void scencd_c ( SpiceInt sc,
ConstSpiceChar * sclkch,
SpiceDouble * sclkdp )
Encode character representation of spacecraft clock time into a
double precision number.
SCLK
CONVERSION
TIME
Variable I/O Description
-------- --- --------------------------------------------------
sc I NAIF spacecraft identification code.
sclkch I Character representation of a spacecraft clock.
sclkdp O Encoded representation of the clock count.
MXPART P Maximum number of spacecraft clock partitions.
sc is the standard NAIF ID of the spacecraft whose clock's
time is being encoded.
sclkch is the character representation of some spacecraft's
clock count.
sclkch will have the following general format:
"pp/sclk_string", or just
"sclk_string"
"pp" is an integer greater than or equal to one
and is called the 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,
using the routine furnsh_c.
The routine scpart_c is used to read the partition
start and stop times, in encoded units of SCLK (called
"ticks" -- see sclkdp below) from the kernel file.
If the partition number is included, it must be
separated from the rest of the string by a "/".
Any number of spaces may separate the partition number,
the "/", and the rest of the clock string.
If the partition number is omitted, a default partition
will be assumed. The default partition is the lowest-
numbered partition that contains the given clock time.
If the clock time does not fall in any of the
partition boundaries then an error is signalled.
"sclk_string" is a spacecraft specific clock string.
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: " " ":" "," "-" "."
Any number of spaces can separate the components and
the delimiters. The presence of the RIM component
is required. Successive components may be omitted, and
in such cases are assumed to represent zero values.
Values for the individual components may exceed the
maximum expected values. For instance, "0:0:0:9" is
an acceptable Galileo clock string, and will convert
to the same number of ticks as "0:0:1:1".
Consecutive delimiters containing no intervening digits
are treated as if they delimit zero components.
Trailing zeros should always be included to match the
length of the counter. For example, a Galileo clock
count of "25684.90" should not be represented as
"25684.9".
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.
sclkdp is the double precision encoding of sclkch.
The encoding is such that order and proximity will be
preserved. That is, if t1, t2, and t3 are spacecraft
clock times, and t1*, t2*, and t3* are their encodings,
then if
t1 < t2 < t3, and
t2 is closer to t1 than to t3, you will have the result
that
t1* < t2* < t3*, and
t2* is closer to t1* than to t3*.
The units of encoded SCLK are "ticks since the start of
the mission", where a "tick" is defined to be the
shortest time increment expressible by a particular
spacecraft's clock.
Each clock string without partition number represents
a certain number of ticks, but you need to include
partition information to determine the relative
position of that time in relation to the start of the
mission.
Since the end time of one partition is coincident
with the begin time of the next, there are two
different representations for this instant, and they
will both yield the same encoding.
For example, if partition 1 has an end time of t1, and
partition 2 has a begin time of t2, then if we executed
the code fragment
scencd_c ( "1/t1", sc, &x );
scencd_c ( "2/t2", sc, &y );
The we would obtain x = y.
The individual routines tiksNN_c, where NN is the
clock type code, contain more detailed information
on the conversion process.
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 the number of partitions in the kernel file for spacecraft
sc exceeds the parameter MXPART, the error
SPICE(TOOMANYPARTS) is signalled.
If a partition number is included in the SCLK string, the
following exceptions may occur:
2) If the partition number cannot be parsed as an integer, the
error SPICE(BADPARTNUMBER) is signalled.
3) If the partition number is not in the range of the number of
partitions found in the kernel pool, the error
SPICE(BADPARTNUMBER) is signalled.
4) If the clock count does not fall within the boundaries of the
specified partition, the error SPICE(NOTINPART) is signalled.
If a partition number is not included in the SCLK string, the
following exception may occur.
5) If the clock count does not fall within the boundaries of any
partition found in the kernel pool, the error SPICE(NOPARTITION)
is signalled.
The following error is signalled by a routine called by scencd_c.
6) If any of the extracted clock components cannot be parsed as
integers, or the string has too many components, or the value
of one of the components is less than the offset value, then
the error SPICE(INVALIDSCLKSTRING) is signalled.
A kernel file containing spacecraft clock partition information
for the desired spaceraft 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. This routine provides a method of assigning a
single double precision number to a spacecraft's clock count,
given one of its character representations.
The routine scdecd_c performs the inverse operation of scencd_c,
converting an encoded double precision number to character format.
To convert the string to ticks since the start of the mission,
scencd_c
1) Converts the non-partition portion of the string to
ticks, using the routine sctiks_c.
2) Determines the partition number for the clock time,
either by getting it directly from the input string, or
determining the default partition if none was specified.
3) Includes partition start and stop times, which are also
measured in ticks, to compute the number of ticks
from the beginning of the mission to the clock time.
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"
None.
None.
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 1.1.1, 14-AUG-2006 (EDW)
Replace mention of ldpool_c with furnsh_c.
-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 1.0.1, 10-MAR-1992 (WLT)
encode spacecraft_clock
Link to routine scencd_c source file scencd_c.c
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