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ckgpav_c
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Procedure
Abstract
Required_Reading
Keywords
Brief_I/O
Detailed_Input
Detailed_Output
Parameters
Exceptions
Files
Particulars
Examples
Restrictions
Literature_References
Author_and_Institution
Version
Index_Entries

Procedure

   void ckgpav_c ( SpiceInt            inst, 
                   SpiceDouble         sclkdp, 
                   SpiceDouble         tol, 
                   ConstSpiceChar    * ref, 
                   SpiceDouble         cmat[3][3], 
                   SpiceDouble         av[3],
                   SpiceDouble       * clkout,  
                   SpiceBoolean      * found      ) 

Abstract

 
   Get pointing (attitude) and angular velocity for a specified 
   spacecraft clock time.
 

Required_Reading

 
   CK 
   SCLK
 

Keywords

 
   POINTING 
 

Brief_I/O

 
   Variable  I/O  Description 
   --------  ---  -------------------------------------------------- 
   inst       I   NAIF ID of instrument, spacecraft, or structure.
   sclkdp     I   Encoded spacecraft clock time. 
   tol        I   Time tolerance. 
   ref        I   Reference frame. 
   cmat       O   C-matrix pointing data. 
   av         O   Angular velocity vector.
   clkout     O   Output encoded spacecraft clock time. 
   found      O   True when requested pointing is available. 
 

Detailed_Input

 
   inst       is the NAIF integer ID for the instrument, spacecraft, or
              other structure for which pointing and angular velocity
              are requested. For brevity we will refer to this object
              as the "instrument," and the frame fixed to this object
              as the "instrument frame" or "instrument-fixed" frame.
 
   sclkdp     is the encoded spacecraft clock time for which 
              pointing and angular velocity are requested.  

              The CSPICE routines scencd_c and sce2c_c respectively
              convert spacecraft clock strings and ephemeris time
              to encoded spacecraft clock.  The inverse conversions
              are performed by scdecd_c and sct2e_c.
 
   tol        is a time tolerance in ticks, the units of encoded 
              spacecraft clock time.  

              The CSPICE routine sctiks_c converts a spacecraft clock
              tolerance duration from its character string
              representation to ticks.  scfmt_c performs the inverse
              conversion.
 
              The C-matrix - angular velocity vector pair returned by
              ckgpav_c is the one whose time tag is closest to `sclkdp'
              and within `tol' units of `sclkdp'.  (More in
              Particulars, below.)
 
              In general, because using a non-zero tolerance 
              affects selection of the segment from which the
              data is obtained, users are strongly discouraged 
              from using a non-zero tolerance when reading CKs 
              with continuous data. Using a non-zero tolerance
              should be reserved exclusively to reading CKs with 
              discrete data because in practice obtaining data 
              from such CKs using a zero tolerance is often not 
              possible due to time round off. 

   ref        is the desired reference frame for the returned pointing
              and angular velocity. The returned C-matrix `cmat' gives
              the orientation of the instrument designated by `inst'
              relative to the frame designated by `ref'.  When a vector
              specified relative to frame `ref' is left-multiplied by
              `cmat', the vector is rotated to the frame associated
              with `inst'.  The returned angular velocity vector `av'
              expresses the angular velocity of the instrument
              designated by `inst' relative to the frame designated by
              `ref'.  See the discussion of `cmat' and `av' below 
              for details.

              Consult the SPICE document "Frames" for a discussion 
              of supported reference frames. 
 

Detailed_Output

 
   cmat       is a rotation matrix that transforms the components of a
              vector expressed in the frame specified by `ref' to
              components expressed in the frame tied to the instrument,
              spacecraft, or other structure at time `clkout' (see
              below). 

              Thus, if a vector v has components x,y,z in the `ref'
              reference frame, then v has components x',y',z' in the 
              instrument fixed frame at time `clkout': 
 
                   [ x' ]     [          ] [ x ] 
                   | y' |  =  |   cmat   | | y | 
                   [ z' ]     [          ] [ z ] 
 
              If you know x', y', z', use the transpose of the 
              C-matrix to determine x, y, z as follows: 
 
                   [ x ]      [          ]T    [ x' ] 
                   | y |  =   |   cmat   |     | y' | 
                   [ z ]      [          ]     [ z' ] 
                           (Transpose of cmat) 
 

   av         is the angular velocity vector.  This is the axis about
              which the reference frame tied to the instrument is
              rotating in the right-handed sense at time `clkout'. The
              magnitude of `av' is the magnitude of the instantaneous
              velocity of the rotation, in radians per second. 
              The components of `av' are given relative to the
              reference frame specified by the input argument `ref'.

   clkout     is the encoded spacecraft clock time associated with
              the returned C-matrix and the returned angular
              velocity vector. This value may differ from the
              requested time, but never by more than the input
              tolerance `tol'.
 
              The particulars section below describes the search 
              algorithm used by ckgpav_c to satisfy a pointing request. 
              This algorithm determines the pointing instance 
              (and therefore the associated time value) that is 
              returned. 
 
   found      is SPICETRUE if a record was found to satisfy the
              pointing request.  `found' will be SPICEFALSE otherwise.
 

Parameters

 
   None. 
 

Exceptions

 
   1)  If a C-kernel file has not been loaded using furnsh_c prior to a
       call to this routine, an error is signaled by a routine in the
       call tree of this routine.
 
   2)  If `tol' is negative, found is set to SPICEFALSE. 
 
   3)  If `ref' is not a supported reference frame, an error is
       signaled by a routine in the call tree of this routine and
       `found' is set to SPICEFALSE.
 

Files

 
   ckgpav_c searches through files loaded by furnsh_c to locate a
   segment that satisfies the request for pointing and angular velocity
   for the instrument `inst' at time `sclkdp'. You must load at least
   one C-kernel file via furnsh_c prior to calling this routine.
 

Particulars

 
   How the tolerance argument is used
   ==================================


   Reading a type 1 CK segment (discrete pointing instances)
   ---------------------------------------------------------

   In the diagram below
  
      - "0" is used to represent discrete pointing instances 
        (quaternions, angular velocity vectors, and associated 
        time tags).

      - "( )" are used to represent the end points of the time
        interval covered by a segment in a CK file.
      
      - `sclkdp' is the time at which you requested pointing.
        The location of `sclkdp' relative to the time tags of the
        pointing instances is indicated by the "+" sign.

      - `tol' is the time tolerance specified in the pointing
        request.  The square brackets "[ ]" represent the 
        endpoints of the time interval 

           sclkdp-tol : sclkdp+tol

      - The quaternions occurring in the segment need not be
        evenly spaced in time.


   Case 1:  pointing is available
   ------------------------------

                            sclkdp 
                                 \   tol 
                                  | / 
                                  |/\ 
   Your request                [--+--] 
                               .  .  . 
   Segment      (0-----0--0--0--0--0--0---0--0------------0--0--0--0) 
                                   ^ 
                                   | 
                       ckgpav_c returns this instance.


   Case 2:  pointing is not available
   ----------------------------------

                                                 sclkdp 
                                                    \   tol 
                                                     | / 
                                                     |/\ 
   Your request                                   [--+--] 
                                                  .  .  . 
   Segment      (0-----0--0--0--0--0--0---0--0--0---------0--0--0--0) 

                      
                       ckgpav_c returns no pointing; the output 
                       `found' flag is set to SPICEFALSE.



   Reading a type 2, 3, 4, or 5 CK segment (continuous pointing)
   -------------------------------------------------------------

   In the diagrams below
  
      - "==" is used to represent periods of continuous pointing. 

      - "--" is used to represent gaps in the pointing coverage. 

      - "( )" are used to represent the end points of the time
        interval covered by a segment in a CK file.
      
      - `sclkdp' is the time at which you requested pointing.
        The location of `sclkdp' relative to the time tags of the
        pointing instances is indicated by the "+" sign.

      - `tol' is the time tolerance specified in the pointing
        request.  The square brackets "[ ]" represent the 
        endpoints of the time interval 

           sclkdp-tol : sclkdp+tol

      - The quaternions occurring in the periods of continuous
        pointing need not be evenly spaced in time.


   Case 1:  pointing is available at the request time
   --------------------------------------------------
 
                           sclkdp 
                                 \   tol 
                                  | / 
                                  |/\ 
   Your request                [--+--] 
                               .  .  . 
                               .  .  . 
                               .  .  . 
   Segment            (==---===========---=======----------===--) 
                                  ^ 
                                  | 

                 The request time lies within an interval where
                 continuous pointing is available. ckgpav_c returns
                 pointing at the requested epoch.


   Case 2:  pointing is available "near" the request time
   ------------------------------------------------------
 
                                  sclkdp 
                                        \   tol 
                                         | / 
                                         |/\ 
   Your request                       [--+--] 
                                      .  .  . 
   Segment            (==---===========----=======---------===--) 
                                           ^ 
                                           | 

                 The request time lies in a gap:  an interval where
                 continuous pointing is *not* available.  ckgpav_c 
                 returns pointing for the epoch closest to the 
                 request time `sclkdp'.

 
   Case 3:  pointing is not available
   ----------------------------------
 
                                               sclkdp 
                                                     \   tol 
                                                      | / 
                                                      |/\ 
   Your request                                    [--+--] 
                                                   .  .  . 
   Segment            (==---===========----=======---------===--) 

                       ckgpav_c returns no pointing; the output 
                       `found' flag is set to SPICEFALSE.



   Tolerance and segment priority
   ==============================

   ckgpav_c searches through loaded C-kernels to satisfy a pointing
   request. Last-loaded files are searched first. Individual files are
   searched in backwards order, so that between competing segments
   (segments containing data for the same object, for overlapping time
   ranges), the one closest to the end of the file has highest
   priority. ckgpav_c considers only those segments that contain both
   pointing and angular velocity data, as indicated by the segment
   descriptor.
 
   The search ends when a segment is found that can provide pointing
   and angular velocity for the specified instrument at a time
   falling within the specified tolerance on either side of the
   request time. Within that segment, the instance closest to the       
   input time is located and returned.
 
   The following four cases illustrate this search procedure. Segments 
   A and B are in the same file, with segment A located further 
   towards the end of the file than segment B. Both segments A and B 
   contain discrete pointing data, indicated by the number 0.
 

   Case 1:  Pointing is available in the first segment searched.
            Because segment A has the highest priority and can
            satisfy the request, segment B is not searched.
            
 
                                sclkdp 
                                      \  tol 
                                       | / 
                                       |/\ 
   Your request                     [--+--] 
                                    .  .  . 
   Segment A          (0-----------------0--------0--0-----0) 
                                         ^ 
                                         | 
                                         | 
                             ckgpav_c returns this instance 
 
   Segment B     (0--0--0--0--0--0--0--0--0--0--0--0--0--0--0--0--0)   



   Case 2:  Pointing is not available in the first segment searched.
            Because segment A cannot satisfy the request, segment B 
            is searched.


                           sclkdp 
                                \   tol 
                                 | / 
                                 |/\ 
   Your request               [--+--] 
                              .  .  . 
   Segment A          (0-----------------0--------0--0-----0) 
                              .  .  . 
   Segment B     (0--0--0--0--0--0--0--0--0--0--0--0--0--0--0--0--0) 
                                 ^ 
                                 | 
                     ckgpav_c returns this instance 
  
 
   Segments that contain continuous pointing data are searched in the
   same manner as segments containing discrete pointing data. For
   request times that fall within the bounds of continuous intervals,
   ckgpav_c will return pointing at the request time. When the request
   time does not fall within an interval, then a time at an endpoint of
   an interval may be returned if it is the closest time in the segment
   to the user request time and is also within the tolerance.
 
   In the following examples, segment A is located further towards the
   end of the file than segment C. Segment A contains discrete pointing
   data and segment C contains continuous data, indicated by the "="
   character.
 

   Case 3:  Pointing is not available in the first segment searched.
            Because segment A cannot satisfy the request, segment C
            is searched.

                           sclkdp 
                                 \  tol 
                                  | / 
                                  |/\ 
   Your request                [--+--] 
                               .  .  . 
                               .  .  . 
   Segment A          (0-----------------0--------0--0-----0) 
                               .  .  . 
                               .  .  . 
   Segment C          (---=============-----====--------==--) 
                                  ^ 
                                  | 
                                  | 
                       ckgpav_c returns this instance 
 
 
   In the next case, assume that the order of segments A and C in the
   file is reversed:  A is now closer to the front, so data from
   segment C are considered first.


   Case 4:  Pointing is available in the first segment searched.
            Because segment C has the highest priority and can
            satisfy the request, segment A is not searched.

                                           sclkdp
                                          /
                                         |  tol
                                         | /
                                         |/\
   Your request                       [--+--]
                                      .  .  .
                                      .  .  .
   Segment C          (---=============-----====--------==--)
                                           ^
                                           |
                              ckgpav_c returns this instance

   Segment A          (0-----------------0--------0--0-----0)
                                         ^
                                         |
                                   "Best" answer


   The next case illustrates an unfortunate side effect of using 
   a non-zero tolerance when reading multi-segment CKs with
   continuous data. In all cases when the look-up interval 
   formed using tolerance overlaps a segment boundary and 
   the request time falls within the coverage of the lower 
   priority segment, the data at the end of the higher priority
   segment will be picked instead of the data from the lower 
   priority segment.


   Case 5:  Pointing is available in the first segment searched.
            Because segment C has the highest priority and can
            satisfy the request, segment A is not searched.

                                           sclkdp
                                          /
                                         |  tol
                                         | /
                                         |/\
   Your request                       [--+--]
                                      .  .  .
                                      .  .  .
   Segment C                                (===============)
                                            ^
                                            |
                              ckgpav_c returns this instance

   Segment A          (=====================)
                                         ^
                                         |
                                   "Best" answer

 

Examples

 
 
   Suppose you have two C-kernel files containing pointing for the
   Voyager 2 narrow angle camera.  One file contains predict (planned)
   values, and the other contains corrected pointing for a selected
   group of images, that is, for a subset of images from the first
   file.
 
   The following example program uses ckgpav_c to get C-matrices and
   associated angular velocity vectors for a set of images whose
   SCLK counts (un-encoded character string versions) are contained
   in the array `sclkch'.

   If available, the program will get the corrected pointing values.
   Otherwise, predict values will be used.

   For each C-matrix, a unit pointing vector is constructed and printed
   along with the angular velocity vector.
 
   Note: if the C-kernels of interest do not contain angular velocity
   data, then the CSPICE routine ckgp_c should be used to read the
   pointing data.  An example program in the header of the CSPICE
   routine ckgp_c demonstrates this.


      #include <stdio.h>
      #include "SpiceUsr.h"
      
      int main ()
      {
         /.
         Constants for this program:
      
         -- The code for the Voyager 2 spacecraft clock is -32
      
         -- The code for the narrow angle camera on the Voyager 2
            spacecraft is -32001.
      
        --  Spacecraft clock times for successive Voyager images always
            differ by more than 0:0:400.  This is an acceptable
            tolerance, and must be converted to "ticks" (units of
            encoded SCLK) for input to ckgpav_c.
      
         -- The reference frame we want is FK4.
      
         -- The narrow angle camera boresight defines the third
            axis of the instrument-fixed reference frame.
            Therefore, the vector ( 0, 0, 1 ) represents
            the boresight direction in the camera-fixed frame.
         ./
         
         #define   SC        -32
         #define   INST      -32001
         #define   REF       "FK4"
         #define   TOLVGR    "0:0:400"
         #define   NPICS     2
         #define   MAXCLK    30
         #define   CKPRED    "voyager2_predict.bc"
         #define   CKCORR    "voyager2_corrected.bc"
         #define   SCLK      "voyager2_sclk.tsc"
         
      
         SpiceBoolean            found;
      
         SpiceChar               sclkch  [NPICS][MAXCLK] = 
         
                                         { { "4/08966:30:768" },
                                           { "4/08970:58:768" } };
                                           
         SpiceChar               clkch   [MAXCLK];
      
         SpiceDouble             av      [3];
         SpiceDouble             cmat    [3][3];
         SpiceDouble             clkout;
         SpiceDouble             sclkdp;
         SpiceDouble             toltik;
         SpiceDouble             vinert  [3];
      
         SpiceInt                i;
      
      
         /.
         Loading the files in this order ensures that the 
         corrected file will get searched first. 
         ./
         furnsh_c ( CKPRED ); 
         furnsh_c ( CKCORR ); 
       
         /.
         Need to load a Voyager 2 SCLK kernel to convert from 
         clock string to ticks.  Although not required for
         the Voyager spacecraft clocks, most modern spacecraft
         clocks require a leapseconds kernel to be loaded in
         addition to an SCLK kernel.
         ./
         furnsh_c ( SCLK );
          
         /.
         Convert tolerance from VGR formatted character string 
         SCLK to ticks, which are units of encoded SCLK. 
         ./
         sctiks_c ( SC, TOLVGR, &toltik ); 
      
         for ( i = 0;  i < NPICS;  i++ )
         {
         
            /.
            ckgpav_c requires encoded spacecraft clock time. 
            ./
            scencd_c ( SC, sclkch[ i ], &sclkdp ); 
      
            ckgpav_c ( INST,  sclkdp,  toltik,   REF, 
                       cmat,  av,      &clkout,  &found ); 
      
            if ( found )
            { 
               /.
               The boresight vector, relative to inertial coordinates, 
               is just the third row of the C-matrix. 
               ./
               vequ_c   ( cmat[2], vinert );

               scdecd_c ( SC, clkout, MAXCLK, clkch  ); 
      

               printf ( "VGR 2 SCLK time: %s\n", clkch  );

               printf ( "VGR 2 NA ISS boresight pointing vector: "
                        "%f %f %f\n", 
                        vinert[0], 
                        vinert[1], 
                        vinert[2]                               ); 

               printf ( "Angular velocity vector: %f %f %f\n", 
                                                  av[0], 
                                                  av[1], 
                                                  av[2]         ); 
            }
            else
            {
               printf ( "Pointing not found for time %s\n", sclkch[i] ); 
            }
      
         }
         
         return ( 0 );
      }
 
 

Restrictions

 
   Only loaded C-kernel segments containing both pointing and angular
   velocity data will be searched by this reader.  Segments containing
   only pointing data will be skipped over.
 

Literature_References

 
   None. 
 

Author_and_Institution

 
   C.H. Acton     (JPL)
   N.J. Bachman   (JPL)
   W.L. Taber     (JPL) 
   J.M. Lynch     (JPL) 
   B.V. Semenov   (JPL)
   M.J. Spencer   (JPL) 
   R.E. Thurman   (JPL) 
   I.M. Underwood (JPL) 
 

Version

 
   -CSPICE Version 1.2.3, 03-JUN-2010 (BVS)  

      Header update: description of the tolerance and Particulars
      section were expanded to address some problems arising from
      using a non-zero tolerance.  

   -CSPICE Version 1.2.2, 29-JAN-2004 (NJB)  

      Header update:  the description of the input argument `ref'
      was expanded.

   -CSPICE Version 1.2.1, 27-JUL-2003 (CHA) (NJB)  

      Various header corrections were made.
   
   -CSPICE Version 1.2.0, 02-SEP-1999 (NJB)  
   
      Local type logical variable now used for found flag used in
      interface of ckgpav_.
            
   -CSPICE Version 1.1.0, 08-FEB-1998 (NJB)  
   
      References to C2F_CreateStr_Sig were removed; code was
      cleaned up accordingly.  String checks are now done using
      the macro CHKFSTR.
       
   -CSPICE Version 1.0.0, 25-OCT-1997 (NJB)
   
      Based on SPICELIB Version 5.0.0, 28-JUL-1997 (WLT)

Index_Entries

 
   get ck pointing and angular velocity 
 
Wed Apr  5 17:54:29 2017