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Index Page spkltc_c

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Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries

#### Procedure

       spkltc_c ( S/P Kernel, light time corrected state )

       void spkltc_c ( SpiceInt           targ,
                       SpiceDouble        et,
                       ConstSpiceChar   * ref,
                       ConstSpiceChar   * abcorr,
                       ConstSpiceDouble   stobs[6],
                       SpiceDouble        starg[6],
                       SpiceDouble      * lt,
                       SpiceDouble      * dlt      )

#### Abstract

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

#### Required_Reading

       FRAMES
       SPK

#### Keywords

       EPHEMERIS

#### 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.
       stobs      I   State of the observer relative to the SSB.
       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 input state `stobs' and the output state `starg' are
                   expressed. `ref' must be recognized by the CSPICE
                   Toolkit. The acceptable frames are listed in the Frames
                   Required Reading, as well as in the CSPICE 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. See the discussion in the -Particulars
                   section for recommendations on how to choose
                   aberration corrections.

                   If `abcorr' includes the stellar aberration correction
                   symbol "+S", this flag is simply ignored. Aside from
                   the possible presence of this symbol, `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 involves
                                 iterative solution of the light time
                                 equation (see -Particulars for details).
                                 The solution invoked by the "LT" option
                                 uses one iteration.

                      "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_c for a discussion of precision of
                                 light time corrections.

                   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'.

                      "XCN"      "Transmission" case: converged
                                 Newtonian light time 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'.

       stobs       is the geometric (uncorrected) state of the observer
                   relative to the solar system barycenter at epoch `et'.
                   `stobs' is a 6-vector: the first three components of
                   `stobs' represent a Cartesian position vector; the last
                   three components represent the corresponding velocity
                   vector. `stobs' is expressed relative to the inertial
                   reference frame designated by `ref'.

                   Units are always km and km/sec.

#### 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 aberration, 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 light time, 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)  For the convenience of the caller, the input aberration
           correction flag can call for stellar aberration correction via
           inclusion of the "+S" suffix. This portion of the aberration
           correction flag is ignored if present.

       2)  If the value of `abcorr' is not recognized, an error
           is signaled by a routine in the call tree of this
           routine.

       3)  If the reference frame requested is not a recognized
           inertial reference frame, the error SPICE(BADFRAME)
           is signaled by a routine in the call tree of this routine.

       4)  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.

       5)  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.

       6)  If a division by zero error would occur in the computation of
           `dlt', the error SPICE(DIVIDEBYZERO) is signaled by a routine in
           the call tree of this routine.

       7)  If any of the `ref' or `abcorr' input string pointers is null,
           the error SPICE(NULLPOINTER) is signaled.

       8)  If any of the `ref' or `abcorr' input strings has zero length,
           the error SPICE(EMPTYSTRING) is signaled.

#### Files

       This routine computes states using SPK files that have been
       loaded into the SPICE system, normally via the kernel loading
       interface routine furnsh_c. 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_c 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_c 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 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: spkltc_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 spkltc_ex1
          ./

          #include <stdio.h>
          #include "SpiceUsr.h"

          int main()
          {
             /.
             Local constants

             The meta-kernel name shown here refers to a file whose contents
             are those shown above. This file and the kernels it references
             must exist in your current working directory.
             ./
             #define META                   "spkltc_ex1.tm"

             /.
             Use a time step of 1 hour; look up 5 states.
             ./
             #define STEP                   3600.0
             #define MAXITR                 5

             /.
             Local variables
             ./
             SpiceDouble             dlt;
             SpiceDouble             et;
             SpiceDouble             et0;
             SpiceDouble             lt;
             SpiceDouble             state  [6];
             SpiceDouble             stobs  [6];
             SpiceInt                i;

             /.
             Load the SPK and LSK kernels via the meta-kernel.
             ./
             furnsh_c ( META );

             /.
             Convert the start time to seconds past J2000 TDB.
             ./
             str2et_c ( "2000 JAN 1 12:00:00 TDB", &et0 );

             /.
             Step through a series of epochs, looking up a
             state vector at each one.
             ./
             for ( i = 0;  i < MAXITR;  i++ )
             {
                et = et0 + i*STEP;

                /.
                Look up a state vector at epoch ET using the
                following inputs:

                   Target:                 Moon (NAIF ID code 301)
                   Reference frame:        J2000
                   Aberration correction:  Light time ('LT')
                   Observer:               Earth (NAIF ID code 399)

                Before we can execute this computation, we'll need
                the geometric state of the observer relative to the
                solar system barycenter at ET, expressed relative
                to the J2000 reference frame:
                ./
                spkssb_c ( 399, et, "j2000", stobs );

                spkltc_c ( 301,   et,    "j2000", "lt",
                           stobs, state, &lt,   &dlt   );

                printf( "et = %20.6f\n",                        et       );
                printf( "J2000 x-position (km):   %20.8f\n",    state[0] );
                printf( "J2000 y-position (km):   %20.8f\n",    state[1] );
                printf( "J2000 z-position (km):   %20.8f\n",    state[2] );
                printf( "J2000 x-velocity (km/s): %20.12f\n",   state[3] );
                printf( "J2000 y-velocity (km/s): %20.12f\n",   state[4] );
                printf( "J2000 z-velocity (km/s): %20.12f\n",   state[5] );
                printf( "One-way light time (s):  %20.12f\n",   lt       );
                printf( "Light time rate:         %20.08e\n\n", dlt      );
             }
             return ( 0 );
          }

          When this program was executed on a Mac/Intel/cc/64-bit
          platform, the output was:

          et =             0.000000
          J2000 x-position (km):       -291569.26541283
          J2000 y-position (km):       -266709.18647826
          J2000 z-position (km):        -76099.15511876
          J2000 x-velocity (km/s):       0.643530613222
          J2000 y-velocity (km/s):      -0.666081817008
          J2000 z-velocity (km/s):      -0.301322831796
          One-way light time (s):        1.342310610325
          Light time rate:               1.07316909e-07

          et =          3600.000000
          J2000 x-position (km):       -289240.78128184
          J2000 y-position (km):       -269096.44087958
          J2000 z-position (km):        -77180.89972576
          J2000 x-velocity (km/s):       0.650062115201
          J2000 y-velocity (km/s):      -0.660162739217
          J2000 z-velocity (km/s):      -0.299642673906
          One-way light time (s):        1.342693954864
          Light time rate:               1.05652599e-07

          et =          7200.000000
          J2000 x-position (km):       -286888.88736709
          J2000 y-position (km):       -271462.30170548
          J2000 z-position (km):        -78256.55568214
          J2000 x-velocity (km/s):       0.656535991543
          J2000 y-velocity (km/s):      -0.654196576804
          J2000 z-velocity (km/s):      -0.297940273074
          One-way light time (s):        1.343071311734
          Light time rate:               1.03990457e-07

          et =         10800.000000
          J2000 x-position (km):       -284513.79173691
          J2000 y-position (km):       -273806.60031034
          J2000 z-position (km):        -79326.04318327
          J2000 x-velocity (km/s):       0.662951900546
          J2000 y-velocity (km/s):      -0.648183807097
          J2000 z-velocity (km/s):      -0.296215779371
          One-way light time (s):        1.343442689069
          Light time rate:               1.02330665e-07

          et =         14400.000000
          J2000 x-position (km):       -282115.70368389
          J2000 y-position (km):       -276129.16976799
          J2000 z-position (km):        -80389.28296571
          J2000 x-velocity (km/s):       0.669309503775
          J2000 y-velocity (km/s):      -0.642124908057
          J2000 z-velocity (km/s):      -0.294469343362
          One-way light time (s):        1.343808095656
          Light time rate:               1.00673404e-07

#### Restrictions

       1)  The routine spkgeo_c should be used instead of this routine
           to compute geometric states. spkgeo_c introduces less
           round-off error when the observer and target have common
           center that is closer to both objects than is the solar
           system barycenter.

       2)  The kernel files to be used by spkltc_c must be loaded
           (normally by the CSPICE kernel loader furnsh_c) before
           this routine is called.

       3)  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

       -CSPICE Version 1.0.2, 10-AUG-2021 (JDR)

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

           Added frames.req to the list of -Required_Reading. Updated
           -Exceptions, -Restrictions and -Literature_References sections.

       -CSPICE Version 1.0.1, 07-JUL-2014 (NJB)

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

       -CSPICE Version 1.0.0, 11-JAN-2008 (NJB)

#### Index_Entries

       low-level light time correction
       light-time corrected state from SPK file
       get light-time corrected state

#### Link to routine spkltc_c source file spkltc_c.c

    Fri Dec 31 18:41:12 2021