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Index Page spkpos_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

       spkpos_c ( S/P Kernel, position )

       void spkpos_c ( ConstSpiceChar   * targ,
                       SpiceDouble        et,
                       ConstSpiceChar   * ref,
                       ConstSpiceChar   * abcorr,
                       ConstSpiceChar   * obs,
                       SpiceDouble        ptarg[3],
                       SpiceDouble      * lt        )

#### Abstract

       Return the position of a target body relative to an observing
       body, optionally corrected for light time (planetary aberration)
       and stellar aberration.

#### Required_Reading

       ABCORR
       SPK
       NAIF_IDS
       FRAMES
       TIME

#### Keywords

       EPHEMERIS

#### Brief_I/O

       VARIABLE  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_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 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_Output

       ptarg       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.

#### Parameters

       None.

#### Exceptions

       1)  If name of target or observer cannot be translated to its NAIF
           ID code, the error SPICE(IDCODENOTFOUND) is signaled by a
           routine in the call tree of this routine.

       2)  If the reference frame `ref' is not a recognized reference
           frame, the error SPICE(UNKNOWNFRAME) is signaled by a routine
           in the call tree of this routine.

       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.

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

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

#### Files

       This routine computes positions using SPK files that have been
       loaded into the SPICE system, normally via the kernel loading
       interface routine furnsh_c. See the routine furnsh_c 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.

#### Particulars

       This 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_c
       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 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.

          /.
             Program spkpos_ex1
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main()
          {

             #define        ABCORR        "NONE"
             #define        FRAME         "J2000"
             #define        SPK           "de421.bsp"

             /.
             ET0 represents the date 2000 Jan 1 12:00:00 TDB.
             ./
             #define        ET0           0.0

             /.
             Use a time step of 1 hour; look up 4 positions.
             ./
             #define        STEP          3600.0
             #define        MAXITR        4

             #define        OBSERVER      "earth"
             #define        TARGET        "moon"

             /.
             Local variables
             ./
             SpiceInt       i;

             SpiceDouble    et;
             SpiceDouble    lt;
             SpiceDouble    pos [3];

             /.
             Load the spk file.
             ./
             furnsh_c ( SPK );

             /.
             Step through a series of epochs, looking up a position vector
             at each one.
             ./
             for ( i = 0;  i < MAXITR;  i++ )
             {
                et  =  ET0 + i*STEP;

                spkpos_c ( TARGET,    et,   FRAME,  ABCORR,
                           OBSERVER,  pos,  &lt             );

                printf( "\net = %20.10f\n\n",                 et     );
                printf( "J2000 x-position (km):   %20.10f\n", pos[0] );
                printf( "J2000 y-position (km):   %20.10f\n", pos[1] );
                printf( "J2000 z-position (km):   %20.10f\n", pos[2] );
             }

             return ( 0 );

          }

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

          et =         0.0000000000

          J2000 x-position (km):     -291608.3853096409
          J2000 y-position (km):     -266716.8329467875
          J2000 z-position (km):      -76102.4871467836

          et =      3600.0000000000

          J2000 x-position (km):     -289279.8983133120
          J2000 y-position (km):     -269104.1084289378
          J2000 z-position (km):      -77184.2420729120

          et =      7200.0000000000

          J2000 x-position (km):     -286928.0014055001
          J2000 y-position (km):     -271469.9902460162
          J2000 z-position (km):      -78259.9083077002

          et =     10800.0000000000

          J2000 x-position (km):     -284552.9026554719
          J2000 y-position (km):     -273814.3097527430
          J2000 z-position (km):      -79329.4060465982

#### Restrictions

       None.

#### Literature_References

       None.

#### Author_and_Institution

       C.H. Acton          (JPL)
       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
       W.L. Taber          (JPL)
       E.D. Wright         (JPL)

#### Version

       -CSPICE Version 3.0.2, 01-NOV-2021 (JDR)

           Edited the header to comply with NAIF standard. Added
           reference to the required SPK for the example code. Reduced
           the number of positions to be computed.

           Moved SPK required reading from -Literature_References to
           -Required_Reading section. Added entries #5 and #6 to -Exceptions
           section.

           Updated -Particulars to refer to Aberration Corrections
           Required Reading document, which was added to
           -Required_Reading list.

       -CSPICE Version 3.0.1, 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 2.0.4, 04-APR-2008 (NJB)

           Corrected minor error in description of XLT+S aberration
           correction.

       -CSPICE Version 2.0.3, 17-APR-2005 (NJB)

           Error was corrected in example program: variable name `state'
           was changed to `pos' in printf calls.

       -CSPICE Version 2.0.2, 13-OCT-2003 (EDW)

           Various minor header changes were made to improve clarity.
           Added mention that 'lt' returns a value in seconds.

       -CSPICE Version 2.0.1, 27-JUL-2003 (NJB) (CHA)

           Various header corrections were made.

       -CSPICE Version 2.0.0, 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.

       -CSPICE Version 1.0.0, 29-MAY-1999 (NJB) (WLT)

#### Index_Entries

       using names get target position relative to an observer
       position relative to observer corrected for aberrations
       read ephemeris data
       read trajectory data

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

    Fri Dec 31 18:41:12 2021