Index Page gffove_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 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 the routine 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       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     indicates 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 "in view" to being "not in view" 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
                  computes a refinement in the times that bracket a
                  transition point. In other words, once a pair of
                  times have been detected such that the system is in
                  different states at each of the two times, `udrefn'
                  selects an intermediate time which should be closer to
                  the transition state than one of the two known times.
                  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 system is in state `s2'. `t2'
                           is expressed as seconds past J2000 TDB. `t2' is
                           assumed to be larger than `t1'.

                     s1    is the visibility state at time at time t1.

                     s2    is the visibility state at time 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 udpref (see descriptions below)
                  are used to report progress.

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

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

                  where

                     cnfine

                  is a 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,

                     "FOV search"
                     "done."

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

                     FOV 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 subroutine that updates the
                  progress report for a search.  The prototype
                  of `udrepu' is

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

                  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'.

                     - Over a search, the sum of the differences

                          ivend - ivbeg

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

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

                     ivbeg : ivend

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

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

       udrepf     is a user-defined subroutine 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). udbail has the
                  prototype

                     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 the user doesn't wish to provide a custom interrupt
                  handling function, the CSPICE routine

                     gfbail_c

                  may be used.

                  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' nmust 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.
#### 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 target intersection with the FOV 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 gffove_c conducts its
                  search.
#### 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, 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 the name of either the target or 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 specified aberration correction is not a supported value
           for the target type (ephemeris object or ray), the error will be
           diagnosed 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, the error will be diagnosed by a routine in the
           call tree of this routine.

       6)  If the target body coincides with the observer body `obsrvr',
           the error will be diagnosed by a routine in the call tree of
           this routine.

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

       10) If the instrument name `inst' does not have corresponding NAIF
           ID code, the error will be diagnosed 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, the error will be be diagnosed by routines
           in the call tree of this routine.

       12) If the FOV boundary has more than SPICE_GF_MAXVRT vertices,
           the error will be be diagnosed by routines 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, the error will be
           diagnosed 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, the error will be diagnosed 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 will be diagnosed 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, the error will be diagnosed
           by a routine in the call tree of this routine.

       17) If the convergence tolerance size is non-positive, the error
           SPICE(INVALIDTOLERANCE) will be signaled.

       18) If the step size is non-positive, the error
           SPICE(INVALIDSTEP) will be signaled.

       19) If the ray's direction vector is zero, the error
           SPICE(ZEROVECTOR) is signaled.

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

       21) If any input string argument other than `tframe', `target',
           or `obsrvr' is empty, the error SPICE(EMPTYSTRING) will be
           signaled.

       22) If any attempt to change the handler for the interrupt
           signal SIGINT fails, the error SPICE(SIGNALFAILURE) is
           signaled.

       23) If operation of this routine is interrupted, the output result
           window will be invalid.
#### 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
                ---------                     --------
                naif0009.tls                  Leapseconds
                cpck05Mar2004.tpc             Satellite orientation and
                                              radii
                981005_PLTEPH-DE405S.bsp      Planetary ephemeris
                020514_SE_SAT105.bsp          Satellite ephemeris
                030201AP_SK_SM546_T45.bsp     Spacecraft ephemeris
                cas_v37.tf                    Cassini FK
                04135_04171pc_psiv2.bc        Cassini bus CK
                cas00084.tsc                  Cassini SCLK kernel
                cas_iss_v09.ti                Cassini IK

             \begindata

                KERNELS_TO_LOAD = ( 'naif0009.tls',
                                    'cpck05Mar2004.tpc',
                                    '981005_PLTEPH-DE405S.bsp',
                                    '020514_SE_SAT105.bsp',
                                    '030201AP_SK_SM546_T45.bsp',
                                    'cas_v37.tf',
                                    '04135_04171pc_psiv2.bc',
                                    'cas00084.tsc',
                                    'cas_iss_v09.ti'            )
             \begintext

          Example code begins here.

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

             int main()
             {
                /.
                PROGRAM EX1
                ./

                /.
                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 PC/Linux/gcc platform, the
          progress report had the format shown below:

             Target visibility search   2.66% done.

          The completion percentage was updated approximately once per
          second.

          When this program completed execution, the output was:

             Instrument: CASSINI_ISS_NAC
             Target:     PHOEBE

             Target visibility search 100.00% done.

               Visibility start time              Stop time
               2004-JUN-11 07:35:49.958590 (TDB)  2004-JUN-11 08:48:27.485965 (TDB)
               2004-JUN-11 09:03:19.767799 (TDB)  2004-JUN-11 09:35:27.634790 (TDB)
               2004-JUN-11 09:50:19.585474 (TDB)  2004-JUN-11 10:22:27.854254 (TDB)
               2004-JUN-11 10:37:19.332696 (TDB)  2004-JUN-11 11:09:28.116016 (TDB)
               2004-JUN-11 11:24:19.049484 (TDB)  2004-JUN-11 11:56:28.380304 (TDB)

       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.

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

             int main()
             {
                /.
                PROGRAM EX2
                ./

                /.
                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: %ld\n",
                         inst,
                         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 PC/Linux/gcc 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
       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)
       E.D. Wright   (JPL)
#### Version
       -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

    Wed Jun  9 13:05:24 2010