Table of contents
SPKEZR ( S/P Kernel, easier reader ) SUBROUTINE SPKEZR ( TARG, ET, REF, ABCORR, OBS, STARG, LT )
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.
IMPLICIT NONE INCLUDE 'zzctr.inc' CHARACTER*(*) TARG DOUBLE PRECISION ET CHARACTER*(*) REF CHARACTER*(*) ABCORR CHARACTER*(*) OBS DOUBLE PRECISION STARG ( 6 ) DOUBLE PRECISION LT
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.
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.
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.
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.
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.
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.
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
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)
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