Index of Functions: A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X 
Index Page

Table of contents


   dskxsi_c (DSK, ray-surface intercept with source information) 

   void dskxsi_c ( SpiceBoolean         pri,
                   ConstSpiceChar     * target,
                   SpiceInt             nsurf,
                   ConstSpiceInt        srflst [],
                   SpiceDouble          et,
                   ConstSpiceChar     * fixref,
                   ConstSpiceDouble     vertex [3],
                   ConstSpiceDouble     raydir [3],
                   SpiceInt             maxd,
                   SpiceInt             maxi,
                   SpiceDouble          xpt    [3],
                   SpiceInt           * handle,
                   SpiceDLADescr      * dladsc,
                   SpiceDSKDescr      * dskdsc,
                   SpiceDouble          dc     [],
                   SpiceInt             ic     [],
                   SpiceBoolean       * found      )


   Compute a ray-surface intercept using data provided by
   multiple loaded DSK segments. Return information about
   the source of the data defining the surface on which the
   intercept was found: DSK handle, DLA and DSK descriptors,
   and DSK data type-dependent parameters.






   --------  ---  --------------------------------------------------
   pri        I   Data prioritization flag.
   target     I   Target body name.
   nsurf      I   Number of surface IDs in list.
   srflst     I   Surface ID list.
   et         I   Epoch, expressed as seconds past J2000 TDB.
   fixref     I   Name of target body-fixed reference frame.
   vertex     I   Vertex of ray.
   raydir     I   Direction vector of ray.
   maxd       I   Size of DC array.
   maxi       I   Size of IC array.
   xpt        O   Intercept point.
   handle     O   Handle of segment contributing surface data.
   dladsc     O   DLA descriptor of segment.
   dskdsc     O   DSK descriptor of segment.
   dc         O   Double precision component of source info.
   ic         O   Integer component of source info.
   found      O   Found flag.
              P    Required size of DC array.
              P    Required size of IC array.


   pri         is a logical flag indicating whether to perform a
               prioritized or unprioritized DSK segment search. In an
               unprioritized search, no segment masks another: data from
               all specified segments are used to define the surface of

               The search is unprioritized if and only if `pri' is set to
               SPICEFALSE. In the N0066 SPICE Toolkit, this is the only
               allowed value.

   target      is the name of the target body on which a surface
               intercept is sought.

   srflst      are, respectively, a count of surface ID codes in a list
               and an array containing the list. Only DSK segments for
               the body designated by `target' and having surface IDs in
               this list will be considered in the intercept
               computation. If the list is empty, all DSK segments for
               `target' will be considered.

   et          is the epoch of the intersection computation, expressed
               as seconds past J2000 TDB. This epoch is used only for
               DSK segment selection. Segments used in the intercept
               computation must include `et' in their time coverage

   fixref      is the name of a body-fixed, body-centered reference
               frame associated with the target. The input ray vectors
               are specified in this frame, as is the output intercept

               The frame designated by `fixref' must have a fixed
               orientation relative to the frame of any DSK segment used
               in the computation.

   raydir      are, respectively, the vertex and direction vector of the
               ray to be used in the intercept computation.

               Both the vertex and ray's direction vector must be
               represented in the reference frame designated by `fixref'.
               The vertex is considered to be an offset from the target

   maxi        are, respectively, the declared sizes of the arrays `dc' and
               `ic'. `maxd' must be at least SPICE_DSKXSI_DCSIZE, while `maxi'
               must be at least SPICE_DSKXSI_ICSIZE. See the -Parameters
               section for details.


   xpt         is the intercept of the input ray on the surface
               specified by the inputs


               if such an intercept exists. If the ray intersects the
               surface at multiple points, the one closest to the ray's
               vertex is selected.

               `xpt' is defined if and only if `found' is SPICETRUE.

               Units are km.

   dskdsc      are, respectively, the DSK file handle, DLA descriptor,
               and DSK descriptor of the DSK file and segment that
               contributed the surface data on which the intercept was

               These outputs are defined if and only if `found' is SPICETRUE.

   ic          are, respectively, double precision and integer arrays
               that may contain additional information associated with
               the segment contributing the surface data on which the
               intercept was found. The information is DSK data

                  For DSK type 2 segments

                     ic[0] is the intercept plate ID. `dc' is unused.

               These outputs are defined if and only if `found' is

               The declared length of `dc' must be at least
               SPICE_DSKXSI_DCSIZE; the declared length of `ic' must be at
               least SPICE_DSKXSI_ICSIZE. See the -Parameters section for

   found       is a logical flag that is set to SPICETRUE if and only if
               and intercept was found.


   See the header file


   for declarations of size parameters for the output arguments


   See the header files


   for declarations of DLA and DSK descriptor sizes and
   documentation of the contents of these descriptors.

   See the header file


   for the values of tolerance parameters used by default by the
   ray-surface intercept algorithm. These are discussed in the
   -Particulars section below.


   1)  If the input prioritization flag `pri' is set to SPICETRUE, the
       error SPICE(BADPRIORITYSPEC) is signaled by a routine in the
       call tree of this routine.

   2)  If the input body name `target' cannot be mapped to an ID code,
       the error SPICE(IDCODENOTFOUND) is signaled by a routine in
       the call tree of this routine.

   3)  If the input frame name `fixref' cannot be mapped to an ID code,
       the error SPICE(IDCODENOTFOUND) is signaled by a routine in
       the call tree of this routine.

   4)  If the frame center associated with `fixref' cannot be
       retrieved, the error SPICE(NOFRAMEINFO) is signaled by a
       routine in the call tree of this routine.

   5)  If the frame center associated with `fixref' is not the target
       body, the error SPICE(INVALIDFRAME) is signaled by a routine
       in the call tree of this routine.

   6)  If `maxd' is less than SPICE_DSKXSI_DCSIZE or `maxi' is less
       signaled by a routine in the call tree of this routine.

   7)  If `nsurf' is less than 0, the error SPICE(INVALIDCOUNT)
       is signaled by a routine in the call tree of this routine.

   8)  If an error occurs during the intercept computation, the error
       is signaled by a routine in the call tree of this routine.

   9)  If any of the `target' or `fixref' input string pointers is
       null, the error SPICE(NULLPOINTER) is signaled.

   10) If any of the `target' or `fixref' input strings has zero
       length, the error SPICE(EMPTYSTRING) is signaled.


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

   The following data are required:

   -  SPK data: ephemeris data for the positions of the centers
      of DSK reference frames relative to the target body are
      required if those frames are not centered at the target
      body center.

      Typically ephemeris data are made available by loading one
      or more SPK files via furnsh_c.

   -  DSK data: DSK files containing topographic data for the
      target body must be loaded. If a surface list is specified,
      data for at least one of the listed surfaces must be loaded.

   -  Frame data: if a frame definition is required to convert
      DSK segment data to the body-fixed frame designated by
      `fixref', the target, that definition must be available in the
      kernel pool. Typically the definitions of frames not already
      built-in to SPICE are supplied by loading a frame kernel.

   -  CK data: if the frame to which `fixref' refers is a CK frame,
      and if any DSK segments used in the computation have a
      different frame, at least one CK file will be needed to
      permit transformation of vectors between that frame and both
      the J2000 and the target body-fixed frames.

   -  SCLK data: if a CK file is needed, an associated SCLK
      kernel is required to enable conversion between encoded SCLK
      (used to time-tag CK data) and barycentric dynamical time

   In all cases, kernel data are normally loaded once per program
   run, NOT every time this routine is called.


   This is the lowest-level public interface for computing
   ray-surface intercepts, where the surface is modeled using
   topographic data provided by DSK files. The highest-level
   interface for this purpose is sincpt_c.

   In cases where the data source information returned by this
   routine are not needed, the routine dskxv_c may be more suitable.

   This routine works with multiple DSK files. It places no
   restrictions on the data types or coordinate systems of the DSK
   segments used in the computation. DSK segments using different
   reference frames may be used in a single computation. The only
   restriction is that any pair of reference frames used directly or
   indirectly are related by a constant rotation.

   This routine enables calling applications to identify the source of
   the data defining the surface on which an intercept was found. The
   file, segment, and segment-specific information such as a DSK type 2
   plate ID are returned.

   This routine can be used for improved efficiency in situations
   in which multiple ray-surface intercepts are to be performed
   using a constant ray vertex.

   Using DSK data

      DSK loading and unloading

      DSK files providing data used by this routine are loaded by
      calling furnsh_c and can be unloaded by calling unload_c or
      kclear_c. See the documentation of furnsh_c for limits on numbers
      of loaded DSK files.

      For run-time efficiency, it's desirable to avoid frequent
      loading and unloading of DSK files. When there is a reason to
      use multiple versions of data for a given target body---for
      example, if topographic data at varying resolutions are to be
      used---the surface list can be used to select DSK data to be
      used for a given computation. It is not necessary to unload
      the data that are not to be used. This recommendation presumes
      that DSKs containing different versions of surface data for a
      given body have different surface ID codes.

      DSK data priority

      A DSK coverage overlap occurs when two segments in loaded DSK
      files cover part or all of the same domain---for example, a
      given longitude-latitude rectangle---and when the time
      intervals of the segments overlap as well.

      When DSK data selection is prioritized, in case of a coverage
      overlap, if the two competing segments are in different DSK
      files, the segment in the DSK file loaded last takes
      precedence. If the two segments are in the same file, the
      segment located closer to the end of the file takes

      When DSK data selection is unprioritized, data from competing
      segments are combined. For example, if two competing segments
      both represent a surface as sets of triangular plates, the
      union of those sets of plates is considered to represent the

      Currently only unprioritized data selection is supported.
      Because prioritized data selection may be the default behavior
      in a later version of the routine, the presence of the `pri'
      argument is required.

      Round-off errors and mitigating algorithms

      When topographic data are used to represent the surface of a
      target body, round-off errors can produce some results that
      may seem surprising.

      Note that, since the surface in question might have mountains,
      valleys, and cliffs, the points of intersection found for
      nearly identical sets of inputs may be quite far apart from
      each other: for example, a ray that hits a mountain side in a
      nearly tangent fashion may, on a different host computer, be
      found to miss the mountain and hit a valley floor much farther
      from the observer, or even miss the target altogether.

      Round-off errors can affect segment selection: for example, a
      ray that is expected to intersect the target body's surface
      near the boundary between two segments might hit either
      segment, or neither of them; the result may be

      A similar situation exists when a surface is modeled by a set
      of triangular plates, and the ray is expected to intersect the
      surface near a plate boundary.

      To avoid having the routine fail to find an intersection when
      one clearly should exist, this routine uses two "greedy"

         1) If the ray passes sufficiently close to any of the
            boundary surfaces of a segment (for example, surfaces of
            maximum and minimum longitude or latitude), that segment
            is tested for an intersection of the ray with the
            surface represented by the segment's data.

            This choice prevents all of the segments from being
            missed when at least one should be hit, but it could, on
            rare occasions, cause an intersection to be found in a
            segment other than the one that would be found if higher
            precision arithmetic were used.

         2) For type 2 segments, which represent surfaces as
            sets of triangular plates, each plate is expanded very
            slightly before a ray-plate intersection test is
            performed. The default plate expansion factor is

               1 + XFRACT

            where XFRACT is declared in


            For example, given a value for XFRACT of 1.e-10, the
            sides of the plate are lengthened by 1/10 of a micron
            per km. The expansion keeps the centroid of the plate

            Plate expansion prevents all plates from being missed
            in cases where clearly at least one should be hit.

            As with the greedy segment selection algorithm, plate
            expansion can occasionally cause an intercept to be
            found on a different plate than would be found if higher
            precision arithmetic were used. It also can occasionally
            cause an intersection to be found when the ray misses
            the target by a very small distance.


   The numerical results shown for this example may differ across
   platforms. The results depend on the SPICE kernels used as
   input, the compiler and supporting libraries, and the machine
   specific arithmetic implementation.

   1) Compute surface intercepts of rays emanating from a set of
      vertices distributed on a longitude-latitude grid. All
      vertices are outside the target body, and all rays point
      toward the target's center.

      Check intercepts against expected values. Indicate the
      number of errors, the number of computations, and the
      number of intercepts found.

      Use the meta-kernel shown below to load example SPICE



         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
            ---------                        --------
            phobos512.bds                    DSK based on
                                             Gaskell ICQ Q=512
                                             plate model

            KERNELS_TO_LOAD = ( 'phobos512.bds' )


         End of meta-kernel

      Example code begins here.

         Program dskxsi_ex1

         Multi-segment spear program.

         This program expects all loaded DSKs
         to represent the same body and surface.
      #include <stdio.h>
      #include <stdlib.h>
      #include "SpiceUsr.h"

      int main( )
         Local constants
         #define  DTOL           1.0e-14
         #define  FILSIZ         256
         #define  FRNMLN         33
         #define  BDNMLN         37
         #define  TYPLEN         5
         #define  INTLEN         12
         #define  MAXN           100000
         #define  MAXSRF         100
         #define  MAXD           SPICE_DSKXSI_DCSIZE
         #define  MAXI           SPICE_DSKXSI_ICSIZE

         Local variables
         SpiceBoolean            found;

         SpiceChar               dsk1   [FILSIZ];
         SpiceChar               filtyp [TYPLEN];
         SpiceChar               fixref [FRNMLN];
         SpiceChar               meta   [FILSIZ];
         SpiceChar               source [FILSIZ];
         SpiceChar               target [BDNMLN];

         SpiceDLADescr           dladsc;
         SpiceDLADescr           xptDLAdsc;

         SpiceDSKDescr           dskdsc;
         SpiceDSKDescr           xptDSKdsc;

         SpiceDouble             d;
         SpiceDouble             dc    [1];
         static SpiceDouble      dirarr[MAXN][3];
         SpiceDouble             et;
         SpiceDouble             lat;
         SpiceDouble             latcrd[3];
         SpiceDouble             latstp;
         SpiceDouble             lon;
         SpiceDouble             lonstp;
         SpiceDouble             polmrg;
         SpiceDouble             r;
         SpiceDouble             radius;
         SpiceDouble             vlat;
         SpiceDouble             vlon;
         SpiceDouble             vrad;
         static SpiceDouble      vtxarr[MAXN][3];
         static SpiceDouble      xpt   [3];
         SpiceDouble             xyzhit[3];

         SpiceInt                bodyid;
         SpiceInt                framid;
         SpiceInt                handle;
         SpiceInt                i;
         SpiceInt                ic    [1];
         SpiceInt                nderr;
         SpiceInt                nhits;
         SpiceInt                nlstep;
         SpiceInt                nrays;
         SpiceInt                nsurf;
         static SpiceInt         srflst [MAXSRF];
         SpiceInt                surfid;
         SpiceInt                xpthan;

         chkin_c ( "spear" );

         Prompt for the name of the meta-kernel.
         prompt_c ( "Enter meta-kernel name >  ", FILSIZ, meta );

         Load the meta-kernel.
         furnsh_c ( meta );

         Get a handle for one of the loaded DSKs,
         then find the first segment and extract
         the body and surface IDs.
         kdata_c ( 0,    "DSK",  FILSIZ, TYPLEN,  FILSIZ,
                   dsk1, filtyp, source, &handle, &found );
         if ( !found )
            sigerr_c ( "SPICE(NOINFO)" );

         dlabfs_c ( handle, &dladsc, &found );

         if ( !found )
            sigerr_c ( "SPICE(NOSEGMENT)" );

         dskgd_c ( handle, &dladsc, &dskdsc );

         bodyid =;
         surfid = dskdsc.surfce;
         framid = dskdsc.frmcde;

         bodc2n_c ( bodyid, BDNMLN, target, &found );

         if ( !found )
            setmsg_c ( "Cannot map body ID # to a name." );
            errint_c ( "#", bodyid                       );
            sigerr_c ( "SPICE(BODYNAMENOTFOUND)"         );

         frmnam_c ( framid, FRNMLN, fixref );

         if ( eqstr_c( fixref, " " ) )
            setmsg_c ( "Cannot map frame ID # to a name." );
            errint_c ( "#", framid                        );
            sigerr_c ( "SPICE(BODYNAMENOTFOUND)"          );

         Set the magnitude of the ray vertices. Use a large
         number to ensure the vertices are outside of
         any realistic target.
         r = 1.0e10;

         Spear the target with rays pointing toward
         the origin.  Use a grid of ray vertices
         located on a sphere enclosing the target.

         The variable `polmrg' ("pole margin") can
         be set to a small positive value to reduce
         the number of intercepts done at the poles.
         This may speed up the computation for
         the multi-segment case, since rays parallel
         to the Z axis will cause all segments converging
         at the pole of interest to be tested for an

         polmrg =    0.5;
         latstp =    1.0;
         lonstp =    2.0;

         nhits  =    0;
         nderr  =    0;

         lon    = -180.0;
         lat    =   90.0;
         nlstep =    0;
         nrays  =    0;

         Set the epoch for interval selection.
         et     = 0.0;

         Generate rays.
         while ( lon < 180.0 )
            while ( nlstep <= 180 )
               if ( lon == 180.0 )
                  lat = 90.0 - nlstep*latstp;
                  if ( nlstep == 0 )
                     lat =  90.0 - polmrg;
                  else if ( nlstep == 180 )
                     lat = -90.0 + polmrg;
                     lat =  90.0 - nlstep*latstp;

               latrec_c ( r,             lon*rpd_c(),
                          lat*rpd_c(),   vtxarr[nrays] );

               vminus_c ( vtxarr[nrays], dirarr[nrays] );

               ++ nrays;
               ++ nlstep;

            lon   += lonstp;
            lat    = 90.0;
            nlstep = 0;

         Assign surface ID list.

         Note that, if we knew that all files had the desired
         surface ID, we could set `nsurf' to 0 and omit the
         initialization of the surface ID list.
         nsurf     = 1;
         srflst[0] = surfid;

         printf ( "\nComputing intercepts...\n" );

         for ( i = 0;  i < nrays;  i++ )
            Find the surface intercept of the ith ray.

            dskxsi_c ( SPICEFALSE, target,     nsurf,      srflst,
                       et,         fixref,     vtxarr[i],  dirarr[i],
                       MAXD,       MAXI,       xpt,        &xpthan,
                       &xptDLAdsc, &xptDSKdsc, dc,         ic,
                       &found                                       );

            if ( found )

               Record that a new intercept was found.
               ++ nhits;

               Check results.

               Compute the latitude and longitude of
               the intercept. Make sure these agree
               well with those of the vertex.
               reclat_c ( xpt, latcrd, latcrd+1, latcrd+2 );
               radius = latcrd[0];

               Recover the vertex longitude and latitude.
               reclat_c ( vtxarr[i], &vrad, &vlon, &vlat  );
               latrec_c ( radius,     vlon,  vlat, xyzhit );

               d = vdist_c( xpt, xyzhit );

               if ( d/r > DTOL )
                  printf ( "===========================\n" );
                  printf ( "Lon = %f;  Lat = %f\n",
                           lon, lat                        );
                  printf ( "Bad intercept\n"               );
                  printf ( "Distance error = %e\n", d      );
                  printf ( "xpt    = (%e %e %e)\n",
                           xpt[0], xpt[1], xpt[2] );
                  printf ( "xyzhit = (%e %e %e)\n",
                           xyzhit[0], xyzhit[1], xyzhit[2] );
                  Display the intercept segment's plate ID if
                  if ( xptDSKdsc.dtype == 2 )
                     printf ( "Plate ID = %d\n", (int)ic[0] );

                  ++ nderr;
               Missing the target entirely is a fatal error.

               This is true only for this program, not in
               general. For example, if the target shape is
               a torus, many rays would miss the target.
               printf ( "===========================\n" );
               printf ( "Lon = %f;  Lat = %f\n",
                        lon, lat                        );
               printf ( "No intercept\n"                );
               exit( 1 );
         printf ( "Done.\n\n" );

         printf( "nrays = %d\n", (int)nrays );
         printf( "nhits = %d\n", (int)nhits );
         printf( "nderr = %d\n", (int)nderr );

         return ( 0 );

      When this program was executed on a Mac/Intel/cc/64-bit
      platform, using as input the meta-kernel, the
      output was:

      Enter meta-kernel name >

      Computing intercepts...

      nrays = 32580
      nhits = 32580
      nderr = 0


   1)  The frame designated by `fixref' must have a fixed
       orientation relative to the frame of any DSK segment
       used in the computation. This routine has no
       practical way of ensuring that this condition is met;
       so this responsibility is delegated to the calling




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


   -CSPICE Version 1.0.1, 06-AUG-2021 (JDR)

       Edited the header to comply with NAIF standard.

       Updated code example to prompt for input meta-kernel name and
       set input time to zero.

       Added exception entry #7.

   -CSPICE Version 1.0.0, 04-APR-2017 (NJB)


   DSK ray-surface intercept with source information
   DSK ray-surface intercept with handle and descriptors
Fri Dec 31 18:41:05 2021