| scdecd_c |
|
Table of contents
Procedure
scdecd_c ( Decode spacecraft clock )
void scdecd_c ( SpiceInt sc,
SpiceDouble sclkdp,
SpiceInt scllen,
SpiceChar * sclkch )
AbstractConvert a double precision encoding of spacecraft clock time into a character representation. Required_ReadingSCLK KeywordsCONVERSION TIME Brief_I/O
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
SPICE_SCLK_MXPART
P Maximum number of spacecraft clock partitions.
sc I NAIF spacecraft identification code.
sclkdp I Encoded representation of a spacecraft clock count.
scllen I Maximum allowed length of output SCLK string.
sclkch O Character representation of a clock count.
Detailed_Input
sc is the NAIF integer code of the spacecraft whose
clock's time is being decoded.
sclkdp is the double precision encoding of a clock time in
units of ticks since the spacecraft clock start time.
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.
scllen is the maximum number of characters that can be
accommodated in the output string. This count
includes room for the terminating null character.
For example, if the maximum allowed length of the
output string, including the terminating null, is 25
characters, then `scllen' should be set to 25.
Detailed_Output
sclkch is the character representation of the clock count.
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 scdecd_c.
The routine scpart_c 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. scdecd_c
always decodes such values into the latter of the
two partitions. In this example,
scdecd_c ( x, sc, scllen, 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.
Parameters
SPICE_SCLK_MXPART
is the maximum number of spacecraft clock partitions
expected in the kernel file for any one spacecraft.
See the header file SpiceSCLK.h for this parameter's
value.
Exceptions
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 a blank string in this
case.
2) If the number of partitions in the kernel file for spacecraft `sc'
exceeds the parameter SPICE_SCLK_MXPART, the error
SPICE(TOOMANYPARTS) is signaled by a routine in the call tree of this
routine. `sclkch' will be returned as a blank 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 a blank string in this case.
4) If the declared length of `sclkch' is not large enough to
contain the output clock string, the error
SPICE(SCLKTRUNCATED) is signaled by either this routine or a
routine in the call tree of this routine. On output `sclkch'
will contain a portion of the truncated clock string.
5) If the `sclkch' output string pointer is null, the error
SPICE(NULLPOINTER) is signaled.
6) If the `sclkch' output string has length less than two
characters, the error SPICE(STRINGTOOSHORT) is signaled, since
the output string is too short to contain one character of
output data plus a null terminator.
FilesA kernel file containing spacecraft clock partition information for the desired spacecraft must be loaded, using the routine furnsh_c, before calling this routine. ParticularsIn general, it is difficult to compare spacecraft clock counts numerically since there are too many clock components for a single comparison. The routine scencd_c 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 scencd_c, 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, scdecd_c: 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" 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) 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.
/.
Program scdecd_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local parameters.
The instrument we want pointing for is the CASSINI
spacecraft. The reference frame we want is
J2000. The spacecraft is CASSINI.
./
#define SC -82
#define INST -82000
#define REF "J2000"
#define CK "04153_04182ca_ISS.bc"
#define SCLK "cas00071.tsc"
#define NPICS 4
#define CLKTOL "1.0"
#define MAXLEN 30
/.
Local variables.
./
SpiceBoolean found;
SpiceChar sclkout[25];
SpiceDouble tol;
SpiceDouble timein;
SpiceDouble timeout;
SpiceDouble cmat [3][3];
SpiceInt i;
/.
Set the input SCLK strings.
./
SpiceChar sclkin [4][25] = { {"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.
./
furnsh_c ( CK );
furnsh_c ( SCLK );
/.
Convert the tolerance string to ticks.
./
sctiks_c ( SC, CLKTOL, &tol );
for ( i = 0; i < NPICS; i++ )
{
scencd_c ( SC, sclkin[i], &timein );
ckgp_c ( INST, timein, tol, REF,
cmat, &timeout, &found );
printf ( "\nInput s/c clock count : %s\n", sclkin[i] );
if ( found )
{
scdecd_c ( SC, timeout, MAXLEN, sclkout );
printf ( "Output s/c clock count: %s\n"
"Output C-Matrix:\n"
" %19.15f %19.15f %19.15f\n"
" %19.15f %19.15f %19.15f\n"
" %19.15f %19.15f %19.15f\n",
sclkout,
cmat[0][0], cmat[0][1], cmat[0][2],
cmat[1][0], cmat[1][1], cmat[1][2],
cmat[2][0], cmat[2][1], cmat[2][2] );
}
else
{
printf ( "No pointing found.\n" );
}
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/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.
Restrictions
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.
Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) E.D. Wright (JPL) Version
-CSPICE Version 1.3.0, 05-AUG-2021 (JDR)
Changed the input argument "lenout" to "scllen" for consistency
with other routines.
Edited the header to comply with NAIF standard. Reformatted
example's output and updated kernel set to work with PDS
archived CASSINI data.
-CSPICE Version 1.2.0, 11-FEB-2008 (NJB)
Definition of constant macro MXPART was deleted.
Documentation was updated to reflect current
MXPART value of 9999.
-CSPICE Version 1.1.2, 14-AUG-2006 (EDW)
Replace mention of ldpool_c with furnsh_c.
-CSPICE Version 1.1.1, 26-MAR-2003 (NJB)
Fixed description of exception (6): replaced "scllen-1"
with "scllen." Corrected spelling of "signaled."
-CSPICE Version 1.1.0, 09-FEB-1998 (NJB)
Re-implemented routine without dynamically allocated, temporary
strings. Added output string length and pointer checks.
-CSPICE Version 1.0.0, 25-OCT-1997 (NJB)
Index_Entriesdecode spacecraft_clock |
Fri Dec 31 18:41:11 2021