spkpos |

Table of contents## ProcedureSPKPOS ( S/P Kernel, position ) SUBROUTINE SPKPOS ( TARG, ET, REF, ABCORR, OBS, PTARG, LT ) ## AbstractReturn the position of a target body relative to an observing body, optionally corrected for light time (planetary aberration) and stellar aberration. ## Required_ReadingABCORR SPK NAIF_IDS FRAMES TIME ## KeywordsEPHEMERIS ## DeclarationsIMPLICIT NONE INCLUDE 'zzctr.inc' CHARACTER*(*) TARG DOUBLE PRECISION ET CHARACTER*(*) REF CHARACTER*(*) ABCORR CHARACTER*(*) OBS DOUBLE PRECISION PTARG ( 3 ) DOUBLE PRECISION LT ## Brief_I/OVARIABLE 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_InputTARG 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_OutputPTARG 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. ## ParametersNone. ## Exceptions1) 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. ## FilesThis 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. ## ParticularsThis 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. ## ExamplesThe 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 ## RestrictionsNone. ## Literature_ReferencesNone. ## Author_and_InstitutionC.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) ## VersionSPICELIB 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) |

Fri Dec 31 18:36:53 2021