| spkpos |
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Table of contents
Procedure
SPKPOS ( S/P Kernel, position )
SUBROUTINE SPKPOS ( TARG, ET, REF, ABCORR, OBS, PTARG, LT )
Abstract
Return the position of a target body relative to an observing
body, optionally corrected for light time (planetary aberration)
and stellar aberration.
Required_Reading
ABCORR
SPK
NAIF_IDS
FRAMES
TIME
Keywords
EPHEMERIS
Declarations
IMPLICIT NONE
INCLUDE 'zzctr.inc'
CHARACTER*(*) TARG
DOUBLE PRECISION ET
CHARACTER*(*) REF
CHARACTER*(*) ABCORR
CHARACTER*(*) OBS
DOUBLE PRECISION PTARG ( 3 )
DOUBLE PRECISION LT
Brief_I/O
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
TARG I Target body name.
ET I Observer epoch.
REF I Reference frame of output position vector.
ABCORR I Aberration correction flag.
OBS I Observing body name.
PTARG O Position of target.
LT O One way light time between observer and target.
Detailed_Input
TARG is the name of a target body. Optionally, you may
supply the integer ID code for the object as an
integer string. For example both 'MOON' and '301'
are legitimate strings that indicate the moon is the
target body.
The target and observer define a position vector
which points from the observer to the target.
ET is 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. ET refers to time at the
observer's location.
REF is the name of the reference frame relative to which
the output position vector should be expressed. This
may be any frame supported by the SPICE system,
including built-in frames (documented in the Frames
Required Reading) and frames defined by a loaded
frame kernel (FK).
When REF designates a non-inertial frame, the
orientation of the frame is evaluated at an epoch
dependent on the selected aberration correction. See
the description of the output position vector PTARG
for details.
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. 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-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 position 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
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
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
below 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
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 computed 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.
OBS is the name of an observing body. Optionally, you
may supply the ID code of the object as an integer
string. For example, both 'EARTH' and '399' are
legitimate strings to supply to indicate the
observer is Earth.
Detailed_Output
PTARG is a Cartesian 3-vector representing the position of
the target body relative to the specified observer.
PTARG is corrected for the specified aberrations, and
is expressed with respect to the reference frame
specified by REF. The three components of PTARG
represent the x-, y- and z-components of the target's
position.
PTARG points from the observer's location at ET to
the aberration-corrected location of the target.
Note that the sense of this position vector is
independent of the direction of radiation travel
implied by the aberration correction.
Units are always km.
Non-inertial frames are treated as follows: letting
LTCENT be the one-way light time between the observer
and the central body associated with the frame, the
orientation of the frame is evaluated at ET-LTCENT,
ET+LTCENT, or ET depending on whether the requested
aberration correction is, respectively, for received
radiation, transmitted radiation, or is omitted.
LTCENT is computed using the method indicated by
ABCORR.
LT is the one-way light time between the observer and
target in seconds. If the target position is
corrected for aberrations, then LT is the one-way
light time between the observer and the light time
corrected target location.
Parameters
None.
Exceptions
1) If name of target or observer cannot be translated to its
NAIF ID code, the error SPICE(IDCODENOTFOUND) is signaled.
2) If the reference frame REF is not a recognized reference
frame, the error SPICE(UNKNOWNFRAME) is signaled.
3) If the loaded kernels provide insufficient data to compute the
requested position vector, an error is signaled by a routine
in the call tree of this routine.
4) If an error occurs while reading an SPK or other kernel file,
the error is signaled by a routine in the call tree
of this routine.
Files
This routine computes positions using SPK files that have been
loaded into the SPICE system, normally via the kernel loading
interface routine FURNSH. See the routine FURNSH and the SPK
and KERNEL Required Reading for further information on loading
(and unloading) kernels.
If the output position PTARG is to be expressed relative to a
non-inertial frame, or 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 the position. These additional kernels may be C-kernels,
PCK files or frame kernels. Any such kernels must already be
loaded at the time this routine is called.
Particulars
This routine is part of the user interface to the SPICE ephemeris
system. It allows you to retrieve position information for any
ephemeris object relative to any other in a reference frame that
is convenient for further computations.
This routine is identical in function to the routine SPKEZP
except that it allows you to refer to ephemeris objects by name
(via a character string).
Please refer to the Aberration Corrections Required Reading
abcorr.req for detailed information describing the nature and
calculation of the applied 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) Load a planetary ephemeris SPK, then look up a series of
geometric positions of the moon relative to the earth,
referenced to the J2000 frame.
Use the SPK kernel below to load the required Earth and
Moon ephemeris data.
de421.bsp
Example code begins here.
IMPLICIT NONE
C
C Local constants
C
CHARACTER*(*) FRAME
PARAMETER ( FRAME = 'J2000' )
CHARACTER*(*) ABCORR
PARAMETER ( ABCORR = 'NONE' )
CHARACTER*(*) SPK
PARAMETER ( SPK = 'de421.bsp' )
C
C ET0 represents the date 2000 Jan 1 12:00:00 TDB.
C
DOUBLE PRECISION ET0
PARAMETER ( ET0 = 0.0D0 )
C
C Use a time step of 1 hour; look up 4 positions.
C
DOUBLE PRECISION STEP
PARAMETER ( STEP = 3600.0D0 )
INTEGER MAXITR
PARAMETER ( MAXITR = 4 )
CHARACTER*(*) OBSRVR
PARAMETER ( OBSRVR = 'Earth' )
CHARACTER*(*) TARGET
PARAMETER ( TARGET = 'Moon' )
C
C Local variables
C
DOUBLE PRECISION ET
DOUBLE PRECISION LT
DOUBLE PRECISION POS ( 3 )
INTEGER I
C
C Load the SPK file.
C
CALL FURNSH ( SPK )
C
C Step through a series of epochs, looking up a
C position vector at each one.
C
DO I = 1, MAXITR
ET = ET0 + (I-1)*STEP
CALL SPKPOS ( TARGET, ET, FRAME, ABCORR, OBSRVR,
. POS, LT )
WRITE (*,*) 'ET = ', ET
WRITE (*,*) ' '
WRITE (*,*) 'J2000 x-position (km): ', POS(1)
WRITE (*,*) 'J2000 y-position (km): ', POS(2)
WRITE (*,*) 'J2000 z-position (km): ', POS(3)
WRITE (*,*) ' '
END DO
END
When this program was executed on a Mac/Intel/gfortran/64-bit
platform, the output was:
ET = 0.0000000000000000
J2000 x-position (km): -291608.38530964090
J2000 y-position (km): -266716.83294678747
J2000 z-position (km): -76102.487146783606
ET = 3600.0000000000000
J2000 x-position (km): -289279.89831331203
J2000 y-position (km): -269104.10842893779
J2000 z-position (km): -77184.242072912006
ET = 7200.0000000000000
J2000 x-position (km): -286928.00140550011
J2000 y-position (km): -271469.99024601618
J2000 z-position (km): -78259.908307700243
ET = 10800.000000000000
J2000 x-position (km): -284552.90265547187
J2000 y-position (km): -273814.30975274299
J2000 z-position (km): -79329.406046598189
Restrictions
None.
Literature_References
None.
Author_and_Institution
C.H. Acton (JPL)
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
B.V. Semenov (JPL)
W.L. Taber (JPL)
E.D. Wright (JPL)
Version
SPICELIB Version 3.2.0, 01-OCT-2021 (JDR) (NJB)
Deleted include statement for frmtyp.inc.
Edited the header to comply with NAIF standard. Added complete
code example from existing fragment.
Updated $Particulars to refer to Aberration Corrections
Required Reading document, which was added to
$Required_Reading list.
SPICELIB Version 3.1.0, 03-JUL-2014 (NJB) (BVS)
Discussion of light time corrections was updated. Assertions
that converged light time corrections are unlikely to be
useful were removed.
Updated to save the input body names and ZZBODTRN state
counters and to do name-ID conversions only if the counters
have changed.
SPICELIB Version 3.0.3, 04-APR-2008 (NJB)
Corrected minor error in description of XLT+S aberration
correction.
SPICELIB Version 3.0.2, 20-OCT-2003 (EDW)
Added mention that LT returns in seconds.
SPICELIB Version 3.0.1, 29-JUL-2003 (NJB) (CHA)
Various minor header changes were made to improve clarity.
SPICELIB Version 3.0.0, 31-DEC-2001 (NJB)
Updated to handle aberration corrections for transmission
of radiation. Formerly, only the reception case was
supported. The header was revised and expanded to explain
the functionality of this routine in more detail.
SPICELIB Version 1.0.0, 03-MAR-1999 (WLT)
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Fri Dec 31 18:36:53 2021