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Index Page gffove_c
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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
       gffove_c ( GF, is target in FOV? )

       void gffove_c ( ConstSpiceChar     * inst,
                       ConstSpiceChar     * tshape,
                       ConstSpiceDouble     raydir [3],
                       ConstSpiceChar     * target,
                       ConstSpiceChar     * tframe,
                       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 a specified target body or ray
       intersects the space bounded by the field-of-view (FOV) of a
       specified instrument. Report progress and handle interrupts if so
       commanded.
#### Required_Reading
       CK
       FRAMES
       GF
       KERNEL
       NAIF_IDS
       PCK
       SPK
       TIME
       WINDOWS
#### Keywords
       EVENT
       FOV
       GEOMETRY
       INSTRUMENT
       SEARCH
       WINDOW
#### Brief_I/O
       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       SPICE_GF_MARGIN
                  P   Minimum complement of FOV cone angle.
       SPICE_GF_CNVTOL
                  P   Convergence tolerance.
       SPICE_GF_MAXVRT
                  P   Maximum number of FOV boundary vertices.
       inst       I   Name of the instrument.
       tshape     I   Type of shape model used for target body.
       raydir     I   Ray's direction vector.
       target     I   Name of the target body.
       tframe     I   Body-fixed, body-centered frame for target body.
       abcorr     I   Aberration correction flag.
       obsrvr     I   Name of the observing body.
       tol        I   Convergence tolerance in seconds.
       udstep     I   Name of 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
       inst        is a string indicates the name of an instrument, such as
                   a spacecraft-mounted framing camera, the field of view
                   (FOV) of which is to be used for a target intersection
                   search: times when the specified target intersects the
                   region of space corresponding to the FOV are sought.

                   `inst' must have a corresponding NAIF ID and a frame
                   defined, as is normally done in a frame kernel. It must
                   also have an associated reference frame and a FOV shape,
                   boresight and boundary vertices (or reference vector and
                   reference angles) defined, as is usually done in an
                   instrument kernel.

                   See the header of the CSPICE routine getfov_c for a
                   description of the required parameters associated with an
                   instrument.

       tshape      is a string indicating the geometric model used to
                   represent the location and shape of the target body. The
                   target body may be represented by either an ephemeris
                   object or a ray emanating from the observer.

                   The supported values of `tshape' are:

                      "ELLIPSOID"     The target is an ephemeris object.

                                      The target's shape is represented
                                      using 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"         The target is an ephemeris object.
                                      The body is treated as a single
                                      point.

                      "RAY"           The target is NOT an ephemeris
                                      object. Instead, the target is
                                      represented by the ray emanating
                                      from the observer's location and
                                      having direction vector `raydir'. The
                                      target is considered to be visible
                                      if and only if the ray is contained
                                      within the space bounded by the
                                      instrument FOV.

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

       raydir      is the direction vector associated with a ray
                   representing the target. `raydir' is used if and only if
                   `tshape' (see description above) indicates the target is
                   modeled as a ray.

       target      is the name of the target body, the appearances of which
                   in the specified instrument's field of view are sought.
                   The body must be an ephemeris object.

                   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 `target'.

                   The input argument `target' is used if and only if the
                   target is NOT modeled as ray, as indicated by the input
                   argument `tshape'.

                   `target' may be set to a blank string if the target is
                   modeled as a ray.

       tframe      is the name of the reference frame associated with the
                   target. Examples of such names are "IAU_SATURN" (for
                   Saturn) and "ITRF93" (for the Earth).

                   If the target is an ephemeris object modeled as an
                   ellipsoid, `tframe' must designate a body-fixed reference
                   frame centered on the target body.

                   If the target is an ephemeris object modeled as a point,
                   `tframe' is ignored; `tframe' should be left blank.

                   If the target is modeled as a ray, `tframe' may designate
                   any reference frame. Since light time corrections are not
                   supported for rays, the orientation of the frame is
                   always evaluated at the epoch associated with the
                   observer, as opposed to the epoch associated with the
                   light-time corrected position of the frame center.

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

       abcorr      is a string indicating the aberration corrections to be
                   applied when computing the target's position and
                   orientation. The supported values of `abcorr' depend on the
                   target representation.

                   If the target is represented by a ray, the aberration
                   correction options are

                      "NONE"       No correction.
                      "S"          Stellar aberration correction, reception
                                   case.
                      "XS"         Stellar aberration correction,
                                   transmission case.

                   If the target is an ephemeris object, the aberration
                   correction options are those supported by the SPICE SPK
                   system. For remote sensing applications, where the
                   apparent position and orientation of the target seen by
                   the observer are desired, normally either of the
                   corrections

                      "LT+S"
                      "CN+S"

                   should be used. These and the other supported options are
                   described below.

                   Supported aberration correction options for observation
                   (the case where radiation is received by observer at ET)
                   are:

                      "NONE"       No correction.
                      "LT"         Light time only
                      "LT+S"       Light time and stellar aberration.
                      "CN"         Converged Newtonian (CN) light time.
                      "CN+S"       CN light time and stellar aberration.

                   Supported aberration correction options for transmission
                   (the case where radiation is emitted from observer at ET)
                   are:

                      "XLT"        Light time only.
                      "XLT+S"      Light time and stellar aberration.
                      "XCN"        Converged Newtonian (CN) light time.
                      "XCN+S"      CN light time and stellar aberration.

                   For detailed information, see the geometry finder
                   required reading, gf.req.

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

       obsrvr      is the name of the body from which the target is
                   observed. The instrument designated by `inst' is treated as
                   if it were co-located with the observer.

                   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 "visible" to being "not visible" or
                   vice versa.

                   This routine relies on `udstep' returning step sizes small
                   enough so that state transitions within the confinement
                   window are not overlooked.

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

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

       rpt         is a logical variable that 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,

                      "Target visibility search"
                      "done."

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

                      Target visibility 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 of the confinement window.

                   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.

                   gffove_c uses `udbail' only when `bail' (see above) is set to
                   SPICETRUE, indicating that interrupt handling is enabled.
                   When interrupt handling is enabled, gffove_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 gffove_c as an input argument. The
                   CSPICE routine

                      gfbail_c

                   may be used for this purpose.

                   The function `udbail' will be 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.
                   So `udbail' must provide a "reset" mechanism."
                   In the case of gfbail_c, the reset function is

                      gfclrh_c

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

                      gfbail_c

                   may be used for this purpose.

                   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 image
                   of the target body is partially or completely within
                   the specified instrument field of view.

                   `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 gffove_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
       All parameters described here are declared in the header file
       SpiceGF.h. See that file for parameter values.

       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_MAXVRT

                  is the maximum number of vertices that may be used
                  to define the boundary of the specified instrument's
                  field of view.

       SPICE_GF_MARGIN

                  is a small positive number used to constrain the
                  orientation of the boundary vectors of polygonal
                  FOVs. Such FOVs must satisfy the following constraints:

                     1)  The boundary vectors must be contained within
                         a right circular cone of angular radius less
                         than than (pi/2) - SPICE_GF_MARGIN radians; in other
                         words, there must be a vector A such that all
                         boundary vectors have angular separation from
                         A of less than (pi/2)-SPICE_GF_MARGIN radians.

                     2)  There must be a pair of boundary vectors U, V
                         such that all other boundary vectors lie in the
                         same half space bounded by the plane containing U
                         and V. Furthermore, all other boundary vectors
                         must have orthogonal projections onto a specific
                         plane normal to this plane (the normal plane
                         contains the angle bisector defined by U and V)
                         such that the projections have angular separation
                         of at least 2*SPICE_GF_MARGIN radians from the
                         plane spanned by U and V.

       See header file SpiceGF.h for declarations and descriptions of
       parameters used throughout the GF system.
#### 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 the name of either the target or 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 specified aberration correction is not a supported
           value for the target type (ephemeris object or ray), an error
           is signaled by a routine in the call tree of this routine.

       5)  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.

       6)  If the target body coincides with the observer body `obsrvr', an
           error is signaled by a routine in the call tree of this
           routine.

       7)  If the body model specifier `tshape' is not recognized, an error
           is signaled by a routine in the call tree of this routine.

       8)  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.

       9)  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.

       10) If the instrument name `inst' does not have corresponding NAIF
           ID code, an error is signaled by a routine in the call
           tree of this routine.

       11) If the FOV parameters of the instrument are not present in
           the kernel pool, an error is signaled by a routine
           in the call tree of this routine.

       12) If the FOV boundary has more than SPICE_GF_MAXVRT vertices, an error
           is signaled by a routine in the call tree of this
           routine.

       13) If the instrument FOV is polygonal, and this routine cannot
           find a ray R emanating from the FOV vertex such that maximum
           angular separation of R and any FOV boundary vector is within
           the limit (pi/2)-SPICE_GF_MARGIN radians, an error is signaled
           by a routine in the call tree of this routine. If the FOV is
           any other shape, the same error check will be applied with the
           instrument boresight vector serving the role of R.

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

       15) 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.

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

       17) 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.

       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 the step size is non-positive, an error is signaled by a
           routine in the call tree of this routine.

       20) If the ray's direction vector is zero, 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 `inst', `tshape', `abcorr', `tframe', `target'
           or `obsrvr' input string pointers is null, the error
           SPICE(NULLPOINTER) is signaled.

       23) If any of the `inst', `tshape' or `abcorr' 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: ephemeris data for target and observer that
          describes the ephemeris of these objects for the period
          defined by the confinement window, `cnfine' must be
          loaded. If aberration corrections are used, the states of
          target and observer relative to the solar system barycenter
          must be calculable from the available ephemeris data.
          Typically ephemeris data are made available by loading one
          or more SPK files via furnsh_c.

       -  Frame data: if a frame definition is required to convert
          the observer and target states to the body-fixed frame of
          the target, that definition must be available in the kernel
          pool. Typically the definitions of frames not already
          built-in to SPICE are supplied by loading a frame kernel.

          Data defining the reference frame associated with the
          instrument designated by `inst' must be available in the kernel
          pool. Additionally the name `inst' must be associated with an
          ID code. Normally these data are  made available by loading
          a frame kernel via furnsh_c.

       -  IK data: the kernel pool must contain data such that
          the CSPICE routine getfov_c may be called to obtain
          parameters for `inst'. Normally such data are provided by
          an IK via furnsh_c.

       The following data may be required:

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

          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.

       -  CK data: if the instrument frame is fixed to a spacecraft,
          at least one CK file will be needed to permit transformation
          of vectors between that frame and both J2000 and the target
          body-fixed 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).

       -  Since the input ray direction may be expressed in any
          frame, FKs, CKs, SCLK kernels, PCKs, and SPKs may be
          required to map the direction to the J2000 frame.

       Kernel data are normally loaded once per program run, NOT every
       time this routine is called.
#### Particulars
       This routine determines a set of one or more time intervals within
       the confinement window when a specified ray or any portion of a
       specified target body appears within the field of view of a
       specified instrument. We'll use the term "visibility event" to
       designate such an appearance. The set of time intervals resulting
       from the search is returned as a SPICE window.

       This routine provides the SPICE GF system's most flexible
       interface for searching for FOV intersection 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 either gftfov_c or gfrfov_c rather than this routine.

       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 visibility events is treated as a search for state
       transitions: times are sought when the state of the target ray or
       body changes from "not visible" to "visible" 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 visibility 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 visibility state is constant:
       the step size should be shorter than the shortest visibility event
       duration and the shortest period between visibility events, within
       the confinement window.

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

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

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

       The times of state transitions are called ``roots.''

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

       The convergence tolerance used by 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.

       Setting the input tolerance `tol' tighter than SPICE_GF_CNVTOL is
       unlikely to be useful, since the results are unlikely to be more
       accurate. Making the tolerance looser will speed up searches
       somewhat, since a few convergence steps will be omitted. However, in
       most cases, the step size is likely to have a much greater effect on
       processing time than would the convergence tolerance.

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

       The simplest use of the confinement window is to specify a time
       interval within which a solution is sought. However, the confinement
       window can, in some cases, be used to make searches more efficient.
       Sometimes it's possible to do an efficient search to reduce the size
       of the time period over which a relatively slow search of interest
       must be performed. For an example, see the program CASCADE in the
       GF Example Programs chapter of the GF Required Reading, gf.req.
#### 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) 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.

          Search for times when Saturn's satellite Phoebe is within
          the FOV of the Cassini narrow angle camera (CASSINI_ISS_NAC).
          To simplify the problem, restrict the search to a short time
          period where continuous Cassini bus attitude data are
          available.

          Use a step size of 1 second to reduce chances of missing
          short visibility events.

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

             KPL/MK

             File name: gffove_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
                -----------------------------   ----------------------
                naif0012.tls                    Leapseconds
                pck00010.tpc                    Satellite orientation
                                                and radii
                041014R_SCPSE_01066_04199.bsp   CASSINI, planetary and
                                                Saturn satellite
                                                ephemeris
                cas_v40.tf                      Cassini FK
                04161_04164ra.bc                Cassini bus CK
                cas00071.tsc                    Cassini SCLK kernel
                cas_iss_v10.ti                  Cassini IK

             \begindata

                KERNELS_TO_LOAD = ( 'naif0012.tls',
                                    'pck00010.tpc',
                                    '041014R_SCPSE_01066_04199.bsp',
                                    'cas_v40.tf',
                                    '04161_04164ra.bc',
                                    'cas00071.tsc',
                                    'cas_iss_v10.ti'            )
             \begintext

             End of meta-kernel

          Example code begins here.

          /.
             Program gffove_ex1
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main()
          {

             /.
             Local constants
             ./
             #define  META           "gffove_ex1.tm"
             #define  TIMFMT         "YYYY-MON-DD HR:MN:SC.######::TDB (TDB)"
             #define  TIMLEN         41
             #define  MAXWIN         10000
             #define  TIMTOL         1.e-6

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

             SpiceBoolean            bail;
             SpiceBoolean            rpt;

             SpiceChar             * abcorr;
             SpiceChar             * inst;
             SpiceChar             * obsrvr;
             SpiceChar             * target;
             SpiceChar             * tframe;
             SpiceChar               timstr  [2][ TIMLEN ];
             SpiceChar             * tshape;

             SpiceDouble             endpt   [2];
             SpiceDouble             et0;
             SpiceDouble             et1;
             SpiceDouble             raydir [3];

             SpiceInt                i;
             SpiceInt                j;
             SpiceInt                n;

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

             /.
             Insert search time interval bounds into the
             confinement window.
             ./
             str2et_c ( "2004 JUN 11 06:30:00 TDB", &et0 );
             str2et_c ( "2004 JUN 11 12:00:00 TDB", &et1 );

             wninsd_c ( et0, et1, &cnfine );

             /.
             Initialize inputs for the search.
             ./
             inst   = "CASSINI_ISS_NAC";
             target = "PHOEBE";
             tshape = "ELLIPSOID";
             tframe = "IAU_PHOEBE";
             abcorr = "LT+S";
             obsrvr = "CASSINI";

             /.
             Select a 1-second step. We'll ignore any target
             appearances lasting less than 1 second.
             ./
             gfsstp_c ( 1.0 );

             printf ( "\n"
                      "Instrument: %s\n"
                      "Target:     %s\n",
                      inst,
                      target            );

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

             /.
             Perform the search.
             ./
             gffove_c ( inst,      tshape,    raydir,    target,    tframe,
                        abcorr,    obsrvr,    TIMTOL,    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
             {

                n = wncard_c ( &result );

                if ( n == 0 )
                {
                   printf (  "No FOV intersection found.\n" );
                }
                else
                {
                   printf ( "  Visibility start time              Stop time\n" );

                   for ( i = 0;  i < n;  i++ )
                   {
                      wnfetd_c ( &result, i, endpt, endpt+1 );

                      for ( j = 0;  j < 2;  j++ )
                      {
                         timout_c ( endpt[j], TIMFMT, TIMLEN, timstr[j] );
                      }

                      printf ( "  %s  %s\n", timstr[0], timstr[1] );
                   }
                }

                printf ( "\n" );

             }

             return ( 0 );
          }

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

          Instrument: CASSINI_ISS_NAC
          Target:     PHOEBE

          Target visibility search 100.00% done.

            Visibility start time              Stop time
            2004-JUN-11 07:35:27.066980 (TDB)  2004-JUN-11 08:48:03.954696 (TDB)
            2004-JUN-11 09:02:56.580045 (TDB)  2004-JUN-11 09:35:04.038509 (TDB)
            2004-JUN-11 09:49:56.476397 (TDB)  2004-JUN-11 10:22:04.242879 (TDB)
            2004-JUN-11 10:36:56.283771 (TDB)  2004-JUN-11 11:09:04.397165 (TDB)
            2004-JUN-11 11:23:56.020645 (TDB)  2004-JUN-11 11:56:04.733536 (TDB)

          Note that the progress report has the format shown below:

             Target visibility search   2.66% done.

          The completion percentage was updated approximately once per
          second.

       2) A variation of example (1): search the same confinement
          window for times when a selected background star is visible.
          We use the FOV of the Cassini ISS wide angle camera
          (CASSINI_ISS_WAC) to enhance the probability of viewing the
          star.

          The star we'll use has catalog number 6000 in the Hipparcos
          Catalog. The star's J2000 right ascension and declination,
          proper motion, and parallax are taken from that catalog.

          Use the meta-kernel from the first example.

          Example code begins here.

          /.
             Program gffove_ex2
          ./
          #include <stdio.h>
          #include <math.h>
          #include "SpiceUsr.h"

          int main()
          {

             /.
             Local constants
             ./
             #define  META           "gffove_ex1.tm"
             #define  TIMFMT         "YYYY-MON-DD HR:MN:SC.######::TDB (TDB)"
             #define  TIMLEN         41
             #define  MAXWIN         10000
             #define  TIMTOL         1.e-6
             #define  AU             149597870.693

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

             SpiceBoolean            bail;
             SpiceBoolean            rpt;

             SpiceChar             * abcorr;
             SpiceChar             * inst;
             SpiceChar             * obsrvr;
             SpiceChar             * rframe;
             SpiceChar             * target;
             SpiceChar               timstr  [2][ TIMLEN ];
             SpiceChar             * tshape;

             SpiceDouble             dec;
             SpiceDouble             decdeg;
             SpiceDouble             decdg0;
             SpiceDouble             decepc;
             SpiceDouble             decpm;
             SpiceDouble             dtdec;
             SpiceDouble             dtra;
             SpiceDouble             endpt   [2];
             SpiceDouble             et0;
             SpiceDouble             et1;
             SpiceDouble             lt;
             SpiceDouble             parlax;
             SpiceDouble             plxdeg;
             SpiceDouble             pos     [3];
             SpiceDouble             pstar   [3];
             SpiceDouble             ra;
             SpiceDouble             radeg0;
             SpiceDouble             radeg;
             SpiceDouble             raepc;
             SpiceDouble             rapm;
             SpiceDouble             raydir  [3];
             SpiceDouble             rstar;
             SpiceDouble             t;

             SpiceInt                catno;
             SpiceInt                i;
             SpiceInt                j;
             SpiceInt                n;

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

             /.
             Insert search time interval bounds into the
             confinement window.
             ./
             str2et_c ( "2004 JUN 11 06:30:00 TDB", &et0 );
             str2et_c ( "2004 JUN 11 12:00:00 TDB", &et1 );

             wninsd_c ( et0, et1, &cnfine );

             /.
             Initialize inputs for the search.
             ./
             inst   = "CASSINI_ISS_WAC";
             target = " ";
             tshape = "RAY";

             /.
             Create a unit direction vector pointing from
             observer to star. We'll assume the direction
             is constant during the confinement window, and
             we'll use et0 as the epoch at which to compute the
             direction from the spacecraft to the star.

             The data below are for the star with catalog
             number 6000 in the Hipparcos catalog. Angular
             units are degrees; epochs have units of Julian
             years and have a reference epoch of J1950.
             The reference frame is J2000.
             ./
             catno  = 6000;

             plxdeg = 0.000001056;

             radeg0 = 19.290789927;
             rapm   = -0.000000720;
             raepc  = 41.2000;

             decdg0 =  2.015271007;
             decpm  =  0.000001814;
             decepc = 41.1300;

             rframe = "j2000";

             /.
             Correct the star's direction for proper motion.

             The argument t represents et0 as Julian years
             past J1950.
             ./
             t      =    ( et0      / jyear_c() )
                      +  ( j2000_c()- j1950_c() ) / 365.25;

             dtra   = t - raepc;
             dtdec  = t - decepc;

             radeg  = radeg0  +  dtra  * rapm;
             decdeg = decdg0  +  dtdec * decpm;

             ra     = radeg  * rpd_c();
             dec    = decdeg * rpd_c();

             radrec_c ( 1.0, ra, dec, pstar );

             /.
             Correct star position for parallax applicable at
             the Cassini orbiter's position. (The parallax effect
             is negligible in this case; we're simply demonstrating
             the computation.)
             ./
             parlax = plxdeg * rpd_c();
             rstar  = AU / tan(parlax);

             /.
             Scale the star's direction vector by its distance from
             the solar system barycenter. Subtract off the position
             of the spacecraft relative to the solar system barycenter;
             the result is the ray's direction vector.
             ./
             vscl_c ( rstar, pstar, pstar );

             spkpos_c ( "cassini", et0, "j2000",  "none",
                        "solar system barycenter", pos,  &lt );

             vsub_c   ( pstar, pos, raydir );

             /.
             Correct the star direction for stellar aberration when
             we conduct the search.
             ./
             abcorr = "S";
             obsrvr = "CASSINI";

             /.
             Select a 1-second step. We'll ignore any target
             appearances lasting less than 1 second.
             ./
             gfsstp_c ( 1.0 );

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

             printf ( "\n"
                      "Instrument:            %s\n"
                      "Star's catalog number: %d\n",
                      inst,
                      (int)catno                        );

             /.
             Perform the search.
             ./
             gffove_c ( inst,      tshape,    raydir,    target,    rframe,
                        abcorr,    obsrvr,    TIMTOL,    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
             {

                n = wncard_c ( &result );

                if ( n == 0 )
                {
                   printf (  "No FOV intersection found.\n" );
                }
                else
                {
                   printf ( "  Visibility start time              Stop time\n" );

                   for ( i = 0;  i < n;  i++ )
                   {
                      wnfetd_c ( &result, i, endpt, endpt+1 );

                      for ( j = 0;  j < 2;  j++ )
                      {
                         timout_c ( endpt[j], TIMFMT, TIMLEN, timstr[j] );
                      }

                      printf ( "  %s  %s\n", timstr[0], timstr[1] );
                   }
                }

                printf ( "\n" );

             }

             return ( 0 );
          }

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

          Instrument:            CASSINI_ISS_WAC
          Star's catalog number: 6000

          Target visibility search 100.00% done.

            Visibility start time              Stop time
            2004-JUN-11 06:30:00.000000 (TDB)  2004-JUN-11 12:00:00.000000 (TDB)
#### Restrictions
       1)  The kernel files to be used by gffove_c must be loaded (normally via
           the CSPICE routine furnsh_c) before gffove_c is called.
#### Literature_References
       None.
#### Author_and_Institution
       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
       E.D. Wright         (JPL)
#### Version
       -CSPICE Version 1.0.2, 06-AUG-2021 (JDR)

           Edited the header to comply to comply with NAIF standard.

           Updated Examples' kernels set to use PDS archived data.

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

           Added entries #17 and #24 in -Exceptions section.

       -CSPICE Version 1.0.1, 12-JUL-2016 (EDW)

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

       -CSPICE Version 1.0.0, 15-APR-2009 (NJB) (EDW)
#### Index_Entries
       GF low-level target in instrument FOV search
#### Link to routine gffove_c source file gffove_c.c

    Fri Dec 31 18:41:07 2021