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 have been loaded to compute
the requested state, the error SPICE(SPKINSUFFDATA) is
spkgeo_c computes the geometric state, T(t), of the target
body and the geometric state, O(t), of the observing body
relative to the first common center of motion. Subtracting
O(t) from T(t) gives the geometric state of the target
body relative to the observer.
CENTER ----- O(t)
| / T(t) - O(t)
The one-way light time, tau, is given by
| T(t) - O(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.
O(t) is the state of -94 relative to 4 and T(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 O(t) would be the state of 399 relative
to 0 and T(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 spklef_c (load
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 following code example computes the geometric
state of the moon with respect to the earth and
then prints the distance of the moon from the
the earth at a number of epochs.
Assume the SPK file sample.bsp contains ephemeris data
for the moon relative to earth over the time interval
from BEGIN to END.
#define EARTH 399
#define MOON 301
#define N 100
#define UTCLEN 35
SpiceChar utc [ 20 ];
SpiceDouble state ;
Load the binary SPK ephemeris file.
furnsh_c ( "sample.bsp" );
Divide the interval of coverage [BEGIN,END] into
N steps. At each step, compute the state, and
print out the epoch in UTC time and position norm.
delta = ( end - begin ) / N;
for ( i = 0; i < N; i++ )
et = begin + i*delta;
spkgeo_c ( MOON, et, "J2000", EARTH, state, < );
et2utc_c ( et, "c", 0, UTCLEN, utc );
printf ( "UTC = %s; ||pos|| = %f\n", utc, vnorm_c(state) );
1) The ephemeris files to be used by spkgeo_c must be loaded
by SPKLEF before spkgeo_c is called.
N.J. Bachman (JPL)
J.E. McLean (JPL)
W.L. Taber (JPL)
-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)
Based on SPICELIB Version 2.2.0, 11-APR-1997 (WLT)
geometric state of one body relative to another