| spkgps_c |
|
Table of contents
Procedure
spkgps_c ( S/P Kernel, geometric position )
void spkgps_c ( SpiceInt targ,
SpiceDouble et,
ConstSpiceChar * ref,
SpiceInt obs,
SpiceDouble pos[3],
SpiceDouble * lt )
AbstractCompute the geometric position of a target body relative to an observing body. Required_ReadingSPK KeywordsEPHEMERIS Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- targ I Target body. et I Target epoch. ref I Target reference frame. obs I Observing body. pos O Position of target. lt O Light time. Detailed_Input
targ is the standard NAIF ID code for a target body.
et is the epoch (ephemeris time) at which the position
of the target body is to be computed.
ref is the name of the reference frame to
which the vectors returned by the routine should
be rotated. This may be any frame supported by
the CSPICE subroutine sxform_c.
obs is the standard NAIF ID code for an observing body.
Detailed_Output
pos contains the position of the target
body, relative to the observing body. This vector is
rotated into the specified reference frame. Units
are always km.
lt is the one-way light time from the observing body
to the geometric position of the target body at the
specified epoch.
ParametersNone. Exceptions
1) If insufficient ephemeris data has been loaded to compute
the necessary positions, the error SPICE(SPKINSUFFDATA) is
signaled by a routine in the call tree of this routine.
2) If the `ref' input string pointer is null, the error
SPICE(NULLPOINTER) is signaled.
3) If the `ref' input string has zero length, the error
SPICE(EMPTYSTRING) is signaled.
FilesSee -Restrictions. Particulars
spkgps_c computes the geometric position, T(t), of the target
body and the geometric position, O(t), of the observing body
relative to the first common center of motion. Subtracting
O(t) from T(t) gives the geometric position of the target
body relative to the observer.
CENTER ----- O(t)
| /
| /
| /
| / T(t) - O(t)
| /
T(t)
The one-way light time, tau, is given by
| T(t) - O(t) |
tau = -----------------
c
For example, if the observing body is -94, the Mars Observer
spacecraft, and the target body is 401, Phobos, then the
first common center is probably 4, the Mars Barycenter.
O(t) is the position of -94 relative to 4 and T(t) is the
position of 401 relative to 4.
The center could also be the Solar System Barycenter, body 0.
For example, if the observer is 399, Earth, and the target
is 299, Venus, then O(t) would be the position of 399 relative
to 0 and T(t) would be the position of 299 relative to 0.
Ephemeris data from more than one segment may be required
to determine the positions of the target body and observer
relative to a common center. spkgps_c reads as many segments
as necessary, from as many files as necessary, using files
that have been loaded by previous calls to spklef_c (load
ephemeris file).
spkgps_c is similar to spkgeo_c but returns geometric positions
only.
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) Return the geometric position vector of Mars (499) as seen from
Earth (399) in the J2000 frame and the one-way light time
between them at the epoch July 4, 2003 11:00 AM PST.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: spkgps_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
--------- --------
de430.bsp Planetary ephemeris
mar097.bsp Mars satellite ephemeris
naif0011.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de430.bsp',
'mar097.bsp',
'naif0011.tls' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program spkgps_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local variables.
./
SpiceChar * epoch;
SpiceChar * reffrm;
SpiceDouble et;
SpiceDouble lt;
SpiceDouble pos [3];
SpiceInt obsrvr;
SpiceInt target;
/.
Load kernels.
./
furnsh_c ( "spkgps_ex1.tm" );
/.
Define parameters for a position lookup:
./
target = 499;
epoch = "July 4, 2003 11:00 AM PST";
reffrm = "J2000";
obsrvr = 399;
/.
Convert the epoch to ephemeris time.
./
str2et_c ( epoch, &et );
/.
Look-up the state for the defined parameters.
./
spkgps_c ( target, et, reffrm, obsrvr, pos, < );
/.
Output...
./
printf( "The position of : %d\n", target );
printf( "As observed from : %d\n", obsrvr );
printf( "In reference frame : %s\n", reffrm );
printf( "At epoch : %s\n", epoch );
printf( "\n" );
printf( " R (km): %17.6f %17.6f %17.6f\n",
pos[0], pos[1], pos[2] );
printf( "\n" );
printf( "Light time (s) between observer and target: %18.6f\n",
lt );
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
The position of : 499
As observed from : 399
In reference frame : J2000
At epoch : July 4, 2003 11:00 AM PST
R (km): 73826216.435288 -27128030.732406 -18741973.868287
Light time (s) between observer and target: 269.702648
Restrictions
1) The ephemeris files to be used by spkgps_c must be loaded
by spklef_c before spkgps_c is called.
Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) J.E. McLean (JPL) W.L. Taber (JPL) Version
-CSPICE Version 1.0.1, 10-AUG-2021 (JDR)
Edited the header to comply to NAIF standard. Added
complete code example, problem statement and solution.
Added entries #2 and #3 in -Exceptions section.
-CSPICE Version 1.0.0, 30-MAY-1999 (NJB) (JEM) (WLT)
Index_Entriesgeometric position of one body relative to another |
Fri Dec 31 18:41:12 2021