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cspice_gfilum

Table of contents
Abstract
I/O
Parameters
Examples
Particulars
Exceptions
Files
Restrictions
Required_Reading
Literature_References
Author_and_Institution
Version
Index_Entries


Abstract


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

I/O


   Given:

      method   scalar string providing parameters defining the computation
               method to be used.

               help, method
                  STRING = Scalar

               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 white space are significant in
               `method'. For example, the string ' eLLipsoid ' is
               valid.

      angtyp   a string specifying the type of illumination angle for which a
               search is to be performed.

               help, angtyp
                  STRING = Scalar

               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 white space are significant in
               `angtyp'. For example, the string ' Incidence ' is
               valid.

      target   scalar string name of a target body.

               help, target
                  STRING = Scalar

               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    scalar string name of the illumination source.

               help, illmn
                  STRING = Scalar

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

      fixref   scalar string name of the body-fixed, body-centered reference
               frame associated with the target body.

               help, fixref
                  STRING = Scalar

               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   scalar string indicating the aberration corrections to apply to
               the observer-target position vector to account for one-way light
               time and stellar aberration.

               help, abcorr
                  STRING = Scalar

               Any "reception" correction accepted by cspice_spkezr can be
               used here. See the header of cspice_spkezr 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   scalar string name of an observing body.

               help, obsrvr
                  STRING = Scalar

               Optionally, you may supply the ID code of the object as an
               integer string. For example, both 'EARTH' and '399' are
               legitimate strings to supply to indicate that the observer is
               the Earth.

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

               help, spoint
                  DOUBLE = Scalar

               `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   scalar string relational operator used to define a constraint on
               a specified illumination angle.

               help, relate
                  STRING = Scalar

               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 region 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   the reference value used together with the argument `relate' to
               define an equality or inequality to be satisfied by the
               specified illumination angle.

               help, refval
                  DOUBLE = Scalar

               See the discussion of `relate' above for further information.

               The units of `refval' are radians.

      adjust   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, cspice_gfilum will find times when the
               observer-target distance is within `adjust' km of the specified
               extreme value.

               help, adjust
                  DOUBLE = Scalar

               If `adjust' is non-zero and a search for an absolute
               minimum `min' is performed, the result window contains
               time intervals when the observer-target distance has
               values between `min' and min+adjust.

               If the search is for an absolute maximum `max', the
               corresponding range is from max-adjust to `max'.

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

      step     the step size to be used in the search.

               help, step
                  DOUBLE = Scalar

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

               help, nintvls
                  LONG = Scalar

               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   a SPICE window that confines the time period over which the
               specified search is conducted.

               help, cnfine
                  STRUCT = cspice_celld(2*N)

               `cnfine' may consist of a single interval or a collection of
               intervals.

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

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

               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.

   the call:

      cspice_gfilum, method, angtyp, target,  illmn,  fixref,                $
                     abcorr, obsrvr, spoint,  relate, refval,                $
                     adjust, step,   nintvls, cnfine, result

   returns:

      result   the SPICE window of intervals, contained within the confinement
               window `cnfine', on which the specified constraint is satisfied.

               help, result
                  STRUCT = cspice_celld(2*R)

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

               `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 constraint
               is satisfied.

               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
               constraint, `result' will return with cardinality zero.

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 Icy include file IcyGF.pro for declarations and descriptions of
   parameters used throughout the GF subsystem.

Examples


   Any numerical results shown for this example may differ between
   platforms as the results depend on the SPICE kernels used as input
   and the machine specific arithmetic implementation.

   1) Determine time intervals over which the planned Mars Science
      Laboratory (MSL) Gale Crater landing site satisfies certain
      constraints on its illumination and visibility as seen from
      the Mars Reconnaissance Orbiter (MRO) spacecraft. The
      observation period will range from slightly before the planned
      landing time to about 10 days later.

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

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


         KPL/MK

         File name: 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
            naif0012.tls      Leapseconds
            mro_psp24.bsp     MRO ephemeris


         \begindata

         KERNELS_TO_LOAD = ( 'de421.bsp',
                             'pck00010.tpc',
                             'naif0012.tls',
                             'mro_psp24.bsp' )
         \begintext

         End of meta-kernel


      Example code begins here.


      PRO gfilum_ex1

         ;;
         ;; Output time format:
         ;;
         TIMFMT = 'YYYY MON DD HR:MN:SC.###### TDB::TDB'

         ;;
         ;; Meta-kernel name:
         ;;
         META = 'gfilum_ex1.tm'

         ;;
         ;; Maximum number of intervals in the windows
         ;; used in this program:
         ;;
         MAXIVL = 1000
         MAXWIN = ( 2 * MAXIVL )

         ;;
         ;; Maximum length of time string:
         ;;
         TIMLEN = 41

         ;;
         ;; Local variables
         ;;
         cnfine = cspice_celld( MAXWIN )
         result = cspice_celld( MAXWIN )
         wnsolr = cspice_celld( MAXWIN )
         r2d    = cspice_dpr()

         ;;
         ;; Initial values
         ;;
         ;; Mars planetodetic coordinates of landing site.
         ;; Angular units are degrees; distance units are km.
         ;;
         gclat  =  -4.543182D
         gclon  = 137.420000D
         gcalt  =  -4.876405D

         ;;
         ;; Load kernels:
         ;;
         cspice_furnsh, META

         ;;
         ;; Convert the landing site location from planetodetic
         ;; to Cartesian coordinates for use with GFILUM.
         ;;
         cspice_bodvrd, 'MARS', 'RADII', 3, radii

         re = radii[0]
         rp = radii[2]

         f  = ( re - rp ) / re

         cspice_georec, gclon * cspice_rpd(), gclat * cspice_rpd(),          $
                        gcalt, re, f, gcpos

         ;;
         ;; Set the search interval:
         ;;
         utcbeg = '2012 AUG 5 00:00:00 UTC'
         cspice_str2et, utcbeg, et0

         utcend = '2012 SEP 15 00:00:00 UTC'
         cspice_str2et, utcend, et1

         cspice_wninsd, 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'

         ;;
         ;; 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 = 45.D0 * cspice_rpd()

         ;;
         ;; 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.D0

         ;;
         ;; Search over the confinement window for times
         ;; when the solar incidence angle is less than
         ;; the reference value.
         ;;
         cspice_gfilum, method, 'INCIDENCE', target, illmn,  fixref,         $
                        abcorr, obsrvr,      gcpos,  '<',    refval,         $
                        adjust, step,        MAXIVL, cnfine, wnsolr

         ;;
         ;; With the search on the incidence angle complete, perform
         ;; a search on the emission angle.
         ;;
         ;; Set the reference value for the emission angle search.
         ;;
         refval = 80.D0 * cspice_rpd()

         ;;
         ;; 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.D0

         ;;
         ;; Search over the previous result window for times when the
         ;; emission angle is less than the reference value.
         ;;
         cspice_gfilum, method, 'EMISSION', target, illmn,  fixref,          $
                        abcorr, obsrvr,     gcpos,  '<',    refval,          $
                        adjust, step,       MAXIVL, wnsolr, result

         ;;
         ;; Display the result window. Show the solar incidence
         ;; and emission angles at the window's interval
         ;; boundaries.
         ;;

         if ( cspice_wncard( result ) eq 0 ) then begin

            printf, '     Window is empty: condition is not met.'

         endif else begin

            print, FORMAT = '(A40,"Solar Incidence   Emission")', ''
            print, FORMAT = '(A40,"    (deg)           (deg)" )', ''

            for i = 0, cspice_wncard( result ) -1 do begin

               cspice_wnfetd, result, i, start, finish

               ;;
               ;; Compute the angles of interest at the boundary
               ;; epochs.
               ;;
               cspice_timout, start, TIMFMT, TIMLEN, timstr

               cspice_ilumin, method, target, start, fixref,                 $
                              abcorr, obsrvr, gcpos, trgepc,                 $
                              srfvec, phase,  solar, emissn

               print, FORMAT = '("Start: ",A32,F14.9,F14.9)',                $
                                timstr,  solar*r2d,  emissn*r2d

               cspice_timout, finish, TIMFMT, TIMLEN, timstr

               cspice_ilumin, method, target, finish, fixref,                $
                              abcorr, obsrvr, gcpos,  trgepc,                $
                              srfvec, phase, solar, emissn

               print, FORMAT = '("Stop:  ",A32,F14.9,F14.9)',                $
                                 timstr,  solar*r2d,  emissn*r2d
               print, ''

             endfor

         endelse

         ;;
         ;; It's always good form to unload kernels after use,
         ;; particularly in IDL due to data persistence.
         ;;
         cspice_kclear

      END


      When this program was executed on a Mac/Intel/IDL8.x/64-bit
      platform, the output was:


                                              Solar Incidence   Emission
                                                  (deg)           (deg)
      Start:  2012 AUG 09 06:14:46.475539 TDB  41.793493032  80.000000000
      Stop:   2012 AUG 09 06:15:29.695045 TDB  41.954623385  80.000000002

      Start:  2012 AUG 14 09:37:47.093234 TDB  42.772767813  80.000000007
      Stop:   2012 AUG 14 09:41:59.554719 TDB  43.729251675  79.999999998

      Start:  2012 AUG 19 13:01:43.056249 TDB  44.000361046  80.000000017
      Stop:   2012 AUG 19 13:06:03.429007 TDB  44.999999999  75.754083310

      Start:  2012 AUG 30 20:10:42.196910 TDB  42.214690783  79.999999993
      Stop:   2012 AUG 30 20:14:47.411493 TDB  43.170768309  79.999999996

      Start:  2012 SEP 04 23:35:53.476437 TDB  43.804510481  79.999999983
      Stop:   2012 SEP 04 23:40:57.001978 TDB  45.000000001  77.221887661

      Start:  2012 SEP 11 03:22:35.751759 TDB  41.115348965  80.000000009
      Stop:   2012 SEP 11 03:24:59.610628 TDB  41.684463728  79.999999996


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 cspice_gfstol, e.g.

      cspice_gfstol, tolerance value in seconds

   Call cspice_gfstol 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.

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 Icy window routine
       cspice_wncond 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 by a routine in the call
       tree of this routine.

   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 input arguments, `method', `angtyp', `target',
       `illmn', `fixref', `abcorr', `obsrvr', `spoint', `relate',
       `refval', `adjust', `step', `nintvls', `cnfine' or `result',
       is undefined, an error is signaled by the IDL error handling
       system.

   18) If any of the input arguments, `method', `angtyp', `target',
       `illmn', `fixref', `abcorr', `obsrvr', `spoint', `relate',
       `refval', `adjust', `step', `nintvls', `cnfine' or `result',
       is not of the expected type, or it does not have the expected
       dimensions and size, an error is signaled by the Icy
       interface.

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 cspice_furnsh.

   -  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 cspice_furnsh.

   -  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 cspice_furnsh.

   -  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.

Restrictions


   1)  The kernel files to be used by this routine must be loaded
       (normally using the Icy routine cspice_furnsh) 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.

Required_Reading


   FRAMES.REQ
   GF.REQ
   ICY.REQ
   NAIF_IDS.REQ
   PCK.REQ
   SPK.REQ
   TIME.REQ

Literature_References


   None.

Author_and_Institution


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

Version


   -Icy Version 1.0.1, 03-NOV-2021 (EDW) (JDR)

       Added -Parameters, -Exceptions, -Files, -Restrictions,
       -Literature_References and -Author_and_Institution sections.

       Updated header to describe use of expanded confinement window.

       Edited the header to comply with NAIF standard. Corrected typos in
       header. Changed argument name "illum" to "illmn" for consistency
       with other procedures. Updated the list of Required Reading and
       -Index_Entries.

       Corrected error in header that listed 'SOLAR INCIDENCE' as an
       allowed angle type rather than the correct value 'INCIDENCE'.

       Removed reference to the routine's corresponding CSPICE header from
       -Abstract section.

       Added arguments' type and size information in the -I/O section.

   -Icy Version 1.0.0, 05-SEP-2012 (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



Fri Dec 31 18:43:05 2021