Table of contents
CSPICE_SPKAPO returns the position 3-vector of a target
body relative to an observer, optionally corrected for
light time and stellar aberration, given the geometric
state 6-vector of the observer relative to the Solar System
Barycenter.
Given:
targ the NAIF ID code for a target body.
help, targ
LONG = Scalar
The target and observer define a position vector which points
from the observer to the target.
et the ephemeris time, expressed as seconds past J2000 TDB, at
which the position of the target body relative to the observer
is to be computed.
help, et
DOUBLE = Scalar
`et' refers to time at the observer's location.
ref the inertial reference frame with respect to which the
observer's state `sobs' is expressed.
help, ref
STRING = Scalar
`ref' must be recognized by the SPICE Toolkit. The acceptable
frames are listed in the Frames Required Reading, as well as in
the SPICELIB routine CHGIRF.
Case and blanks are not significant in the string `ref'.
sobs the geometric (uncorrected) state of the observer relative to
the solar system barycenter at epoch `et'.
help, sobs
DOUBLE = Array[6]
`sobs' is a 6-vector: the first three components of `sobs'
represent a Cartesian position vector; the last three components
represent the corresponding velocity vector. `sobs' is expressed
relative to the inertial reference frame designated by `ref'.
Units are always km and km/sec.
abcorr indicates the aberration corrections to be applied to the
position of the target body to account for one-way light time
and stellar aberration.
help, abcorr
STRING = Scalar
See the discussion in the -Particulars section for
recommendations on how to choose aberration corrections.
`abcorr' may be any of the following:
'NONE' Apply no correction. Return the
geometric position 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-ltime 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 position 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.
'LT+S' Correct for one-way light time and
stellar aberration using a Newtonian
formulation. This option modifies the
position obtained with the 'LT' option
to account for the observer's velocity
relative to the solar system
barycenter. The result is the apparent
position of the target---the position
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 cspice_spkezr 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+ltime:
'XLT' "Transmission" case: correct for
one-way light time using a Newtonian
formulation. This correction yields the
position 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
position obtained with the 'XLT' option
to account for the observer's velocity
relative to the solar system
barycenter. The target position
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'.
the call:
cspice_spkapo, targ, et, ref, sobs, abcorr, ptarg, ltime
returns:
ptarg a Cartesian 3-vector representing the position of the target
body relative to the specified observer.
help, ptarg
DOUBLE = Array[3]
`ptarg' is corrected for the specified aberrations, and is
expressed with respect to the specified inertial reference
frame. The components of `ptarg' represent the x-, y- and
z-components of the target's position.
The vector `ptarg' points from the observer's position
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.
ltime the one-way light time between the observer and target in
seconds.
help, ltime
DOUBLE = Scalar
If the target position is corrected for aberrations, then
`ltime' is the one-way light time between the observer and the
light time corrected target location.
None.
Any numerical results shown for this example may differ between
platforms as the results depend on the SPICE kernels used as input
and the machine specific arithmetic implementation.
1) Compute the position of Mars relative to the Moon, corrected
for one-way light time and stellar aberration, given the
geometric state of the Moon relative to the Solar System
Barycenter.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: spkapo_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
--------- --------
de421.bsp Planetary ephemeris
naif0009.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'naif0009.tls' )
\begintext
End of meta-kernel
Example code begins here.
PRO spkapo_ex1
;;
;; Define the needed parameters.
;;
MARS = 499
MOON = 301
EPOCH = 'Jan 1 2004 5:00 PM'
REF = 'J2000'
ABCORR = 'LT+S'
;;
;; Load the kernels.
;;
cspice_furnsh, 'spkapo_ex1.tm'
;;
;; Convert the time string to ephemeris time J2000.
;;
cspice_str2et, EPOCH, et
;;
;; Compute the geometric state of the Moon wrt the
;; the Solar System barycenter.
;;
cspice_spkssb, MOON, et, REF, state
;;
;; Apply an aberration correction for a position vector
;; of the MOON as seen from Mars.
;;
cspice_spkapo, MARS, et, REF, state, ABCORR, pos_vec, ltime
print, 'Position vector of : ' + string(MARS)
print, '...as seen from : ' + string(MOON)
print, '...in frame : ' + REF
print, '...at time : ' + EPOCH
print, '...with correction : ' + ABCORR
print, FORMAT='(A,3F15.4)', '...equals (km): ', pos_vec
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_kclear
END
When this program was executed on a Mac/Intel/IDL8.x/64-bit
platform, the output was:
Position vector of : 499
...as seen from : 301
...in frame : J2000
...at time : Jan 1 2004 5:00 PM
...with correction : LT+S
...equals (km): 164534472.3125 25121994.3686 11145412.8385
Note that this code example could be replaced by the following
cspice_spkpos call:
cspice_spkpos, 'MARS', et, REF, ABCORR, 'MOON', pos_vec, ltime
Note: NAIF recommends the use of cspice_spkpos instead of cspice_spkapo
to calculate observer to target position vectors.
Please refer to the Aberation Corrections Required Reading (abcorr.req)
for detailed information describing the nature and calculation of the
applied corrections.
1) If the value of `abcorr' is not recognized, the error
SPICE(SPKINVALIDOPTION) is signaled by a routine in the call
tree of this routine.
2) 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.
3) If the position 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.
4) If any of the input arguments, `targ', `et', `ref', `sobs' or
`abcorr', is undefined, an error is signaled by the IDL error
handling system.
5) If any of the input arguments, `targ', `et', `ref', `sobs' or
`abcorr', is not of the expected type, or it does not have the
expected dimensions and size, an error is signaled by the Icy
interface.
6) If any of the output arguments, `ptarg' or `ltime', is not a
named variable, an error is signaled by the Icy interface.
This routine computes positions using SPK files that have been
loaded into the SPICE system, normally via the kernel loading
interface routine cspice_furnsh. 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 cspice_furnsh and the SPK and KERNEL Required Reading
for further information on loading (and unloading) kernels.
If any of the ephemeris data used to compute `ptarg' 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 `ptarg'. Normally
these additional kernels are PCK files or frame kernels. Any
such kernels must already be loaded at the time this routine is
called.
1) The ephemeris files to be used by cspice_spkapo must be loaded
(normally by the Icy kernel loader cspice_furnsh) before
this routine is called.
2) Unlike most other SPK position computation routines, this
routine requires that the input state be relative to an
inertial reference frame. Non-inertial frames are not
supported by this routine.
3) In a future version of this routine, the implementation
of the aberration corrections may be enhanced to improve
accuracy.
ICY.REQ
ABCORR.REQ
SPK.REQ
NAIF_IDS.REQ
FRAMES.REQ
TIME.REQ
None.
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
E.D. Wright (JPL)
-Icy Version 1.0.5, 10-AUG-2021 (JDR)
Edited the header to comply with NAIF standard. Added example's
problem statement and meta-kernel. Reformatted example's output and
added call to cspice_kclear.
Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections.
Removed reference to the routine's corresponding CSPICE header from
-Abstract section.
Added arguments' type and size information in the -I/O section.
-Icy Version 1.0.4, 01-APR-2015 (EDW)
Corrected typo in -Version section. The 07-NOV-2013
version entry lacked a version ID.
Particulars updated to refer to Aberration Corrections
Required Reading document.
-Icy Version 1.0.3, 10-JUL-2014 (NJB) (EDW)
Discussion of light time corrections was updated. Assertions
that converged light time corrections are unlikely to be
useful were removed.
-Icy Version 1.0.2, 07-NOV-2013 (EDW)
Improvement of -I/O section descriptions. Added aberration
algorithm explanation to -Particulars section.
-Icy Version 1.0.1, 23-SEP-2008 (EDW)
Eliminated error in English.
-Icy Version 1.0.0, 16-JUN-2003 (EDW)
apparent position from SPK file
get apparent position
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