| spkaps_c |
|
Table of contents
Procedure
spkaps_c ( SPK, apparent state )
void spkaps_c ( SpiceInt targ,
SpiceDouble et,
ConstSpiceChar * ref,
ConstSpiceChar * abcorr,
ConstSpiceDouble stobs [6],
ConstSpiceDouble accobs[3],
SpiceDouble starg [6],
SpiceDouble * lt,
SpiceDouble * dlt )
AbstractReturn the state (position and velocity) of a target body relative to an observer specified by its state and acceleration relative to the solar system barycenter. The returned state may be optionally corrected for light time and stellar aberration. All input and output vectors are expressed relative to an inertial reference frame. This routine supersedes spkapp_c. SPICE users normally should call the high-level API routines spkezr_c or spkez_c rather than this routine. Required_ReadingSPK KeywordsEPHEMERIS Brief_I/OVARIABLE 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. accobs I Acceleration 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. Detailed_Input
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', the input acceleration `accobs',
and the output state `starg' are expressed. `ref' must be
recognized by the SPICE 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 and stellar aberration. See the discussion in
the -Particulars section of spkezr_c for recommendations
on how to choose aberration corrections.
`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 uses an
iterative solution of the light time
equation (see -Particulars for details).
The solution invoked by the "LT" option
uses one iteration.
"LT+S" Correct for one-way light time and
stellar aberration using a Newtonian
formulation. This option modifies the
state obtained with the "LT" option to
account for the observer's velocity
relative to the solar system
barycenter. The result is the apparent
state of the target---the position and
velocity of the target as seen by the
observer.
"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.
"CN+S" Converged Newtonian light time
correction and stellar aberration
correction.
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'.
"XLT+S" "Transmission" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation This option modifies the
state obtained with the "XLT" option to
account for the observer's velocity
relative to the solar system
barycenter. The position component of
the computed target state indicates the
direction that photons emitted from the
observer's location must be "aimed" to
hit the target.
"XCN" "Transmission" case: converged
Newtonian light time correction.
"XCN+S" "Transmission" case: converged
Newtonian light time correction and
stellar aberration 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'.
accobs is the geometric acceleration of the observer
relative to the solar system barycenter at `et'. This
is the derivative with respect to time of the
velocity portion of `stobs'. `accobs' is expressed
relative to the reference frame designated by `ref'.
`accobs' is used for computing stellar aberration
corrected velocity. If stellar aberration corrections
are not specified by `abcorr', `accobs' is ignored; the
caller need not provide a valid input value in this
case.
Detailed_Output
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.
ParametersNone. Exceptions
1) If the value of `abcorr' is not recognized, an error is signaled
by a routine in the call tree of this routine.
2) If `abcorr' calls for stellar aberration but not light
time corrections, the error SPICE(NOTSUPPORTED) is
signaled by a routine in the call tree of this routine.
3) If `abcorr' calls for relativistic light time corrections, the
error SPICE(NOTSUPPORTED) is signaled by a routine in the call
tree of this routine.
4) If the reference frame requested is not a recognized
inertial reference frame, the error SPICE(BADFRAME)
is signaled by a routine in the call tree of this routine.
5) If the state of the target relative to the solar system
barycenter cannot be computed, an error is signaled by a
routine in the call tree of this routine.
6) 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.
7) If any of the `ref' or `abcorr' input string pointers is null,
the error SPICE(NULLPOINTER) is signaled.
8) If any of the `ref' or `abcorr' input strings has zero length,
the error SPICE(EMPTYSTRING) is signaled.
FilesThis 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. Particulars
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. However, this routine can improve run-time efficiency
in situations where many targets are observed from the same
location at the same time. In such cases, the state and
acceleration of the observer relative to the solar system
barycenter need be computed only once per look-up epoch.
When apparent positions, rather than apparent states, are
required, consider using the high-level position-only API
routines
spkpos_c
spkezp_c
or the low-level, position-only analog of this routine
spkapo_c
In general, the position-only routines are more efficient.
See the header of the routine spkezr_c for a detailed discussion
of aberration corrections.
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) Look up a sequence of states of the Moon as seen from the
Earth. Use light time and stellar aberration 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 meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: spkaps_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
--------- --------
de418.bsp Planetary ephemeris
pck00008.tpc Planet orientation and
radii
naif0008.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de418.bsp',
'pck00008.tpc',
'naif0008.tls' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program spkaps_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
#include "SpiceZfc.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 "spkaps_ex1.tm"
/.
Use a time step of 1 hour; look up 5 states.
./
#define STEP 3600.0
#define MAXITR 5
/.
Local variables
./
SpiceDouble acc [3];
SpiceDouble dlt;
SpiceDouble et;
SpiceDouble et0;
SpiceDouble lt;
SpiceDouble state [6];
SpiceDouble state0 [6];
SpiceDouble state2 [6];
SpiceDouble stobs [6];
SpiceDouble tdelta;
SpiceInt dim;
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 and stellar
aberration ('LT+S')
Observer: Earth (NAIF ID code 399)
Before we can execute this computation, we'll need the
geometric state and acceleration of the observer relative to
the solar system barycenter at ET, expressed relative to the
J2000 reference frame. First find the state:
./
spkssb_c ( 399, et, "j2000", stobs );
/.
Next compute the acceleration. We numerically differentiate
the velocity using a quadratic approximation.
./
tdelta = 1.0;
spkssb_c ( 399, et-tdelta, "j2000", state0 );
spkssb_c ( 399, et+tdelta, "j2000", state2 );
dim = 3;
qderiv_c ( dim, state0+3, state2+3, tdelta, acc );
/.
Now compute the desired state vector:
./
spkaps_c ( 301, et, "j2000", "lt+s",
stobs, acc, 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 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
et = 0.000000
J2000 x-position (km): -291584.61369498
J2000 y-position (km): -266693.40583163
J2000 z-position (km): -76095.65320924
J2000 x-velocity (km/s): 0.643439157435
J2000 y-velocity (km/s): -0.666065873657
J2000 z-velocity (km/s): -0.301310063429
One-way light time (s): 1.342310610325
Light time rate: 1.07316909e-07
et = 3600.000000
J2000 x-position (km): -289256.45942322
J2000 y-position (km): -269080.60545908
J2000 z-position (km): -77177.35277130
J2000 x-velocity (km/s): 0.649970320169
J2000 y-velocity (km/s): -0.660148253293
J2000 z-velocity (km/s): -0.299630417907
One-way light time (s): 1.342693954864
Light time rate: 1.05652599e-07
et = 7200.000000
J2000 x-position (km): -286904.89654240
J2000 y-position (km): -271446.41676468
J2000 z-position (km): -78252.96553362
J2000 x-velocity (km/s): 0.656443883155
J2000 y-velocity (km/s): -0.654183552046
J2000 z-velocity (km/s): -0.297928532945
One-way light time (s): 1.343071311734
Light time rate: 1.03990457e-07
et = 10800.000000
J2000 x-position (km): -284530.13302757
J2000 y-position (km): -273790.67111559
J2000 z-position (km): -79322.41170392
J2000 x-velocity (km/s): 0.662859504730
J2000 y-velocity (km/s): -0.648172246851
J2000 z-velocity (km/s): -0.296204558469
One-way light time (s): 1.343442689069
Light time rate: 1.02330665e-07
et = 14400.000000
J2000 x-position (km): -282132.37807792
J2000 y-position (km): -276113.20159697
J2000 z-position (km): -80385.61203056
J2000 x-velocity (km/s): 0.669216846492
J2000 y-velocity (km/s): -0.642114815280
J2000 z-velocity (km/s): -0.294458644904
One-way light time (s): 1.343808095656
Light time rate: 1.00673404e-07
Restrictions
1) This routine should not be used to compute geometric states.
Instead, use spkezr_c, spkez_c, or spkgeo_c. spkgeo_c, which is called
by spkezr_c and spkez_c, introduces less round-off error when the
observer and target have a common center that is closer to
both objects than is the solar system barycenter.
2) The kernel files to be used by spkaps_c must be loaded
(normally by the CSPICE kernel loader furnsh_c) before
this routine is called.
3) Unlike most other SPK state computation routines, this
routine requires that the output state be relative to an
inertial reference frame.
Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) Version
-CSPICE Version 1.1.0, 01-NOV-2021 (JDR)
Fixed size of "accobs" argument: it was 6 when it should be 3.
Edited the header and meta-kernel in -Examples section to comply with
NAIF standard. Updated code example comments.
Updated code example to use qderiv_c instead of f2c'ed routine.
-CSPICE Version 1.0.1, 07-JUL-2014 (NJB)
Descriptions of aberration correction choices that include
stellar aberration were missing. These have been added.
Erroneous claim that stellar aberration specifiers (instances
of "+S") in `abcorr' are ignored was deleted.
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)
Index_Entrieslow-level aberration-corrected state computation low-level light time and stellar aberration correction |
Fri Dec 31 18:41:12 2021