Table of contents
spkgeo_c ( S/P Kernel, geometric state )
void spkgeo_c ( SpiceInt targ,
ConstSpiceChar * ref,
SpiceDouble * lt )
Compute the geometric state (position and velocity) of a target
body relative to an observing body.
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
targ I Target body.
et I Target epoch.
ref I Target reference frame.
obs I Observing body.
state O State of target.
lt O Light time.
targ is the standard NAIF ID code for a target body.
et is the epoch (ephemeris time) at which the state
of the target body is to be computed.
ref is the name of the reference frame to which the state
vector returned by the routine should be rotated. This
may be any frame supported by the SPICELIB subroutine
FRMCHG. See also the Frames Required Reading for a list
of supported frames.
obs is the standard NAIF ID code for an observing body.
state contains the geometric position and velocity of the
target body, relative to the observing body, at epoch
`et'. `state' has six elements: the first three contain
the target's position; the last three contain the
target's velocity. These vectors are transformed into
the specified reference frame.
Units are always km and km/sec.
lt is the one-way light time from the observing body
to the geometric position of the target body
in seconds at the specified epoch.
1) If insufficient ephemeris data has been loaded to compute
the necessary states, 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.
spkgeo_c computes the geometric state, targ(t), of the target
body and the geometric state, obs(t), of the observing body
relative to the first common center of motion. Subtracting
obs(t) from targ(t) gives the geometric state of the target
body relative to the observer.
center ----- obs(t)
| / targ(t) - obs(t)
The one-way light time, tau, is given by
| targ(t) - obs(t) |
tau = ----------------------
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.
obs(t) is the state of -94 relative to 4 and targ(t) is the
state 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 obs(t) would be the state of 399 relative
to 0 and targ(t) would be the state of 299 relative to 0.
Ephemeris data from more than one segment may be required
to determine the states of the target body and observer
relative to a common center. spkgeo_c reads as many segments
as necessary, from as many files as necessary, using files
that have been loaded by previous calls to furnsh_c or
spklef_c (load ephemeris file).
spkgeo_c is similar to spkez_c but returns geometric states
only, with no option to make planetary (light-time) nor
stellar aberration corrections. The geometric states
returned by spkez_c and spkgeo_c are the same.
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 state 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
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
KERNELS_TO_LOAD = ( 'de430.bsp',
End of meta-kernel
Example code begins here.
int main( )
SpiceChar * epoch;
SpiceChar * reffrm;
SpiceDouble state ;
Load a set of kernels. Use a meta
kernel for convenience.
furnsh_c ( "spkgeo_ex1.tm" );
Define parameters for a state 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.
spkgeo_c ( target, et, reffrm, obsrvr, state, < );
printf( "The position of : %2d\n", target );
printf( "As observed from : %2d\n", obsrvr );
printf( "In reference frame : %s\n", reffrm );
printf( "At epoch : %s\n", epoch );
printf( " \n" );
The first three entries of state contain the
X, Y, Z position components. The final three contain
the Vx, Vy, Vz velocity components.
printf( "R (km): %17.5f %17.5f %17.5f\n",
state, state, state );
printf( "V (km/s): %17.5f %17.5f %17.5f\n",
state, state, state );
printf( " \n" );
printf( "Light time (s) between observer and target:"
" %18.13f\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): 73826216.43529 -27128030.73241 -18741973.86829
V (km/s): -6.80950 7.51381 3.00129
Light time (s) between observer and target: 269.7026477631753
1) The ephemeris files to be used by spkgeo_c must be loaded
by furnsh_c or spklef_c before spkgeo_c is called.
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
J.E. McLean (JPL)
B.V. Semenov (JPL)
W.L. Taber (JPL)
E.D. Wright (JPL)
-CSPICE Version 1.1.3, 10-AUG-2021 (JDR)
Edited the header to comply with NAIF standard.
Added complete code example to -Examples section.
Added reference to furnsh_c in -Particulars and -Restrictions
Added entries #2 and #3 in -Exceptions section.
-CSPICE Version 1.1.2, 08-JAN-2014 (BVS)
Fixed description of "state" in -Detailed_Output. Replaced
spklef_c with furnsh_c and fixed errors in -Examples.
-CSPICE Version 1.1.1, 13-OCT-2003 (EDW)
Various minor header changes were made to improve clarity.
Added mention that 'lt' returns a value in seconds.
-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) (JEM) (WLT)
Based on SPICELIB Version 2.2.0, 11-APR-1997 (WLT)
geometric state of one body relative to another