void scdecd_c ( SpiceInt sc,
SpiceDouble sclkdp,
SpiceInt lenout,
SpiceChar * sclkch )
Convert double precision encoding of spacecraft clock time into
a character representation.
SCLK
CONVERSION
TIME
Variable I/O Description
-------- --- --------------------------------------------------
sc I NAIF spacecraft identification code.
sclkdp I Encoded representation of a spacecraft clock count.
lenout I Maximum allowed length of output SCLK string.
sclkch O Character representation of a clock count.
MXPART P Maximum number of spacecraft clock partitions.
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.
lenout 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 lenout should be set to 25.
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, MAXLEN, 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.
MXPART is the maximum number of spacecraft clock partitions
expected in the kernel file for any one spacecraft.
MXPART is currently set to 9999.
1) If kernel variables required by this routine are unavailable,
the error will be diagnosed by routines called by 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 MXPART, the error
SPICE(TOOMANYPARTS) is signaled. 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.
sclkch will be returned as a blank string in this case.
4) If the output string pointer is null, the error SPICE(NULLPOINTER)
is signaled.
5) If the output string has length less than two characters, it
is too short to contain one character of output data plus a null
terminator, so it cannot be passed to the underlying Fortran
routine. In this event, the error SPICE(STRINGTOOSHORT) is
signaled.
6) If the length of sclkch (indicated by lenout) is at least two
characters but not large enough to contain the output clock
string, the error SPICE(SCLKTRUNCATED) is signaled either by the
underlying Fortran routine or by a routine called by that routine.
On output sclkch will contain a portion of the truncated clock
string.
A kernel file containing spacecraft clock partition information
for the desired spacecraft must be loaded, using the routine
furnsh_c, before calling this routine.
In 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"
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 Voyager 2 narrow angle camera 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.
#include <stdio.h>
#include "SpiceUsr.h"
void main()
{
/.
The instrument we want pointing for is the Voyager 2
narrow angle camera. The reference frame we want is
J2000. The spacecraft is Voyager 2.
./
#define SC -32
#define INST -32001
#define REF "J2000"
#define CK "/kernels/voyager2/ck/vg2_jup_qmw_na.bc"
#define SCLK "/kernels/voyager2/sclk/vg200004.tsc"
#define NPICS 4
#define CLKTOL "0:01:001"
#define MAXLEN 30
SpiceBoolean found;
SpiceChar sclkin [4][25] = { {"2 / 20538:39:768"},
{"2 / 20543:21:768"},
{"2 / 20550:37" },
{"2 / 20561:59" } };
SpiceChar sclkout[25];
SpiceDouble tol;
SpiceDouble timein;
SpiceDouble timeout;
SpiceDouble cmat [3][3];
SpiceInt handle;
SpiceInt i;
/.
Load the appropriate files. We need
1) CK file containing pointing data.
2) Spacecraft clock kernel file, for scencd_c and SCDECD.
./
cklpf_c ( CK, &handle );
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 );
scdecd_c ( SC, timeout, MAXLEN, sclkout );
if ( found )
{
printf ( "\n"
"Input s/c clock count: %s\n"
"Output s/c clock count: %s\n"
"Output C-Matrix: \n"
"%25.16f %25.16f %25.16f\n"
"%25.16f %25.16f %25.16f\n"
"%25.16f %25.16f %25.16f\n"
"\n",
sclkin[i],
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 ( "\n"
"Input s/c clock count: %s\n"
"No pointing found.\n",
sclkin[i] );
}
}
}
The output from such a program might look like:
Input s/c clock count: 2 / 20538:39:768
Output s/c clock count: 2/20538:39:768
Output C-Matrix: "first C-matrix"
Input s/c clock count: 2 / 20543:21:768
Output s/c clock count: 2/20543:22:768
Output C-Matrix: "second C-matrix"
Input s/c clock count: 2 / 20550:37
Output s/c clock count: 2/20550:36:768
Output C-Matrix: "third C-matrix"
Input s/c clock count: 2 / 20561:59
Output s/c clock count: 2/20561:58:768
Output C-Matrix: "fourth C-matrix"
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.
None.
N.J. Bachman (JPL)
J.M. Lynch (JPL)
R.E. Thurman (JPL)
-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 "lenout-1"
with "lenout." 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)
decode spacecraft_clock
Link to routine scdecd_c source file scdecd_c.c
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