)  2008 NOV 13 02:26:18.227357 (TDB)
               2008 NOV 28 20:31:28.522707 (TDB)  2008 NOV 29 02:31:33.691598 (TDB)
               2008 DEC 14 19:48:27.094229 (TDB)  2008 DEC 15 02:01:36.360243 (TDB)
               2008 DEC 30 18:44:23.485898 (TDB)  2008 DEC 31 00:59:17.030568 (TDB)

          3) Find occultations of the Mars Reconaissance Orbiter (MRO)
             by Mars, or transits of the MRO spacecraft across Mars,
             as seen from the DSN station DSS-14 over a period of a
             few hours on FEB 28 2015.

             Use both ellipsoid and DSK shape models for Mars.

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

             We select a step size of 3 minutes, which means we ignore
             occultation events lasting less than 3 minutes, if any exist.

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

               KPL/MK

               File: gfoclt_ex3.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
                  ---------                        --------
                  de410.bsp                        Planetary ephemeris
                  mar063.bsp                       Mars satellite ephemeris
                  pck00010.tpc                     Planet orientation and
                                                   radii
                  naif0011.tls                     Leapseconds
                  earthstns_itrf93_050714.bsp      DSN station ephemeris
                  earth_latest_high_prec.bpc       Earth orientation
                  mro_psp34.bsp                    MRO ephemeris
                  megr90n000cb_plate.bds           Plate model based on
                                                   MEGDR DEM, resolution
                                                   4 pixels/degree.

               \begindata

                  PATH_SYMBOLS    = ( 'MRO', 'GEN' )

                  PATH_VALUES     = (
                                      '/ftp/pub/naif/pds/data+'
                                      '/mro-m-spice-6-v1.0/+'
                                      'mrosp_1000/data/spk',
                                      '/ftp/pub/naif/generic_kernels'
                                    )

                  KERNELS_TO_LOAD = ( '$MRO/de410.bsp',
                                      '$MRO/mar063.bsp',
                                      '$MRO/mro_psp34.bsp',
                                      '$GEN/spk/stations/+'
                                      'earthstns_itrf93_050714.bsp',
                                      '$GEN/pck/earth_latest_high_prec.bpc',
                                      'pck00010.tpc',
                                      'naif0011.tls',
                                      'megr90n000cb_plate.bds'
                                    )
               \begintext

            Example code begins here.

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

               int main()
               {
                  /.
                  Local constants
                  ./

                  #define META            "gfoclt_ex3.tm"
                  #define TIMFMT          "YYYY MON DD HR:MN:SC" \
                                          ".###### (TDB)::TDB"
                  #define MAXWIN          200
                  #define TIMLEN          41

                  /.
                  Local variables
                  ./
                  SPICEDOUBLE_CELL      ( cnfine, MAXWIN );
                  SPICEDOUBLE_CELL      ( result, MAXWIN );

                  SpiceChar             * abcorr;
                  SpiceChar             * back;
                  SpiceChar               begstr [ TIMLEN ];
                  SpiceChar             * bframe;
                  SpiceChar             * bshape;
                  SpiceChar               endstr [ TIMLEN ];
                  SpiceChar             * fframe;
                  SpiceChar             * front;
                  SpiceChar             * fshape;
                  SpiceChar             * obsrvr;
                  SpiceChar             * occtyp;
                  SpiceChar             * win0;
                  SpiceChar             * win1;

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

                  SpiceInt                i;
                  SpiceInt                j;
                  SpiceInt                k;

                  /.
                  Load kernels.
                  ./
                  furnsh_c ( META );

                  /.
                  Set the observer and aberration correction.
                  ./
                  obsrvr = "DSS-14";
                  abcorr = "CN";

                  /.
                  Set the occultation type.
                  ./
                  occtyp = "ANY";

                  /.
                  Set the TDB time bounds of the confinement
                  window, which is a single interval in this case.
                  ./
                  win0 = "2015 FEB 28 07:00:00 TDB";
                  win1 = "2015 FEB 28 12:00:00 TDB";

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

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

                  /.
                  Select a 3-minute step. We'll ignore any occultations
                  lasting less than 3 minutes.  Units are TDB seconds.
                  ./
                  step = 180.0;

                  /.
                  Perform both spacecraft occultation and spacecraft
                  transit searches.
                  ./
                  for ( i = 0;  i < 2;  i++ )
                  {
                     if ( i == 0 )
                     {
                        /.
                        Perform a spacecraft occultation search.
                        ./
                        front  = "MARS";
                        fframe = "IAU_MARS";

                        back   = "MRO";
                        bshape = "POINT";
                        bframe = " ";
                     }
                     else
                     {
                        /.
                        Perform a spacecraft transit search.
                        ./
                        front  = "MRO";
                        fframe = " ";
                        fshape = "POINT";

                        back   = "MARS";
                        bframe = "IAU_MARS";
                     }

                     for ( j = 0;  j < 2;  j++ )
                     {
                        if ( j == 0 )
                        {
                           /.
                           Model the planet shape as an ellipsoid.
                           ./
                           if ( i == 0 )
                           {
                              fshape = "ELLIPSOID";
                           }
                           else
                           {
                              bshape = "ELLIPSOID";
                           }
                        }
                        else
                        {
                           /.
                           Model the planet shape using DSK data.
                           ./
                           if ( i == 0 )
                           {
                              fshape = "DSK/UNPRIORITIZED";
                           }
                           else
                           {
                              bshape = "DSK/UNPRIORITIZED";
                           }
                        }
                        /.
                        Perform the spacecraft occultation or
                        transit search.
                        ./
                        printf ( "\n" );

                        if ( i == 0 )
                        {
                           printf ( "Using shape model %s\n"
                                    "Starting occultation search...\n",
                                    fshape                             );
                        }
                        else
                        {
                           printf ( "Using shape model %s\n"
                                    "Starting transit search...\n",
                                    bshape                             );
                        }

                        gfoclt_c ( occtyp,
                                   front,   fshape,  fframe,
                                   back,    bshape,  bframe,
                                   abcorr,  obsrvr,  step,
                                   &cnfine, &result          );

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

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

                              printf ( "Interval %d\n"
                                       "   Start time: %s\n"
                                       "   Stop time:  %s\n",
                                       (int)k, begstr, endstr );
                           }
                        }
                     }
                     /.
                     End of the target shape loop.
                     ./
                  }
                  /.
                  End of the occultation vs transit loop.
                  ./
                  printf ( "\n" );
                  return ( 0 );
               }

            When this program was executed on a PC/Linux/gcc 64-bit
            platform, the output was:

               Using shape model ELLIPSOID
               Starting occultation search...
               Interval 0
                  Start time: 2015 FEB 28 07:17:35.379879 (TDB)
                  Stop time:  2015 FEB 28 07:50:37.710284 (TDB)
               Interval 1
                  Start time: 2015 FEB 28 09:09:46.920140 (TDB)
                  Stop time:  2015 FEB 28 09:42:50.497193 (TDB)
               Interval 2
                  Start time: 2015 FEB 28 11:01:57.845730 (TDB)
                  Stop time:  2015 FEB 28 11:35:01.489716 (TDB)

               Using shape model DSK/UNPRIORITIZED
               Starting occultation search...
               Interval 0
                  Start time: 2015 FEB 28 07:17:38.130608 (TDB)
                  Stop time:  2015 FEB 28 07:50:38.310802 (TDB)
               Interval 1
                  Start time: 2015 FEB 28 09:09:50.314903 (TDB)
                  Stop time:  2015 FEB 28 09:42:55.369626 (TDB)
               Interval 2
                  Start time: 2015 FEB 28 11:02:01.756296 (TDB)
                  Stop time:  2015 FEB 28 11:35:08.368384 (TDB)

               Using shape model ELLIPSOID
               Starting transit search...
               Interval 0
                  Start time: 2015 FEB 28 08:12:21.112018 (TDB)
                  Stop time:  2015 FEB 28 08:45:48.401746 (TDB)
               Interval 1
                  Start time: 2015 FEB 28 10:04:32.682324 (TDB)
                  Stop time:  2015 FEB 28 10:37:59.920302 (TDB)
               Interval 2
                  Start time: 2015 FEB 28 11:56:39.757564 (TDB)
                  Stop time:  2015 FEB 28 12:00:00.000000 (TDB)

               Using shape model DSK/UNPRIORITIZED
               Starting transit search...
               Interval 0
                  Start time: 2015 FEB 28 08:12:15.750020 (TDB)
                  Stop time:  2015 FEB 28 08:45:43.406870 (TDB)
               Interval 1
                  Start time: 2015 FEB 28 10:04:29.031706 (TDB)
                  Stop time:  2015 FEB 28 10:37:55.565509 (TDB)
               Interval 2
                  Start time: 2015 FEB 28 11:56:34.634642 (TDB)
                  Stop time:  2015 FEB 28 12:00:00.000000 (TDB)

#### Restrictions

      None.

#### Literature_References

      None.

#### Author_and_Institution

      N. J. Bachman  (JPL)
      L. S. Elson    (JPL)
      E. D. Wright   (JPL)

#### Version

       -CSPICE Version 2.0.0, 12-JUL-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, 07-APR-2009 (NJB) (LSE) (EDW)

#### Index_Entries

       GF occultation search

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

    Wed Apr  5 17:54:36 2017