| spkpvn_c |
|
Table of contents
Procedure
spkpvn_c ( S/P Kernel, position and velocity in native frame )
void spkpvn_c ( SpiceInt handle,
ConstSpiceDouble descr [5],
SpiceDouble et,
SpiceInt * ref,
SpiceDouble state [6],
SpiceInt * center )
AbstractReturn, for a specified SPK segment and time, the state (position and velocity) of the segment's target body relative to its center of motion. Required_ReadingSPK KeywordsEPHEMERIS Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- handle I File handle. descr I Segment descriptor. et I Evaluation epoch. ref O Segment reference frame ID code. state O Output state vector. center O Center of state. Detailed_Input
handle,
descr are, respectively, the file handle assigned to a SPK
file and the descriptor for a segment within the file.
Together they determine the ephemeris data from which
the state of the body is to be computed.
et is the epoch (ephemeris time) at which the state
is to be computed. `et' is expressed as seconds
past J2000 TDB.
Detailed_Output
ref is the ID code of the reference frame relative to which
the state returned by the routine is expressed.
state contains the position and velocity, at epoch `et',
for the body covered by the specified segment.
`state' has six elements: the first three contain the
body's position; the last three contain the body's
velocity. These vectors are expressed into the
specified reference frame. Units are always km and
km/sec.
center is the integer ID code of the center of motion for
the state.
ParametersNone. Exceptions
1) If the segment type is not supported by the current
version of spkpvn_c, the error SPICE(SPKTYPENOTSUPP)
is signaled by a routine in the call tree of this routine.
FilesSee argument `handle'. Particulars
For most user applications, the high-level SPK APIs
spkezr_c
spkez_c
spkpos_c
spkezp_c
should be used instead of this routine.
The f2c'd routine spkpvn_ called by spkpvn_c is the most basic of
the SPK readers, the reader upon which spkezr_c and spkgeo_c, etc.
are built. spkpvn_c normally should not be called directly except in
cases where some optimization is required. (That is, where the
calling program has prior knowledge of the center-barycenter shifts
to be performed, or a non-standard method of determining the files
and segments to be used when computing states.)
This is the only reader that makes distinctions between the
various segment types in the SPK format. The complete list
of types currently supported is shown below.
Type Description
---- -------------------------
1 Modified Difference Array
2 Chebyshev (P)
3 Chebyshev (P,V)
5 Two body propagation between discrete states
8 Lagrange interpolation, equally spaced discrete states
9 Lagrange interpolation, unequally spaced discrete states
12 Hermite interpolation, equally spaced discrete states
13 Hermite interpolation, unequally spaced discrete states
14 Chebyshev Unequally spaced
15 Precessing Ellipse
17 Equinoctial Elements
18 ESOC/DDID Hermite/Lagrange Interpolation
19 ESOC/DDID Piecewise Interpolation
21 Extended Modified Difference Array
The maximum record lengths for each data type currently
supported are as follows:
Data type Maximum record length
--------- ---------------------
1 71
2 87
3 171
5 15
8 171
9 197
12 87
13 99
14 Variable
15 16
17 12
18 198
19 198
21 96
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) In the following code example, an SPK file is searched for
a segment containing ephemeris data for the Jupiter system
barycenter at a particular epoch. Using this segment,
states of the Jupiter system barycenter relative to the
solar system barycenter are evaluated at a sequence of times.
This method of state computation minimizes the number of
segment searches required to obtain requested data, but
it bypasses the SPK subsystem's state chaining mechanism.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: spkpvn_ex1.tm
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
The names and contents of the kernels referenced
by this meta-kernel are as follows:
File name Contents
--------- --------
de421.bsp Planetary ephemeris
naif0010.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'naif0010.tls' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program spkpvn_ex1
./
#include <stdio.h>
#include <stdlib.h>
#include "SpiceUsr.h"
int main()
{
/.
Local constants
./
#define FRNMLN 33
#define META "spkpvn_ex1.tm"
#define DSCSIZ 5
#define ND 2
#define NI 6
#define SIDLEN 41
#define TIMFMT "YYYY MON DD HR:MN:SC.######::TDB TDB"
#define TIMLEN 41
/.
Local variables
./
SpiceBoolean found;
SpiceChar frname [ FRNMLN ];
SpiceChar segid [ SIDLEN ];
SpiceChar * timstr;
SpiceChar outstr [ TIMLEN ];
SpiceDouble dc [ ND ] ;
SpiceDouble descr [ DSCSIZ ];
SpiceDouble et;
SpiceDouble et0;
SpiceDouble state [ 6 ];
SpiceInt body;
SpiceInt center;
SpiceInt handle;
SpiceInt i;
SpiceInt ic [ NI ];
SpiceInt refID;
/.
Load meta-kernel.
./
furnsh_c ( META );
/.
Convert starting time to seconds past J2000 TDB.
./
timstr = "2012 APR 27 00:00:00.000 TDB";
str2et_c ( timstr, &et0 );
/.
Find a loaded segment for the Jupiter barycenter
that covers `et0'.
./
body = 5;
spksfs_c ( body, et0, SIDLEN,
&handle, descr, segid, &found );
if ( !found )
{
setmsg_c ( "No SPK segment available for body # at "
"time #." );
errint_c ( "#", body );
errch_c ( "#", timstr );
sigerr_c ( "SPICE(NOSEGMENT)" );
}
/.
Unpack the descriptor of the current segment.
./
dafus_c ( descr, 2, 6, dc, ic );
frmnam_c ( ic[2], FRNMLN, frname );
printf ( "\n"
" Body = %d\n"
" Center = %d\n"
" Frame = %s\n"
" Data type = %d\n"
" Start ET = %24.17e\n"
" Stop ET = %24.17e\n"
" Segment ID = %s\n",
(int)ic[0],
(int)ic[1],
frname,
(int)ic[3],
dc[0],
dc[1],
segid );
/.
Evaluate states at 10-second steps, starting at `et0'
and continuing for 20 seconds.
./
for ( i = 0; i < 3; i++ )
{
et = et0 + ( 10.0 * i );
/.
Convert `et' to a string for display.
./
timout_c ( et, TIMFMT, TIMLEN, outstr );
/.
Attempt to compute a state only if the segment's
coverage interval contains `et'.
./
if ( et <= dc[1] )
{
/.
This segment has data at `et'. Evaluate the
state of the target relative to its center
of motion.
./
spkpvn_c ( handle, descr, et, &refID, state, ¢er );
/.
Display the time and state.
./
printf ( "\n"
" %s\n"
" Position X (km): %24.17e\n"
" Position Y (km): %24.17e\n"
" Position Z (km): %24.17e\n"
" Velocity X (km/s): %24.17e\n"
" Velocity Y (km/s): %24.17e\n"
" Velocity Z (km/s): %24.17e\n",
outstr,
state[0], state[1], state[2],
state[3], state[4], state[5] );
}
else
{
printf ( "\nNo data for body %d found at %s.\n",
(int)body,
outstr );
exit(0);
}
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Body = 5
Center = 0
Frame = J2000
Data type = 2
Start ET = -3.16919520000000000e+09
Stop ET = 1.69685280000000000e+09
Segment ID = DE-0421LE-0421
2012 APR 27 00:00:00.000000 TDB
Position X (km): 4.64528993982164860e+08
Position Y (km): 5.41513126156852007e+08
Position Z (km): 2.20785135624629408e+08
Velocity X (km/s): -1.03868564830765493e+01
Velocity Y (km/s): 7.95324700713742416e+00
Velocity Z (km/s): 3.66185835431306517e+00
2012 APR 27 00:00:10.000000 TDB
Position X (km): 4.64528890113592625e+08
Position Y (km): 5.41513205689313412e+08
Position Z (km): 2.20785172243209451e+08
Velocity X (km/s): -1.03868579616041927e+01
Velocity Y (km/s): 7.95324528430304944e+00
Velocity Z (km/s): 3.66185765185608103e+00
2012 APR 27 00:00:20.000000 TDB
Position X (km): 4.64528786245005608e+08
Position Y (km): 5.41513285221757650e+08
Position Z (km): 2.20785208861782461e+08
Velocity X (km/s): -1.03868594401314791e+01
Velocity Y (km/s): 7.95324356146845002e+00
Velocity Z (km/s): 3.66185694939899253e+00
RestrictionsNone. Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) W.L. Taber (JPL) E.D. Wright (JPL) Version
-CSPICE Version 1.0.2, 27-AUG-2021 (JDR)
Edited the header to comply with NAIF standard.
-CSPICE Version 1.0.1, 12-JUL-2016 (EDW)
Edit to example program to use "%d" with explicit casts
to int for printing SpiceInts with printf.
-CSPICE Version 1.0.0, 05-OCT-2012 (NJB) (WLT)
Index_Entriesposition and velocity from ephemeris SPK file position and velocity |
Fri Dec 31 18:41:12 2021