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spkacs

Table of contents
Procedure
Abstract
Required_Reading
Keywords
Declarations
Brief_I/O
Detailed_Input
Detailed_Output
Parameters
Exceptions
Files
Particulars
Examples
Restrictions
Literature_References
Author_and_Institution
Version

Procedure

     SPKACS ( SPK, aberration corrected state )

     SUBROUTINE SPKACS ( TARG, ET, REF, ABCORR, OBS, STARG, LT, DLT )

Abstract

     Return the state (position and velocity) of a target body
     relative to an observer, optionally corrected for light time
     and stellar aberration, expressed relative to an inertial
     reference frame.

Required_Reading

     SPK

Keywords

     EPHEMERIS

Declarations

     IMPLICIT NONE

     INCLUDE 'zzabcorr.inc'

     INTEGER               TARG
     DOUBLE PRECISION      ET
     CHARACTER*(*)         REF
     CHARACTER*(*)         ABCORR
     INTEGER               OBS
     DOUBLE PRECISION      STARG    ( 6 )
     DOUBLE PRECISION      LT
     DOUBLE PRECISION      DLT

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     TARG       I   Target body.
     ET         I   Observer epoch.
     REF        I   Inertial reference frame of output state.
     ABCORR     I   Aberration correction flag.
     OBS        I   Observer.
     STARG      O   State of target.
     LT         O   One way light time between observer and target.
     DLT        O   Derivative of light time with respect to time.

Detailed_Input

     TARG     is the NAIF ID code for a 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 inertial reference frame with respect to which
              the output state STARG is expressed. 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.

     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 of
                            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+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.


     OBS      is the NAIF ID code for the observer body. The
              target and observer define a state vector whose
              position component points from the observer to the
              target.

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 specified inertial reference frame. The first
              three components of STARG represent the x-, y- and
              z-components of the target's position; 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.

              Units are always km and km/sec.

     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.

     DLT      is the derivative with respect to barycentric
              dynamical time of the one way light time between
              target and observer:

                 DLT = d(LT)/d(ET)

              DLT can also be described as the rate of change of
              one way light time. DLT is unitless, since LT and
              ET both have units of TDB seconds.

              If the observer and target are at the same position,
              then DLT is set to zero.

Parameters

     None.

Exceptions

     1)  If the value of ABCORR is not recognized, an error 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.

     3)  If the state 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 the observer and target are at the same position,
         then DLT is set to zero. This situation could arise,
         for example, when the observer is Mars and the target
         is the Mars barycenter.

Files

     This routine computes states using SPK files that have been
     loaded into the SPICE system, normally via the kernel loading
     interface routine 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 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 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. Normally
     these additional kernels are PCK files or frame kernels. Any
     such kernels must already be loaded at the time this routine is
     called.

Particulars

     This routine supports higher-level SPK API routines that can
     perform both light time and stellar aberration corrections.
     User applications normally will not need to call this routine
     directly.

     See the header of the routine SPKEZR for a detailed discussion
     of aberration 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) Look up a sequence of states of the Moon as seen from the
        Earth. Use light time and stellar aberration corrections.
        Compute the first state for the epoch 2000 JAN 1 12:00:00 TDB;
        compute subsequent states at intervals of 1 hour. For each
        epoch, display the states, the one way light time between
        target and observer, and the rate of change of the one way
        light time.

        Use the meta-kernel shown below to load the required SPICE
        kernels.


           KPL/MK

           File name: spkacs_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
              ---------                     --------
              de418.bsp                     Planetary ephemeris
              pck00008.tpc                  Planet orientation and
                                            radii
              naif0008.tls                  Leapseconds


           \begindata

              KERNELS_TO_LOAD = ( 'de418.bsp',
                                  'pck00008.tpc',
                                  'naif0008.tls'  )

           \begintext

           End of meta-kernel


        Example code begins here.


              PROGRAM SPKACS_EX1
              IMPLICIT NONE
        C
        C     Local constants
        C
        C     The meta-kernel name shown here refers to a file whose
        C     contents are those shown above. This file and the
        C     kernels it references must exist in your current working
        C     directory.
        C
        C
              CHARACTER*(*)         META
              PARAMETER           ( META   = 'spkacs_ex1.tm' )
        C
        C     Use a time step of 1 hour; look up 5 states.
        C
              DOUBLE PRECISION      STEP
              PARAMETER           ( STEP   = 3600.0D0 )

              INTEGER               MAXITR
              PARAMETER           ( MAXITR = 5 )
        C
        C     Local variables
        C
              DOUBLE PRECISION      DLT
              DOUBLE PRECISION      ET
              DOUBLE PRECISION      ET0
              DOUBLE PRECISION      LT
              DOUBLE PRECISION      STATE ( 6 )
              INTEGER               I

        C
        C     Load the SPK and LSK kernels via the meta-kernel.
        C
              CALL FURNSH ( META )
        C
        C     Convert the start time to seconds past J2000 TDB.
        C
              CALL STR2ET ( '2000 JAN 1 12:00:00 TDB', ET0 )
        C
        C     Step through a series of epochs, looking up a
        C     state vector at each one.
        C
              DO I = 1, MAXITR

                 ET = ET0 + (I-1)*STEP
        C
        C        Look up a state vector at epoch ET using the
        C        following inputs:
        C
        C           Target:                 Moon (NAIF ID code 301)
        C           Reference frame:        J2000
        C           Aberration correction:  Light time and stellar
        C                                   aberration ('LT+S')
        C           Observer:               Earth (NAIF ID code 399)
        C

                 CALL SPKACS ( 301, ET,    'J2000', 'LT+S',
             .                 399, STATE, LT,      DLT     )

                 WRITE (*,*) 'ET = ', ET
                 WRITE (*,*) 'J2000 x-position (km):   ', STATE(1)
                 WRITE (*,*) 'J2000 y-position (km):   ', STATE(2)
                 WRITE (*,*) 'J2000 z-position (km):   ', STATE(3)
                 WRITE (*,*) 'J2000 x-velocity (km/s): ', STATE(4)
                 WRITE (*,*) 'J2000 y-velocity (km/s): ', STATE(5)
                 WRITE (*,*) 'J2000 z-velocity (km/s): ', STATE(6)
                 WRITE (*,*) 'One-way light time (s):  ', LT
                 WRITE (*,*) 'Light time rate:         ', DLT
                 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):     -291584.61369497533
         J2000 y-position (km):     -266693.40583162551
         J2000 z-position (km):     -76095.653209237149
         J2000 x-velocity (km/s):   0.64343915743508395
         J2000 y-velocity (km/s):  -0.66606587365741410
         J2000 z-velocity (km/s):  -0.30131006342946742
         One-way light time (s):     1.3423106103251679
         Light time rate:            1.0731690869897750E-007

         ET =    3600.0000000000000
         J2000 x-position (km):     -289256.45942322229
         J2000 y-position (km):     -269080.60545907740
         J2000 z-position (km):     -77177.352771302132
         J2000 x-velocity (km/s):   0.64997032016926526
         J2000 y-velocity (km/s):  -0.66014825329341664
         J2000 z-velocity (km/s):  -0.29963041790724715
         One-way light time (s):     1.3426939548635302
         Light time rate:            1.0565259895222426E-007

         ET =    7200.0000000000000
         J2000 x-position (km):     -286904.89654239739
         J2000 y-position (km):     -271446.41676468350
         J2000 z-position (km):     -78252.965533623050
         J2000 x-velocity (km/s):   0.65644388315539315
         J2000 y-velocity (km/s):  -0.65418355204586442
         J2000 z-velocity (km/s):  -0.29792853294482308
         One-way light time (s):     1.3430713117337547
         Light time rate:            1.0399045689875861E-007

         ET =    10800.000000000000
         J2000 x-position (km):     -284530.13302756584
         J2000 y-position (km):     -273790.67111559171
         J2000 z-position (km):     -79322.411703917489
         J2000 x-velocity (km/s):   0.66285950473048116
         J2000 y-velocity (km/s):  -0.64817224685146524
         J2000 z-velocity (km/s):  -0.29620455846903732
         One-way light time (s):     1.3434426890693671
         Light time rate:            1.0233066524342374E-007

         ET =    14400.000000000000
         J2000 x-position (km):     -282132.37807791750
         J2000 y-position (km):     -276113.20159697317
         J2000 z-position (km):     -80385.612030562901
         J2000 x-velocity (km/s):   0.66921684649247459
         J2000 y-velocity (km/s):  -0.64211481528028158
         J2000 z-velocity (km/s):  -0.29445864490384888
         One-way light time (s):     1.3438080956559786
         Light time rate:            1.0067340363005083E-007

Restrictions

     1)  The kernel files to be used by SPKACS must be loaded
         (normally by the SPICELIB kernel loader FURNSH) before
         this routine is called.

     2)  Unlike most other SPK state computation routines, this
         routine requires that the output state be relative to an
         inertial reference frame.

Literature_References

     None.

Author_and_Institution

     N.J. Bachman       (JPL)
     J. Diaz del Rio    (ODC Space)

Version

    SPICELIB Version 1.1.0, 23-AUG-2021 (JDR) (NJB)

        Edited the header to comply with NAIF standard. Updated
        example's meta-kernel.

        Bug fix: ABCORR now is parsed using ZZVALCOR. This improves
        error checking.

    SPICELIB Version 1.0.1, 04-JUL-2014 (NJB)

        Discussion of light time corrections was updated. Assertions
        that converged light time corrections are unlikely to be
        useful were removed.

    SPICELIB Version 1.0.0, 11-JAN-2008 (NJB)
Fri Dec 31 18:36:50 2021