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Index Page ckgp_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

       ckgp_c ( C-kernel, get pointing )

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

#### Abstract

       Get pointing (attitude) 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.
       clkout     O   Output encoded spacecraft clock time.
       found      O   SPICETRUE when requested pointing is available.

#### Detailed_Input

       inst        is the NAIF integer ID for the instrument, spacecraft, or
                   other structure for which pointing is being 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 is being 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 returned by ckgp_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.
                   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'. See the
                   discussion of `cmat' 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:

                      .-   -.      .-        -.T  .-   -.
                      |  x  |      |          |   |  x' |
                      |  y  |  =   |   cmat   |   |  y' |
                      |  z  |      |          |   |  z' |
                      '-   -'      '-        -'   '-   -'

                             (Transpose of `cmat')

       clkout      is the encoded spacecraft clock time associated with
                   the returned C-matrix. 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 ckgp_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.

       4)  If the `ref' input string pointer is null, the error
           SPICE(NULLPOINTER) is signaled.

       5)  If the `ref' input string has zero length, the error
           SPICE(EMPTYSTRING) is signaled.

#### Files

       ckgp_c searches through files loaded by furnsh_c to locate a segment
       that satisfies the request for pointing 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 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)
                                       ^
                                       |
                           ckgp_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)

                           ckgp_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. ckgp_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. ckgp_c
                     returns pointing for the epoch closest to the
                     request time `sclkdp'.

       Case 3:  pointing is not available
       ----------------------------------

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

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

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

       ckgp_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.

       The search ends when a segment is found that can provide pointing
       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)
                                             ^
                                             |
                                             |
                                 ckgp_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)
                                     ^
                                     |
                         ckgp_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,
       ckgp_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          (---=============-----====--------==--)
                                      ^
                                      |
                                      |
                           ckgp_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          (---=============-----====--------==--)
                                               ^
                                               |
                                  ckgp_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                                (===============)
                                                ^
                                                |
                                  ckgp_c returns this instance

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

#### Examples

       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) The following example program uses ckgp_c to get C-matrices 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.

          For each C-matrix, a unit pointing vector is constructed and
          printed.

          Use the CK kernel below to load the CASSINI image navigated
          spacecraft pointing and orientation data.

             04153_04182ca_ISS.bc

          Use the SCLK kernel below to load the CASSINI spacecraft clock
          time correlation data required for the conversion between
          spacecraft clock string representation and double precision
          encoding of spacecraft clock counts.

             cas00071.tsc

          Example code begins here.

          /.
             Program ckgp_ex1
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main ()
          {
             /.
             Constants for this program:

             -- The code for the CASSINI spacecraft clock is -82.

             -- The code for CASSINI spacecraft reference frame is -82000.

             -- Spacecraft clock times for successive CASSINI navigation
                images always differ by more than 1.0 seconds. This is an
                acceptable tolerance, and must be converted to "ticks"
                (units of encoded SCLK) for input to ckgp_c.

             -- The reference frame we want is J2000.

             -- The CASSINI ISS camera boresight in the spacecraft
                frame is (0.0005760, -0.99999982, -0.0001710).
             ./

             #define   SC        -82
             #define   INST      -82000
             #define   REF       "J2000"
             #define   TOLISS    "1.0"
             #define   NPICS     2
             #define   MAXCLK    30
             #define   CK        "04153_04182ca_ISS.bc"
             #define   SCLK      "cas00071.tsc"

             SpiceBoolean         found;

             SpiceChar            sclkch  [NPICS][MAXCLK] =

                                             { { "1465644281.0" },
                                               { "1465644351.0" } };

             SpiceChar            clkch   [MAXCLK];

             SpiceDouble          cmat    [3][3];
             SpiceDouble          clkout;
             SpiceDouble          issfix [3] = { 0.00057600,
                                                -0.99999982,
                                                -0.00017100 };
             SpiceDouble          sclkdp;
             SpiceDouble          toltik;
             SpiceDouble          vinert  [3];

             SpiceInt             i;

             /.
             Load the CK file.
             ./
             furnsh_c ( CK );

             /.
             Need to load a CASSINI SCLK kernel to convert from
             clock string to ticks.  Although not required for
             the CASSINI spacecraft clock, most modern spacecraft
             clocks require a leapseconds kernel to be loaded in
             addition to an SCLK kernel.
             ./
             furnsh_c ( SCLK );

             /.
             Convert tolerance from CASSINI formatted character string
             SCLK to ticks, which are units of encoded SCLK.
             ./
             sctiks_c ( SC, TOLISS, &toltik );

             for ( i = 0;  i < NPICS;  i++ )
             {

                /.
                ckgp_c requires encoded spacecraft clock time.
                ./
                scencd_c ( SC, sclkch[ i ], &sclkdp );

                ckgp_c ( INST,  sclkdp,  toltik, REF,
                         cmat,  &clkout, &found       );

                if ( found )
                {
                   /.
                   Use the transpose of the C-matrix to transform the
                   boresight vector from camera-fixed to reference
                   coordinates.
                   ./
                   mtxv_c ( cmat, issfix, vinert );

                   scdecd_c ( SC, clkout, MAXCLK, clkch  );

                   printf ( "Requested SCLK time : %s\n", sclkch[i] );
                   printf ( "   CASSINI SCLK time: %s\n", clkch     );
                   printf ( "   CASSINI ISS boresight: "
                            "%10.7f %10.7f %10.7f\n\n",
                            vinert[0],
                            vinert[1],
                            vinert[2]                            );
                }
                else
                {
                   printf ( "Pointing not found for time %s\n", sclkch[i] );
                }

             }

             return ( 0 );
          }

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

          Requested SCLK time : 1465644281.0
             CASSINI SCLK time: 1/1465644281.171
             CASSINI ISS boresight:  0.9376789  0.3444125  0.0462419

          Requested SCLK time : 1465644351.0
             CASSINI SCLK time: 1/1465644351.071
             CASSINI ISS boresight:  0.9376657  0.3444504  0.0462266

#### Restrictions

       None.

#### Literature_References

       None.

#### Author_and_Institution

       C.H. Acton          (JPL)
       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
       J.M. Lynch          (JPL)
       B.V. Semenov        (JPL)
       W.L. Taber          (JPL)
       R.E. Thurman        (JPL)
       I.M. Underwood      (JPL)

#### Version

       -CSPICE Version 1.2.4, 10-AUG-2021 (JDR)

           Edited the header to comply with NAIF standard.

           Updated the code, input times and kernel set to work with PDS
           archived CASSINI data.

       -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: description of 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 ckgp_.

       -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) (RET) (IMU) (JML) (WLT)

           Based on SPICELIB Version 3.0.0, 19-SEP-1994 (WLT)

#### Index_Entries

       get CK pointing

#### Link to routine ckgp_c source file ckgp_c.c

    Fri Dec 31 18:41:02 2021