| spkezr |
|
Table of contents
Procedure
SPKEZR ( S/P Kernel, easier reader )
SUBROUTINE SPKEZR ( TARG, ET, REF, ABCORR, OBS, STARG, LT )
Abstract
Return the state (position and velocity) of a target body
relative to an observing body, optionally corrected for light
time (planetary aberration) and stellar aberration.
Required_Reading
ABCORR
FRAMES
NAIF_IDS
SPK
TIME
Keywords
EPHEMERIS
Declarations
IMPLICIT NONE
INCLUDE 'zzctr.inc'
CHARACTER*(*) TARG
DOUBLE PRECISION ET
CHARACTER*(*) REF
CHARACTER*(*) ABCORR
CHARACTER*(*) OBS
DOUBLE PRECISION STARG ( 6 )
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 state vector.
ABCORR I Aberration correction flag.
OBS I Observing body name.
STARG O State 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 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 name of the reference frame relative to which
the output state 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 state vector STARG
for details.
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 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
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
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.
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
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 aberrations, and is expressed with respect
to the reference frame specified by REF. The first
three components of STARG represent the x-, y- and
z-components of the target's position; the 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.
The velocity component of STARG is the derivative
with respect to time of the position component of
STARG.
Units are always km and km/sec.
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 state 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 state 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 states 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 state STARG is to be expressed relative to a
non-inertial frame, or 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. 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 state 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 SPKEZ 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
state of the MARS BARYCENTER relative to EARTH
in the J2000 frame with aberration correction LT+S.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: spkezr_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
--------- --------
de430.bsp Planetary ephemeris
pck00010.tpc Planet orientation and
radii
naif0011.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de430.bsp',
'pck00010.tpc',
'naif0011.tls' )
\begintext
End of meta-kernel
Example code begins here.
PROGRAM SPKEZR_EX1
IMPLICIT NONE
C
C Local variables
C
CHARACTER*(32) FRAME
CHARACTER*(32) ABCORR
CHARACTER*(36) OBS
CHARACTER*(36) TARGET
CHARACTER*(36) EPOCH
DOUBLE PRECISION ET
DOUBLE PRECISION LT
DOUBLE PRECISION STATE ( 6 )
INTEGER I
C
C Load a set of kernels: an SPK file, a PCK
C file and a leapseconds file. Use a meta-kernel.
C
CALL FURNSH( 'spkezr_ex1.tm' )
C
C Define parameters for a state lookup:
C
C Return the state vector of Mars Barycenter (4) as seen
C from Earth (399) in the J2000 frame using aberration
C correction LT+S (light time plus stellar aberration)
C at the epoch JAN 1 2015 12:00:00.
C
TARGET = 'MARS BARYCENTER'
EPOCH = 'JAN 1 2015 12:00:00'
FRAME = 'J2000'
ABCORR = 'LT+S'
OBS = 'EARTH'
C
C Convert the epoch to ephemeris time.
C
CALL STR2ET( EPOCH, ET )
C
C Look-up the state for the defined parameters.
C
CALL SPKEZR( TARGET, ET, FRAME, ABCORR, OBS, STATE, LT)
C
C Output...
C
WRITE(*,*) 'The position of : ', TARGET
WRITE(*,*) 'As observed from : ', OBS
WRITE(*,*) 'In reference frame : ', FRAME
WRITE(*,*) 'At epoch : ', EPOCH
WRITE(*,*) ' '
C
C The first three entries of state contain the
C X, Y, Z position components. The final three contain
C the Vx, Vy, Vz velocity components.
C
WRITE(*,*) 'R (kilometers) : '
WRITE(*,'(3F20.6)') (STATE(I), I=1,3 )
WRITE(*,*) 'V (kilometers/sec) : '
WRITE(*,'(3F20.6)') (STATE(I), I=4,6 )
WRITE(*,*) 'Light time (secs) : ', LT
END
When this program was executed on a Mac/Intel/gfortran/64-bit
platform, the output was:
The position of : MARS BARYCENTER
As observed from : EARTH
In reference frame : J2000
At epoch : JAN 1 2015 12:00:00
R (kilometers) :
229953013.746498 -167125346.211588 -78800343.963573
V (kilometers/sec) :
35.380861 28.653402 12.861524
Light time (secs) : 983.97882466162321
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 4.2.0, 01-OCT-2021 (JDR) (NJB)
Deleted include statement for frmtyp.inc.
Edited the header to comply with NAIF standard. Changed output
format in code example to comply with maximum line length for
header comments. Added ABCORR to $Required_Reading.
SPICELIB Version 4.1.1, 19-JAN-2016 (EDW)
Example code replaced with a complete program and
the corresponding output.
$Particulars updated to refer to Aberration Corrections
Required Reading document.
SPICELIB Version 4.1.0, 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 4.0.0, 27-DEC-2007 (NJB)
This routine was upgraded to more accurately compute
aberration-corrected velocity, and in particular, make it
more consistent with observer-target positions.
When light time corrections are used, the derivative of light
time with respect to time is now accounted for in the
computation of observer-target velocities. When the reference
frame associated with the output state is time-dependent, the
derivative of light time with respect to time is now accounted
for in the computation of the rate of change of orientation of
the reference frame.
When stellar aberration corrections are used, velocities
now reflect the rate of range of the stellar 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 2.0.0, 21-FEB-1997 (WLT)
Extended the functionality of the routine. Users may
now entered the id code of an object as an ascii string
and the string will be converted to the corresponding
integer representation.
SPICELIB Version 1.1.0, 09-JUL-1996 (WLT)
Corrected the description of LT in the Detailed Output
section of the header.
SPICELIB Version 1.0.0, 25-SEP-1995 (BVS)
|
Fri Dec 31 18:36:52 2021