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

Table of contents

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

       gfevnt_c (GF, geometric event finder )

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

                       void             ( * udrefn ) ( SpiceDouble       t1,
                                                       SpiceDouble       t2,
                                                       SpiceBoolean      s1,
                                                       SpiceBoolean      s2,
                                                       SpiceDouble     * t    ),
                       ConstSpiceChar     * gquant,
                       SpiceInt             qnpars,
                       SpiceInt             lenvals,
                       const void         * qpnams,
                       const void         * qcpars,
                       ConstSpiceDouble   * qdpars,
                       ConstSpiceInt      * qipars,
                       ConstSpiceBoolean  * qlpars,
                       ConstSpiceChar     * op,
                       SpiceDouble          refval,
                       SpiceDouble          tol,
                       SpiceDouble          adjust,
                       SpiceBoolean         rpt,

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

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

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

#### Abstract

       Determine time intervals when a specified geometric quantity
       satisfies a specified mathematical condition.

#### Required_Reading

       GF
       WINDOWS

#### Keywords

       EVENT
       GEOMETRY
       SEARCH
       WINDOW

#### Brief_I/O

       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       SPICE_GFEVNT_MAXPAR
                  P   Maximum number of parameters required to define
                      any quantity.
       SPICE_GF_CNVTOL
                  P   Default convergence tolerance.
       udstep     I   Name of the routine that computes and returns a
                      time step.
       udrefn     I   Name of the routine that computes a refined time.
       gquant     I   Type of geometric quantity.
       qnpars     I   Number of quantity definition parameters.
       lenvals    I   Length of strings in 'qpnams' and 'qcpars'.
       qpnams     I   Names of quantity definition parameters.
       qcpars     I   Array of character quantity definition parameters.
       qdpars     I   Array of double precision quantity definition
                      parameters.
       qipars     I   Array of integer quantity definition parameters.
       qlpars     I   Array of logical quantity definition parameters.
       op         I   Operator that either looks for an extreme value
                      (max, min, local, absolute) or compares the
                      geometric quantity value and a number.
       refval     I   Reference value.
       tol        I   Convergence tolerance in seconds
       adjust     I   Absolute extremum adjustment value.
       rpt        I   Progress reporter on (SPICETRUE) or off (SPICEFALSE).
       udrepi     I   Function that initializes progress reporting.
       udrepu     I   Function that updates the progress report.
       udrepf     I   Function that finalizes progress reporting.
       nintvls    I   Workspace window interval count
       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

       udstep      is an externally specified routine that computes a
                   time step in an attempt to find a transition of the
                   state being considered. In the context of this
                   routine's algorithm, a "state transition" occurs where
                   the geometric state changes from being in the desired
                   geometric condition event to not, or vice versa.

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

                   The prototype for `udstep' is

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

                   where:

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

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

                   If a constant step size is desired, the CSPICE routine

                      gfstep_c

                   may be used as the step size function. This is the
                   default option. If gfstep_c is used, the step size must be
                   set by calling

                      gfsstp_c ( step );

                   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 system is in state `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 state of the system at time `t1'.

                      s2    is the state of the system at time `t2'.

                   `udrefn' may use or ignore the `s1' and `s2' values.

                   The output is:

                      t     is next time to check for a state transition.
                            `t' has value 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. This is the
                   default option.

       gquant      is a string containing the name of a geometric quantity.
                   The times when this quantity satisfies a condition
                   specified by the arguments `op' and `adjust' (described
                   below) are to be found.

                   Each quantity is specified by the quantity name given in
                   argument `gquant', and by a set of parameters specified by
                   the arguments

                      qnpars
                      qpnams
                      qcpars
                      qdpars
                      qipars
                      qlpars

                   For each quantity listed here, we also show how to set up
                   these input arguments to define the quantity. See the
                   detailed discussion of these arguments below for further
                   information.

                   `gquant' may be any of the strings:

                      "ANGULAR SEPARATION"
                      "COORDINATE"
                      "DISTANCE"
                      "ILLUMINATION ANGLE"
                      "PHASE ANGLE"
                      "RANGE RATE"

                   `gquant' strings are case insensitive. Values, meanings,
                   and associated parameters are discussed below.

                   The aberration correction parameter indicates the
                   aberration corrections to be applied to the state of the
                   target body to account for one-way light time and stellar
                   aberration. If relevant, it applies to the rotation of
                   the target body as well.

                   Supported aberration correction options for observation
                   (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
                   (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
                   aberration correction parameter strings.

                   ANGULAR SEPARATION

                      is the apparent angular separation of two target
                      bodies as seen from an observing body.

                      Quantity Parameters:

                         qnpars    = 8;
                         SpiceChar qpnams[qnpars][LNSIZE] =
                                         { "TARGET1",
                                           "FRAME1",
                                           "SHAPE1",
                                           "TARGET2",
                                           "FRAME2",
                                           "SHAPE2",
                                           "OBSERVER",
                                           "ABCORR"    };

                         SpiceChar qcpars[qnpars][LNSIZE] =
                                         { <name of first target>,
                                           <name of body-fixed frame
                                                     of first target>,
                                           <shape of first target>,
                                           <name of second target>,
                                           <name of body-fixed frame
                                                    of second target>,
                                           <shape of second target>,
                                           <name of observer>,
                                           <aberration correction>     };

                      The target shape model specifiers may be set to either
                      of the values

                         "POINT"
                         "SPHERE"

                      The shape models for the two bodies need not match.

                      Spherical models have radii equal to the longest
                      equatorial radius of the PCK-based tri-axial
                      ellipsoids used to model the respective bodies. When
                      both target bodies are modeled as spheres, the angular
                      separation between the bodies is the angle between the
                      closest points on the limbs of the spheres, as viewed
                      from the vantage point of the observer. If the limbs
                      overlap, the angular separation is negative.

                      (In this case, the angular separation is the angle
                      between the centers of the spheres minus the sum of
                      the apparent angular radii of the spheres.)

                   COORDINATE

                      is a coordinate of a specified vector in a specified
                      reference frame and coordinate system. For example, a
                      coordinate can be the Z component of the earth-sun
                      vector in the J2000 reference frame, or the latitude
                      of the nearest point on Mars to an orbiting
                      spacecraft, expressed relative to the IAU_MARS
                      reference frame.

                      The method by which the vector is defined is indicated
                      by the

                         "VECTOR DEFINITION"

                      parameter. Allowed values and meanings of this
                      parameter are:

                         "POSITION"

                            The vector is defined by the position of a
                            target relative to an observer.

                         "SUB-OBSERVER POINT"

                            The vector is the sub-observer point on a
                            specified target body.

                         "SURFACE INTERCEPT POINT"

                            The vector is defined as the intercept point of
                            a vector from the observer to the target body.

                      Some vector definitions, such as the sub-observer
                      point, may be specified by a variety of methods, so a
                      parameter is provided to select the computation
                      method. The computation method parameter name is

                         "METHOD"

                      If the vector definition is

                         "POSITION"

                      the "METHOD" parameter must be set to blank:

                         " "

                      If the vector definition is

                         "SUB-OBSERVER POINT"

                      the "METHOD" parameter must be set to either:

                         "Near point: ellipsoid"
                         "Intercept: ellipsoid"

                      If the vector definition is

                         "SURFACE INTERCEPT POINT"

                      the "METHOD" parameter must be set to:

                         "Ellipsoid"

                            The intercept computation uses a triaxial
                            ellipsoid to model the surface of the target
                            body. The ellipsoid's radii must be available in
                            the kernel pool.

                      The supported coordinate systems and coordinate names:

                         Coordinate System  Coordinates        Range
                         -----------------  -----------------  ------------

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

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

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

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

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

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

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

                      When geodetic coordinates are selected, the radii used
                      are those of the central body associated with the
                      reference frame. For example, if IAU_MARS is the
                      reference frame, then geodetic coordinates are
                      calculated using the radii of Mars taken from a SPICE
                      planetary constants kernel. One cannot ask for
                      geodetic coordinates for a frame which doesn't have an
                      extended body as its center.

                      Reference frame names must be recognized by the SPICE
                      frame subsystem.

                      Quantity Parameters:

                         qnpars = 10;
                         SpiceChar qpnams[qnpars][LNSIZE] =
                                         { "TARGET",
                                           "OBSERVER",
                                           "ABCORR",
                                           "COORDINATE SYSTEM",
                                           "COORDINATE",
                                           "REFERENCE FRAME",
                                           "VECTOR DEFINITION",
                                           "METHOD",
                                           "DREF",
                                           "DVEC"               };

                      Only "SURFACE INTERCEPT POINT" searches make use of
                      the "DREF" and "DVEC" parameters.

                         SpiceChar qcpars[qnpars][LNSIZE] =
                                         { <name of first target>,
                                           <name of observer>,
                                           <aberration correction>,
                                           <coordinate system name>,
                                           <coordinate name>,
                                           <target reference frame name>,
                                           <vector definition>,
                                           <computation method>,
                                           <reference frame of DVEC pointing
                                                   vector, defined in qdpars> };

                         qdpars[0] = <DVEC pointing vector x component
                                                           from observer>
                         qdpars[1] = <DVEC pointing vector y component
                                                           from observer>
                         qdpars[2] = <DVEC pointing vector z component
                                                           from observer>

                   DISTANCE

                      is the apparent distance between a target body and an
                      observing body. Distances are always measured between
                      centers of mass.

                      Quantity Parameters:

                         qnpars    = 3;
                         SpiceChar qpnams[qnpars][LNSIZE] =
                                         { "TARGET",
                                           "OBSERVER",
                                           "ABCORR"    };

                         SpiceChar qcpars[qnpars][LNSIZE] =
                                         { <name of target>,
                                           <name of observer>,
                                           <aberration correction> };

                   ILLUMINATION ANGLE

                      is any of the illumination angles

                         emission
                         phase
                         solar incidence

                      defined at a surface point on a target body. These
                      angles are defined as in the CSPICE routine ilumin_c.

                      Quantity Parameters:

                         qnpars    = 8;
                         SpiceChar qpnams[qnpars][LNSIZE] =
                                         { "TARGET",
                                           "ILLUM",
                                           "OBSERVER",
                                           "ABCORR",
                                           "FRAME",
                                           "ANGTYP",
                                           "METHOD",
                                           "SPOINT"    };

                         SpiceChar qcpars[qnpars][LNSIZE] =
                                         { <name of target>,
                                           <name of illumination source>,
                                           <name of observer>,
                                           <aberration correction>,
                                           <target body-fixed frame>,
                                           <type of illumination angle>,
                                           <computation method>          };

                      The surface point is specified using rectangular
                      coordinates in the specified body-fixed frame.

                         qdpars[0] =  <X coordinate of surface point>
                         qdpars[1] =  <Y coordinate of surface point>
                         qdpars[2] =  <Z coordinate of surface point>

                   PHASE ANGLE

                      is the apparent phase angle between a target body
                      center and an illuminating body center as seen from an
                      observer.

                      Quantity Parameters:

                         qnpars    = 4;
                         SpiceChar qpnams[qnpars][LNSIZE] =
                                         { "TARGET",
                                           "OBSERVER",
                                           "ABCORR",
                                           "ILLUM"    };

                         SpiceChar qcpars[qnpars][LNSIZE] =
                                         { <name of target>,
                                           <name of observer>,
                                           <aberration correction>,
                                           <name of illuminating body> };

                   RANGE RATE

                      is the apparent range rate between a target body and
                      an observing body.

                      Quantity Parameters:

                         qnpars    = 3;
                         SpiceChar qpnams[qnpars][LNSIZE] =
                                         { "TARGET",
                                           "OBSERVER",
                                           "ABCORR"   };

                         SpiceChar qcpars[qnpars][LNSIZE] =
                                         { <name of target>,
                                           <name of observer>,
                                           <aberration correction> };

       qnpars      is the count of quantity parameter definition parameters.
                   These parameters supply the quantity-specific information
                   needed to fully define the quantity used in the search
                   performed by this routine.

       lenvals     is the length of the string in arrays `qpnames' and `qcpars',
                   including the null terminators.

       qpnams      is an array of names of quantity definition parameters.
                   The names occupy elements 0:qnpars-1 of this array. The
                   value associated with the ith element of `qpnams' is
                   located in element `i' of the parameter value argument
                   having data type appropriate for the parameter:

                      Data Type                      Argument
                      ---------                      --------
                      Character strings              qcpars
                      Double precision numbers       qdpars
                      Integers                       qipars
                      Logicals                       qlpars

                   The order in which the parameter names are listed is
                   unimportant, as long as the corresponding parameter
                   values are listed in the same order.

                   The names in `qpnams' are case-insensitive.

                   See the description of the input argument `gquant' for a
                   discussion of the parameter names and values associated
                   with a given quantity.

       qcpars,
       qdpars,
       qipars,
       qlpars      are, respectively, parameter value arrays of types

                       const void         * qcpars;
                       ConstSpiceDouble   * qdpars;
                       ConstSpiceInt      * qipars;
                       ConstSpiceBoolean  * qlpars;

                   The value associated with the ith name in the array
                   `qpnams' resides in the ith element of whichever of these
                   arrays has the appropriate data type.

                   All of these arrays should be declared with dimension at
                   least `qnpars'. `qcpars' should have the same dimension
                   and shape as `qpnams'.

                   The names in the array `qcpars' are case-insensitive.

                   Note that there is no required order for qpnams/q*pars
                   pairs.

                   See the description of the input argument `gquant' for a
                   discussion of the parameter names and values associated
                   with a given quantity.

       op          is a scalar string comparison operator indicating the
                   numeric constraint of interest. Values are:

                      ">"        value of geometric quantity greater than
                                 some reference (refval).

                      "="        value of geometric quantity equal to some
                                 reference (refval).

                      "<"        value of geometric quantity less than some
                                 reference (refval).

                      "ABSMAX"   The geometric quantity is at an absolute
                                 maximum.

                      "ABSMIN"   The geometric quantity is at an absolute
                                 minimum.

                      "LOCMAX"   The geometric quantity is at a local
                                 maximum.

                      "LOCMIN"   The geometric quantity is at a local
                                 minimum.

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

                   Local extrema are considered to exist only in the
                   interiors of the intervals comprising the confinement
                   window: a local extremum cannot exist at a boundary point
                   of the confinement window.

                   Case is not significant in the string `op'.

       refval      is the reference value used to define an equality or
                   inequality to be satisfied by the geometric quantity. The
                   units of `refval' are radians, radians/sec, km, or km/sec
                   as appropriate.

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

       adjust      is the amount by which the quantity is allowed to vary
                   from an absolute extremum.

                   If the search is for an absolute minimum is performed,
                   the resulting window contains time intervals when the
                   geometric quantity `gquant' has values between `absmin' and
                   absmin + adjust.

                   If the search is for an absolute maximum, the
                   corresponding range is  between absmax - adjust and
                   `absmax'.

                   `adjust' is not used for searches for local extrema,
                   equality or inequality conditions and must have value
                   zero for such searches. `adjust' must not be negative.

       rpt         is a logical variable which controls whether the progress
                   reporter 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 the name of the user specified 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 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
                   if `srcpre' and `srcsuf' are, respectively,

                      "Occultation/transit search"
                      "done."

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

                      Occultation/transit search 100.00% done.

                   If the user doesn't wish to provide a custom set of
                   progress reporting functions, the CSPICE routine

                      gfrepi_c

                   may be used.

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

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

                   where `et' is an epoch belonging to the confinement window,
                   `ivbeg' and `ivend' are the start and stop times,
                   respectively of the current confinement window interval.
                   The ratio of the measure of the portion of `cnfine' that
                   precedes `et' to the measure of `cnfine' would be a logical
                   candidate for the searches completion percentage; however
                   the method of measurement is up to the user.

                   If the user doesn't wish to provide a custom set of
                   progress reporting functions, the CSPICE routine

                      gfrepu_c

                   may be used.

       udrepf      is the name of the user specified routine that finalizes
                   a progress report. `udrepf' has no arguments.

                   If the user doesn't wish to provide a custom set of
                   progress reporting functions, the CSPICE routine

                      gfrepf_c

                   may be used.

       nintvls     is an integer parameter specifying the number of intervals
                   that can be accommodated by each of the dynamically allocated
                   workspace windows used internally by this routine.

                   In many cases, it's not necessary to compute an accurate
                   estimate of how many intervals are needed; rather, the user
                   can pick a size considerably larger than what's really
                   required.

                   However, since excessively large arrays can prevent
                   applications from compiling, linking, or running properly,
                   sometimes `nintvls' must be set according to the actual
                   workspace requirement. A rule of thumb for the number of
                   intervals needed is

                      nintvls  =  2*n  +  ( m / step )

                   where

                      n      is the number of intervals in the confinement
                             window.

                      m      is the measure of the confinement window, in units
                             of seconds.

                      step   is the search step size in seconds.

       bail        is a logical flag indicating whether or not interrupt
                   signaling 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 the user specified routine 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.

                   gfevnt_c uses `udbail' only when `bail' (see above) is set
                   to SPICETRUE, indicating that interrupt handling is
                   enabled. When interrupt handling is enabled, gfevnt_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' 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 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.

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

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

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

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

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

                      SPICEDOUBLE_CELL        ( cnfine, CNFINESZ );

                   where CNFINESZ is the maximum capacity of `cnfine'.

#### Detailed_Output

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

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

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

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

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

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

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

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

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

                      SPICEDOUBLE_CELL        ( result, RESULTSZ );

                   where RESULTSZ is the maximum capacity of `result'.

#### Parameters

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

       SPICE_GFEVNT_MAXPAR

                   is the maximum number of parameters required to define
                   any quantity. SPICE_GFEVNT_MAXPAR may grow if new quantities
                   require more parameters.

       SPICE_GF_CNVTOL

                   is the default convergence tolerance used by the high-level
                   GF search API routines. This tolerance is used to terminate
                   searches for binary state transitions: when the time at which
                   a transition occurs is bracketed by two times that differ by
                   no more than SPICE_GF_CNVTOL, the transition time is considered
                   to have been found.

#### Exceptions

       1)  There are varying requirements on how distinct the three
           objects, `qcpars', must be. If the requirements are not met, an,
           an error is signaled by a routine in the call tree of this
           routine.

           When `gquant' has value "ANGULAR SEPARATION" then all three
           must be distinct.

           When `gquant' has value of either

              "DISTANCE"
              "COORDINATE"
              "RANGE RATE"

           the qcpars[0] and qcpars[1] objects must be distinct.

       2)  If any of the bodies involved do not have NAIF ID codes, an
           error is signaled by a routine in the call tree of this
           routine.

       3)  If the value of `gquant' is not recognized as a valid value, the
           error SPICE(NOTRECOGNIZED) is signaled by a routine in the
           call tree of this routine.

       4)  If the number of quantity definition parameters, `qnpars' is
           greater than the maximum allowed value, SPICE_GFEVNT_MAXPAR,
           the error SPICE(INVALIDCOUNT) is signaled.

       5)  If the proper required parameters are not supplied in `qnpars',
           the error SPICE(MISSINGVALUE) is signaled by a routine in the
           call tree of this routine.

       6)  If the comparison operator, `op', is not recognized, the error
           SPICE(NOTRECOGNIZED) is signaled by a routine in the call tree
           of this routine.

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

       8)  If `tol' is not greater than zero, an error is signaled by a
           routine in the call tree of this routine.

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

       10) If `adjust' has a non-zero value when `op' has any value other
           than "ABSMIN" or "ABSMAX", an error is signaled by a routine
           in the call tree of this routine.

       11) The user must take care when searching for an extremum
           ("ABSMAX", "ABSMIN", "LOCMAX", "LOCMIN") of an angular
           quantity. Problems are most common when using the "COORDINATE"
           value of `gquant' with "LONGITUDE" or "RIGHT ASCENSION" values
           for the coordinate name. Since these quantities are cyclical,
           rather than monotonically increasing or decreasing, an
           extremum may be hard to interpret. In particular, if an
           extremum is found near the cycle boundary (-Pi for
           "LONGITUDE", 2*Pi for "RIGHT ASCENSION") it may not be
           numerically reasonable. For example, the search for times when
           a longitude coordinate is at its absolute maximum may result
           in a time when the longitude value is -Pi, due to roundoff
           error.

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

       13) If any of the `qpnams', `qcpars', `gquant' or `op' input
           string pointers is null, the error SPICE(NULLPOINTER) is
           signaled.

       14) If any of the `qpnams', `qcpars', `gquant' or `op' input
           strings has zero length, the error SPICE(EMPTYSTRING) is
           signaled.

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

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

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

#### Files

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

       The following data are required:

       -  SPK data: ephemeris data for target, source and observer that
          describes the ephemeris of these objects for the period
          defined by the confinement window, `udbail' 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.

       -  PCK data: bodies are assumed to be spherical and must have a
          radius loaded from the kernel pool. Typically this is done by
          loading a text PCK file via furnsh_c. If the bodies are
          triaxial, the largest radius is chosen as that of the
          equivalent spherical body.

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

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

#### Particulars

       This routine provides the SPICE GF subsystem's general interface
       to determines time intervals when the value of some
       geometric quantity related to one or more objects and an observer
       satisfies a user specified constraint. It puts these times in a
       result window called `cnfine'. It does this by first finding
       windows when the quantity of interest is either monotonically
       increasing or decreasing. These windows are then manipulated to
       give the final result.

       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 a high level geometry quantity routine rather than
       this routine.

       The Search Process
       ==================

       Regardless of the type of constraint selected by the caller, this
       routine starts the search for solutions by determining the time
       periods, within the confinement window, over which the specified
       geometric quantity function is monotone increasing and monotone
       decreasing. Each of these time periods is represented by a SPICE
       window. Having found these windows, all of the quantity
       function's local extrema within the confinement window are known.
       Absolute extrema then can be found very easily.

       Within any interval of these "monotone" windows, there will be at
       most one solution of any equality constraint. Since the boundary
       of the solution set for any inequality constraint is contained in
       the union of

       -  the set of points where an equality constraint is met

       -  the boundary points of the confinement window

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

       Step Size
       =========

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

       Note that the optimal choice of step size depends on the lengths
       of the intervals over which the quantity function is monotone:
       the step size should be shorter than the shortest of these
       intervals (within the confinement window).

       The optimal step size is *not* necessarily related to the lengths
       of the intervals comprising the result window. For example, if
       the shortest monotone interval has length 10 days, and if the
       shortest result window interval has length 5 minutes, a step size
       of 9.9 days is still adequate to find all of the intervals in the
       result window. In situations like this, the technique of using
       monotone windows yields a dramatic efficiency improvement over a
       state-based search that simply tests at each step whether the
       specified constraint is satisfied. The latter type of search can
       miss solution intervals if the step size is longer than the
       shortest solution interval.

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

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

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

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

       The GF subsystem defines a parameter, SPICE_GF_CNVTOL (from SpiceGF.h),
       as a default tolerance. This represents a "tight" tolerance value
       so that the tolerance doesn't become the limiting factor in the
       accuracy of solutions found by this routine. In general the
       accuracy of input data will be the limiting factor.

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

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

       The simplest use of the confinement window is to specify a time
       interval within which a solution is sought. However, the
       confinement window can, in some cases, be used to make searches
       more efficient. Sometimes it's possible to do an efficient search
       to reduce the size of the time period over which a relatively
       slow search of interest must be performed.

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

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

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

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

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

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

#### Examples

       The numerical results shown for this example may differ across
       platforms. The results depend on the SPICE kernels used as
       input, the compiler and supporting libraries, and the machine
       specific arithmetic implementation.

       1) Conduct a DISTANCE search using the default GF progress reporting
          capability.

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

          The program will find local maximums of the distance from earth to
          Moon with light time and stellar aberration corrections to model
          the apparent positions of the Moon.

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

             KPL/MK

             File name: gfevnt_ex1.tm

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

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

             The names and contents of the kernels referenced
             by this meta-kernel are as follows:

                File name                     Contents
                ---------                     --------
                de414.bsp                     Planetary ephemeris
                pck00008.tpc                  Planet orientation and
                                              radii
                naif0009.tls                  Leapseconds

             \begindata

                KERNELS_TO_LOAD = ( 'de414.bsp',
                                    'pck00008.tpc',
                                    'naif0009.tls'  )

             \begintext

             End of meta-kernel

          Example code begins here.

          /.
             Program gfevnt_ex1
          ./
          #include "SpiceUsr.h"
          #include <stdio.h>
          #include <signal.h>

          int main()
             {

             /.
             Constants
             ./
             #define  TIMFMT  "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND"
             #define  MAXVAL  10000
             #define  STRSIZ  41
             #define  LNSIZE  81

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

             /.
             Confining window beginning and ending time strings.
             ./
             SpiceChar               begstr [LNSIZE] = "2001 jan 01 00:00:00.000";
             SpiceChar               endstr [LNSIZE] = "2001 dec 31 00:00:00.000";
             SpiceChar               event  []       = "DISTANCE";
             SpiceChar               relate []       = "LOCMAX";

             /.
             Declare qpnams and qcpars with the same dimensions.
             SPICE_GFEVNT_MAXPAR is defined in SpiceGF.h.
             ./
             SpiceChar  qpnams[SPICE_GFEVNT_MAXPAR][LNSIZE] = { "TARGET",
                                                                "OBSERVER",
                                                                "ABCORR" };

             SpiceChar  qcpars[SPICE_GFEVNT_MAXPAR][LNSIZE] = { "MOON",
                                                                "EARTH",
                                                                "LT+S"  };

             SpiceDouble             qdpars[SPICE_GFEVNT_MAXPAR];
             SpiceInt                qipars[SPICE_GFEVNT_MAXPAR];
             SpiceBoolean            qlpars[SPICE_GFEVNT_MAXPAR];

             SPICEDOUBLE_CELL      ( cnfine, MAXVAL );
             SPICEDOUBLE_CELL      ( result, MAXVAL );

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

             SpiceInt                lenvals;
             SpiceInt                nintvls;
             SpiceInt                count;
             SpiceInt                qnpars;
             SpiceInt                i;

             /.
             Load leapsecond and spk kernels. The name of the
             meta kernel file shown here is fictitious; you
             must supply the name of a file available
             on your own computer system.
             ./
             furnsh_c ( "gfevnt_ex1.tm" );

             /.
             Set a beginning and end time for confining window.
             ./

             str2et_c ( begstr, &begtim );
             str2et_c ( endstr, &endtim );

             /.
             Add 2 points to the confinement interval window.
             ./
             wninsd_c ( begtim, endtim, &cnfine );

             /.
             Check the number of intervals in confining window.
             ./
             count = wncard_c( &cnfine );
             printf( "Found %d intervals in cnfine\n", (int)count );

             /.
             Set the step size to 1/1000 day and convert to seconds.
             One day would be a reasonable step size for this
             search, but the run would not last long enough to issue
             an interrupt.
             ./
             step = 0.001 * spd_c();
             gfsstp_c ( step );

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

             lenvals= LNSIZE;
             qnpars = 3;
             tol    = SPICE_GF_CNVTOL;
             refval = 0.;
             adjust = 0.;
             nintvls= MAXVAL;

             /.
             Perform the search.
             ./
             gfevnt_c ( gfstep_c,
                        gfrefn_c,
                        event,
                        qnpars,
                        lenvals,
                        qpnams,
                        qcpars,
                        qdpars,
                        qipars,
                        qlpars,
                        relate,
                        refval,
                        tol,
                        adjust,
                        rpt,
                        &gfrepi_c,
                        gfrepu_c,
                        gfrepf_c,
                        nintvls,
                        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
                {
                count = wncard_c( &result);
                printf( "Found %d intervals in result\n", (int)count );

                /.
                List the beginning and ending points in each interval.
                ./
                for( i=0; i<count; i++ )
                   {
                   wnfetd_c( &result, i, &beg, &end );

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

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

                }

             return ( 0 );
             }

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

          Found 1 intervals in cnfine

          Distance pass 1 of 1 100.00% done.

          Found 13 intervals in result
          Interval 0
          Beginning TDB 2001-JAN-24 19:22:01.436672 (TDB)
          Ending TDB    2001-JAN-24 19:22:01.436672 (TDB)
          Interval 1
          Beginning TDB 2001-FEB-20 21:52:07.914964 (TDB)
          Ending TDB    2001-FEB-20 21:52:07.914964 (TDB)
          Interval 2
          Beginning TDB 2001-MAR-20 11:32:03.182345 (TDB)
          Ending TDB    2001-MAR-20 11:32:03.182345 (TDB)
          Interval 3
          Beginning TDB 2001-APR-17 06:09:00.877038 (TDB)
          Ending TDB    2001-APR-17 06:09:00.877038 (TDB)
          Interval 4
          Beginning TDB 2001-MAY-15 01:29:28.532819 (TDB)
          Ending TDB    2001-MAY-15 01:29:28.532819 (TDB)
          Interval 5
          Beginning TDB 2001-JUN-11 19:44:10.855458 (TDB)
          Ending TDB    2001-JUN-11 19:44:10.855458 (TDB)
          Interval 6
          Beginning TDB 2001-JUL-09 11:14:45.082568 (TDB)
          Ending TDB    2001-JUL-09 11:14:45.082568 (TDB)
          Interval 7
          Beginning TDB 2001-AUG-05 20:51:11.781173 (TDB)
          Ending TDB    2001-AUG-05 20:51:11.781173 (TDB)
          Interval 8
          Beginning TDB 2001-SEP-01 23:10:57.488901 (TDB)
          Ending TDB    2001-SEP-01 23:10:57.488901 (TDB)
          Interval 9
          Beginning TDB 2001-SEP-29 05:23:55.442354 (TDB)
          Ending TDB    2001-SEP-29 05:23:55.442354 (TDB)
          Interval 10
          Beginning TDB 2001-OCT-26 20:08:15.029871 (TDB)
          Ending TDB    2001-OCT-26 20:08:15.029871 (TDB)
          Interval 11
          Beginning TDB 2001-NOV-23 15:45:23.027511 (TDB)
          Ending TDB    2001-NOV-23 15:45:23.027511 (TDB)
          Interval 12
          Beginning TDB 2001-DEC-21 13:04:47.124241 (TDB)
          Ending TDB    2001-DEC-21 13:04:47.124241 (TDB)

          Note that the progress report has the format shown below:

             Distance pass 1 of 1   6.02% done.

          The completion percentage was updated approximately once per
          second.

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

             Distance pass 1 of 1  26.74% done.
             Search was interrupted.

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

#### Restrictions

       1)  The kernel files to be used by gfevnt_c must be loaded (normally
           via the CSPICE routine furnsh_c) before calling gfevnt_c.

       2)  If using the default, constant step size routine, gfstep_c, the
           the caller must set the step size by calling the entry point
           gfsstp_c before calling gfevnt_c. The call syntax for gfsstp_c:

              gfsstp_c ( step );

#### Literature_References

       None.

#### Author_and_Institution

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

#### Version

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

           Added check for error condition "nintvls" less than one.

           Added check for `qnpars' out of range.

           Bug fix: moved creation of Fortran-style arrays for parameter
           names and string parameter values to right before the call to
           gfevnt_. This prevents a memory leak that could occur due to
           the prior placement of this code before checking macros that
           can execute return statements.

           Bug fix: changed input void array checks from using CHKFSTR to
           CHKOSTR. The previous checks did not inspect the input `lenvals'.

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

           Updated header to describe use of expanded confinement window.

           Edited the header to comply with NAIF standard.

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

           Updates in -Exceptions section: Added entries #13 to #15, fixed
           short error message in entries #16 and #17, replaced entry #9 by new
           entries #9 and #10.

           Parameter MAXPAR has been replaced with parameter SPICE_GFEVNT_MAXPAR.

           Added descriptions of SPICE_GFEVNT_MAXPAR and SPICE_GF_CNVTOL
           to the -Brief_I/O and -Parameters sections.

       -CSPICE Version 1.0.2, 12-JUL-2016 (EDW)

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

       -CSPICE Version 1.0.1, 24-APR-2010 (EDW)

           Minor edit to code comments eliminating typo.

       -CSPICE Version 1.0.0, 11-MAR-2009 (EDW) (NJB)

#### Index_Entries

       determine when a geometric quantity satisfies a condition

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

    Fri Dec 31 18:41:07 2021