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spkcpt_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

   void spkcpt_c ( ConstSpiceDouble       trgpos [3],
                   ConstSpiceChar       * trgctr,
                   ConstSpiceChar       * trgref,
                   SpiceDouble            et,
                   ConstSpiceChar       * outref,
                   ConstSpiceChar       * refloc,
                   ConstSpiceChar       * abcorr,
                   ConstSpiceChar       * obsrvr,
                   SpiceDouble            state  [6],
                   SpiceDouble          * lt          )

Abstract

 
   Return the state, relative to a specified observer, of a target
   having constant position in a specified reference frame. The
   target's position is provided by the calling program rather than by
   loaded SPK files.
 

Required_Reading

 
   FRAMES 
   PCK 
   SPK 
   TIME 
 

Keywords

 
   EPHEMERIS 
 

Brief_I/O

 
   Variable  I/O  Description 
   --------  ---  -------------------------------------------------- 
   trgpos     I   Target position relative to center of motion. 
   trgctr     I   Center of motion of target. 
   trgref     I   Frame of target position. 
   et         I   Observation epoch. 
   outref     I   Reference frame of output state. 
   refloc     I   Output reference frame evaluation locus. 
   abcorr     I   Aberration correction. 
   obsrvr     I   Name of observing ephemeris object. 
   state      O   State of target with respect to observer. 
   lt         O   One way light time between target and 
                  observer. 
 

Detailed_Input

 
   trgpos      is the fixed (constant) position of a target relative to
               its "center of motion" `trgctr', expressed in the
               reference frame `trgref'.
 
               Units are always km.
 
 
   trgctr      is the name of the center of motion of `trgpos'. The
               ephemeris of `trgctr' is provided by loaded SPK files.
 
               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 center of motion.
 
               Case and leading and trailing blanks are not significant
               in the string `trgctr'.
 
 
   trgref      is the name of the reference frame relative to which the
               input position `trgpos' is expressed. The target has
               constant position relative to its center of motion in
               this reference frame.
 
               Case and leading and trailing blanks are not significant
               in the string `trgref'.
 
 
   et          is the ephemeris time at which the state of the target
               relative to the observer is to be computed. `et' is
               expressed as seconds past J2000 TDB. `et' refers to time
               at the observer's location.
 
               `et' is independent of the target epoch `trgepc'. 
 
 
   outref      is the name of the reference frame with respect to which
               the output state is expressed.
 
               When `outref' is time-dependent (non-inertial), its
               orientation relative to the J2000 frame is evaluated in
               the manner commanded by the input argument `refloc' (see
               description below).
 
               Case and leading and trailing blanks are not significant
               in the string `outref'.
  
 
   refloc      is a string indicating the output reference frame 
               evaluation locus: this is the location associated 
               with the epoch at which this routine is to evaluate 
               the orientation, relative to the J2000 frame, of the 
               output frame `outref'. The values and meanings of 
               `refloc' are: 
 
                  "OBSERVER"  Evaluate `outref' at the observer's 
                              epoch `et'. 
 
                              Normally the locus "OBSERVER" should 
                              be selected when `outref' is centered 
                              at the observer. 
 
 
                  "TARGET"    Evaluate `outref' at the target epoch; 
                              letting `lt' be the one-way light time  
                              between the target and observer, the 
                              target epoch is 
 
                                 et-lt  if reception aberration 
                                        corrections are used 
 
                                 et+lt  if transmission aberration 
                                        corrections are used 
 
                                 et     if no aberration corrections 
                                        are used 
 
                              Normally the locus "TARGET" should 
                              be selected when `outref' is `trgref', 
                              the frame in which the target position 
                              is specified. 
 
 
                  "CENTER"    Evaluate the frame `outref' at the epoch 
                              associated its center. This epoch, 
                              which we'll call `etctr', is determined 
                              as follows: 
 
                                 Let `ltctr' be the one-way light time 
                                 between the observer and the center  
                                 of `outref'. Then `etctr' is 
 
                                    et-ltctr  if reception 
                                              aberration corrections 
                                              are used 
 
                                    et+ltctr  if transmission 
                                              aberration corrections 
                                              are used 
 
                                    et        if no aberration 
                                              corrections are used 
 
 
                              The locus "CENTER" should be selected 
                              when the user intends to obtain 
                              results compatible with those produced 
                              by spkezr_c.  
  
               When `outref' is inertial, all choices of `refloc'  
               yield the same results. 
    
               Case and leading and trailing blanks are not 
               significant in the string `refloc'. 
 
 
   abcorr      indicates the aberration corrections to be applied to
               the observer-target state to account for one-way light
               time and stellar aberration.
                 
               `abcorr' may be any of the following: 
 
                  "NONE"     Apply no correction. Return the  
                             geometric state of the target  
                             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. 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. 
 
                  "CN+S"     Converged Newtonian light time 
                             and stellar aberration 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'. 
 
                  "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 and stellar  
                             aberration corrections. 
 
               Neither special nor general relativistic effects are 
               accounted for in the aberration corrections applied 
               by this routine. 
 
               Case and leading and trailing blanks are not 
               significant in the string `abcorr'. 
 
 
   obsrvr      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. 
 
               Case and leading and trailing blanks are not 
               significant in the string `obsrvr'. 
 
                                

Detailed_Output

 
   state       is a Cartesian state vector representing the position 
               and velocity of the target relative to the specified 
               observer. `state' is corrected for the specified 
               aberrations and is expressed with respect to the 
               reference frame specified by `outref'. The first three 
               components of `state' 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 `state' 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 `state' is the derivative 
               with respect to time of the position component of 
               `state'. 
 
               Units are always km and km/sec. 
 
               When `state' is expressed in a time-dependent 
               (non-inertial) output frame, the orientation of that 
               frame relative to the J2000 frame is evaluated in the 
               manner indicated by the input argument `refloc' (see 
               description above). 
 
 
   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 either the name of the center of motion or the observer 
       cannot be translated to its NAIF ID code, the error will 
       be diagnosed by a routine in the call tree of this routine.
 
   2)  If the reference frame `outref' is unrecognized, the error will
       be diagnosed by a routine in the call tree of this routine.
 
   3)  If the reference frame `trgref' is unrecognized, the error will
       be diagnosed by a routine in the call tree of this routine.
 
   4)  If the frame evaluation locus `refloc' is not recognized, the
       error will be diagnosed by a routine in the call tree of this
       routine.
 
   5)  If the loaded kernels provide insufficient data to compute 
       the requested state vector, the deficiency will be diagnosed 
       by a routine in the call tree of this routine. 
 
   6)  If an error occurs while reading an SPK or other kernel file, 
       the error will be diagnosed by a routine in the call tree of 
       this routine. 
 
   7)  If the aberration correction `abcorr' is not recognized,  
       the error will be diagnosed by a routine in the call tree of 
       this routine. 
 
   8)  If any input string pointer is null, or if the output state
       pointer is null, the error SPICE(NULLPOINTER) will be signaled.
 
   9)  If any input string has length zero, the error SPICE(EMPTYSTRING)
       will be signaled.
  

Files

 
   Appropriate kernels must be loaded by the calling program before 
   this routine is called. 
 
   The following data are required: 
 
      -  SPK data: ephemeris data for target center and observer 
         must be loaded. If aberration corrections are used, the 
         states of target center and observer relative to the solar 
         system barycenter must be calculable from the available 
         ephemeris data. Typically ephemeris data are made available 
         by loading one or more SPK files using furnsh_c. 
 
   The following data may be required: 
 
      -  PCK data: if the target frame is a PCK frame, rotation data 
         for the target frame must be loaded. These may be provided 
         in a text or binary PCK file. 
 
      -  Frame data: if a frame definition not built into SPICE is
         required, for example to convert the observer-target state to
         the output frame, that definition must be available in the
         kernel pool. Typically frame definitions are supplied by
         loading a frame kernel using furnsh_c.
 
      -  Additional kernels: if any frame used in this routine's 
         state computation is a CK frame, then at least one CK and 
         corresponding SCLK kernel is required. If dynamic frames 
         are used, additional SPK, PCK, CK, or SCLK kernels may be 
         required. 
 
   In all cases, kernel data are normally loaded once per program 
   run, NOT every time this routine is called. 
 

Particulars

 
   This routine computes observer-target states for targets whose 
   trajectories are not provided by SPK files. 
 
   Targets supported by this routine must have constant position 
   with respect to a specified center of motion, expressed in a 
   caller-specified reference frame. The state of the center of 
   motion relative to the observer must be computable using  
   loaded SPK data. 
 
   For applications in which the target has non-zero, constant velocity
   relative to its center of motion, the CSPICE routine
 
      spkcvt_c     { SPK, constant velocity target } 
 
   can be used.
    
   This routine is suitable for computing states of landmarks on the 
   surface of an extended object, as seen by a specified observer, 
   in cases where no SPK data are available for those landmarks. 
 
   This routine's treatment of the output reference frame differs 
   from that of the principal SPK API routines 
 
      spkezr_c 
      spkez_c 
      spkpos_c 
      spkezp_c 
 
   which require both observer and target ephemerides to be provided 
   by loaded SPK files: 
 
      The SPK API routines listed above evaluate the orientation of 
      the output reference frame (with respect to the J2000 frame) 
      at an epoch corrected for one-way light time between the 
      observer and the center of the output frame. When the center 
      of the output frame is not the target (for example, when the 
      target is on the surface of Mars and the output frame is 
      centered at Mars' center), the epoch of evaluation may not 
      closely match the light-time corrected epoch associated with 
      the target itself. 
  
      This routine allows the caller to dictate how the orientation 
      of the output reference frame is to be evaluated. The caller 
      passes to this routine an input string called the output 
      frame's evaluation "locus." This string specifies the location 
      associated with the output frame's evaluation epoch. The three 
      possible values of the locus are 
 
         "TARGET" 
         "OBSERVER" 
         "CENTER"         
 
      The choice of locus has an effect when aberration corrections 
      are used and the output frame is non-inertial. 
 
      When the locus is "TARGET" and light time corrections are used,
      the orientation of the output frame is evaluated at the epoch
      obtained by correcting the observation epoch `et' for one-way
      observer-target light time `lt'. The evaluation epoch will be
      either et-lt or et+lt for reception or transmission corrections
      respectively.
 
      For remote sensing applications where the target is a surface 
      point on an extended object, and the orientation of that 
      object should be evaluated at the emission time, the locus 
      "TARGET" should be used. 
 
      When the output frame's orientation should be evaluated at 
      the observation epoch `et', which is the case when the  
      output frame is centered at the observer, the locus  
      "OBSERVER" should be used. 
       
      The locus option "CENTER" is provided for compatibility 
      with existing SPK state computation APIs such as spkezr_c. 
 
      Note that the output frame evaluation locus does not affect 
      the computation of light time between the target and 
      observer.  
 
 
   The SPK routines that compute observer-target states for
   combinations of objects having ephemerides provided by SPK files and
   objects having constant position or constant velocity are
 
      spkcpo_c {SPK, Constant position observer} 
      spkcpt_c {SPK, Constant position target} 
      spkcvo_c {SPK, Constant velocity observer} 
      spkcvt_c {SPK, Constant velocity target} 
 

Examples

 
   The numerical results shown for these examples 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) Demonstrate use of this routine; in particular demonstrate 
      applications of the output frame evaluation locus.  
    
      The following program is not necessarily realistic: for 
      brevity, it combines several unrelated computations. 
 
      Task Description 
      ================ 
 
      Find the state of a given surface point on earth, corrected 
      for light time and stellar aberration, relative to the Mars 
      Global Surveyor spacecraft, expressed in the earth fixed 
      reference frame ITRF93. The selected point is the position 
      of the DSN station DSS-14. 
 
      Contrast the states computed by setting the output frame 
      evaluation locus to "TARGET" and to "CENTER". Show that the 
      latter choice produces results very close to those that 
      can be obtained using spkezr_c. 
 
      Also compute the state of a selected Mars surface point as 
      seen from MGS. The point we'll use is the narrow angle MOC 
      boresight surface intercept corresponding to the chosen 
      observation time. Express the state in a spacecraft-centered 
      reference frame. Use the output frame evaluation locus 
      "OBSERVER" for this computation. 
  
      The observation epoch is 2003 OCT 13 06:00:00 UTC. 
 
 
      Kernels 
      =======      
 
      Use the meta-kernel shown below to load the required SPICE 
      kernels. 
 
         KPL/MK 
 
         File name: spkcpt.tm 
 
         This is the meta-kernel file for the header code example for 
         the subroutine spkcpt_c. The kernel files referenced by this 
         meta-kernel can be found on the NAIF website. 
 
         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 
            ---------                        -------- 
            de421.bsp                        Planetary ephemeris 
            pck00010.tpc                     Planet orientation and 
                                             radii 
            naif0010.tls                     Leapseconds 
            earth_720101_070426.bpc          Earth historical 
                                             binary PCK 
            earthstns_itrf93_050714.bsp      DSN station SPK 
            mgs_moc_v20.ti                   MGS MOC instrument 
                                             parameters 
            mgs_sclkscet_00061.tsc           MGS SCLK coefficients 
            mgs_sc_ext12.bc                  MGS s/c bus attitude 
            mgs_ext12_ipng_mgs95j.bsp        MGS ephemeris 
 
         \begindata 
 
         KERNELS_TO_LOAD = ( 'de421.bsp', 
                             'pck00010.tpc', 
                             'naif0010.tls', 
                             'earth_720101_070426.bpc', 
                             'earthstns_itrf93_050714.bsp', 
                             'mgs_moc_v20.ti', 
                             'mgs_sclkscet_00061.tsc', 
                             'mgs_sc_ext12.bc', 
                             'mgs_ext12_ipng_mgs95j.bsp'  ) 
 
         \begintext 
 
         End of meta-kernel. 
 
 
      Example code begins here. 
    

         /.
            Program spkcpt_ex1 


               This program demonstrates the use of spkcpt_c.
               Computations are performed using all three possible
               values of the output frame evaluation locus `refloc':

                  "TARGET"
                  "OBSERVER"
                  "CENTER"

               Several unrelated computations are performed in this
               program. In particular, computations involving a surface
               point on Mars are included simply to demonstrate use of
               the "OBSERVER" option.
         ./

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

         int main()
         {   
            /.
            Local constants 
            ./

            #define  CAMERA   "MGS_MOC_NA"
            #define  MAXBND   100
            #define  META     "spkcpt.tm"
            #define  FRNMLN   33
            #define  SHAPLN   33
            #define  TIMFMT   "YYYY MON DD HR:MN:SC.###### UTC"
            #define  TIMLEN   41

            /.
            Local variables 
            ./   
            SpiceBoolean            found;

            SpiceChar             * abcorr;
            SpiceChar               camref  [ FRNMLN ];
            SpiceChar               emitim  [ TIMLEN ];
            SpiceChar             * refloc;
            SpiceChar             * obsrvr;
            SpiceChar             * obstim;
            SpiceChar             * outref;
            SpiceChar               shape   [ SHAPLN ];
            SpiceChar             * target;
            SpiceChar             * trgctr;
            SpiceChar             * trgref;

            SpiceDouble             bounds [ MAXBND ] [ 3 ];
            SpiceDouble             bsight [ 3 ];
            SpiceDouble             et;
            SpiceDouble             lt0;
            SpiceDouble             lt1;
            SpiceDouble             lt2;
            SpiceDouble             lt3;
            SpiceDouble             spoint [ 3 ];
            SpiceDouble             srfvec [ 3 ];
            SpiceDouble             state0 [ 6 ];
            SpiceDouble             state1 [ 6 ];
            SpiceDouble             state2 [ 6 ];
            SpiceDouble             state3 [ 6 ];
            SpiceDouble             trgepc ;
            SpiceDouble             trgpos [ 3 ];

            SpiceInt                camid;
            SpiceInt                n;

            /.
            Load SPICE kernels. 
            ./
            furnsh_c ( META );

            /.
            Convert the observation time to seconds past J2000 TDB.
            ./
            obstim = "2003 OCT 13 06:00:00.000000 UTC";

            str2et_c ( obstim, &et );

            /.
            Set the observer, target center, and target frame.
            ./
            obsrvr = "MGS";
            trgctr = "EARTH";
            trgref = "ITRF93";

            /.
            Set the state of DSS-14 relative to the earth's 
            center at the J2000 epoch, expressed in the 
            ITRF93 reference frame. Values come from the 
            earth station SPK specified in the meta-kernel.

            The actual station velocity is non-zero due
            to tectonic plate motion; we ignore the motion
            in this example. See the routine spkcvt_c for an
            example in which the plate motion is accounted for.
            ./
            trgpos[0] =  -2353.6213656676991;
            trgpos[1] =  -4641.3414911499403;
            trgpos[2] =   3677.0523293197439;

            /.
            Find the apparent state of the station relative
            to the spacecraft in the ITRF93 reference frame. 
            Evaluate the earth's orientation, that is the 
            orientation of the ITRF93 frame relative to the
            J2000 frame, at the epoch obtained by correcting 
            the observation time for one-way light time. This 
            correction is obtained by setting `refloc' to "TARGET".
            ./
            outref = "ITRF93";
            abcorr = "CN+S";

            refloc = "TARGET";

            /.
            Compute the observer-target state. 
            ./
            spkcpt_c ( trgpos, trgctr, trgref, 
                       et,     outref, refloc, abcorr,
                       obsrvr, state0, &lt0            );

            /. 
            Display the computed state and light time.
            ./
            timout_c ( et-lt0, TIMFMT, TIMLEN, emitim );

            printf ( "\n"
                     " Frame evaluation locus:   %s\n"
                     "\n"
                     " Observer:                 %s\n"
                     " Observation time:         %s\n"
                     " Target center:            %s\n"
                     " Target frame:             %s\n"
                     " Emission time:            %s\n"
                     " Output reference frame:   %s\n"
                     " Aberration correction:    %s\n"
                     "\n"
                     " Observer-target position (km):\n"
                     "   %20.8f %20.8f %20.8f\n"
                     " Observer-target velocity (km/s):\n"
                     "   %20.8f %20.8f %20.8f\n"
                     " Light time (s):   %20.8f\n",

                     refloc,    obsrvr,    obstim,    trgctr, 
                     trgref,    emitim,    outref,    abcorr,  
                     state0[0], state0[1], state0[2],
                     state0[3], state0[4], state0[5], lt0   );

            /.
            Repeat the computation, this time evaluating the
            earth's orientation at the epoch obtained by
            subtracting from the observation time the one way
            light time from the earth's center.

            This is equivalent to looking up the observer-target
            state using spkezr_c.
            ./   
            refloc = "CENTER";

            spkcpt_c ( trgpos, trgctr, trgref, 
                       et,     outref, refloc, abcorr,
                       obsrvr, state1, &lt1            );

            /. 
            Display the computed state and light time.
            ./
            timout_c ( et-lt1, TIMFMT, TIMLEN, emitim );

            printf ( "\n\n"
                     " Frame evaluation locus:   %s\n"
                     "\n"
                     " Observer:                 %s\n"
                     " Observation time:         %s\n"
                     " Target center:            %s\n"
                     " Target frame:             %s\n"
                     " Emission time:            %s\n"
                     " Output reference frame:   %s\n"
                     " Aberration correction:    %s\n"
                     "\n"
                     " Observer-target position (km):\n"
                     "   %20.8f %20.8f %20.8f\n"
                     " Observer-target velocity (km/s):\n"
                     "   %20.8f %20.8f %20.8f\n"
                     " Light time (s):   %20.8f\n",

                     refloc,    obsrvr,    obstim,    trgctr, 
                     trgref,    emitim,    outref,    abcorr, 
                     state1[0], state1[1], state1[2],
                     state1[3], state1[4], state1[5], lt0   );

            printf ( "\n"
                     " Distance between above positions (km):  "
                     "    %20.8f\n"
                     " Velocity difference magnitude  (km/s):"
                     "      %20.8f\n",
                     vdist_c( state0,   state1   ),
                     vdist_c( state0+3, state1+3 )                     );

            /.
            Check: compare the state computed directly above
            to one produced by spkezr_c: 
            ./
            target = "DSS-14";

            spkezr_c ( target,  et,      outref,  abcorr, 
                       obsrvr,  state2,  &lt2            );

            printf ( "\n\n"
                     " State computed using spkezr_c:\n"
                     "\n"
                     " Observer:               %s\n"
                     " Observation time:       %s\n"
                     " Target:                 %s\n"
                     " Output reference frame: %s\n"
                     " Aberration correction:  %s\n"
                     "\n"
                     " Observer-target position (km):\n"
                     "   %20.8f %20.8f %20.8f\n"
                     " Observer-target velocity (km/s):\n"
                     "   %20.8f %20.8f %20.8f\n"
                     " Light time (s): %20.8f\n",

                     obsrvr,    obstim,    target,
                     outref,    abcorr,
                     state2[0], state2[1], state2[2],
                     state2[3], state2[4], state2[5], lt2   );

            printf ( "\n"
                     " Distance between last two "
                     "positions (km):   %20.8f\n"
                     " Velocity difference magnitude    "
                     " (km/s):   %20.8f\n",
                     vdist_c( state1,   state2   ),       
                     vdist_c( state1+3, state2+3 )          );

            /.
            Finally, compute an observer-target state in
            a frame centered at the observer.
            The reference frame will be that of the 
            MGS MOC NA camera.

            In this case we'll use as the target the surface
            intercept on Mars of the camera boresight. This
            allows us to easily verify the correctness of
            the results returned by spkcpt_c.

            Get camera frame and FOV parameters. We'll need
            the camera ID code first.
            ./
            bodn2c_c ( CAMERA, &camid, &found );

            if ( !found )
            {
               printf ( "Camera name could not be mapped "
                        "to an ID code.\n"              );
               exit( 1 );
            }

            /.
            getfov_c will return the name of the camera-fixed frame
            in the string `camref', the camera boresight vector in
            the array `bsight', and the FOV corner vectors in the
            array `bounds'. All we're going to use are the camera
            frame name and camera boresight.
            ./
            getfov_c ( camid, MAXBND, SHAPLN, FRNMLN, 
                       shape, camref, bsight, &n,     bounds );

            /.
            Find the camera boresight surface intercept.
            ./

            trgctr = "MARS";
            trgref = "IAU_MARS";

            sincpt_c ( "Ellipsoid",  trgctr,  et,     trgref,
                       abcorr,       obsrvr,  camref, bsight, 
                       spoint,       &trgepc, srfvec, &found  );

            outref = camref;

            refloc = "OBSERVER";

            spkcpt_c ( spoint, trgctr, trgref, 
                       et,     outref, refloc, abcorr,
                       obsrvr, state3, &lt3           );

            /.
            Convert the emission time and the target state
            evaluation epoch to strings for output. 
            ./
            timout_c ( et-lt3, TIMFMT, TIMLEN, emitim );

            printf ( "\n\n"
                     " Frame evaluation locus:   %s\n"
                     "\n"
                     " Observer:                 %s\n"
                     " Observation time:         %s\n"
                     " Target center:            %s\n"
                     " Target frame:             %s\n"
                     " Emission time:            %s\n"
                     " Output reference frame:   %s\n"
                     " Aberration correction:    %s\n"
                     "\n"
                     " Observer-target position (km):\n"
                     "   %20.8f %20.8f %20.8f\n"
                     " Observer-target velocity (km/s):\n"
                     "   %20.8f %20.8f %20.8f\n"
                     " Light time (s): %20.8f\n"
                     " Target range from sincpt_c (km): "
                     "           %20.8f\n",

                     refloc,    obsrvr,    obstim,    trgctr,
                     trgref,    emitim,    outref,    abcorr, 
                     state3[0], state3[1], state3[2],
                     state3[3], state3[4], state3[5], lt3,
                     vnorm_c( srfvec )                          );

            return ( 0 );
         }


      When this program was executed on a PC/Linux/gcc
      platform, the output was: 
 

         Frame evaluation locus:   TARGET

         Observer:                 MGS
         Observation time:         2003 OCT 13 06:00:00.000000 UTC
         Target center:            EARTH
         Target frame:             ITRF93
         Emission time:            2003 OCT 13 05:55:44.232914 UTC
         Output reference frame:   ITRF93
         Aberration correction:    CN+S

         Observer-target position (km):
              52746468.84243592    52367725.79653772    18836142.68957234
         Observer-target velocity (km/s):
                  3823.39593314       -3840.60002121           2.21337692
         Light time (s):           255.76708533


         Frame evaluation locus:   CENTER

         Observer:                 MGS
         Observation time:         2003 OCT 13 06:00:00.000000 UTC
         Target center:            EARTH
         Target frame:             ITRF93
         Emission time:            2003 OCT 13 05:55:44.232914 UTC
         Output reference frame:   ITRF93
         Aberration correction:    CN+S

         Observer-target position (km):
              52746419.34648802    52367775.65036674    18836142.68969753
         Observer-target velocity (km/s):
                  3823.40103499       -3840.59789000           2.21337692
         Light time (s):           255.76708533

         Distance between above positions (km):               70.25135676
         Velocity difference magnitude  (km/s):                0.00552910


         State computed using spkezr_c:

         Observer:               MGS
         Observation time:       2003 OCT 13 06:00:00.000000 UTC
         Target:                 DSS-14
         Output reference frame: ITRF93
         Aberration correction:  CN+S

         Observer-target position (km):
              52746419.34641990    52367775.65039122    18836142.68968301
         Observer-target velocity (km/s):
                  3823.40103499       -3840.59789000           2.21337692
         Light time (s):         255.76708533

         Distance between last two positions (km):             0.00007383
         Velocity difference magnitude     (km/s):             0.00000000


         Frame evaluation locus:   OBSERVER

         Observer:                 MGS
         Observation time:         2003 OCT 13 06:00:00.000000 UTC
         Target center:            MARS
         Target frame:             IAU_MARS
         Emission time:            2003 OCT 13 05:59:59.998702 UTC
         Output reference frame:   MGS_MOC_NA
         Aberration correction:    CN+S

         Observer-target position (km):
                     0.00000001          -0.00000001         388.97573572
         Observer-target velocity (km/s):
                     2.91968665           0.15140014           0.92363513
         Light time (s):           0.00129748
         Target range from sincpt_c (km):                    388.97573572

Restrictions

 
   1)  This routine may not be suitable for work with stars or other 
       objects having large distances from the observer, due to loss 
       of precision in position vectors. 
 

Literature_References

 
   None. 
 

Author_and_Institution

 
   N.J. Bachman    (JPL) 
   S.C. Krening    (JPL) 
   B.V. Semenov    (JPL) 
 

Version

 
   -CSPICE Version 1.0.1, 09-SEP-2015 (NJB)

       The Exceptions section of the header was updated
       to mention exceptions involving null pointers and 
       empty input strings.

   -CSPICE Version 1.0.0, 27-MAR-2012 (NJB) (SCK) (BVS)

Index_Entries

 
   state of constant_velocity_target  
   state of surface_point on extended_object 
   state of landmark on extended_object 
 
Wed Apr  5 17:54:43 2017