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cspice_subsol_pl02

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

Abstract


   Deprecated: This routine has been superseded by the Mice routine
   cspice_subslr. This routine is supported for purposes of backward
   compatibility only.

   CSPICE_SUBSOL_PL02 returns the rectangular coordinates of the sub-solar
   point on a target body in the body-fixed frame associated with the body, 
   at a particular epoch, optionally corrected for light time and stellar 
   aberration. In addition, it returns return the observer's distance from 
   the sub-solar point.

   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.

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

               This kernel must contain a type 2 segment that
               provides a plate model representing the entire surface
               of the target body.

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

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

      method   a short string specifying the computation method
               to be used.

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

                  or

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

               The choices are:

                  'Intercept'        The sub-solar point is defined as
                                     the plate model surface intercept
                                     of the ray starting at the Sun and
                                     passing through the target's
                                     center.

                  'Ellipsoid
                   near point'       The sub-solar point is defined as
                                     the plate model surface intercept
                                     of the ray starting at the Sun and
                                     passing through the nearest point
                                     to the observer on a reference
                                     ellipsoid associated with the
                                     target body.

                                     This option requires that the
                                     reference ellipsoid's radii be
                                     available in the kernel pool.

               Neither case nor white space are significant in the
               string `method'. For example, the string

                  '  ellipsoidNEARPOINT'

               is valid.

      target   the name of the target body.

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

                  or

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

               `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 by a plate model, and that a DSK file
               containing the plate model has been loaded via cspice_dasopr.

      et       the epoch, represented  as seconds past J2000 TDB, at
               which the sub-solar point on the target body is to be
               computed.

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

               When aberration corrections are used, `et' refers to the epoch
               at which radiation is received at the observer.

      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.

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

                  or

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

               See the discussion in the -Particulars section for
               recommendations on how to choose aberration corrections.

               `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 the
                             sub-solar point 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.

               [1,c4] = size(obsrvr); char = class(obsrvr)

                  or

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

               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.

   the call:

      [spoint, dist, pltid] = cspice_subsol_pl02( handle, dladsc,         ...
                                                    method, target,       ...
                                                    et,     abcorr,       ...
                                                    obsrvr         )

   returns:

      spoint   the sub-solar point on the target body expressed
               relative to the body-fixed reference frame of the target
               body.

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

               The definition of sub-solar point depends on the
               selected computation method. See the description of the
               input argument `method' for details.

               The target body-fixed frame, which is time-dependent, is
               evaluated at `et' if `abcorr' is 'NONE'; otherwise the
               frame is evaluated at et-lt, where `lt' is the one-way
               light time from target to observer.

               The position and orientation of the target body and the
               position of the Sun are corrected for aberration as
               specified by `abcorr'; the corrected positions and
               orientation are used in the computation of `spoint'.

      dist     the distance between the observer and the sub-solar
               point.

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

               The observer is presumed to be outside the
               target body, so `dist' is always non-negative.

      pltid    the integer ID code of the plate on which the
               sub-solar point is located.

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

               This ID code can be use to look up data associated with the
               plate, such as the plate's vertices or outward normal vector.

Parameters


   None.

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) Find the sub-solar point on Phobos as seen from Earth for a
      specified sequence of times. Perform the computation twice,
      using both the "intercept" and "ellipsoid near point"
      options. Compute the corresponding sub-solar point values
      using an ellipsoidal surface for comparison.


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


         KPL/MK

         File: subsol_pl02_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
            ---------                        --------
            mar097.bsp                       Mars satellite ephemeris
            pck00010.tpc                     Planet orientation and
                                             radii
            naif0010.tls                     Leapseconds


         \begindata

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

         End of meta-kernel


      Use the DSK kernel below to provide the plate model representation
      of the surface of Phobos.

         phobos_3_3.bds



      Example code begins here.


      function subsol_pl02_ex1( meta, dsknam )

         %
         % Constants
         %
         NCORR       = 2;
         NSAMP       = 3;
         NMETHOD     = 2;
         TIMLEN      = 40;
         FIXREF      = 'IAU_PHOBOS';
         TOL         = 1.d-12;

         %
         % Initial values
         %
         abcorrs     = { 'NONE', 'CN+S' };
         emethods    = { 'Intercept: ellipsoid', 'Near point: ellipsoid' };
         plmethods   = { 'Intercept', 'Ellipsoid near point' };

         obsrvr      = 'Earth';
         target      = 'Phobos';

         %
         % Load the metakernel.
         %
         cspice_furnsh( meta )

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

         %
         % 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', dsknam )
            return

         end

         %
         % If we made it this far, `dladsc' is the
         % DLA descriptor of the first segment.
         %
         % Now compute sub-solar 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( '\n  abcorr = %s\n', char(abcorr) );

               for  midx = 1:NMETHOD

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

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

                  %
                  % 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                    );

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

                  fprintf(                                                ...
                  '\n    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( '      Radius                    (km):  %f\n', ...
                                                                    xr )
                  fprintf( '      Observer distance         (km):  %f\n', ...
                                                                  dist )
                  fprintf( '      ID of surface point plate:       %d\n', ...
                                                                  plid )

                  %
                  % Compute the sub-solar point using an ellipsoidal
                  % representation of the target's surface.
                  %
                  method = emethods( midx );

                  [xpt, trgepc, srfvec] = cspice_subslr( method, target,  ...
                                                         et,     FIXREF,  ...
                                                         abcorr, obsrvr );

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

                  fprintf( '    Sub-solar point on ellipsoidal surface:\n' )
                  fprintf( '      Planetocentric Longitude (deg):  %f\n', ...
                                                    xlon * cspice_dpr() )
                  fprintf( '      Planetocentric Latitude  (deg):  %f\n', ...
                                                    xlat * cspice_dpr() )
                  fprintf( '      Radius                    (km):  %f\n', ...
                                                                     xr )
                  fprintf( '      Observer distance         (km):  %f\n', ...
                                                   cspice_vnorm(srfvec) )

               end

            end

         end

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

         cspice_kclear


      When this program was executed on a Mac/Intel/Octave6.x/64-bit
      platform, with the following variables as inputs

         meta   = 'subsol_pl02_ex1.tm';
         dsknam = 'phobos_3_3.bds';

      the output was:


      Observation epoch:  2000-JAN-01 12:00:00.000 (TDB)

        abcorr = NONE

          Method = Intercept

          Sub-solar point on plate model surface:
            Planetocentric Longitude (deg):  102.413905
            Planetocentric Latitude  (deg):  -24.533127
            Radius                    (km):  11.612325
            Observer distance         (km):  276700026.580116
            ID of surface point plate:       164811
          Sub-solar point on ellipsoidal surface:
            Planetocentric Longitude (deg):  102.413905
            Planetocentric Latitude  (deg):  -24.533127
            Radius                    (km):  10.922580
            Observer distance         (km):  276700027.168434

          Method = Ellipsoid near point

          Sub-solar point on plate model surface:
            Planetocentric Longitude (deg):  105.857346
            Planetocentric Latitude  (deg):  -16.270558
            Radius                    (km):  11.645162
            Observer distance         (km):  276700027.058857
            ID of surface point plate:       192093
          Sub-solar point on ellipsoidal surface:
            Planetocentric Longitude (deg):  105.973365
            Planetocentric Latitude  (deg):  -15.976232
            Radius                    (km):  11.249340
            Observer distance         (km):  276700027.400706

        abcorr = CN+S

          Method = Intercept

          Sub-solar point on plate model surface:
            Planetocentric Longitude (deg):  114.623420
            Planetocentric Latitude  (deg):  -24.533628
            Radius                    (km):  11.411417
            Observer distance         (km):  276710249.789163
            ID of surface point plate:       170492
          Sub-solar point on ellipsoidal surface:
            Planetocentric Longitude (deg):  114.623420
            Planetocentric Latitude  (deg):  -24.533628
            Radius                    (km):  11.046740
            Observer distance         (km):  276710250.099485

          Method = Ellipsoid near point

          Sub-solar point on plate model surface:
            Planetocentric Longitude (deg):  120.870428
            Planetocentric Latitude  (deg):  -15.247903
            Radius                    (km):  11.350346
            Observer distance         (km):  276710250.680859
            ID of surface point plate:       205719
          Sub-solar point on ellipsoidal surface:
            Planetocentric Longitude (deg):  120.795481
            Planetocentric Latitude  (deg):  -15.366726
            Radius                    (km):  11.494153
            Observer distance         (km):  276710250.555135


      Observation epoch:  2000-JAN-13 01:46:40.000 (TDB)

        abcorr = NONE

          Method = Intercept

          Sub-solar point on plate model surface:
            Planetocentric Longitude (deg):  4.432684
            Planetocentric Latitude  (deg):  -24.281966
            Radius                    (km):  12.888491
            Observer distance         (km):  286106845.772886
            ID of surface point plate:       122143
          Sub-solar point on ellipsoidal surface:
            Planetocentric Longitude (deg):  4.432684
            Planetocentric Latitude  (deg):  -24.281966
            Radius                    (km):  11.980161
            Observer distance         (km):  286106846.561862

          Method = Ellipsoid near point

          Sub-solar point on plate model surface:
            Planetocentric Longitude (deg):  3.418663
            Planetocentric Latitude  (deg):  -12.568166
            Radius                    (km):  12.783152
            Observer distance         (km):  286106846.122051
            ID of surface point plate:       149271
          Sub-solar point on ellipsoidal surface:
            Planetocentric Longitude (deg):  3.411488
            Planetocentric Latitude  (deg):  -12.479982
            Radius                    (km):  12.689005
            Observer distance         (km):  286106846.205591

        abcorr = CN+S

          Method = Intercept

      [...]


      Warning: incomplete output. Only 100 out of 189 lines have been
      provided.


Particulars


   cspice_subsol_pl02 computes the sub-solar point on a target body.
   cspice_subsol_pl02 also determines the distance of the observer from the
   sub-solar point.

   Sub-point Definitions
   =====================

   This routine offers two ways of defining the sub-solar point:

      - The 'intercept' method. In general, this definition
        calls for defining a ray emanating from the Sun and
        passing through the center of the target body. The intercept
        on the first plate (the one closest to the observer) hit by this
        ray is the sub-point.

      - The 'ellipsoid near point' method. When a target's surface is
        modeled by a set of triangular plates, the notion of "dropping
        a perpendicular segment to the surface," which makes sense for
        convex surfaces, becomes problematic: there need not be any
        plate whose normal vector is parallel to a segment from the Sun
        to some point on that plate, or there could be more than one
        such plate. If such a plate exists, it might be located
        anywhere on the visible surface---not necessarily "below" the
        Sun.

        To work around these problems, the ellipsoid near point method
        uses a reference ellipsoid to define a preliminary sub-solar
        point: this is the unique point on the ellipsoid's surface at
        which the outward surface normal points toward the Sun. Then
        the plate model sub-solar point is defined as the plate
        intercept closest to the Sun of a ray emanating from the Sun
        and passing through the preliminary sub-solar point on the
        ellipsoid.

   For a large target such as Mars, or for any target whose reference
   ellipsoid deviates significantly from spherical, the results
   obtained using the two sub-point definitions can be quite different.
   The example program provided below demonstrates this fact; Phobos is
   the target body in this case. Some analysis on the user's part will
   be needed to select the "best" definition for a given application.

   When comparing sub-solar point computations with results from
   sources other than SPICE, it's essential to make sure the same
   geometric definitions are used.


   Aberration Corrections
   ======================

   Below, we indicate the aberration corrections to use for some
   common applications:

      1) Compute the sub-solar point using the apparent direction
         and orientation of a target. This is the most common case for
         a remote-sensing observation. When the observer's altitude
         is more than one target radius above the surface:

            Use 'LT+S': apply both light time and stellar
            aberration corrections.

         Note that when the observer is close to the target surface,
         this choice may yield inaccurate results, since light time is
         measured between the observer and the target center. When the
         observer has altitude of less than one target radius above the
         surface, aberration corrections should be omitted, so in this
         case abcorr should be set to:

            'NONE'

         Note that this selection calls for using the geometric position
         of the Sun.


      2) Use a geometric position vector and uncorrected target
         orientation as low-accuracy estimates for an application where
         execution speed is critical.

            Use 'NONE'.

   See the header of the CSPICE routine spkezr_c for a detailed
   discussion of aberration corrections.

Exceptions


   If any of the listed errors occur, the output arguments are
   left unchanged.

   1)  If the input argument `method' is not recognized, the error
       SPICE(DUBIOUSMETHOD) is signaled by a routine in the call
       tree of this routine.

   2)  If either of the input body names `target' or `obsrvr' cannot be
       mapped to NAIF integer codes, the error SPICE(IDCODENOTFOUND)
       is signaled by a routine in the call tree of this routine.

   3)  If `obsrvr' and `target' map to the same NAIF integer ID codes, the
       error SPICE(BODIESNOTDISTINCT) is signaled by a routine in the call
       tree of this routine.

   4)  If frame definition data enabling the evaluation of the state
       of the target relative to the observer in target body-fixed
       coordinates have not been loaded prior to calling cspice_subsol_pl02,
       an error is signaled by a routine in the call tree of this
       routine.

   5)  If the specified aberration correction is not recognized, an
       error is signaled by a routine in the call tree of this
       routine.

   6)  If insufficient ephemeris data have been loaded prior to
       calling cspice_subsol_pl02, an error is signaled by a
       routine in the call tree of this routine.

   7)  If a DSK providing a DSK type 2 plate model has not been
       loaded prior to calling cspice_subsol_pl02, an error is signaled by a
       routine in the call tree of this routine.

   8)  If the computation method is "near point" and radii of the
       target body have not been loaded into the kernel pool, an
       error is signaled by a routine in the call tree of this
       routine.

   9)  If PCK data supplying a rotation model for the target body
       have not been loaded prior to calling cspice_subsol_pl02, an error is
       signaled by a routine in the call tree of this routine.

   10) If the segment associated with the input DLA descriptor does not
       contain data for the designated target, the error
       SPICE(TARGETMISMATCH) is signaled by a routine in the call tree
       of this routine.

   11) If the segment associated with the input DLA descriptor is not
       of data type 2, the error SPICE(WRONGDATATYPE) is signaled by
       a routine in the call tree of this routine.

   12) If the sub-solar point cannot be computed because the ray from
       the observer to the aim point designated by `method' fails to
       intersect the target surface as defined by the plate model, the
       error SPICE(NOINTERCEPT) is signaled by a routine in the call
       tree of this routine.

   13) Use of transmission-style aberration corrections is not
       permitted. If abcorr specified such a correction, the
       error SPICE(NOTSUPPORTED) is signaled by a routine in the call
       tree of this routine.

   14) The observer is presumed to be outside the target body; no
       checks are made to verify this.

   15) If any of the input arguments, `handle', `dladsc', `method', `target'
       `et', `abcorr' or `obsrvr', is undefined, an error is signaled by the
       Matlab error handling system.

   16) If any of the input arguments, `handle', `dladsc', `method', `target'
       `et', `abcorr' or `obsrvr', is not of the expected type, or it does
       not have the expected dimensions and size, an error is signaled by
       the Mice interface.

Files


   Appropriate DSK, SPK, PCK, and frame data must be available to
   the calling program before this routine is called. Typically
   the data are made available by loading kernels; however the
   data may be supplied via subroutine interfaces if applicable.

   The following data are required:

   -  DSK data:  a DSK file containing a plate model representing the
      target body's surface must be loaded. This kernel must contain
      a type 2 segment that contains data for the entire surface of
      the target body.

   -  SPK data: ephemeris data for target, observer, and Sun must be
      loaded. If aberration corrections are used, the states of
      target and observer relative to the solar system barycenter
      must be calculable from the available ephemeris data. Typically
      ephemeris data are made available by loading one or more SPK
      files via cspice_furnsh.

   -  PCK data: triaxial radii for the target body must be loaded
      into the kernel pool if the "Near Point" method is selected.
      Typically these data are made available 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.
      Either type of file may be loaded via cspice_furnsh.

   -  Frame data: if a frame definition 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 all cases, kernel data are normally loaded once per program
   run, NOT every time this routine is called.

Restrictions


   1)  This routine assumes that the origin of the body-fixed reference
       frame associated with the target body is located in the interior
       of that body.

Required_Reading


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

Literature_References


   None.

Author_and_Institution


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

Version


   -Mice Version 1.1.0, 26-OCT-2021 (EDW) (JDR)

       Edited the header to comply with NAIF standard. Changed the output
       argument name "plateID" to "pltid" for consistency with other
       functions.

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

       Eliminated use of "lasterror" in rethrow.

       Removed reference to the function's corresponding CSPICE header from
       -Required_Reading section.

       Index lines now state that this routine is deprecated.

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

Index_Entries


   DEPRECATED sub-solar point using plate_model
   DEPRECATED sub-solar point using type_2 DSK


Fri Dec 31 18:44:27 2021