Index Page gfevnt_c

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

#### Procedure

       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
       --------  ---  --------------------------------------------------
       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 (.TRUE.) or off (.FALSE.)
       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. 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 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.

       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:

                     "COORDINATE"
                     "DISTANCE"
                     "ANGULAR SEPARATION"

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

                  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 should be set to
                              blank:

                                 " "

                              If the vector definition is

                                 "SUB-OBSERVER POINT"

                              the METHOD parameter should be set to
                              either:

                                 "Near point: ellipsoid"
                                 "Intercept: ellipsoid"

                              If the vector definition is

                                 "SURFACE INTERCEPT POINT"

                              the METHOD parameter should 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 defined
                              using the radii of Mars.  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[SPICE_GFEVNT_MAXPAR][LNSIZE] =
                                                        { "TARGET",
                                                          "OBSERVER",
                                                          "ABCORR",
                                                          "COORDINATE SYSTEM",
                                                          "COORDINATE",
                                                          "REFERENCE FRAME",
                                                          "VECTOR DEFINITION",
                                                          "METHOD",
                                                          "DREF",
                                                          "DVEC" };

                              Only "SUB-OBSERVER POINT" searches make
                              use of the "DREF" and "DVEC" parameters.

                              Only "SUB-OBSERVER POINT" searches make
                              use of the "DREF" and "DVEC" parameters.

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

                                 qdpars[0] = <pointing vector x from observer>
                                 qdpars[1] = <pointing vector y from observer>
                                 qdpars[2] = <pointing vector x 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[SPICE_GFEVNT_MAXPAR][LNSIZE] =
                                                         { "TARGET",
                                                           "OBSERVER",
                                                           "ABCORR" };

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

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

                              Quantity Parameters:

                                 qnpars    = 8

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

                                 SpiceChar qcpars[SPICE_GFEVNT_MAXPAR][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.)

                  A note on aberration correction parameters:  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.

       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    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 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     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
                  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 `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 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 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      )

                  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 a user-defined subroutine 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    an integer value specifying the number of intervals in the
                  the internal workspace array used by this routine. 'nintvls'
                  should be at least as large as the number of intervals
                  within the search region on which the specified geometric
                  event function is monotone increasing or decreasing.
                  It does no harm to pick a value of 'nintvls' larger than the
                  minimum required to execute the specified search, but if chosen
                  too small, the search will fail.

       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.

                  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.

#### 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.

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

#### Parameters

       None.

#### Exceptions

       1)  There are varying requirements on how distinct the three
           objects, QCPARS, must be. If the requirements are not met,
           the error, SPICE(BODIESNOTDISTINCT) will signal from
           this routine.

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

           When 'gquant' has value "DISTANCE" or "COORDINATE" then

              The 'qcpas[0]' and 'qcpas[1]' objects must be distinct.

       2)  If any of the bodies involved do not have NAIF ID codes, the
           error SPICE(IDCODENOTFOUND) will signal from this routine.

       3)  If the value of 'gquant' is not recognized as a valid value,
           the error SPICE(NOTRECOGNIZED) will signal from this routine.

       4)  If the number of quantity definition parameters, QNPARS is
           greater than the maximum allowed value, MAXPAR, the error
           SPICE(INVALIDCOUNT) will signal from this routine.

       5)  If the proper required parameters, 'qpars', are not supplied,
           the error SPICE(MISSINGVALUE) will signal from this routine.

       6)  If the comparison operator, 'op', is not recognized, the error
           SPICE(NOTRECOGNIZED) will signal from this routine.

       7)  If the sizes of the workspace windows are too small,
           the error SPICE(ARRAYTOOSMALL) will signal from routines
           called by this routine.

       8)  If 'tol' is not greater than zero, the error
           SPICE(VALUEOUTOFRANGE) will signal from routines called by
           this routine.

       9)  If 'adjust' is negative, the error SPICE(VALUEOUTOFRANGE) will
           signal from routines called by this routine. A non-zero
           value for 'adjust' when 'op' has any value other than
           "ABSMIN" or "ABSMAX" causes routines called by this
           routine to signal the error SPICE(INVALIDVALUE).

      10)  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.

       11) If the required amount of workspace memory cannot be
           allocated, the error SPICE(MALLOCFAILURE) will signal
           from this routine.

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

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

#### Files

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

       The following data are required:

          - SPK data: the calling application must load ephemeris data
            for the target, source and observer that cover the time
            period specified by the window `cnfine'. 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 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.

       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 determine 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 'result'. 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 gfsep_c, gfdist_c, gfposc_c, gfsubc_c, or gfsntc_c 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 the set
       of points where an equality constraint is met, 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 shorter 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.

#### 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.

       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: standard.tm

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

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

          \begindata

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

          \begintext

       Code:

          #include "SpiceUsr.h"
          #include "SpiceGF.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
             #define  MAXPAR  10

             /.
             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 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 ( "standard.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 stepsize 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 );
             }

       The program compiled on OS X with gcc:

          The run output; the progress report had the format shown below:

             Distance pass 1 of 3  49.60% done.

             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 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)

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

             Distance pass 1 of 3  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)
       L.S. Elson     (JPL)
       W.L. Taber     (JPL)
       I.M. Underwood (JPL)
       E.D. Wright    (JPL)

#### Version

       -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)

#### Index_Entries

       determine when a geometric quantity satisfies a condition

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

    Wed Apr  5 17:54:35 2017