void ckgp_c ( SpiceInt inst,
SpiceDouble sclkdp,
SpiceDouble tol,
ConstSpiceChar * ref,
SpiceDouble cmat[3][3],
SpiceDouble * clkout,
SpiceBoolean * found )
Get pointing (attitude) for a specified spacecraft clock time.
CK
SCLK
POINTING
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 True when requested pointing is available.
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.
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)
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.
None.
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.
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.
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
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 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.
Otherwise, predict values will be used.
For each C-matrix, a unit pointing vector is constructed and
printed.
#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 ckgp_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 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++ )
{
/.
ckgp_c requires encoded spacecraft clock time.
./
scencd_c ( SC, sclkch[ i ], &sclkdp );
ckgp_c ( INST, sclkdp, toltik, REF,
cmat, &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] );
}
else
{
printf ( "Pointing not found for time %s\n", sclkch[i] );
}
}
return ( 0 );
}
None.
None.
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)
-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)
Based on SPICELIB Version 3.0.0, 19-SEP-1994 (WLT)
get ck pointing
Link to routine ckgp_c source file ckgp_c.c
|