void spkltc_c ( SpiceInt targ,
SpiceDouble et,
ConstSpiceChar * ref,
ConstSpiceChar * abcorr,
ConstSpiceDouble stobs[6],
SpiceDouble starg[6],
SpiceDouble * lt,
SpiceDouble * dlt )
Return the state (position and velocity) of a target body
relative to an observer, optionally corrected for light time,
expressed relative to an inertial reference frame.
SPK
EPHEMERIS
Variable I/O Description
-------- --- --------------------------------------------------
targ I Target body.
et I Observer epoch.
ref I Inertial reference frame of output state.
abcorr I Aberration correction flag.
stobs I State of the observer relative to the SSB.
starg O State of target.
lt O One way light time between observer and target.
dlt O Derivative of light time with respect to time.
targ is the NAIF ID code for a target body. The target
and observer define a state vector whose position
component points from the observer to the target.
et is the ephemeris time, expressed as seconds past
J2000 TDB, at which the state of the target body
relative to the observer is to be computed. `et'
refers to time at the observer's location.
ref is the inertial reference frame with respect to which
the input state `stobs' and the output state `starg' are
expressed. `ref' must be recognized by the CSPICE
Toolkit. The acceptable frames are listed in the Frames
Required Reading, as well as in the CSPICE routine
chgirf_.
Case and blanks are not significant in the string
`ref'.
abcorr indicates the aberration corrections to be applied to
the state of the target body to account for one-way
light time. See the discussion in the Particulars
section for recommendations on how to choose
aberration corrections.
If `abcorr' includes the stellar aberration correction
symbol "+S", this flag is simply ignored. Aside from
the possible presence of this symbol, `abcorr' may be
any of the following:
"NONE" Apply no correction. Return the
geometric state of the target body
relative to the observer.
The following values of `abcorr' apply to the
"reception" case in which photons depart from the
target's location at the light-time corrected epoch
et-lt and *arrive* at the observer's location at `et':
"LT" Correct for one-way light time (also
called "planetary aberration") using a
Newtonian formulation. This correction
yields the state of the target at the
moment it emitted photons arriving at
the observer at `et'.
The light time correction involves
iterative solution of the light time
equation (see Particulars for details).
The solution invoked by the "LT" option
uses one iteration.
"CN" Converged Newtonian light time
correction. In solving the light time
equation, the "CN" correction iterates
until the solution converges (three
iterations on all supported platforms).
Whether the "CN+S" solution is
substantially more accurate than the
"LT" solution depends on the geometry
of the participating objects and on the
accuracy of the input data. In all
cases this routine will execute more
slowly when a converged solution is
computed. See the Particulars section of
spkezr_c for a discussion of precision of
light time corrections.
The following values of `abcorr' apply to the
"transmission" case in which photons *depart* from
the observer's location at `et' and arrive at the
target's location at the light-time corrected epoch
et+lt:
"XLT" "Transmission" case: correct for
one-way light time using a Newtonian
formulation. This correction yields the
state of the target at the moment it
receives photons emitted from the
observer's location at `et'.
"XCN" "Transmission" case: converged
Newtonian light time correction.
Neither special nor general relativistic effects are
accounted for in the aberration corrections applied
by this routine.
Case and blanks are not significant in the string
`abcorr'.
stobs is the geometric state of the observer relative
to the solar system barycenter at `et'. The
target and observer define a state vector whose
position component points from the observer to the
target. `stobs' is expressed relative to the reference
frame designated by `ref'.
starg is a Cartesian state vector representing the position
and velocity of the target body relative to the
specified observer. `starg' is corrected for the
specified aberration, and is expressed with respect
to the specified inertial reference frame. The first
three components of `starg' represent the x-, y- and
z-components of the target's position; last three
components form the corresponding velocity vector.
The position component of `starg' points from the
observer's location at `et' to the aberration-corrected
location of the target. Note that the sense of the
position vector is independent of the direction of
radiation travel implied by the aberration
correction.
Units are always km and km/sec.
lt is the one-way light time between the observer and
target in seconds. If the target state is corrected
for light time, then `lt' is the one-way light time
between the observer and the light time-corrected
target location.
dlt is the derivative with respect to barycentric
dynamical time of the one way light time between
target and observer:
dlt = d(lt)/d(et)
`dlt' can also be described as the rate of change of
one way light time. `dlt' is unitless, since `lt' and
`et' both have units of TDB seconds.
If the observer and target are at the same position,
then `dlt' is set to zero.
None.
1) For the convenience of the caller, the input aberration
correction flag can call for stellar aberration correction via
inclusion of the '+S' suffix. This portion of the aberration
correction flag is ignored if present.
2) If `abcorr' calls for stellar aberration but not light
time corrections, the error SPICE(NOTSUPPORTED) is
signaled.
3) If `abcorr' calls for relativistic light time corrections, the
error SPICE(NOTSUPPORTED) is signaled.
4) If the value of `abcorr' is not recognized, the error
is diagnosed by routines in the call tree of this routine.
5) If the reference frame requested is not a recognized
inertial reference frame, the error SPICE(BADFRAME)
is signaled.
6) If the state of the target relative to the solar system
barycenter cannot be computed, the error is diagnosed
by routines in the call tree of this routine.
7) If the observer and target are at the same position,
then `dlt' is set to zero. This situation could arise,
for example, when the observer is Mars and the target
is the Mars barycenter.
8) If a division by zero error would occur in the computation
of `dlt', the error SPICE(DIVIDEBYZERO) is signaled.
9) The error SPICE(EMPTYSTRING) is signaled if either of the input
strings `ref' or `abcorr' do not contain at least one character,
since such an input string cannot be converted to a
Fortran-style string.
10) The error SPICE(NULLPOINTER) is signaled if either of the input
string pointers `ref' or `abcorr' are null.
This routine computes states using SPK files that have been
loaded into the SPICE system, normally via the kernel loading
interface routine furnsh_c. Application programs typically load
kernels once before this routine is called, for example during
program initialization; kernels need not be loaded repeatedly.
See the routine furnsh_c and the SPK and KERNEL Required Reading
for further information on loading (and unloading) kernels.
If any of the ephemeris data used to compute `starg' are expressed
relative to a non-inertial frame in the SPK files providing those
data, additional kernels may be needed to enable the reference
frame transformations required to compute the state. Normally
these additional kernels are PCK files or frame kernels. Any
such kernels must already be loaded at the time this routine is
called.
This routine supports higher-level SPK API routines that can
perform both light time and stellar aberration corrections.
User applications normally will not need to call this routine
directly.
See the header of the routine spkezr_c for a detailed discussion
of aberration corrections.
1) Look up a sequence of states of the Moon as seen from the
Earth. Use light time corrections. Compute the first state for
the epoch 2000 JAN 1 12:00:00 TDB; compute subsequent states at
intervals of 1 hour. For each epoch, display the states, the one
way light time between target and observer, and the rate of
change of the one way light time.
Use the following meta-kernel to specify the kernels to
load:
KPL/MK
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.
\begindata
KERNELS_TO_LOAD = ( 'de418.bsp',
'pck00008.tpc',
'naif0008.tls' )
\begintext
The code example follows:
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local constants
The meta-kernel name shown here refers to a file whose contents
are those shown above. This file and the kernels it references
must exist in your current working directory.
./
#define META "example.mk"
/.
Use a time step of 1 hour; look up 100 states.
./
#define STEP 3600.0
#define MAXITR 5
/.
Local variables
./
SpiceDouble dlt;
SpiceDouble et;
SpiceDouble et0;
SpiceDouble lt;
SpiceDouble state [6];
SpiceDouble stobs [6];
SpiceInt i;
/.
Load the SPK and LSK kernels via the meta-kernel.
./
furnsh_c ( META );
/.
Convert the start time to seconds past J2000 TDB.
./
str2et_c ( "2000 JAN 1 12:00:00 TDB", &et0 );
/.
Step through a series of epochs, looking up a
state vector at each one.
./
for ( i = 0; i < MAXITR; i++ )
{
et = et0 + i*STEP;
/.
Look up a state vector at epoch ET using the
following inputs:
Target: Moon (NAIF ID code 301)
Reference frame: J2000
Aberration correction: Light time ('LT')
Observer: Earth (NAIF ID code 399)
Before we can execute this computation, we'll need
the geometric state of the observer relative to the
solar system barycenter at ET, expressed relative
to the J2000 reference frame:
./
spkssb_c ( 399, et, "j2000", stobs );
spkltc_c ( 301, et, "j2000", "lt",
stobs, state, <, &dlt );
printf( "et = %20.6f\n", et );
printf( "J2000 x-position (km): %20.8f\n", state[0] );
printf( "J2000 y-position (km): %20.8f\n", state[1] );
printf( "J2000 z-position (km): %20.8f\n", state[2] );
printf( "J2000 x-velocity (km/s): %20.12f\n", state[3] );
printf( "J2000 y-velocity (km/s): %20.12f\n", state[4] );
printf( "J2000 z-velocity (km/s): %20.12f\n", state[5] );
printf( "One-way light time (s): %20.12f\n", lt );
printf( "Light time rate: %20.08e\n\n", dlt );
}
return ( 0 );
}
The output produced by this program will vary somewhat as
a function of the platform on which the program is built and
executed. On a PC/Linux/gcc platform, the following output
was produced:
et = 0.000000
J2000 x-position (km): -291569.26541283
J2000 y-position (km): -266709.18647826
J2000 z-position (km): -76099.15511876
J2000 x-velocity (km/s): 0.643530613222
J2000 y-velocity (km/s): -0.666081817008
J2000 z-velocity (km/s): -0.301322831796
One-way light time (s): 1.342310610325
Light time rate: 1.07316909e-07
et = 3600.000000
J2000 x-position (km): -289240.78128184
J2000 y-position (km): -269096.44087958
J2000 z-position (km): -77180.89972576
J2000 x-velocity (km/s): 0.650062115201
J2000 y-velocity (km/s): -0.660162739217
J2000 z-velocity (km/s): -0.299642673906
One-way light time (s): 1.342693954864
Light time rate: 1.05652599e-07
et = 7200.000000
J2000 x-position (km): -286888.88736709
J2000 y-position (km): -271462.30170548
J2000 z-position (km): -78256.55568214
J2000 x-velocity (km/s): 0.656535991543
J2000 y-velocity (km/s): -0.654196576804
J2000 z-velocity (km/s): -0.297940273074
One-way light time (s): 1.343071311734
Light time rate: 1.03990457e-07
et = 10800.000000
J2000 x-position (km): -284513.79173691
J2000 y-position (km): -273806.60031034
J2000 z-position (km): -79326.04318327
J2000 x-velocity (km/s): 0.662951900546
J2000 y-velocity (km/s): -0.648183807097
J2000 z-velocity (km/s): -0.296215779371
One-way light time (s): 1.343442689069
Light time rate: 1.02330665e-07
et = 14400.000000
J2000 x-position (km): -282115.70368389
J2000 y-position (km): -276129.16976799
J2000 z-position (km): -80389.28296571
J2000 x-velocity (km/s): 0.669309503775
J2000 y-velocity (km/s): -0.642124908057
J2000 z-velocity (km/s): -0.294469343362
One-way light time (s): 1.343808095656
Light time rate: 1.00673404e-07
1) The kernel files to be used by spkltc_c must be loaded
(normally by the CSPICE kernel loader furnsh_c) before
this routine is called.
2) Unlike most other SPK state computation routines, this
routine requires that the output state be relative to an
inertial reference frame.
SPK Required Reading.
N.J. Bachman (JPL)
-CSPICE Version 1.0.1, 07-JUL-2014 (NJB)
Discussion of light time corrections was updated. Assertions
that converged light time corrections are unlikely to be
useful were removed.
-CSPICE Version 1.0.0, 11-JAN-2008 (NJB)
low-level light time correction
light-time corrected state from spk file
get light-time corrected state
Link to routine spkltc_c source file spkltc_c.c
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