void spkgps_c ( SpiceInt targ,
ConstSpiceChar * ref,
SpiceDouble * lt )
Compute the geometric position of a target body relative to an
Variable 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.
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.
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
1) If insufficient ephemeris data has been loaded to compute
the necessary positions, the error SPICE(SPKINSUFFDATA) is
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)
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 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
spkgps_c is similar to spkgeo_c but returns geometric positions
The following code example computes the geometric
position 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
whose endpoints are represented by the variables begin and
#define EARTH 399
#define MOON 301
#define N 100
#define TIMLEN 30
SpiceChar utc [TIMLEN];
SpiceDouble pos ;
Load the binary SPK ephemeris file.
spklef_c ( "SAMPLE.BSP", &handle );
Divide the interval of coverage [begin,end] into
n steps. At each step, compute the position, 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;
spkgps_c ( MOON, et, "J2000", EARTH, pos, < );
et2utc_c ( et, "C", 0, utc );
printf ( "%s %25.15e\n", utc, vnorm_c(pos) );
return ( 0 );
1) The ephemeris files to be used by spkgps_c must be loaded
by spklef_c before spkgps_c is called.
N.J. Bachman (JPL)
J.E. McLean (JPL)
W.L. Taber (JPL)
-CSPICE Version 1.0.0, 30-MAY-1999 (NJB) (JEM) (WLT)
geometric position of one body relative to another