Table of contents
CSPICE_SCDECD converts a double precision encoding of spacecraft clock
time into a character representation.
Given:
sc the NAIF integer code of the spacecraft whose clock's time is
being decoded.
help, sc
LONG = Scalar
sclkdp the double precision encoding of a clock time in units of
ticks since the spacecraft clock start time, on an N-vector
of encodings.
help, sclkdp
DOUBLE = Scalar or DOUBLE = Array[N]
This value does reflect partition information.
An analogy may be drawn between a spacecraft clock
and a standard wall clock. The number of ticks
corresponding to the wall clock string
hh:mm:ss
would be the number of seconds represented by that
time.
For example:
Clock string Number of ticks
------------ ---------------
00:00:10 10
00:01:00 60
00:10:00 600
01:00:00 3600
If `sclkdp' contains a fractional part the result
is the same as if `sclkdp' had been rounded to the
nearest whole number.
the call:
cspice_scdecd, sc, sclkdp, sclkch
returns:
sclkch the character representation of the clock count, on an N-vector
of representations.
help, sclkch
STRING = Scalar or STRING = Array[N]
The exact form that `sclkch' takes depends on the spacecraft.
Nevertheless, `sclkch' will have the following general
format:
'pp/sclk_string'
'pp' is an integer greater than or equal to one and
represents a "partition number".
Each mission is divided into some number of partitions.
A new partition starts when the spacecraft clock
resets, either to zero, or to some other
value. Thus, the first partition for any mission
starts with launch, and ends with the first clock
reset. The second partition starts immediately when
the first stopped, and so on.
In order to be completely unambiguous about a
particular time, you need to specify a partition number
along with the standard clock string.
Information about when partitions occur for different
missions is contained in a spacecraft clock kernel
file which needs to be loaded into the kernel pool
before calling cspice_scdecd.
The routine cspice_scpart may be used to read the partition
start and stop times, in encoded units of ticks, from
the kernel file.
Since the end time of one partition is coincident with
the begin time of the next, two different time strings
with different partition numbers can encode into the
same value.
For example, if partition 1 ends at time t1, and
partition 2 starts at time t2, then
'1/t1' and '2/t2'
will be encoded into the same value, say `x'. cspice_scdecd
always decodes such values into the latter of the
two partitions. In this example,
cspice_scdecd, x, sc, clkstr
will result in
clkstr = '2/t2'.
'sclk_string' is a spacecraft specific clock string,
typically consisting of a number of components
separated by delimiters.
Using Galileo as an example, the full format is
wwwwwwww:xx:y:z
where z is a mod-8 counter (values 0-7) which
increments approximately once every 8 1/3 ms., y is a
mod-10 counter (values 0-9) which increments once
every time z turns over, i.e., approximately once every
66 2/3 ms., xx is a mod-91 (values 0-90) counter
which increments once every time y turns over, i.e.,
once every 2/3 seconds. wwwwwwww is the Real-Time Image
Count (RIM), which increments once every time xx turns
over, i.e., once every 60 2/3 seconds. The roll-over
expression for the RIM is 16777215, which corresponds
to approximately 32 years.
wwwwwwww, xx, y, and z are referred to interchangeably
as the fields or components of the spacecraft clock.
SCLK components may be separated by any of these five
characters: ' ' ':' ',' '-' '.'
The delimiter used is determined by a kernel pool
variable and can be adjusted by the user.
Some spacecraft clock components have offset, or
starting, values different from zero. For example,
with an offset value of 1, a mod 20 counter would
cycle from 1 to 20 instead of from 0 to 19.
See the SCLK required reading for a detailed
description of the Voyager and Mars Observer clock
formats.
`sclkch' returns with the same measure of vectorization (N)
as `sclkdp'.
MXPART is the maximum number of spacecraft clock partitions
expected in the kernel file for any one spacecraft.
MXPART is currently set to 9999.
Any numerical results shown for these examples may differ between
platforms as the results depend on the SPICE kernels used as input
and the machine specific arithmetic implementation.
1) Double precision encodings of spacecraft clock counts are used
to tag pointing data in the C-kernel.
In the following example, pointing for a sequence of images
from the CASSINI Imaging Science Subsystem (ISS) is requested
from the C-kernel using an array of character spacecraft clock
counts as input. The clock counts attached to the output are
then decoded to character and compared with the input strings.
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.
PRO scdecd_ex1
;;
;; Local parameters.
;;
;; The instrument we want pointing for is the CASSINI
;; spacecraft. The reference frame we want is
;; J2000. The spacecraft is CASSINI.
;;
SC = -82L
INST = -82000L
REF = 'J2000'
CK = '04153_04182ca_ISS.bc'
SCLK = 'cas00071.tsc'
NPICS = 4L
CLKTOL = '1.0'
;;
;; Set the input SCLK strings.
;;
sclkin = ['1/1465644279.0', '1/1465644281.0', $
'1/1465644351.0', '1/1465644361.0']
;;
;; Load the appropriate files. We need
;;
;; 1. CK file containing pointing data.
;; 2. Spacecraft clock kernel file.
;;
cspice_furnsh, CK
cspice_furnsh, SCLK
;;
;; Convert the tolerance string to ticks.
;;
cspice_sctiks, SC, CLKTOL, tol
for i=0L, NPICS-1L do begin
cspice_scencd, SC, sclkin[i], timein
cspice_ckgp, INST, timein, tol, REF, cmat, timeout, found
print
print, format='(2A)', 'Input s/c clock count : ', sclkin[i]
if ( found ) then begin
cspice_scdecd, SC, timeout, sclkout
print, format='(2A)', 'Output s/c clock count: ', sclkout
print, format='(A)', 'Output C-Matrix:'
print, format='(3F21.15)', cmat
endif else begin
print, format='(A)', 'No pointing found.'
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:
Input s/c clock count : 1/1465644279.0
No pointing found.
Input s/c clock count : 1/1465644281.0
Output s/c clock count: 1/1465644281.171
Output C-Matrix:
-0.335351455948710 0.864374440205611 0.374694846658341
-0.937887426812980 -0.343851965210223 -0.046184419961653
0.088918927227039 -0.366909598048763 0.925997176691424
Input s/c clock count : 1/1465644351.0
Output s/c clock count: 1/1465644351.071
Output C-Matrix:
-0.335380929397586 0.864363638262230 0.374693385378623
-0.937874292008090 -0.343889838107825 -0.046169163264003
0.088946301703530 -0.366899550417080 0.925998528787713
Input s/c clock count : 1/1465644361.0
No pointing found.
2) Convert a series of CASSINI encoded SCLK values to their
corresponding character representation of spacecraft clock
time, and convert back that SCLK string to double precision
form.
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.
PRO scdecd_ex2
;;
;; Assign values for the spacecraft ID (CASSINI),
;; the SCLK kernel, and a double precision
;; encodings of SCLK strings
;;
SC = -82
SCLK = 'cas00071.tsc'
timein = 197483587237.d0
;;
;; Load the kernel file.
;;
cspice_furnsh, SCLK
;;
;; Convert the CASSINI encoded SCLK to an
;; SCLK string.
;;
cspice_scdecd, SC, timein, sclkch
;;
;; Convert the SCLK string to double precision form.
;; The output value should match the original for
;; encoded SCLKs that are integers, and be truncated
;; to an integer for encoded SCLKs that have a non-zero
;; fractional part.
;;
cspice_scencd, SC, sclkch, sclkdp
print, 'Scalar:'
print, format='(A10,F21.8)', 'Original: ', timein
print, 'SCLKCH : ', sclkch
print, format='(A10,F21.8)', 'Decoded : ', sclkdp
print
;;
;; Convert a vector of SCLK values.
;;
timein = [ 197483587237.D0, $
197483587250.D0, $
197485901583.201D0, $
197486447183.D0, $
198136032015.4D0]
;;
;; Convert the SCLK dps to the string representation,
;; then convert to the dp form. As before,
;; the output value should match the original for
;; encoded SCLKs that are integers, and be truncated
;; to an integer for encoded SCLKs that have a non-zero
;; fractional part.
;;
cspice_scdecd, SC, timein, sclkch
cspice_scencd, SC, sclkch, sclkdp
print, 'Vector:'
for i=0, n_elements(timein)-1 do begin
print, format='(A10,F21.8)', 'Original: ', timein[i]
print, 'SCLKCH : ', sclkch[i]
print, format='(A10,F21.8)', 'Decoded : ', sclkdp[i]
print
endfor
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_unload, SCLK
END
When this program was executed on a Mac/Intel/IDL8.x/64-bit
platform, the output was:
Scalar:
Original: 197483587237.00000000
SCLKCH : 1/1465644281.165
Decoded : 197483587237.00000000
Vector:
Original: 197483587237.00000000
SCLKCH : 1/1465644281.165
Decoded : 197483587237.00000000
Original: 197483587250.00000000
SCLKCH : 1/1465644281.178
Decoded : 197483587250.00000000
Original: 197485901583.20098877
SCLKCH : 1/1465653322.015
Decoded : 197485901583.00000000
Original: 197486447183.00000000
SCLKCH : 1/1465655453.079
Decoded : 197486447183.00000000
Original: 198136032015.39999390
SCLKCH : 1/1468192894.015
Decoded : 198136032015.00000000
In general, it is difficult to compare spacecraft clock counts
numerically since there are too many clock components for a
single comparison. The routine cspice_scencd provides a method of
assigning a single double precision number to a spacecraft's
clock count, given one of its character representations.
This routine performs the inverse operation to cspice_scencd, converting
an encoded double precision number to character format.
To convert the number of ticks since the start of the mission to
a clock format character string, cspice_scdecd:
1) Determines the spacecraft clock partition that TICKS falls
in.
2) Subtracts off the number of ticks occurring in previous
partitions, to get the number of ticks since the beginning
of the current partition.
3) Converts the resulting ticks to clock format and forms the
string
'partition_number/clock_string'
1) If kernel variables required by this routine are unavailable,
an error is signaled by a routine in the call tree of this
routine. `sclkch' will be returned as an empty string in this
case.
2) If the number of partitions in the kernel file for spacecraft
`sc' exceeds the parameter MXPART, the error SPICE(TOOMANYPARTS)
is signaled by a routine in the call tree of this routine.
`sclkch' will be returned as an empty string in this case.
3) If the encoded value does not fall in the boundaries of the
mission, the error SPICE(VALUEOUTOFRANGE) is signaled by a
routine in the call tree of this routine. `sclkch' will be
returned as an empty string in this case.
4) If any of the input arguments, `sc' or `sclkdp', is undefined,
an error is signaled by the IDL error handling system.
5) If any of the input arguments, `sc' or `sclkdp', 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 the output argument `sclkch' is not a named variable, an
error is signaled by the Icy interface.
A kernel file containing spacecraft clock partition information
for the desired spacecraft must be loaded, using the routine
cspice_furnsh, before calling this routine.
1) Assumes that an SCLK kernel file appropriate for the clock
designated by `sc' is loaded in the kernel pool at the time
this routine is called.
ICY.REQ
SCLK.REQ
None.
J. Diaz del Rio (ODC Space)
E.D. Wright (JPL)
-Icy Version 1.1.2, 13-AUG-2021 (JDR)
Edited the header to comply with NAIF standard. Extended -I/O
section to provide further description of arguments. Added
complete examples; second one based on existing fragment using
CASSINI PDS archived data.
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 1.1.1, 05-FEB-2008 (EDW)
Edited -I/O section, replaced comment
"returns with the same order"
with
"returns with the same measure of vectorization"
-Icy Version 1.1.0, 12-SEP-2004 (EDW)
Added capability to process vector 'sclkdp' as
input returning vector 'sclkch' on output.
-Icy Version 1.0.0, 16-JUN-2003 (EDW)
decode spacecraft_clock
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