| sctiks_c |
|
Table of contents
Procedure
sctiks_c ( Convert spacecraft clock string to ticks. )
void sctiks_c ( SpiceInt sc,
ConstSpiceChar * clkstr,
SpiceDouble * ticks )
AbstractConvert a spacecraft clock format string to number of "ticks". Required_ReadingSCLK KeywordsCONVERSION TIME Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- sc I NAIF spacecraft identification code. clkstr I Character representation of a spacecraft clock. ticks O Number of ticks represented by the clock string. Detailed_Input
sc is the NAIF ID number for the spacecraft whose clock
string is being converted.
clkstr is a character string representing a spacecraft clock
time, WITHOUT PARTITION NUMBER.
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 the
following characters: " " "." ":" "," "-"
Any number of spaces may 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.
Detailed_Output
ticks is the number of ticks represented by the spacecraft
clock string. A tick is defined to be the smallest
time increment expressible by the spacecraft clock.
An analogy may be drawn between a spacecraft clock
and a standard wall clock, measuring hours, minutes
and seconds. The number of ticks represented by the
wall clock string
hh:mm:ss
would be the number of seconds represented by that
time.
For example:
00:00:10 would convert to 10
00:01:00 would convert to 60
00:10:00 would convert to 600
01:00:00 would convert to 3600
01:01:00 would convert to 3660
See the -Examples section below for examples for
actual spacecraft clocks.
ParametersNone. Exceptions
1) If the spacecraft clock type is not supported, the error
SPICE(NOTSUPPORTED) is signaled by a routine in the call tree
of this routine.
2) 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,
an error is signaled by a routine in the call tree of this
routine.
3) Invalid spacecraft ID's are not diagnosed.
4) If the `clkstr' input string pointer is null, the error
SPICE(NULLPOINTER) is signaled.
5) If the `clkstr' input string has zero length, the error
SPICE(EMPTYSTRING) is signaled.
FilesNone. ParticularsEach spacecraft is assigned a clock type code in the kernel file. sctiks_c calls the function SCTYPE to determine this value. If the clock type is supported by SPICE, then the SPICELIB routine TIKSnn is called to handle the actual conversion from clock format to number of ticks. The nn in TIKSnn refers to the spacecraft clock type code. Different spacecraft have distinct clock formats but can still be of the same clock type. The TIKSnn routines are routines to the routines SCLKnn, which also contain the ticks-to-clock format conversion routines FMTnn. FMTnn is called by the routine scfmt_c, which performs the inverse operation to sctiks_c. Note the important difference between scencd_c and sctiks_c. scencd_c converts a clock string to the number of ticks it represents since the beginning of the mission, and so uses partition information. sctiks_c just converts to absolute ticks. Examples
The numerical results shown for these examples 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) sctiks_c is used as part of the process of encoding spacecraft
clock by scencd_c, though sctiks_c does not process any partition
information.
Another use of sctiks_c, however, is to convert a clock
measurement to ticks for use as a tolerance for the CK reader
ckgp_c.
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 sctiks_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
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
SpiceBoolean found;
SpiceChar sclkin [4][25] = { {"1/1465644279.0"},
{"1/1465644281.0"},
{"1/1465644351.0"},
{"1/1465644361.0"} };
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.
./
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"
" %19.16f %19.16f %19.16f\n"
" %19.16f %19.16f %19.16f\n"
" %19.16f %19.16f %19.16f\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] );
}
}
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:
-0.3353514559487096 0.8643744402056108 0.3746948466583409
-0.9378874268129802 -0.3438519652102230 -0.0461844199616532
0.0889189272270388 -0.3669095980487626 0.9259971766914239
Input s/c clock count: 1/1465644351.0
Output s/c clock count: 1/1465644351.071
Output C-Matrix:
-0.3353809293975862 0.8643636382622295 0.3746933853786226
-0.9378742920080898 -0.3438898381078248 -0.0461691632640035
0.0889463017035304 -0.3668995504170804 0.9259985287877129
Input s/c clock count: 1/1465644361.0
No pointing found.
2) Below are some examples illustrating various clock string inputs
and the resulting outputs for the Galileo spacecraft. See the
SCLK required reading for a detailed description of the Galileo
clock format.
CLKSTR TICKS
---------------- --------------------
"0:0:0:1" 1
"0:0:1" 8
"0:1" 80
"1" 7280
"1 0 0 0" 7280
"1,0,0,0" 7280
"1:90" 14480
"1:9" 8000
"1:09" 8000
"0-0-10" 80 |-- Third component is supposed
"0-1-0" 80 | to be a mod-10 count.
"0/1/0" Error: "/" is not an accepted delimiter.
"1: 00 : 0 : 1" 7281
"1:::1" 7281
"1.1.1.1.1" Error: Too many components
"1.1.1.1." Error: The last delimiter signals that
a fifth component will follow.
The following examples are for the Voyager 2 spacecraft. Note
that the last component of the Voyager clock has an offset
value of 1.
CLKSTR TICKS
---------------- --------------------
"0.0.001" 0
"0:0:002" 1
"0:01" 800
"1" 48000
"1.0" 48000
"1.0.0" Error: The 3rd component is never 0.
"0.0:100" 99
"0-60-1" 48000
"1-1-1" 48800
"1-1-2" 48801
RestrictionsNone. Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) J.M. Lynch (JPL) W.L. Taber (JPL) R.E. Thurman (JPL) E.D. Wright (JPL) Version
-CSPICE Version 1.1.2, 01-NOV-2021 (JDR)
Edited the header to comply with NAIF standard.
Created complete example from existing code fragment and added
input kernels set using PDS archived CASSINI data.
Added entries #4 and #5 in -Exceptions section.
-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) (JML) (RET) (WLT)
Based on SPICELIB Version 1.0.1, 10-MAR-1992 (WLT)
Index_Entriesconvert spacecraft_clock string to ticks |
Fri Dec 31 18:41:12 2021