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Index Page spkezr_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

       spkezr_c ( S/P Kernel, easier reader )

       void spkezr_c ( ConstSpiceChar     *targ,
                       SpiceDouble         et,
                       ConstSpiceChar     *ref,
                       ConstSpiceChar     *abcorr,
                       ConstSpiceChar     *obs,
                       SpiceDouble         starg[6],
                       SpiceDouble        *lt        )

#### Abstract

       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.

#### Required_Reading

       ABCORR
       FRAMES
       NAIF_IDS
       SPK
       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 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.

#### 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 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.

#### 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 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.

#### 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 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.

       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 states 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 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.

#### Particulars

       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_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 states 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 spkezr_ex1
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main()
          {

             #define        ABCORR        "NONE"
             #define        FRAME         "J2000"

             /.
             The name of the SPK file shown here is fictitious;
             you must supply the name of an SPK file available
             on your own computer system.
             ./
             #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 states.
             ./
             #define        STEP          3600.0
             #define        MAXITR        4

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

             /.
             Local variables
             ./
             SpiceInt       i;

             SpiceDouble    et;
             SpiceDouble    lt;
             SpiceDouble    state [6];

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

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

                spkezr_c ( TARGET,    et,     FRAME,  ABCORR,
                           OBSERVER,  state,  &lt             );

                printf( "\net = %20.10f\n\n",                 et       );
                printf( "J2000 x-position (km):   %20.10f\n", state[0] );
                printf( "J2000 y-position (km):   %20.10f\n", state[1] );
                printf( "J2000 z-position (km):   %20.10f\n", state[2] );
                printf( "J2000 x-velocity (km/s): %20.10f\n", state[3] );
                printf( "J2000 y-velocity (km/s): %20.10f\n", state[4] );
                printf( "J2000 z-velocity (km/s): %20.10f\n", state[5] );
             }

             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
          J2000 x-velocity (km/s):         0.6435313868
          J2000 y-velocity (km/s):        -0.6660876862
          J2000 z-velocity (km/s):        -0.3013257043

          et =      3600.0000000000

          J2000 x-position (km):     -289279.8983133120
          J2000 y-position (km):     -269104.1084289378
          J2000 z-position (km):      -77184.2420729120
          J2000 x-velocity (km/s):         0.6500629244
          J2000 y-velocity (km/s):        -0.6601685834
          J2000 z-velocity (km/s):        -0.2996455351

          et =      7200.0000000000

          J2000 x-position (km):     -286928.0014055001
          J2000 y-position (km):     -271469.9902460162
          J2000 z-position (km):      -78259.9083077002
          J2000 x-velocity (km/s):         0.6565368360
          J2000 y-velocity (km/s):        -0.6542023962
          J2000 z-velocity (km/s):        -0.2979431229

          et =     10800.0000000000

          J2000 x-position (km):     -284552.9026554719
          J2000 y-position (km):     -273814.3097527430
          J2000 z-position (km):      -79329.4060465982
          J2000 x-velocity (km/s):         0.6629527800
          J2000 y-velocity (km/s):        -0.6481896017
          J2000 z-velocity (km/s):        -0.2962186180

#### Restrictions

       None.

#### Literature_References

       None.

#### Author_and_Institution

       C.H. Acton          (JPL)
       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
       B.V. Semenov        (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 states 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 3.0.0, 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.

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

           Added mention that 'lt' returns a value in seconds.

       -CSPICE Version 2.0.1, 29-JUL-2003 (NJB) (CHA)

           Various minor header changes were made to improve clarity.

       -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 1.2.0, 29-MAY-1999 (NJB) (BVS)

           Comment correction: the name spkez_c was changed to spkezr_c.

#### Index_Entries

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

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

    Fri Dec 31 18:41:12 2021