Navigation and Ancillary Information Facility
NAIF
 Camera-matrix  Kernel
CK
(Orientation or Attitude Kernel)
Emphasis on CK files for Cassini
October 2004
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 2
NAIF Scope
  This tutorial covers use of predict and
reconstruction CKs produced by the Cassini
AACS Team.
  It does not cover use of  image navigated
(sometimes called  c-smithed ) CKs made by the
Imaging Team, or any other CKs.
  Those topics are best presented by the producers of such CK
files.
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 3
NAIF Cassini CK Contents - 1
  A CK file holds orientation data for the Cassini
spacecraft .
   Orientation data   quaternions that may be used to rotate
vectors from a base reference frame to a target frame
  For Cassini:
  the base frame is the Earth Equator and Equinox of 2000 inertial frame
 
  SPICE frame name is  J2000
  the target reference frame is the Cassini spacecraft
  SPICE frame name is  CASSINI_SC_COORD
  A CK file also contains angular velocity of the target
frame with respect to the base frame.
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 4
NAIF Cassini CK Contents - 2
  CK files also contain comments (metadata).
  For reconstruction CK files, the comments include:
  parameters used in the creation of the file.
  segment summaries showing time bounds for the:
  entire segment
  interpolation intervals within each segment
  For predict CK files, the comments include:
  names of products used in making the predict CK
  These comments may be displayed on your computer using
the COMMNT utility program supplied with the SPICE Toolkit.
  To display comments, run COMMNT as follows:
% commnt -r <binary CK file name> | more
  See COMMNT User s Guide (toolkit/doc/commnt.ug) for complete usage
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 5
NAIF Cassini CK Files-1
Cassini CK files produced at JPL are either  predict  or
 reconstruction
 Predict
  Predict CK files are made using information that predicts the
orientation some time in the future, as generated by Cassini s
Kinematic Predictor Tool (KPT).
  Due to their high quality (good fidelity and, by definition, no data
gaps) they may be used to fill  gaps  in reconstructed CKs.
  Normally a predict CK covers one sequence.
  To achieve the best possible accuracy a predict CK should be used
with the same SCLK data used in the CK generation.
  This is found in the SCLK used in generation of the predict CK.
  This may* also be found in a later-made SCLK.
* see later discussion about re-made predict CKs
  Latest file naming schema: [start]_[end][p][version][_xxxxxx].[ext]
  E.G. 03215_03220pa_port1.bc
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 6
NAIF Cassini CK Files-2
  Reconstruction
  Reconstruction CK files are made from orientation telemetry
returned from the Cassini spacecraft.
  To achieve the best possible accuracy, a reconstruction CK should
be used with an SCLK produced after the last data in the CK.
  File naming schema: [start]_[end][r][version][_xxxxxx].[ext]
optional
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 7
NAIF
While the Cassini CK naming scheme may suggest that a CK file provides
continuous coverage for the interval of time specified in the file name,
 in reality
this is rarely the case for  reconstruction  CK files   they almost alwa
ys contain
gaps in coverage. Below is an example of this.
Coverage of .
CK file
as suggested by file name
CK file segments
as appears in ckbrief and spacit
summaries, and in comment area
CK segments containing
 continuous  data
this is what is actually available to
the SPICE high-level interfaces
Black lines represent interpolation intervals within which pointing
data may be returned. White spaces represent gaps in the data.
CK segments may overlap slightly (not shown here).
Cassini Reconstruction CK Coverage - 1
A segment boundary
Interpolation intervals (typical of many)
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 8
NAIF Cassini Reconstruction CK Coverage - 2
  All Cassini CK files are of Type 3, in which
segments contain discrete pointing instances.
  In  reconstruction  CK files, the nominal spacing between
orientation data points extracted from telemetry is 4 seconds.
  These pointing instances are partitioned into
groups, called  interpolation intervals,  within
which linear interpolation between adjacent
instances is valid and is done by SPICE code.
  Gaps in coverage in  reconstruction  CKs result
from insufficient source orientation data, i.e.
telemetry.
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 9
NAIF Cassini Reconstruction CK Coverage - 3
  Interpolation intervals can be thought of as
representing intervals of time over which the pointing
of the spacecraft is available continuously.
  In Cassini reconstruction CK files, the intervals are constrained such
 
that interpolation is valid between any two adjacent points that are
less than or exactly 16 seconds apart. This means that interpolation
will be done if up to three adjacent pointing instances are missing. If
four instances are missing, there will be a gap in the segment.
  SPICE CK reader routines compute and return
pointing using linear interpolation between the
attitude data points that bound the user s pointing
request time as long as that time is within an
interpolation interval.
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 10
NAIF Cassini Reconstruction CK Coverage - 4
  The time periods between interpolation intervals are
gaps during which the CK readers are not able to
compute pointing
  Information about the start and stop time for each
interpolation interval can be obtained using two
methods
  by examining a  reconstruction  CK file s comments using the
COMMNT utility
  by summarizing a CK file using CKBRIEF with the  -dump  option
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 11
NAIF Cassini Reconstruction CK Coverage - 5
  To view the interpolation intervals table provided in a
reconstruction CK file s comments using COMMNT:
% commnt -r 04059_04066ra.bc
...
************************************************************************
**
INTERPOLATION INTERVALS IN THE FILE SEGMENTS:
************************************************************************
**
SEG.SUMMARY: ID -82000, COVERG: 2004-02-28T13:00:00 2004-03-03T22:27:22
------------------------------------------------------------------------
----
2004-02-28T13:00:00.989 2004-02-29T14:16:28.395
2004-03-01T07:05:43.750 2004-03-01T14:52:19.568
...
  To view a summary of interpolation intervals in a
reconstruction CK using CKBRIEF:
% ckbrief 04215_04220ra.bc cas00085.tsc naif0007.tls -utc -dump
...
Segment No.: 1
Object: -82000
Interval Begin UTC Interval End UTC AV
------------------------ ------------------------ ---
2004-AUG-02 00:00:01.275 2004-AUG-02 01:56:53.479 Y
2004-AUG-02 01:59:21.478 2004-AUG-04 11:28:19.880 Y
...
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 12
NAIF Cassini Predict CK Coverage
  Cassini  predict  CK files are also of Type 3, in
which segments contain discrete pointing
instances.
  Spacing between points varies depending on the simulated
attitude mode and the actual method of generation.
  By definition, predict CK files have no data gaps
and thus provide continuous coverage
throughout the entire span of the file.
   Continuous coverage  means you always get a pointing
solution.
  The coverage of a predict CK file may be seen
using the ckbrief utility.
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 13
NAIF When Are Cassini CKs Made?
1. A predict CK for an entire sequence is made
prior to uplink of the sequence.
  There may be multiple instances of this; if you use it be sure
to get the very last version prior to uplink
2. During execution of the sequence,
reconstruction CKs are made about every week.
3. After completion of the sequence, the predict CK
for that sequence may be re-made using the
latest SCLK, and
4. After completion of the sequence, a single
reconstruct CK covering the entire sequence is
made using the latest SCLK.
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 14
NAIF Using Predict CKs To Fill Gaps - 1
  You may require continuous spacecraft
orientation data may to process science data.
  Reconstruction CKs have gaps and therefore
cannot provide 100% continuous coverage.
  Predict CKs have no gaps and are of very high
fidelity; these can be used to  fill  the gaps in
reconstruction CKs.
  Therefore, for continuous coverage, both predict
and reconstruction CKs must be loaded into a
user s program.
  To make sure that reconstruction data is used
when available, the predict CK should be loaded
first and reconstruction CK(s) loaded afterwards.
  SPICE gives the latest loaded file the highest priority and will
therefore take data from the latest file, if possible.
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 15
NAIF Using Predict CKs To Fill Gaps-2
  Since only one SCLK file can be loaded, it is
important to use the correct file for this procedure.
  If processing data from a sequence while that
sequence is still active:
1. Load the latest predict CK made for that sequence (the one
uplinked).
2. Load one or more  weekly  reconstruction CK(s).
3. Load the SCLK used in making the aforementioned predict CK.
  If processing data from a sequence that has finished
executing:
1. Load the latest predict CK for that sequence, made perhaps after
the sequence is finished.
2. Load the single reconstruction CK for that sequence, made after the
sequence is finished.
3. Load the SCLK used in making the aforementioned predict and
reconstruction CKs, or use any later-made SCLK.
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 16
NAIF Validation of Predict CKs
  AACS validates each predict CK by comparing it
with the reconstruction CKs for the same time
period.
  The results of this validation are presented as a
set of plots showing the difference between
predict and reconstruction orientation (where it is
available) broken down into rotational deltas
about individual axes of the s/c frame.
  See slides 25-30 for examples.
  These reports will be available on the Web. (mid-
October)
  Currently they are being stored in AACS AFS space.
  (/afs/jpl/home/b/burkt/public/ck_compare)
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 17
NAIF
 To get the orientation (rotation matrix) and angular
velocity of the spacecraft, one usually needs four
SPICE file types: CK, SCLK, LSK and FK.
  Spacecraft orientation kernel (CK).
  Spacecraft Clock Kernel (SCLK) and Leapseconds Kernel (LSK).
  Used to convert between ephemeris time (ET) and spacecraft clock time
(SCLK).
  Frames Kernel (FK), when high level SPICE interfaces are used to
access CK data.
  Used to associate e.g., an instrument frame with the CK (e.g. spacecra
ft)
frame.
  See the next page for examples of high level SPICE interfaces.
Accessing Cassini CKs - 1
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 18
NAIF
  The high-level SPICELIB routines should normally
be used to access CK data. These high-level
routines are:
  Position or state transformation matrix determination
  PXFORM: return a rotation matrix (3x3) from one frame to another,
either of which can be a CK frame
  SXFORM: return a state transformation matrix (6x6) from one
frame to another, either of which can be a CK frame
  Position or state vector vector determination
  SPKPOS: return a position vector (1x3) in a specified frame, which
can be a CK frame or have a CK frame as one of the  links  in its
chain
  SPKEZR: return a state vector (1x6) in a specified frame, which
can be a CK frame or have a CK frame as one of the  links  in its
chain
  The above mentioned routines are discussed in the frames
(fk.*, using_frames. *) and spk (spk. *) tutorials.
Accessing Cassini CKs - 2
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 19
NAIF
Initialization ... typically once per program run
Tell your program which SPICE files to use ( loading  files).
CALL FURNSH( 'lsk_file_name' )
CALL FURNSH( 'sclk_file_name' )
CALL FURNSH( 'ck_file_name' )
CALL FURNSH(  fk_file_name' )
Loop ... do as often as you need
Convert UTC to Ephemeris Time (et)
CALL STR2ET( 'utc_string', et )
Get 3x3 position vector rotation matrix (cmat), or 6x6 state vector rota
tion
matrix (camat) at your requested time.
CALL PXFORM (  from_frame',  to_frame', et, cmat )
CALL SXFORM (  from_frame',  to_frame', et, camat )
inputs output
or
Better yet, use a  furnsh kernel
to load all the needed files.
Accessing Cassini CKs - 3
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 20
NAIF Accessing Cassini CKs - 4
  The original CK access routines, CKGP and
CKGPAV, are also available but not recommended
for use because they don t have the ability to
combine transformations provided by the high
level interfaces.
  The exception to this is if one wants to know if pointing is
available for a particular time prior to calling one of the highlevel
routines, in order to prevent those from signaling an
error.
  In this case CKGP or CKGPAV can be called before calling
high-level routines, and the FOUND flag returned by this call
can be checked to see if pointing is available.
  Use the Cassini spacecraft ID (-82000), encoded Cassini SCLK,
tolerance 0.0 and reference frame  J2000  when calling CKGP or
CKGPAV for that purpose
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 21
NAIF
PROGRAM READ_CASSINI_CK_EXAMPLE
IMPLICIT NONE
CC Declare variables
C
CHARACTER*(32) UTCBEG
CHARACTER*(32) UTCEND
DOUBLE PRECISION ET
DOUBLE PRECISION ETBEG
DOUBLE PRECISION ETEND
DOUBLE PRECISION ETSTEP
DOUBLE PRECISION SCLK
DOUBLE PRECISION HMAT ( 3, 3 )
DOUBLE PRECISION CMAT ( 3, 3 )
LOGICAL FOUND
CC Load needed kernels -- LSK, SCLK, CK and FK.
C
CALL FURNSH ( 'naif0007.tls' )
CALL FURNSH ( 'cas00084.tsc' )
CALL FURNSH ( '04059_04066ra.bc' )
CALL FURNSH ( 'cas_v38.tf' )
Accessing Cassini CKs - 5
A small FORTRAN program below illustrates a way of computing
transformation from the ISS NAC frame to the J2000 inertial frame,
incorporating pointing data from Cassini spacecraft CKs.
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 22
NAIF
CC Set begin and end UTC and time step. Convert UTCs to ETs.
C
UTCBEG = '2004-FEB-29 14:10'
UTCEND = '2004-FEB-29 14:30
ETSTEP = 60.D0
CALL STR2ET( UTCBEG, ETBEG )
CALL STR2ET( UTCEND, ETEND )
CC Loop over times.
C
ET = ETBEG
DO WHILE( ET .LE. ETEND )
CC Convert ET to Cassini encoded SCLK and call CKGP to verify
C that s/c pointing is available in the loaded Cassini CK(s).
C
CALL SCE2T( -82, ET, SCLK )
CALL CKGP ( -82000, SCLK, 0.D0,  J2000 , CMAT, SCLK, FOUND )
C
C If pointing is available, compute matrix rotating vectors
C from ISS NAC frame to J2000 inertial frame.
C
IF ( FOUND ) THEN
CALL PXFORM ( 'CASSINI_ISS_NAC', 'J2000', ET, CMAT )
WRITE (*,*) ' ET, Rotation matrix: ', ET, CMAT
ELSEWRITE (*,*)  S/C POINTING IS NOT FOUND AT ET:  , ET
END IF
ET = ET + ETSTEP
END DO
END
Accessing Cassini CKs - 6
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 23
NAIF Common Error When Reading CK
  The file, or files, you loaded do not cover the time at
which you requested orientation
  You may get an error message like:
========================================================================
========
Toolkit version: N0057
SPICE(NOFRAMECONNECT) --
There is insufficient information available to ROTform from -82360
(CASSINI_ISS_NAC) to frame 1 (J2000). Frame -82360 could be transformed
to
-82000 (CASSINI_SC_COORD). Frame 1 could be transformed to 1 (J2000).'
A traceback follows. The name of the highest level module is first.
RADEC --> GBV --> PXFORM --> REFCHG
Oh, by the way: The SPICELIB error handling actions are USER-TAILORABLE.
 You
can choose whether the Toolkit aborts or continues when errors occur, wh
ich
error messages to output, and where to send the output. Please read the
ERROR
"Required Reading" file, or see the routines ERRACT, ERRDEV, and ERRPRT.
 
========================================================================
========
  Check file coverage on the segment level by summarizing the
file(s) using CKBRIEF or SPACIT.
  Check interpolation interval coverage using CKBRIEF with option
 -dump,  or by examining comments provided in the comment
area of the file   you may be asking for data within a coverage gap
(outside of interpolation intervals).
 
Navigation and Ancillary Information Facility
Cassini C-Kernel 24
NAIF
  For more information about CK, look at the following
documents
  Headers for PXFORM, SXFORM, SPKPOS and SPKEZR
  If needed, see headers for CKGP and CKGPAV
  Generic CK Tutorial
  Frames tutorial
  CK and Frames Required Reading
  CKBRIEF and SPACIT User s Guides
  Most Useful SPICELIB Routines
  Porting_Kernels tutorial
  Related documents
  SCLK Required Reading
  Time Required Reading
  Frames Required Reading
  NAIF_IDS Required Reading
  Rotations Required Reading
  For a general guide to locating documents, see
ftp://naif/pub/naif/toolkit_docs/Tutorials/pdf/individual_docs/
39_finding_components.pdf
Additional Information on CK
 
AACS CK-Compare Reports
  Each report covers 5-7 days of attitude data
  Each report includes:
  Interval and gap times in SCET format (e.g. 2004-DOYT00:00:00.000)
  24-hours of data per plot
  Tabular data of mean and 1-sigma attitude difference, each axis, for t
hat 24-hour
period (also tabulated on a continuous trend report)
  The plots contain:
  Page 1 -- Attitude difference data expressed as a rotation (in mrads)
about the S/C axes
(X, Y, and Z)  from  the predicted attitude  to  the reconstructed attit
ude
  Page 2 -- Predicted S/C body rates (X, Y, and Z) in mrads/sec
  Page 3 -- Data present indication
  0 = All data is present
  1 = Gap in Recon CK
  2 = Real-time activity (predict not applicable)
 
Navigation and Ancillary Information Facility
26
NAIF
 
Navigation and Ancillary Information Facility
27
NAIF
 
Navigation and Ancillary Information Facility
28
NAIF
 
AACS Proposed C-Kernel Prediction/Reconstruction Comparison
Recon CK: /home2/gsw/dbeach/utils/ckernel_dir/04136_04142ra.bc
PRODUCT_CREATION_TIME = 2004-145T17:37:57
QUERY_START_TIME = 2004-136T13:00:01
QUERY_STOP_TIME = 2004-142T23:58:57
INTERVALS = 20
Interval Gap
------------------------------------------- ----------------------------
------
1. 2004-136T13:00:01 2004-136T14:25:45
2004-136T14:25:45 2004-136T14:27:17
2. 2004-136T14:27:17 2004-137T10:54:57
2004-137T10:54:57 2004-137T11:11:09
3. 2004-137T11:11:09 2004-138T17:39:52
2004-138T17:39:52 2004-138T17:40:44
4. 2004-138T17:40:44 2004-138T18:31:52
2004-138T18:31:52 2004-138T18:33:08
5. 2004-138T18:33:08 2004-138T22:38:32
2004-138T22:38:32 2004-138T22:38:52
6. 2004-138T22:38:52 2004-138T22:41:20
2004-138T22:41:20 2004-138T22:42:36
7. 2004-138T22:42:36 2004-138T22:56:08
2004-138T22:56:08 2004-138T22:56:52
8. 2004-138T22:56:52 2004-138T23:09:28
2004-138T23:09:28 2004-138T23:09:56
9. 2004-138T23:09:56 2004-138T23:13:12
2004-138T23:13:12 2004-138T23:14:12
10. 2004-138T23:14:12 2004-138T23:34:48
2004-138T23:34:48 2004-138T23:35:40
11. 2004-138T23:35:40 2004-139T09:30:32
2004-139T09:30:32 2004-139T09:33:00
12. 2004-139T09:33:00 2004-140T06:41:27
2004-140T06:41:27 2004-140T06:45:31
13. 2004-140T06:45:31 2004-140T07:11:19
2004-140T07:11:19 2004-140T07:14:51
14. 2004-140T07:14:51 2004-140T14:54:51
2004-140T14:54:51 2004-140T14:56:59
15. 2004-140T14:56:59 2004-141T07:37:34
2004-141T07:37:34 2004-141T07:37:54
 
Day No 1 (DOY 136-137)
2004-136T13:00:01.675 to 2004-137T13:00:01.675
Maximum Error
X 1.26 mrad at 2004-136T16:37:26.675
Y 1.80 mrad at 2004-136T15:19:11.675
Z 1.01 mrad at 2004-136T16:24:36.675
Mean Deviation
X 0.17 mrad
Y 0.15 mrad
Z 0.13 mrad
Day No 2 (DOY 137-138)
2004-137T13:00:01.675 to 2004-138T13:00:01.675
Maximum Error
X 2.98 mrad at 2004-137T20:17:36.675
Y 2.86 mrad at 2004-137T23:42:41.675
Z 3.07 mrad at 2004-137T20:40:56.675
Mean Deviation
X 0.59 mrad
Y 0.57 mrad
Z 0.69 mrad
Day No 3 (DOY 138-139)
2004-138T13:00:01.675 to 2004-139T13:00:01.675
Maximum Error
X 1.10 mrad at 2004-138T18:35:21.675
Y 1.39 mrad at 2004-138T19:50:11.675
Z 0.66 mrad at 2004-138T18:35:56.675
Mean Deviation
X 0.11 mrad
Y 0.11 mrad
Z 0.07 mrad