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Abstract
I/O
Examples
Particulars
Required Reading
Version
Index_Entries

Abstract


   Deprecated: This routine has been superseded by the CSPICE routines
   cspice_ilumin, cspice_illumg and cspice_illumf. This routine is
   supported for purposes of backward compatibility only.

   CSPICE_ILLUM_PL02 returns the illumination angles---phase, solar
   incidence, and emission---at a specified point on a target body at a
   particular epoch, optionally corrected for light time and stellar
   aberration. The target body's surface is represented by a triangular
   plate model contained in a type 2 DSK segment.

I/O


   Given:

      handle      the DAS file handle of a DSK file open for read
                  access. This kernel must contain a type 2 segment
                  that provides a plate model representing the entire
                  surface of the target body.

                  [1,1] = size(handle); int32 = class(handle)

      dladsc      the DLA descriptor of a DSK segment representing
                  the surface of the target body.

                  [SPICE_DLA_DSCSIZ,1]  = size(dladsc)
                                int32 = class(dladsc)

      target      the name of the target body. `target' is
                  case-insensitive, and leading and trailing blanks in
                  `target' are not significant. Optionally, you may supply
                  a string containing the integer ID code for the object.
                  For example both 'MOON' and '301' are legitimate strings
                  that indicate the moon is the target body.

                  This routine assumes that the target body's surface is
                  represented using a plate model, and that a DSK file
                  containing the plate model has been loaded via cspice_dasopr.

                  [1,c1] = size(target); char = class(target)

                     or

                  [1,1] = size(target); cell = class(target)

      et          the epoch, represented as seconds past J2000 TDB, at
                  which the illumination angles are to be computed. When
                  aberration corrections are used, `et' refers to the
                  epoch at which radiation is received at the observer.

                  [1,1] = size(et); double = class(et)

      abcorr      indicates the aberration corrections to be applied to
                  the position and orientation of the target body and the
                  position of the Sun to account for one-way light time
                  and stellar aberration. See the discussion in the
                  Particulars section for recommendations on how to choose
                  aberration corrections.

                  [1,c2] = size(abcorr); char = class(abcorr)

                     or

                  [1,1] = size(abcorr); cell = class(abcorr)

                  `abcorr' may be any of the following:

                     'NONE'     Apply no correction. Use the geometric
                                positions of the Sun and target body
                                relative to the observer; evaluate the
                                target body's orientation at `et'.

                  The following values of `abcorr' apply to the
                  "reception" case in which photons depart from the
                  target's location at the light-time corrected epoch
                  et-lt and *arrive* at the observer's location at
                  `et':

                     'LT'       Correct for one-way light time (also
                                called "planetary aberration") using a
                                Newtonian formulation. This correction
                                uses the position and orientation of the
                                target at the moment it emitted photons
                                arriving at the observer at `et'. The
                                position of the Sun relative to the
                                target is corrected for the one-way light
                                time from the Sun to the target.

                                The light time correction uses an
                                iterative solution of the light time
                                equation (see Particulars for details).
                                The solution invoked by the 'LT' option
                                uses one iteration.

                     'LT+S'     Correct for one-way light time and stellar
                                aberration using a Newtonian formulation.
                                This option modifies the positions
                                obtained with the 'LT' option to account
                                for the observer's velocity relative to
                                the solar system barycenter (note the
                                target plays the role of "observer" in the
                                computation of the aberration-corrected
                                target-Sun vector). The result is that the
                                illumination angles are computed using
                                apparent position and orientation of the
                                target as seen by the observer and the
                                apparent position of the Sun as seen by
                                the target.

                     'CN'       Converged Newtonian light time correction.
                                In solving the light time equation, the
                                'CN' correction iterates until the
                                solution converges (three iterations on
                                all supported platforms).

                     'CN+S'     Converged Newtonian light time
                                and stellar aberration corrections.

      obsrvr      the name of the observing body. This is typically a
                  spacecraft, the earth, or a surface point on the earth.
                  `obsrvr' is case-insensitive, and leading and trailing
                  blanks in `obsrvr' are not significant. Optionally, you
                  may supply a string containing the integer ID code for
                  the object. For example both 'EARTH' and '399' are
                  legitimate strings that indicate the earth is the
                  observer.

                  [1,c3] = size(obsrvr); char = class(obsrvr)

                     or

                  [1,1] = size(obsrvr); cell = class(obsrvr)

      spoint      a surface point on the target body, expressed in
                  rectangular body-fixed (body equator and prime meridian)
                  coordinates. `spoint' need not be visible from the
                  observer's location at time `et'.

                  [3,1] = size(spoint); double = class(spoint)

   the call:

      [phase, solar, emissn] =                            ...
               cspice_illum_pl02( handle, dladsc, target, ...
                                  et,     abcorr, obsrvr, ...
                                  spoint )

   returns:

               For all of the angles below, if `spoint' does not lie on
               one of the *exterior* plates comprising the DSK type 2
               surface representation, the "intercept" style
               "sub-observer point" corresponding to `spoint' is used
               in the illumination angle computations in place of
               `spoint'. The selected point will always be on the
               *outermost* plate intersected by a ray emanating from
               the target body's center and passing through `spoint'.

               See the header of CSPICE_SUBPT_PL02 for details
               concerning the definition of the sub-observer point.

               In all cases, the normal vector is taken from the plate
               on which the sub-point corresponding to `spoint' lies.
               If this sub-point lies on an edge or vertex, a normal
               vector for one of the bordering plates is selected.


   phase       the phase angle at `spoint', as seen from `obsrvr' at
               time `et'. This is the angle between the spoint-obsrvr
               vector and the spoint-sun vector. Units are radians. The
               range of `phase' is [0, pi].

               [1,1] = size(phase); double = class(phase)

   solar       the solar incidence angle at `spoint', as seen from
               `obsrvr' at time `et'. This is the angle between the
               surface normal vector at `spoint' and the spoint-sun
               vector. Units are radians. The range of `solar' is [0,
               pi].

               [1,1] = size(solar); double = class(solar)

   emissn      the emission angle at `spoint', as seen from `obsrvr'
               at time `et'. This is the angle between the surface
               normal vector at `spoint' and the spoint-observer
               vector. Units are radians. The range of `emissn' is
               is [0, pi].

               [1,1] = size(emissn); double = class(emissn)

               See Particulars below for a detailed discussion of the
               definitions of these angles.

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.

      Find the illumination angles at both the sub-observer point and
      sub-solar point on Phobos as seen from Mars for a specified
      sequence of times. Perform each computation twice, using both the
      "intercept" and "ellipsoid near point" options for the sub-observer
      point and sub-solar point computations. Compute the corresponding
      illumination angles using an ellipsoidal surface for comparison.
      (Note that the surface points on the plate model generally will
      not lie on the ellipsoid's surface, so the emission and solar
      incidence angles won't generally be zero at the sub-observer
      and sub-solar points, respectively.)

      In the following example program, the file

         phobos_3_3.bds

      is a DSK file containing a type 2 segment that provides a plate model
      representation of the surface of Phobos.  The file

         mar097.bsp

      is a binary SPK file containing data for Phobos, Mars, and the
      Sun for a time interval starting at the date

         2000 JAN 1 12:00:00 TDB.

      pck00010.tpc is a planetary constants kernel file containing radii
      and rotation model constants.  naif0010.tls is a leapseconds kernel.

      All of the kernels other than the DSK file should be loaded via
      a meta-kernel.  An example of the contents of such a kernel is:

          KPL/MK

          File name: illum.tm

          \begindata

             KERNELS_TO_LOAD = ( 'naif0010.tls'
                                 'pck00010.tpc'
                                 'mar097.bsp' )
          \begintext

   Example:

      function illum_pl02_t( meta, dsk )
         %
         % Constants
         %
         NCORR       = 2;
         NSAMP       = 3;
         NMETHOD     = 2;
         FIXREF      = 'IAU_PHOBOS';
         ILUM_METHOD = 'ELLIPSOID';
         TOL         = 1.d-12;

         %
         % Initial values
         %
         abcorrs     = { 'NONE', 'CN+S' };
         methods     = { 'Intercept', 'Ellipsoid near point' };

         obsrvr      = 'Mars';
         target      = 'Phobos';

         %
         % Load the meta kernel.
         %
         cspice_furnsh( meta )

         %
         % Open the DSK file for read access.
         % We use the DAS-level interface for
         % this function.
         %
         handle = cspice_dasopr( dsk );

         %
         % Begin a forward search through the
         % kernel, treating the file as a DLA.
         % In this example, it's a very short
         % search.
         %
         [dladsc, found] = cspice_dlabfs( handle );

         if ~found

            %
            % We arrive here only if the kernel
            % contains no segments. This is
            % unexpected, but we're prepared for it.
            %
            fprintf( 'No segments found in DSK file %s\n', dsk )
            return

         end

         %
         % If we made it this far, `dladsc' is the
         % DLA descriptor of the first segment.
         %
         % Now compute sub-points using both computation
         % methods. We'll vary the aberration corrections
         % and the epochs.
         %

         et0      = 0.0;
         stepsize = 1.d6;


         for  i = 0:(NSAMP-1)

            %
            % Set the computation time for the ith sample.
            %

            et = et0 + i * stepsize;

            timstr = cspice_timout( et,                                    ...
                                    'YYYY-MON-DD HR:MN:SC.### ::TDB(TDB)' );


            fprintf( '\n\nObservation epoch:  %s\n', timstr )


            for  coridx = 1:NCORR

               abcorr = abcorrs( coridx );

               fprintf( '   abcorr = %s\n', char(abcorr) );

               for  midx = 1:NMETHOD

                  %
                  % Select the computation method.
                  %
                  method = methods( midx );

                  fprintf( '\n     Method =%s\n ', char(method) )

                  %
                  % Compute the sub-observer point using a plate
                  % model representation of the target's surface.
                  %
                  [xpt, alt, plid] = ...
                        cspice_subpt_pl02( handle, dladsc, method, ...
                                           target, et,     abcorr, ...
                                           obsrvr                    );

                  %
                  % Compute the illumination angles at the
                  % sub-observer point.
                  %
                  [phase,  solar,  emissn] = cspice_illum_pl02( handle, ...
                                                  dladsc, target, et, ...
                                                  abcorr, obsrvr, xpt );

                  %
                  %  Represent the surface point in latitudinal
                  % coordinates.
                  %
                  [ xr, xlon, xlat] = cspice_reclat( xpt );

                  fprintf(                                                ...
                  '\n     Sub-observer point on plate model surface:\n' )
                  fprintf( '       Planetocentric Longitude (deg):  %f\n', ...
                                                    xlon * cspice_dpr() )
                  fprintf( '       Planetocentric Latitude  (deg):  %f\n', ...
                                                    xlat * cspice_dpr() )

                  fprintf(                                              ...
                   '\n         Illumination angles derived using a\n' )
                  fprintf( '         plate model surface:\n' )
                  fprintf(                                              ...
                    '             Phase angle              (deg): %f\n', ...
                                                  phase  * cspice_dpr() )
                  fprintf(                                              ...
                   '             Solar incidence angle    (deg): %f\n', ...
                                                  solar  * cspice_dpr() )
                  fprintf(                                              ...
                   '             Emission angle           (deg): %f\n\n', ...
                                                  emissn * cspice_dpr() )
                  %
                  % Compute the illumination angles using an ellipsoidal
                  % representation of the target's surface. The role of
                  % this representation is to provide an outward surface
                  % normal.
                  %

                  [trgepc, srfvec, phase,  solar,  emissn] = ...
                                           cspice_ilumin( ILUM_METHOD,  ...
                                               target, et,     FIXREF,  ...
                                               abcorr, obsrvr, xpt);


                  fprintf(                                              ...
                   '         Illumination angles derived using an\n' )
                  fprintf( '         ellipsoidal reference surface:':\n' )
                  fprintf(                                              ...
                   '             Phase angle              (deg): %f\n', ...
                                                  phase  * cspice_dpr() )
                  fprintf(                                              ...
                   '             Solar incidence angle    (deg): %f\n', ...
                                                  solar  * cspice_dpr() )
                  fprintf(                                              ...
                   '             Emission angle           (deg): %f\n\n', ...
                                                  emissn * cspice_dpr() )



                  %
                  % Now repeat our computations using the
                  % sub-solar point.
                  %
                  % Compute the sub-solar point using a plate model
                  % representation of the target's surface.
                  %

                  [xpt, dist, plid] = ...
                  cspice_subsol_pl02( handle, dladsc, method, ...
                                      target, et,     abcorr, ...
                                      obsrvr                    );

                  %
                  % Compute the illumination angles at the
                  % sub-solar point.
                  %

                  [phase,  solar,  emissn] = cspice_illum_pl02( handle, ...
                                                    dladsc, target, et, ...
                                                    abcorr, obsrvr, xpt );

                  %
                  %  Represent the surface point in latitudinal
                  % coordinates.
                  %
                  [ xr, xlon, xlat] = cspice_reclat( xpt );

                  fprintf( '     Sub-solar point on plate model surface:\n' )
                  fprintf( '       Planetocentric Longitude (deg):  %f\n', ...
                                                     xlon * cspice_dpr() )
                  fprintf( '       Planetocentric Latitude  (deg):  %f\n', ...
                                                     xlat * cspice_dpr() )

                  fprintf(                                              ...
                   '\n         Illumination angles derived using a\n' )
                  fprintf( '         plate model surface:\n' )
                  fprintf(                                              ...
                   '             Phase angle              (deg): %f\n', ...
                                                  phase  * cspice_dpr() )
                  fprintf(                                              ...
                   '             Solar incidence angle    (deg): %f\n', ...
                                                  solar  * cspice_dpr() )
                  fprintf(                                              ...
                   '             Emission angle           (deg): %f\n\n', ...
                                                  emissn * cspice_dpr() )

                  %
                  % Compute the illumination angles using an ellipsoidal
                  % representation of the target's surface. The role of
                  % this representation is to provide an outward surface
                  % normal.
                  %

                  [trgepc, srfvec, phase,  solar,  emissn] = ...
                                           cspice_ilumin( ILUM_METHOD, ...
                                           target, et,     FIXREF,     ...
                                           abcorr, obsrvr, xpt);

                  fprintf(                                              ...
                   '         Illumination angles derived using an\n' )
                  fprintf( '         ellipsoidal surface:\n' )
                  fprintf(                                              ...
                   '             Phase angle              (deg): %f\n', ...
                                                  phase  * cspice_dpr() )
                  fprintf(                                              ...
                   '             Solar incidence angle    (deg): %f\n', ...
                                                  solar  * cspice_dpr() )
                  fprintf(                                              ...
                   '             Emission angle           (deg): %f\n\n', ...
                                                  emissn * cspice_dpr() )

               end

            end

         end

         %
         % Close the DSK file. Unload all other kernels as well.
         %
         cspice_dascls( handle )

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

   MATLAB outputs:

      >> illum_pl02_t( 'illum_pl02.tm' ,'phobos_3_3.bds')

      Observation epoch:  2000-JAN-01 12:00:00.000 (TDB)
         abcorr = NONE

           Method =Intercept

           Sub-observer point on plate model surface:
             Planetocentric Longitude (deg):  -0.348118
             Planetocentric Latitude  (deg):  0.008861

               Illumination angles derived using a
               plate model surface:
                   Phase angle              (deg): 101.596824
                   Solar incidence angle    (deg): 98.376877
                   Emission angle           (deg): 9.812914

               Illumination angles derived using an
               ellipsoidal surface:
                   Phase angle              (deg): 101.596824
                   Solar incidence angle    (deg): 101.695444
                   Emission angle           (deg): 0.104977

           Sub-solar point on plate model surface:
             Planetocentric Longitude (deg):  102.413905
             Planetocentric Latitude  (deg):  -24.533127

               Illumination angles derived using a
               plate model surface:
                   Phase angle              (deg): 101.665306
                   Solar incidence angle    (deg): 13.068798
                   Emission angle           (deg): 98.408735

               Illumination angles derived using an
               ellipsoidal surface:
                   Phase angle              (deg): 101.665306
                   Solar incidence angle    (deg): 11.594741
                   Emission angle           (deg): 98.125499


           Method =Ellipsoid near point

           Sub-observer point on plate model surface:
             Planetocentric Longitude (deg):  -0.264850
             Planetocentric Latitude  (deg):  0.004180

               Illumination angles derived using a
               plate model surface:
                   Phase angle              (deg): 101.596926
                   Solar incidence angle    (deg): 98.376877
                   Emission angle           (deg): 9.812985

               Illumination angles derived using an
               ellipsoidal surface:
                   Phase angle              (deg): 101.596926
                   Solar incidence angle    (deg): 101.593324
                   Emission angle           (deg): 0.003834

           Sub-solar point on plate model surface:
             Planetocentric Longitude (deg):  105.857346
             Planetocentric Latitude  (deg):  -16.270558

               Illumination angles derived using a
               plate model surface:
                   Phase angle              (deg): 101.663675
                   Solar incidence angle    (deg): 16.476730
                   Emission angle           (deg): 118.124981

               Illumination angles derived using an
               ellipsoidal surface:
                   Phase angle              (deg): 101.663675
                   Solar incidence angle    (deg): 0.422781
                   Emission angle           (deg): 101.541470

            ...

Particulars


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


      solar incidence angle    Angle between the surface normal at the
                               specified surface point and the vector
                               from the surface point to the Sun.

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

      phase angle              Angle between the vectors from the
                               surface point to the observing body and
                               from the surface point to the Sun.


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


                                                    *
                                                   Sun

                  surface normal vector
                            ._                 _.
                            |\                 /|  Sun vector
                              \    phase      /
                               \   .    .    /
                               .            .
                                 \   ___   /
                            .     \/     \/
                                  _\ s.i./
                           .    /   \   /
                           .   |  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-sun vector are coplanar, then
   phase is the sum of incidence and emission. This is rarely true;
   usually

      phase angle  <  solar 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. The way aberration corrections
   are used is described below.

   Care must be used in computing light time corrections. The
   guiding principle used here is "describe what appears in
   an image."  We ignore differential light time; the light times
   from all points on the target to the observer are presumed to be
   equal.


      Observer-target body 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 body (we use the term "emitted"
      loosely here).

      The correct observer-target vector points from the observer's
      location at `et' to the target body's location at et - lt.
      The target-observer vector points in the opposite direction.

      Since light time corrections are not symmetric, the correct
      target-observer vector CANNOT be found by computing the light
      time corrected position of the observer as seen from the
      target body.


      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 -- Sun vector
      -------------------------

      All surface features on the target body will appear in a
      measurement made at `et' as they were at the target at epoch
      et-lt. In particular, lighting on the target body is dependent
      on the apparent location of the Sun as seen from the target body
      at et-lt. So, a second light time correction is used in finding
      the apparent location of the Sun.


   Stellar aberration corrections, when used, are applied as follows:


      Observer-target body vector
      ---------------------------

      In addition to light time correction, stellar aberration is
      used in computing the apparent target body position as seen
      from the observer's location at time `et'. This apparent
      position defines the observer-target body vector.


      Target body-Sun vector
      ----------------------

      The target body-Sun vector is the apparent position of the Sun,
      corrected for light time and stellar aberration, as seen from
      the target body at time et-lt. Note that the target body's
      position is not affected by the stellar aberration correction
      applied in finding its apparent position as seen by the
      observer.


   Once all of the vectors, as well as the target body's
   orientation, have been computed with the proper aberration
   corrections, the element of time is eliminated from the
   computation. The problem becomes a purely geometric one,
   and is described by the diagram above.

Required Reading


   For important details concerning this module's function, please refer to
   the CSPICE routine illum_pl02.

   MICE.REQ
   ABCORR.REQ
   DSK.REQ
   PCK.REQ
   SPK.REQ
   TIME.REQ

Version


   -Mice Version 1.0.0, 25-JUL-2016, NJB (JPL), EDW (JPL)

Index_Entries


   illumination angles using dsk triangular plate_model
   lighting angles using dsk triangular plate_model
   illumination angles using dsk type_2 plate_model
   lighting angles using dsk type_2 plate_model
   phase angle using dsk triangular plate_model
   emission angle using dsk triangular plate_model
   solar incidence angle using dsk triangular plate_model
   phase angle using dsk type_2 plate_model
   emission angle using dsk type_2 plate_model
   solar incidence angle using dsk type_2 plate_model


Wed Apr  5 18:00:32 2017