Index Page spkacs_c

A B C D E F G H I J K L M N O P Q R S T U V W X

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

       void spkacs_c ( SpiceInt           targ,
                       SpiceDouble        et,
                       ConstSpiceChar   * ref,
                       ConstSpiceChar   * abcorr,
                       SpiceInt           obs,
                       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
       and stellar aberration, expressed relative to an inertial
       reference frame.

#### Required_Reading

       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.
       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 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 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_c 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, the error is
          diagnosed 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, the error will be diagnosed
          by routines 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.

       6) The error SPICE(EMPTYSTRING) is signaled if either of the input
          strings `ref' or `abcorr' do not contain at least one character,
          since such an input string cannot be converted to a Fortran-style
          string.

       7) The error SPICE(NULLPOINTER) is signaled if either of the input
          string pointers `ref' or `abcorr' are null.

#### 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

       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 following meta-kernel to specify the kernels to
          load:

             KPL/MK

             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.

             \begindata

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

             \begintext

          The code example follows:

             #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                   "example.mk"

                /.
                Use a time step of 1 hour; look up 100 states.
                ./
                #define STEP                   3600.0
                #define MAXITR                 5

                /.
                Local variables
                ./
                SpiceDouble             dlt;
                SpiceDouble             et;
                SpiceDouble             et0;
                SpiceDouble             lt;
                SpiceDouble             state  [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 and stellar
                                              aberration ('LT+S')
                      Observer:               Earth (NAIF ID code 399)
                   ./
                   spkacs_c ( 301, et,    "j2000", "lt+s",
                              399, 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 );
             }

          The output produced by this program will vary somewhat as
          a function of the platform on which the program is built and
          executed. On a PC/Linux/gcc platform, the following output
          was produced:

             et =             0.000000
             J2000 x-position (km):       -291584.61369498
             J2000 y-position (km):       -266693.40583163
             J2000 z-position (km):        -76095.65320924
             J2000 x-velocity (km/s):       0.643439157435
             J2000 y-velocity (km/s):      -0.666065873657
             J2000 z-velocity (km/s):      -0.301310063429
             One-way light time (s):        1.342310610325
             Light time rate:               1.07316909e-07

             et =          3600.000000
             J2000 x-position (km):       -289256.45942322
             J2000 y-position (km):       -269080.60545908
             J2000 z-position (km):        -77177.35277130
             J2000 x-velocity (km/s):       0.649970320169
             J2000 y-velocity (km/s):      -0.660148253293
             J2000 z-velocity (km/s):      -0.299630417907
             One-way light time (s):        1.342693954864
             Light time rate:               1.05652599e-07

             et =          7200.000000
             J2000 x-position (km):       -286904.89654240
             J2000 y-position (km):       -271446.41676468
             J2000 z-position (km):        -78252.96553362
             J2000 x-velocity (km/s):       0.656443883155
             J2000 y-velocity (km/s):      -0.654183552046
             J2000 z-velocity (km/s):      -0.297928532945
             One-way light time (s):        1.343071311734
             Light time rate:               1.03990457e-07

             et =         10800.000000
             J2000 x-position (km):       -284530.13302756
             J2000 y-position (km):       -273790.67111559
             J2000 z-position (km):        -79322.41170392
             J2000 x-velocity (km/s):       0.662859504730
             J2000 y-velocity (km/s):      -0.648172246851
             J2000 z-velocity (km/s):      -0.296204558469
             One-way light time (s):        1.343442689069
             Light time rate:               1.02330665e-07

             et =         14400.000000
             J2000 x-position (km):       -282132.37807792
             J2000 y-position (km):       -276113.20159697
             J2000 z-position (km):        -80385.61203056
             J2000 x-velocity (km/s):       0.669216846492
             J2000 y-velocity (km/s):      -0.642114815280
             J2000 z-velocity (km/s):      -0.294458644904
             One-way light time (s):        1.343808095656
             Light time rate:               1.00673404e-07

#### Restrictions

       1) The kernel files to be used by spkacs_c must be loaded
          (normally by the CSPICE kernel loader furnsh_c) 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

       SPK Required Reading.

#### Author_and_Institution

       N.J. Bachman    (JPL)

#### Version

       -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 aberration correction
       aberration-corrected state from spk file
       get light time and stellar aberration-corrected state

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

    Wed Apr  5 17:54:43 2017