Table of contents
CSPICE_CKGPAV returns pointing (attitude) and angular velocity
for a specified spacecraft clock time.
Given:
inst the scalar NAIF integer ID for the instrument, spacecraft, or
other structure for which pointing is being requested.
help, inst
LONG = Scalar
sclkdp the double precision scalar encoded spacecraft clock time for
which pointing is being requested.
help, sclkdp
DOUBLE = Scalar
tol the double precision scalar time tolerance in ticks (+/-), the
units of encoded spacecraft clock time, about 'sclkdp'.
help, tol
DOUBLE = Scalar
The C-matrix returned by cspice_ckgpav is the one whose
time tag is closest to 'sclkdp' and within 'tol' units
of 'sclkdp'.
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 the scalar string name of the desired reference frame for the
returned pointing.
help, ref
STRING = Scalar
the call:
cspice_ckgpav, inst , sclkdp, tol, ref, cmat, av, $
clkout, found
returns:
cmat a double precision, 3x3 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'.
help, cmat
DOUBLE = Array[3,3]
av the double precision angular velocity 3-vector measured in
radians per second (this is the axis about which the reference
frame tied to the instrument is rotating in the right-handed
sense at time 'clkout').
help, av
DOUBLE = Array[3]
clkout the scalar double precision 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').
help, clkout
DOUBLE = Scalar
found a scalar boolean with value TRUE when requested pointing is
found.
help, found
BOOLEAN = Scalar
None.
Any numerical results shown for this example may differ between
platforms as the results depend on the SPICE kernels used as input
and the machine specific arithmetic implementation.
1) The following example program uses cspice_ckgpav to get C-matrices and
associated angular velocity vectors for a set of images whose
SCLK counts (un-encoded character string versions) are known.
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.
We need to load also an SCLK kernel to convert from clock string
to "ticks." Although not required for older spacecraft clocks,
most modern spacecraft ones require a leapseconds kernel to be
loaded in addition to an SCLK kernel.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: ckgpav_ex1.tm
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
-------------------- -----------------------
cas00071.tsc CASSINI SCLK
04153_04182ca_ISS.bc CASSINI image navigated
spacecraft CK
\begindata
KERNELS_TO_LOAD = ( 'cas00071.tsc'
'04153_04182ca_ISS.bc' )
\begintext
End of meta-kernel
Example code begins here.
PRO ckgpav_ex1
;;
;; 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 cspice_ckgp.
;;
;; -- The reference frame we want is J2000.
;;
;; -- The CASSINI ISS camera boresight in the spacecraft
;; frame is (0.0005760, -0.99999982, -0.0001710).
;;
SC = -82
INST = -82000
REF = 'J2000'
TOL = '1.0'
NPICS = 2
BORE = [ 0.0005760d, -0.99999982d, -0.0001710d]
SCLKCH = [ '1465644281.0' , '1465644351.0' ]
;;
;; Load the CK pointing file. Need to load a Cassini SCLK
;; kernel to convert from clock string to ticks and the
;; leapseconds kernel.
;;
cspice_furnsh, 'ckgpav_ex1.tm'
;;
;; Convert tolerance from CASSINI formatted character string
;; SCLK to ticks, which are units of encoded SCLK.
;;
cspice_sctiks, SC, TOL, toltik
for i = 0, (NPICS-1) do begin
;;
;; cspice_ckgp requires encoded spacecraft clock time.
;;
cspice_scencd, SC, SCLKCH[i], sclkdp
;;
;; Retrieve the 'REF' reference frame to 'INST' reference frame
;; transformation matrix at time sclkdp with a tolerance
;; 'toltik'.
;;
;; [INST] = [cmat][ref]
;;
cspice_ckgpav, INST, sclkdp, toltik, REF, cmat, av, clkout, found
;;
;; Did we find pointing information within (+/-) 'toltik' of the
;; requested time.
;;
if ( found ) then begin
;;
;;
;; Transform the 'BORE' vector from 'INST' reference frame to
;; 'REF' frame.
;; T
;; [ref] = [cmat] [INST]
;;
cspice_mtxv, cmat, BORE, bore_ref
cspice_scdecd, SC, clkout, clkch
print, 'Requested SCLK time : ', sclkch[i]
print, ' CASSINI SCLK time: ', clkch
print, FORMAT='(A,3F11.7)', $
' CASSINI ISS boresight :', bore_ref
print, FORMAT='(A,3F11.7)', $
' Angular velocity vector:', av
print, ' '
endif else begin
print, 'Pointing not found for SCLK time: ', sclkch[i]
endelse
endfor
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_kclear
END
When this program was executed on a Mac/Intel/IDL8.x/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
Angular velocity vector: 0.0000000 0.0000000 0.0000000
Requested SCLK time : 1465644351.0
CASSINI SCLK time: 1/1465644351.071
CASSINI ISS boresight : 0.9376657 0.3444504 0.0462266
Angular velocity vector: 0.0000000 0.0000000 0.0000000
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)
^
|
cspice_ckgpav 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)
cspice_ckgpav 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. cspice_ckgpav 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. cspice_ckgpav
returns pointing for the epoch closest to the
request time `sclkdp'.
Case 3: pointing is not available
----------------------------------
sclkdp
\ tol
| /
|/\
Your request [--+--]
. . .
Segment (==---===========----=======---------===--)
cspice_ckgpav returns no pointing; the output
`found' flag is set to SPICEFALSE.
Tolerance and segment priority
==============================
cspice_ckgpav 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. cspice_ckgpav 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)
^
|
|
cspice_ckgpav 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)
^
|
cspice_ckgpav 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,
cspice_ckgpav 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 (---=============-----====--------==--)
^
|
|
cspice_ckgpav 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 (---=============-----====--------==--)
^
|
cspice_ckgpav 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 (===============)
^
|
cspice_ckgpav returns this instance
Segment A (=====================)
^
|
"Best" answer
1) If a C-kernel file has not been loaded using cspice_furnsh 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 False.
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 False.
4) If any of the input arguments, `inst', `sclkdp', `tol' or
`ref', is undefined, an error is signaled by the IDL error
handling system.
5) If any of the input arguments, `inst', `sclkdp', `tol' or
`ref', is not of the expected type, or it does not have the
expected dimensions and size, an error is signaled by the Icy
interface.
6) If any of the output arguments, `cmat', `av', `clkout' or
`found', is not a named variable, an error is signaled by the
Icy interface.
cspice_ckgpav searches through files loaded by cspice_furnsh to locate a
segment that can satisfy the request for pointing and angular
velocity for instrument `inst' at time `sclkdp'. You must load a
C-kernel file using cspice_furnsh prior to calling this routine.
1) 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.
ICY.REQ
CK.REQ
SCLK.REQ
None.
J. Diaz del Rio (ODC Space)
B.V. Semenov (JPL)
E.D. Wright (JPL)
-Icy Version 1.0.3, 10-AUG-2021 (JDR)
Edited -Examples section to comply with NAIF standard. Added
example's problem statement and meta-kernel. Updated input
SCLK times and kernel set to work with PDS archived CASSINI
data. Added call to cspice_kclear in code example.
Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections, and
completed -Particulars section.
Removed reference to the routine's corresponding CSPICE header from
-Abstract section.
Added arguments' type and size information in the -I/O section.
-Icy Version 1.0.2, 03-JUN-2010 (BVS)
Edits to header. Added warning regarding non-zero tolerance.
-Icy Version 1.0.1, 09-JUN-2006 (EDW)
Edits to header. Improved argument descriptions.
-Icy Version 1.0.0, 16-JUN-2003 (EDW)
get CK pointing and angular velocity
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