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Index Page gfilum_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

       gfilum_c ( GF, illumination angle search )

       void gfilum_c ( ConstSpiceChar     * method,
                       ConstSpiceChar     * angtyp,
                       ConstSpiceChar     * target,
                       ConstSpiceChar     * illmn,
                       ConstSpiceChar     * fixref,
                       ConstSpiceChar     * abcorr,
                       ConstSpiceChar     * obsrvr,
                       ConstSpiceDouble     spoint [3],
                       ConstSpiceChar     * relate,
                       SpiceDouble          refval,
                       SpiceDouble          adjust,
                       SpiceDouble          step,
                       SpiceInt             nintvls,
                       SpiceCell          * cnfine,
                       SpiceCell          * result     )

#### Abstract

       Return the time window over which a specified constraint on
       the observed phase, solar incidence, or emission angle at
       a specified target body surface point is met.

#### Required_Reading

       GF
       FRAMES
       NAIF_IDS
       PCK
       SPK
       TIME

#### Keywords

       ANGLE
       EPHEMERIS
       ILLUMINATION
       LIGHTING
       SEARCH

#### Brief_I/O

       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       SPICE_GF_CNVTOL
                  P   Convergence tolerance.
       SPICE_GF_NWILUM
                  P   Number of workspace windows for angle search.
       method     I   Computation method.
       angtyp     I   Type of illumination angle.
       target     I   Name of the target body.
       illmn      I   Name of the illumination source.
       fixref     I   Body-fixed, body-centered target body frame.
       abcorr     I   Aberration correction flag.
       obsrvr     I   Name of the observing body.
       spoint     I   Body-fixed coordinates of a target surface point.
       relate     I   Relational operator.
       refval     I   Reference value.
       adjust     I   Adjustment value for absolute extrema searches.
       step       I   Step size used for locating extrema and roots.
       nintvls    I   Workspace window interval count.
       cnfine    I-O  SPICE window to which the search is confined.
       result     O   SPICE window containing results.

#### Detailed_Input

       method      is a short string providing parameters defining the
                   computation method to be used. Parameters include, but
                   are not limited to, the shape model used to represent the
                   surface of the target body.

                   The only choice currently supported is

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

                   Neither case nor whitespaces are significant in `method'.
                   For example, the string " eLLipsoid " is valid.

       angtyp      is a string specifying the type of illumination angle for
                   which a search is to be performed. The possible values of
                   `angtyp' are

                      "PHASE"
                      "INCIDENCE"
                      "EMISSION"

                   When the illumination source is the sun, the incidence
                   angle is commonly called the "solar incidence angle."

                   See the -Particulars section below for a detailed
                   description of these angles.

                   Neither case nor whitespaces are significant in `angtyp'.
                   For example, the string " Incidence " is valid.

       target      is the name of a target body. The point at which the
                   illumination angles are defined is located on the surface
                   of this body.

                   Optionally, you may supply the integer ID code for the
                   object as an integer string. For example both "MOON" and
                   "301" are legitimate strings that indicate the moon is
                   the target body.

                   Neither case nor leading and trailing blanks are
                   significant in `target'. For example, the string
                   " Incidence " is valid. Sequences of embedded blanks
                   are treated as a single blank.

       illmn       is the name of the illumination source. This source may
                   be any ephemeris object. Case, blanks, and numeric values
                   are treated in the same way as for the input `target'.

       fixref      is the name of the body-fixed, body-centered reference
                   frame associated with the target body. The input surface
                   point `spoint' is expressed relative to this reference
                   frame, and this frame is used to define the orientation
                   of the target body as a function of time.

                   The string `fixref' is case-insensitive, and leading
                   and trailing blanks in `fixref' are not significant.

       abcorr      indicates the aberration corrections to be applied to the
                   observer-surface point vector, the surface point-
                   illumination source vector, and the target body
                   orientation to account for one-way light time and stellar
                   aberration.

                   Any "reception" correction accepted by spkezr_c can be used
                   here. See the header of spkezr_c for a detailed description
                   of the aberration correction options. For convenience,
                   the options are listed below:

                      "NONE"     Apply no correction.

                      "LT"       "Reception" case: correct for
                                 one-way light time using a Newtonian
                                 formulation.

                      "LT+S"     "Reception" case: correct for
                                 one-way light time and stellar
                                 aberration using a Newtonian
                                 formulation.

                      "CN"       "Reception" case: converged
                                 Newtonian light time correction.

                      "CN+S"     "Reception" case: converged
                                 Newtonian light time and stellar
                                 aberration corrections.

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

       obsrvr      is the name of an observing body. Case, blanks, and
                   numeric values are treated in the same way as for the
                   input `target'.

       spoint      is a surface point on the target body, expressed in
                   Cartesian coordinates, relative to the body-fixed target
                   frame designated by `fixref'.

                   `spoint' need not be visible from the observer's location
                   in order for the constraint specified by `relate' and
                   `refval' (see descriptions below) to be satisfied.

                   The components of `spoint' have units of km.

       relate      is a relational operator used to define a constraint on a
                   specified illumination angle. The result window found by
                   this routine indicates the time intervals where the
                   constraint is satisfied. Supported values of `relate' and
                   corresponding meanings are shown below:

                      ">"      The angle is greater than the reference
                               value `refval'.

                      "="      The angle is equal to the reference
                               value `refval'.

                      "<"      The angle is less than the reference
                               value `refval'.

                     "ABSMAX"  The angle is at an absolute maximum.

                     "ABSMIN"  The angle is at an absolute minimum.

                     "LOCMAX"  The angle is at a local maximum.

                     "LOCMIN"  The angle is at a local minimum.

                   The caller may indicate that the window of interest is
                   the set of time intervals where the angle is within a
                   specified separation from an absolute extremum. The
                   argument `adjust' (described below) is used to specify this
                   separation.

                   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 `relate'.

       refval      is the reference value used together with the argument
                   `relate' to define an equality or inequality to be
                   satisfied by the specified illumination angle. See the
                   discussion of `relate' above for further information.

                   The units of `refval' are radians.

       adjust      is a parameter used to modify searches for absolute
                   extrema: when `relate' is set to "ABSMAX" or "ABSMIN" and
                   `adjust' is set to a positive value, gfilum_c will find times
                   when the specified illumination angle is within `adjust'
                   radians of the specified extreme value.

                   If `adjust' is non-zero and a search for an absolute
                   minimum is performed, the result window contains time
                   intervals when the specified illumination angle has
                   values between the absolute minimum `absmin' and
                   absmin + adjust radians.

                   If `adjust' is non-zero and the search is for an absolute
                   maximum, the corresponding angle is between the absolute
                   maximum `absmax' and absmax - adjust radians.

                   `adjust' is not used for searches for local extrema,
                   equality or inequality conditions.

       step        is the step size to be used in the search. `step' must be
                   short enough for a search using this step size to locate
                   the time intervals where the specified illumination angle
                   is monotone increasing or decreasing. However, `step' must
                   not be *too* short, or the search will take an
                   unreasonable amount of time.

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

                   `step' has units of seconds.

       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.

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

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

                   In some cases the confinement window can be used to
                   greatly reduce the time window 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 the SPICE window of intervals, contained within the
                   confinement window `cnfine', on which the specified
                   constraint is satisfied.

                   `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 gfilum_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

       SPICE_GF_CNVTOL   is the default convergence tolerance used for finding
                         endpoints of the intervals comprising the result
                         window. SPICE_GF_CNVTOL is also used for finding
                         intermediate results; in particular, SPICE_GF_CNVTOL is
                         used for finding the windows on which the specified
                         illumination angle is increasing or decreasing.
                         SPICE_GF_CNVTOL is used to determine when binary
                         searches for roots should terminate: when a root is
                         bracketed within an interval of length SPICE_GF_CNVTOL,
                         the root is considered to have been found.

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

                         The calling program can reset the convergence
                         tolerance; see the -Particulars section below for
                         further information.

       SPICE_GF_NWILUM   is the number of workspace windows required by
                         this routine.

       See header file SpiceGF.h for declarations and descriptions of
       parameters used throughout the GF subsystem.

#### Exceptions

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

           This routine does not diagnose invalid step sizes, except that
           if the step size is non-positive, the error SPICE(INVALIDSTEP)
           is signaled by a routine in the call tree of this routine.

       2)  Due to numerical errors, in particular,

              - Truncation error in time values
              - Finite tolerance value
              - Errors in computed geometric quantities

           it is *normal* for the condition of interest to not always be
           satisfied near the endpoints of the intervals comprising the
           result window.

           The result window may need to be contracted slightly by the
           caller to achieve desired results. The SPICE window routine
           wncond_c can be used to contract the result window.

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

       4)  If an error (typically cell overflow) occurs while performing
           window arithmetic, the error is signaled by a routine
           in the call tree of this routine.

       5)  If the output SPICE window `result' has size less than 2, the
           error SPICE(INVALIDDIMENSION) is signaled by a routine in the
           call tree of this routine.

       6)  If the output SPICE window `result' has insufficient capacity to
           hold the set of intervals on which the specified illumination
           angle condition is met, an error is signaled by a routine in
           the call tree of this routine.

       7)  If the input target body-fixed frame `fixref' is not
           recognized, an error is signaled by a routine in the call
           tree of this routine. A frame name may fail to be recognized
           because a required frame specification kernel has not been
           loaded; another cause is a misspelling of the frame name.

       8)  If the input frame `fixref' is not centered at the target body,
           an error is signaled by a routine in the call tree of this
           routine.

       9)  If the input argument `method' is not recognized, an error is
           signaled by a routine in the call tree of this routine.

       10) If the illumination angle type `angtyp' is not recognized,
           an error is signaled by a routine in the call tree
           of this routine.

       11) If the relational operator `relate' is not recognized, an
           error is signaled by a routine in the call tree of this
           routine.

       12) If the aberration correction specifier contains an
           unrecognized value, an error is signaled by a routine in the
           call tree of this routine.

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

       14) If any of the input body names do not map to NAIF ID
           codes, an error is signaled by a routine in the call tree of
           this routine.

       15) If the target coincides with the observer or the illumination
           source, an error is signaled by a routine in the call tree
           of this routine.

       16) If required ephemerides or other kernel data are not
           available, an error is signaled by a routine in the call tree
           of this routine.

       17) If any of the `method', `angtyp', `target', `illmn', `fixref',
           `abcorr', `obsrvr' or `relate' input string pointers is null,
           the error SPICE(NULLPOINTER) is signaled.

       18) If any of the `method', `angtyp', `target', `illmn', `fixref',
           `abcorr', `obsrvr' or `relate' input strings has zero length,
           the error SPICE(EMPTYSTRING) is signaled.

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

       20) 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.

#### Files

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

       The following data are required:

       -  SPK data: ephemeris data for target, observer, and the
          illumination source must be loaded. If aberration
          corrections are used, the states of target, observer, and
          the illumination source 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: if the target body shape is modeled as an
          ellipsoid (currently no other shapes are supported),
          triaxial radii for the target body must be loaded
          into the kernel pool. Typically this is done by loading a
          text PCK file via furnsh_c.

       -  Further PCK data: rotation data for the target body must be
          loaded. These may be provided in a text or binary PCK file.

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

       -  In some cases the observer's state may be computed at times
          outside of `cnfine' 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 determines a set of one or more time intervals
       within the confinement window when the specified illumination
       angle satisfies a caller-specified constraint. The resulting set
       of intervals is returned as a SPICE window.

       The term "illumination angles" refers to the following set of
       angles:

          phase angle              Angle between the vectors from the
                                   surface point to the observer and
                                   from the surface point to the
                                   illumination source.

          incidence angle          Angle between the surface normal at
                                   the specified surface point and the
                                   vector from the surface point to the
                                   illumination source. When the sun is
                                   the illumination source, this angle is
                                   commonly called the "solar incidence
                                   angle."

          emission angle           Angle between the surface normal at
                                   the specified surface point and the
                                   vector from the surface point to the
                                   observer.

       The diagram below illustrates the geometric relationships
       defining these angles. The labels for the incidence, emission,
       and phase angles are "inc.", "e.", and "phase".

                                                        *
                                                illumination source

                      surface normal vector
                                ._                 _.
                                |\                 /|  illumination
                                  \    phase      /    source vector
                                   \   .    .    /
                                   .            .
                                     \   ___   /
                                .     \/     \/
                                      _\ inc./
                               .    /   \   /
                               .   |  e. \ /
           *             <--------------- *  surface point on
        viewing            vector            target body
        location           to viewing
        (observer)         location

       Note that if the target-observer vector, the target normal vector
       at the surface point, and the target-illumination source vector
       are coplanar, then phase is the sum of the incidence and emission
       angles. This rarely occurs; usually

          phase angle  <  incidence angle + emission angle

       All of the above angles can be computed using light time
       corrections, light time and stellar aberration corrections, or no
       aberration corrections. In order to describe apparent geometry as
       observed by a remote sensing instrument, both light time and
       stellar aberration corrections should be used.

       The way aberration corrections are applied by this routine
       is described below.

          Light time corrections
          ======================

             Observer-target surface point vector
             ------------------------------------

             Let `et' be the epoch at which an observation or remote
             sensing measurement is made, and let et - lt (`lt' stands
             for "light time") be the epoch at which the photons
             received at `et' were emitted from the surface point `spoint'.
             Note that the light time between the surface point and
             observer will generally differ from the light time between
             the target body's center and the observer.

             Target body's orientation
             -------------------------

             Using the definitions of `et' and `lt' above, the target body's
             orientation at et - lt is used. The surface normal is
             dependent on the target body's orientation, so the body's
             orientation model must be evaluated for the correct epoch.

             Target body -- illumination source vector
             -----------------------------------------

             The surface features on the target body near `spoint' will
             appear in a measurement made at `et' as they were at et-lt.
             In particular, lighting on the target body is dependent on
             the apparent location of the illumination source as seen
             from the target body at et-lt. So, a second light time
             correction is used to compute the position of the
             illumination source relative to the surface point.

          Stellar aberration corrections
          ==============================

          Stellar aberration corrections are applied only if
          light time corrections are applied as well.

             Observer-target surface point body vector
             -----------------------------------------

             When stellar aberration correction is performed, the
             observer-to-surface point direction vector, which we'll
             call SRFVEC, is adjusted so as to point to the apparent
             position of `spoint': considering `spoint' to be an ephemeris
             object, SRFVEC points from the observer's position at `et' to
             the light time and stellar aberration
             corrected position of `spoint'.

             Target body-illumination source vector
             --------------------------------------

             The target body-illumination source vector is the apparent
             position of the illumination source, corrected for light
             time and stellar aberration, as seen from the surface point
             `spoint' at time et-lt.

       Below we discuss in greater detail aspects of this routine's
       solution process that are relevant to correct and efficient
       use of this routine in user applications.

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

       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
       illumination angle is monotone increasing and monotone decreasing.
       Each of these time periods is represented by a SPICE window.
       Having found these windows, all of the illumination angle'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 via 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 the illumination angle 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 rate of change of the selected
       illumination angle is zero can be found by a refinement process,
       for example, via binary search.

       Note that the optimal choice of step size depends on the lengths
       of the intervals over which the illumination angle 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 target,
       observer, and illumination source 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
       =====================

       As described above, the root-finding process used by this routine
       involves first bracketing roots and then using a search process
       to locate them. "Roots" are both times when local extrema are
       attained and times when the illumination angle is equal to a
       reference value. All endpoints of the intervals comprising the
       result window are either endpoints of intervals of the
       confinement window or roots.

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

       The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
       tolerance doesn't 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.

       The user may change the convergence tolerance from the default
       SPICE_GF_CNVTOL value by calling the routine gfstol_c, e.g.

          gfstol_c ( tolerance value in seconds );

       Call gfstol_c prior to calling this routine. All subsequent
       searches will use the updated tolerance value.

       Searches over time windows of long duration may require use of
       larger tolerance values than the default: the tolerance must be
       large enough so that it, when added to or subtracted from the
       confinement window's lower and upper bounds, yields distinct time
       values.

       Setting 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.

       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 `cnfine'. The time window
       that is actually used is the result of "expanding" `cnfine' by a
       specified amount "T": each time interval of `cnfine' 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 `cnfine' 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) Determine time intervals over which the MER-1 ("Opportunity")
          rover's location satisfies certain constraints on its
          illumination and visibility as seen from the Mars
          Reconnaissance Orbiter (MRO) spacecraft.

          In this case we require the emission angle to be less than
          20 degrees and the solar incidence angle to be less than
          60 degrees.

          The reader can verify that the observation start times of the
          MRO HIRISE images

             Product ID              Image start time
             ----------              ----------------
             TRA_000873_1780_RED     2006-10-03T12:44:13.425
             PSP_001414_1780_RED     2006-11-14T15:39:55.373
             PSP_001612_1780_RED     2006-11-30T01:38:34.390

          are contained within the result window found by the
          example program shown below.

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

             KPL/MK

             File: gfilum_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
                ---------                     --------
                de421.bsp                     Planetary ephemeris
                pck00010.tpc                  Planet orientation
                                              and radii
                naif0010.tls                  Leapseconds
                mer1_surf_rover_ext10_v1.bsp  MER-1 ephemeris
                mer1_surf_rover_ext11_v1.bsp  MER-1 ephemeris
                mer1_ls_040128_iau2000_v1.bsp MER-1 landing site
                                              ephemeris
                mro_psp1.bsp                  MRO ephemeris
                mer1_v10.tf                   MER-1 frame kernel

             \begindata

                KERNELS_TO_LOAD = ( 'de421.bsp',
                                    'pck00010.tpc',
                                    'naif0010.tls',
                                    'mro_psp1.bsp',
                                    'mer1_surf_rover_ext10_v1.bsp',
                                    'mer1_surf_rover_ext11_v1.bsp',
                                    'mer1_ls_040128_iau2000_v1.bsp',
                                    'mro_psp1.bsp',
                                    'mer1_v10.tf'                    )
             \begintext

          Example code begins here.

          /.
             Program gfilum_ex1
          ./

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

          int main()
          {
             /.
             Output time format:
             ./
             #define  TIMFMT  "YYYY MON DD HR:MN:SC.### UTC"

             /.
             Meta-kernel name:
             ./
             #define  META  "gfilum_ex1.tm"

             /.
             Maximum number of intervals in the windows
             used in this program:
             ./
             #define  MAXIVL  1000
             #define  MAXWIN  ( 2 * MAXIVL )

             /.
             Maximum length of time string:
             ./
             #define  TIMLEN  41

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

             SpiceChar             * abcorr;
             SpiceChar             * fixref;
             SpiceChar             * illmn;
             SpiceChar             * method;
             SpiceChar             * obsrvr;
             SpiceChar             * target;
             SpiceChar               timstr [ TIMLEN ];
             SpiceChar             * utcbeg;
             SpiceChar             * utcend;

             SpiceDouble             adjust;
             SpiceDouble             emissn;
             SpiceDouble             et0;
             SpiceDouble             et1;
             SpiceDouble             finish;
             SpiceDouble             phase;
             SpiceDouble             refval;
             SpiceDouble             rovlt;
             SpiceDouble             rovpos [ 3 ];
             SpiceDouble             solar;
             SpiceDouble             srfvec [ 3 ];
             SpiceDouble             start;
             SpiceDouble             step;
             SpiceDouble             trgepc;

             SpiceInt                i;

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

             /.
             Set the search interval:
             ./
             utcbeg = "2006 OCT 02 00:00:00 UTC";
             str2et_c ( utcbeg, &et0 );

             utcend = "2006 NOV 30 12:00:00 UTC";
             str2et_c ( utcend, &et1 );

             wninsd_c ( et0, et1, &cnfine );

             /.
             Set observer, target, aberration correction, and the
             Mars body-fixed, body-centered reference frame. The
             lighting source is the sun.

             Aberration corrections are set for remote observations.
             ./
             illmn  = "sun";
             obsrvr = "mro";
             target = "mars";
             abcorr = "cn+s";
             fixref = "iau_mars";

             /.
             Use the rover position at the start of
             the search interval as the surface point.
             ./
             spkpos_c ( "MER-1", et0,    fixref,
                        "NONE",  target, rovpos, &rovlt );

             /.
             Initialize the adjustment value for absolute
             extremum searches. We're not performing
             such searches in this example, but this input
             to GFILUM must still be set.
             ./
             adjust = 0.0;

             /.
             The computation uses an ellipsoidal model for the
             target body shape.
             ./
             method = "Ellipsoid";

             /.
             Set the reference value to use for the solar
             incidence angle search.
             ./
             refval = 60.0 * rpd_c();

             /.
             Since the period of the solar incidence angle
             is about one Martian day, we can safely use 6 hours
             as the search step.
             ./
             step   = 21600.0;

             /.
             Search over the confinement window for times
             when the solar incidence angle is less than
             the reference value.
             ./
             gfilum_c ( method,  "INCIDENCE", target, illmn,
                        fixref,  abcorr,      obsrvr, rovpos,
                        "<",     refval,      adjust, step,
                         MAXIVL, &cnfine,     &wnsolr       );
             /.
             Set the reference value for the emission angle search.
             ./
             refval = 20.0 * rpd_c();

             /.
             We'll use 15 minutes as the search step. This step
             is small enough to be suitable for Mars orbiters.
             Units are seconds.
             ./
             step   = 900.0;

             /.
             Search over the previous result window for times when the
             emission angle is less than the reference value.
             ./
             gfilum_c ( method,  "EMISSION", target, illmn,
                        fixref,  abcorr,     obsrvr, rovpos,
                        "<",     refval,     adjust, step,
                        MAXIVL,  &wnsolr,    &result        );

             /.
             Display the result window. Show the solar incidence
             and emission angles at the window's interval
             boundaries.
             ./
             printf( "\n" );

             if ( wncard_c( &result ) == 0 )
             {
                printf( "     Window is empty: condition "
                        "is not met.\n"                    );
             }
             else
             {
                printf (   "                                 "
                           "   Solar Incidence   Emission\n"
                           "                                 "
                           "         (deg)         (deg)\n"
                           "\n"                                   );

                for ( i = 0;  i < wncard_c( &result );  i++ )
                {

                   wnfetd_c ( &result, i, &start, &finish );

                   /.
                   Compute the angles of interest at the boundary
                   epochs.
                   ./
                   timout_c ( start, TIMFMT, TIMLEN, timstr );

                   ilumin_c ( method, target, start,  fixref,
                              abcorr, obsrvr, rovpos, &trgepc,
                              srfvec, &phase, &solar, &emissn );

                   printf ( "Start: %s %13.8f %13.8f\n",
                            timstr,  solar*dpr_c(),  emissn*dpr_c() );

                   timout_c ( finish, TIMFMT, TIMLEN, timstr );

                   ilumin_c ( method, target, finish, fixref,
                              abcorr, obsrvr, rovpos, &trgepc,
                              srfvec, &phase, &solar, &emissn );

                   printf ( "Stop:  %s %13.8f %13.8f\n",
                            timstr,  solar*dpr_c(),  emissn*dpr_c() );

                   printf ( "\n" );
                }
             }

             return ( 0 );
          }

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

                                              Solar Incidence   Emission
                                                    (deg)         (deg)

          Start: 2006 OCT 03 12:43:46.949 UTC   56.10415019   20.00000019
          Stop:  2006 OCT 03 12:44:42.288 UTC   56.29996181   20.00000015

          Start: 2006 OCT 08 16:03:33.956 UTC   56.48955485   20.00000021
          Stop:  2006 OCT 08 16:04:29.495 UTC   56.68754510   19.99999997

          Start: 2006 OCT 13 19:23:24.634 UTC   56.88741059   19.99999988
          Stop:  2006 OCT 13 19:24:12.492 UTC   57.05931857   20.00000017

          Start: 2006 OCT 18 22:43:21.631 UTC   57.30924467   20.00000012
          Stop:  2006 OCT 18 22:43:47.966 UTC   57.40457272   20.00000004

          Start: 2006 NOV 14 15:39:44.153 UTC   54.32875839   19.99999994
          Stop:  2006 NOV 14 15:40:10.446 UTC   54.42668077   19.99999990

          Start: 2006 NOV 19 18:59:10.190 UTC   54.63096111   20.00000007
          Stop:  2006 NOV 19 18:59:54.776 UTC   54.79840753   19.99999985

          Start: 2006 NOV 24 22:18:38.342 UTC   54.94960000   19.99999982
          Stop:  2006 NOV 24 22:19:30.964 UTC   55.14883883   20.00000003

          Start: 2006 NOV 30 01:38:07.309 UTC   55.28054784   19.99999983
          Stop:  2006 NOV 30 01:39:03.296 UTC   55.49418925   19.99999999

#### Restrictions

       1)  The kernel files to be used by this routine must be loaded
           (normally using the CSPICE routine furnsh_c) before this
           routine is called.

       2)  This routine has the side effect of re-initializing the
           illumination angle utility package. Callers may
           need to re-initialize the package after calling this routine.

#### Literature_References

       None.

#### Author_and_Institution

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

#### Version

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

           Updated short error messages for consistency within CSPICE wrapper
           interface: MALLOCFAILURE -> MALLOCFAILED, and INVALIDDIMENSION ->
           VALUEOUTOFRANGE.

           Updated header to describe use of expanded confinement window.

           Edited the header to comply with NAIF standard.

           Changed code example for the solution to fit within the -Examples
           section without modifications.

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

           Added entry #20 in -Exceptions section.

       -CSPICE Version 1.0.0, 27-FEB-2014 (NJB) (BVS) (EDW)

#### Index_Entries

       solve for illumination_angle constraints
       solve for phase_angle constraints
       solve for solar_incidence_angle constraints
       solve for incidence_angle constraints
       solve for emission_angle constraints
       search using illumination_angle constraints
       search using lighting_angle constraints

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

    Fri Dec 31 18:41:07 2021