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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 subpnt_c ( ConstSpiceChar       * method,
                   ConstSpiceChar       * target,
                   SpiceDouble            et,
                   ConstSpiceChar       * fixref,
                   ConstSpiceChar       * abcorr,
                   ConstSpiceChar       * obsrvr,
                   SpiceDouble            spoint [3],
                   SpiceDouble          * trgepc,
                   SpiceDouble            srfvec [3] )

Abstract

 
   Compute the rectangular coordinates of the sub-observer point on 
   a target body at a specified epoch, optionally corrected for 
   light time and stellar aberration. 
 
   The surface of the target body may be represented by a triaxial 
   ellipsoid or by topographic data provided by DSK files. 
 
   This routine supersedes subpt_c. 
 

Required_Reading

 
   DSK 
   FRAMES 
   NAIF_IDS 
   PCK 
   SPK 
   TIME 
 

Keywords

 
   GEOMETRY 
 

Brief_I/O

 
   Variable  I/O  Description 
   --------  ---  -------------------------------------------------- 
   method     I   Computation method. 
   target     I   Name of target body. 
   et         I   Epoch in TDB seconds past J2000 TDB. 
   fixref     I   Body-fixed, body-centered target body frame. 
   abcorr     I   Aberration correction flag. 
   obsrvr     I   Name of observing body. 
   spoint     O   Sub-observer point on the target body. 
   trgepc     O   Sub-observer point epoch. 
   srfvec     O   Vector from observer to sub-observer point. 
 

Detailed_Input

 
 
   method   is a short string providing parameters defining 
            the computation method to be used. In the syntax 
            descriptions below, items delimited by brackets 
            are optional. 
 
            `method' may be assigned the following values: 
 
               "NEAR POINT/ELLIPSOID" 
 
                  The sub-observer point computation uses a 
                  triaxial ellipsoid to model the surface of the 
                  target body. The sub-observer point is defined 
                  as the nearest point on the target relative to 
                  the observer. 
 
                  The word "NADIR" may be substituted for the phrase 
                  "NEAR POINT" in the string above.  
    
                  For backwards compatibility, the older syntax 
 
                     "Near point: ellipsoid" 
 
                  is accepted as well. 
 
 
               "INTERCEPT/ELLIPSOID" 
 
                  The sub-observer point computation uses a 
                  triaxial ellipsoid to model the surface of the 
                  target body. The sub-observer point is defined 
                  as the target surface intercept of the line 
                  containing the observer and the target's 
                  center. 
 
                  For backwards compatibility, the older syntax 
 
                     "Intercept: ellipsoid" 
 
                  is accepted as well. 
 
 
               "NADIR/DSK/UNPRIORITIZED[/SURFACES = <surface list>]" 
 
                  The sub-observer point computation uses DSK data 
                  to model the surface of the target body. The 
                  sub-observer point is defined as the intercept, on 
                  the surface represented by the DSK data, of the 
                  line containing the observer and the nearest point 
                  on the target's reference ellipsoid. If multiple 
                  such intercepts exist, the one closest to the 
                  observer is selected. 
 
                  Note that this definition of the sub-observer 
                  point is not equivalent to the "nearest point on 
                  the surface to the observer." The phrase "NEAR 
                  POINT" may NOT be substituted for "NADIR" in the 
                  string above. 
 
                  The surface list specification is optional. The 
                  syntax of the list is 
 
                     <surface 1> [, <surface 2>...] 
 
                  If present, it indicates that data only for the 
                  listed surfaces are to be used; however, data 
                  need not be available for all surfaces in the 
                  list. If absent, loaded DSK data for any surface 
                  associated with the target body are used. 
 
                  The surface list may contain surface names or 
                  surface ID codes. Names containing blanks must 
                  be delimited by escaped double quotes, for example 
 
                     "SURFACES = \"Mars MEGDR 128 PIXEL/DEG\""
 
                  If multiple surfaces are specified, their names 
                  or IDs must be separated by commas. 
 
                  See the Particulars section below for details 
                  concerning use of DSK data. 
 
 
               "INTERCEPT/DSK/UNPRIORITIZED[/SURFACES = <surface list>]" 
 
                  The sub-observer point computation uses DSK data 
                  to model the surface of the target body. The 
                  sub-observer point is defined as the target 
                  surface intercept of the line containing the 
                  observer and the target's center. 
 
                  If multiple such intercepts exist, the one closest 
                  to the observer is selected. 
 
                  The surface list specification is optional. The 
                  syntax of the list is identical to that for the 
                  NADIR option described above. 
 
 
               Neither case nor white space are significant in 
               `method', except within double-quoted strings. For 
               example, the string " eLLipsoid/nearpoint " is valid. 
 
               Within double-quoted strings, blank characters are 
               significant, but multiple consecutive blanks are 
               considered equivalent to a single blank. Case is  
               not significant. So 
 
                  "Mars MEGDR 128 PIXEL/DEG" 
 
               is equivalent to  
 
                  " mars megdr  128  pixel/deg " 
 
               but not to 
 
                  "MARS MEGDR128PIXEL/DEG" 
 
                
   target      is the name of the target body. The target body is  
               an ephemeris object (its trajectory is given by 
               SPK data), and is an extended object. 
 
               The string `target' is case-insensitive, and leading 
               and trailing blanks in `target' are not significant. 
               Optionally, you may supply a string containing the 
               integer ID code for the object. For example both 
               "MOON" and "301" are legitimate strings that indicate 
               the moon is the target body. 
 
               When the target body's surface is represented by a 
               tri-axial ellipsoid, this routine assumes that a 
               kernel variable representing the ellipsoid's radii is 
               present in the kernel pool. Normally the kernel 
               variable would be defined by loading a PCK file. 
 
 
   et          is the epoch of participation of the observer, 
               expressed as TDB seconds past J2000 TDB: `et' is 
               the epoch at which the observer's state is computed. 
 
               When aberration corrections are not used, `et' is also 
               the epoch at which the position and orientation of 
               the target body are computed. 
 
               When aberration corrections are used, the position 
               and orientation of the target body are computed at 
               et-lt or et+lt, where `lt' is the one-way light time 
               between the sub-observer point and the observer, and 
               the sign applied to `lt' depends on the selected 
               correction. See the description of `abcorr' below for 
               details. 
 
 
   fixref      is the name of a body-fixed reference frame centered 
               on the target body. `fixref' may be any such 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). The 
               string `fixref' is case-insensitive, and leading and 
               trailing blanks in `fixref' are not significant. 
 
               The output sub-observer point `spoint' and the 
               observer-to-sub-observer point vector `srfvec' will be 
               expressed relative to this reference frame. 
 
   abcorr      indicates the aberration corrections to be applied 
               when computing the target's position and orientation. 
        
               For remote sensing applications, where the apparent 
               sub-observer point seen by the observer is desired, 
               normally either of the corrections  
             
                  "LT+S"  
                  "CN+S" 
    
               should be used. These and the other supported options 
               are described below. `abcorr' may be any of the  
               following: 
 
                  "NONE"     Apply no correction. Return the  
                             geometric sub-observer point on the 
                             target body. 
 
               Let `lt' represent the one-way light time between the 
               observer and the sub-observer point (note: NOT 
               between the observer and the target body's center). 
               The following values of `abcorr' apply to the 
               "reception" case in which photons depart from the 
               sub-observer point'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 location of sub-observer 
                             point 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. 
 
                             Both the target position as seen by the 
                             observer, and rotation of the target 
                             body, are corrected for light time. 
 
                  "LT+S"     Correct for one-way light time and 
                             stellar aberration using a Newtonian 
                             formulation. This option modifies the 
                             sub-observer point obtained with the 
                             "LT" option to account for the 
                             observer's velocity relative to the 
                             solar system barycenter. These 
                             corrections yield the apparent 
                             sub-observer point. 
 
                  "CN"       Converged Newtonian light time 
                             correction. In solving the light time 
                             equation, the "CN" correction iterates 
                             until the solution converges. Both the 
                             position and rotation of the target 
                             body are corrected for light time. 
 
                  "CN+S"     Converged Newtonian light time and 
                             stellar aberration corrections. This 
                             option produces a solution that is at 
                             least as accurate at that obtainable 
                             with the "LT+S" option. Whether the 
                             "CN+S" solution is substantially more 
                             accurate 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. 
 
 
               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 
               sub-observer point 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 
                             sub-observer location at the moment it 
                             receives photons emitted from the 
                             observer's location 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. 
 
                             Both the target position as seen by the 
                             observer, and rotation of the target 
                             body, are corrected for light time. 
 
                  "XLT+S"    "Transmission" case: correct for 
                             one-way light time and stellar 
                             aberration using a Newtonian 
                             formulation  This option modifies the 
                             sub-observer point obtained with the 
                             "XLT" option to account for the 
                             observer's velocity relative to the 
                             solar system barycenter. 
 
                  "XCN"      Converged Newtonian light time 
                             correction. This is the same as "XLT" 
                             correction but with further iterations 
                             to a converged Newtonian light time 
                             solution.  
 
                  "XCN+S"    "Transmission" case: converged  
                             Newtonian light time and stellar  
                             aberration corrections. 
 
 
               Neither case nor white space are significant in 
               `abcorr'. For example, the string  
 
                 "Lt + s" 
 
               is valid. 
 
 
   obsrvr      is the name of the observing body. The observing body 
               is an ephemeris object: it typically is a spacecraft, 
               the earth, or a surface point on the earth. `obsrvr' is 
               case-insensitive, and leading and trailing blanks in 
               `obsrvr' are not significant. Optionally, you may 
               supply a string containing the integer ID code for 
               the object. For example both "MOON" and "301" are 
               legitimate strings that indicate the moon is the 
               observer. 
 

Detailed_Output

 
 
   spoint      is the sub-observer point on the target body. 
 
               For target shapes modeled by ellipsoids, the 
               sub-observer point is defined either as the point on 
               the target body that is closest to the observer, or 
               the target surface intercept of the line from the 
               observer to the target's center. 
 
               For target shapes modeled by topographic data 
               provided by DSK files, the sub-observer point is 
               defined as the target surface intercept of the line 
               from the observer to either the nearest point on the 
               reference ellipsoid, or to the target's center. If 
               multiple such intercepts exist, the one closest to 
               the observer is selected. 
 
               The input argument `method' selects the target shape 
               model and sub-observer point definition to be used. 
 
               `spoint' is expressed in Cartesian coordinates, 
               relative to the body-fixed target frame designated by 
               `fixref'. The body-fixed target frame is evaluated at 
               the sub-observer epoch `trgepc' (see description below). 
 
               When light time correction is used, the duration of 
               light travel between `spoint' to the observer is 
               considered to be the one way light time. 
 
               When aberration corrections are used, `spoint' is 
               computed using target body position and orientation 
               that have been adjusted for the corrections 
               applicable to `spoint' itself rather than to the target 
               body's center. In particular, if the stellar 
               aberration correction applicable to `spoint' is 
               represented by a shift vector S, then the light-time 
               corrected position of the target is shifted by S 
               before the sub-observer point is computed. 
                
               The components of `spoint' have units of km. 
 
 
   trgepc      is the "sub-observer point epoch." `trgepc' is defined 
               as follows: letting `lt' be the one-way light time 
               between the observer and the sub-observer point, 
               `trgepc' is the epoch et-lt, et+lt, or `et' depending on 
               whether the requested aberration correction is, 
               respectively, for received radiation, transmitted 
               radiation, or omitted. `lt' is computed using the 
               method indicated by `abcorr'. 
 
               `trgepc' is expressed as seconds past J2000 TDB. 
 
 
   SRFVEC      is the vector from the observer's position at `et' to 
               the aberration-corrected (or optionally, geometric) 
               position of `spoint', where the aberration corrections 
               are specified by `abcorr'. `srfvec' is expressed in the 
               target body-fixed reference frame designated by 
               `fixref', evaluated at `trgepc'. 
 
               The components of `srfvec' are given in units of km. 
 
               One can use the CSPICE function vnorm_c to obtain the 
               distance between the observer and `spoint': 
 
                  dist = vnorm_c ( srfvec ); 
 
               The observer's position OBSPOS, relative to the 
               target body's center, where the center's position is 
               corrected for aberration effects as indicated by 
               `abcorr', can be computed via the call: 
 
                  vsub_c ( spoint, srfvec, obspos );
 
               To transform the vector `srfvec' from a reference frame 
               `fixref' at time `trgepc' to a time-dependent reference 
               frame REF at time `et', the routine pxfrm2_c should be 
               called. Let `xform' be the 3x3 matrix representing the 
               rotation from the reference frame `fixref' at time 
               `trgepc' to the reference frame REF at time `et'. Then 
               `srfvec' can be transformed to the result `refvec' as 
               follows: 
 
                   pxfrm2_c ( fixref, ref,    trgepc, et, xform );
                   mxv_c    ( xform,  srfvec, refvec );
 
               The second example in the Examples header section 
               below presents a complete program that demonstrates 
               this procedure. 
 

Parameters

 
   None. 
 

Exceptions

 
   1)  If the specified aberration correction is unrecognized, the 
       error will be diagnosed and signaled by a routine in the call 
       tree of this routine. 
 
   2)  If either the target or observer input strings cannot be 
       converted to an integer ID code, the error 
       SPICE(IDCODENOTFOUND) is signaled. 
 
   3)  If `obsrvr' and `target' map to the same NAIF integer ID code, 
       the error SPICE(BODIESNOTDISTINCT) is signaled. 
 
   4)  If the input target body-fixed frame `fixref' is not 
       recognized, the error SPICE(NOFRAME) is signaled. A frame 
       name may fail to be recognized because a required frame 
       specification kernel has not been loaded; another cause is a 
       misspelling of the frame name. 
 
   5)  If the input frame `fixref' is not centered at the target body, 
       the error SPICE(INVALIDFRAME) is signaled. 
 
   6)  If the input argument `method' is not recognized, the error 
       SPICE(INVALIDMETHOD) is signaled by this routine, or the 
       error is diagnosed by a routine in the call tree of this 
       routine. 
 
       If the sub-observer point type is not specified or is not 
       recognized, the error SPICE(INVALIDSUBTYPE) is signaled. 
 
   7)  If the target and observer have distinct identities but are 
       at the same location (for example, the target is Mars and the 
       observer is the Mars barycenter), the error 
       SPICE(NOSEPARATION) is signaled. 
 
   8)  If insufficient ephemeris data have been loaded prior to 
       calling subpnt_c, the error will be diagnosed and signaled by a 
       routine in the call tree of this routine. Note that when 
       light time correction is used, sufficient ephemeris data must 
       be available to propagate the states of both observer and 
       target to the solar system barycenter. 
 
   9)  If the computation method specifies an ellipsoidal target 
       shape and triaxial radii of the target body have not been 
       loaded into the kernel pool prior to calling subpnt_c, the 
       error will be diagnosed and signaled by a routine in the call 
       tree of this routine. 
 
   10)  The target must be an extended body, and must have a shape 
        for which a sub-observer point can be defined. 
 
        If the target body's shape is modeled as an ellipsoid, and 
        if any of the radii of the target body are non-positive, the 
        error will be diagnosed and signaled by routines in the call 
        tree of this routine. 
 
        If the target body's shape is modeled by DSK data, the shape 
        must be such that the specified sub-observer point 
        definition is applicable. For example, if the target shape 
        is a torus, both the NADIR and INTERCEPT definitions might 
        be inapplicable, depending on the relative locations of the 
        observer and target. 
 
   11)  If PCK or CK data specifying the target body-fixed frame 
        orientation have not been loaded prior to calling subpnt_c, 
        the error will be diagnosed and signaled by a routine in the 
        call tree of this routine. 
 
   12) If `method' specifies that the target surface is represented by 
       DSK data, and no DSK files are loaded for the specified 
       target, the error is signaled by a routine in the call tree 
       of this routine. 
        
   13) If `method' specifies that the target surface is represented 
       by DSK data, and the ray from the observer to the 
       sub-observer point doesn't intersect the target body's 
       surface, the error SPICE(SUBPOINTNOTFOUND) will be signaled. 
 
   14) In some very rare cases, the surface intercept on the  
       target body's reference ellipsoid of the observer to target 
       center vector may not be computable. In these cases the 
       error SPICE(DEGENERATECASE) is 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 and observer must be 
        loaded. If aberration corrections are used, the states of 
        target 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 via furnsh_c. 
 
      - Target body orientation data: these may be provided in a text or 
        binary PCK file. In some cases, target body orientation may
        be provided by one more more CK files. In either case, data
        are made available by loading the files via furnsh_c.
 
      - Shape data for the target body: 
               
          PCK data:  
 
             If the target body shape is modeled as an ellipsoid, 
             triaxial radii for the target body must be loaded into 
             the kernel pool. Typically this is done by loading a 
             text PCK file via furnsh_c. 
 
             Triaxial radii are also needed if the target shape is 
             modeled by DSK data, but the DSK NADIR method is 
             selected. 
 
          DSK data:  
 
             If the target shape is modeled by DSK data, DSK files 
             containing topographic data for the target body must be 
             loaded. If a surface list is specified, data for at 
             least one of the listed surfaces must be loaded. 
 
   The following data may be required: 
 
      - Frame data: if a frame definition is required to convert the 
        observer and target states to the body-fixed frame of the 
        target, that definition must be available in the kernel 
        pool. Typically the definition is supplied by loading a 
        frame kernel via furnsh_c. 
 
      - Surface name-ID associations: if surface names are specified 
        in `method', the association of these names with their 
        corresponding surface ID codes must be established by  
        assignments of the kernel variables 
 
           NAIF_SURFACE_NAME 
           NAIF_SURFACE_CODE 
           NAIF_SURFACE_BODY 
 
        Normally these associations are made by loading a text 
        kernel containing the necessary assignments. An example 
        of such assignments is 
 
           NAIF_SURFACE_NAME += 'Mars MEGDR 128 PIXEL/DEG' 
           NAIF_SURFACE_CODE += 1                     
           NAIF_SURFACE_BODY += 499                     
 
      - SCLK data: if the target body's orientation is provided by
        CK files, an associated SCLK kernel must be loaded.

   In all cases, kernel data are normally loaded once per program 
   run, NOT every time this routine is called. 
 

Particulars

 
   For ellipsoidal target bodies, there are two different popular 
   ways to define the sub-observer point: "nearest point on the 
   target to the observer" or "target surface intercept of the line 
   containing observer and target." These coincide when the target 
   is spherical and generally are distinct otherwise. 
 
   For target body shapes modeled using topographic data provided by 
   DSK files, the "surface intercept" notion is valid, but the 
   "nearest point on the surface" computation is both inefficient to 
   execute and may fail to yield a result that is "under" the 
   observer in an intuitively clear way. The NADIR option for DSK 
   shapes instead finds the surface intercept of a ray that passes 
   through the nearest point on the target reference ellipsoid. For 
   shapes modeled using topography, there may be multiple 
   ray-surface intercepts; the closest one to the observer is 
   selected. 
 
   The NADIR definition makes sense only if the target shape is 
   reasonably close to the target's reference ellipsoid. If the 
   target is very different---the nucleus of comet 
   Churyumov-Gerasimenko is an example---the intercept definition 
   should be used. 
 
   This routine computes light time corrections using light time 
   between the observer and the sub-observer point, as opposed to 
   the center of the target. Similarly, stellar aberration 
   corrections done by this routine are based on the direction of 
   the vector from the observer to the light-time corrected 
   sub-observer point, not to the target center. This technique 
   avoids errors due to the differential between aberration 
   corrections across the target body. Therefore it's valid to use 
   aberration corrections with this routine even when the observer 
   is very close to the sub-observer point, in particular when the 
   observer to sub-observer point distance is much less than the 
   observer to target center distance. 
    
   When comparing sub-observer point computations with results from 
   sources other than SPICE, it's essential to make sure the same 
   geometric definitions are used. 
 
 
   Using DSK data 
   ============== 
 
      DSK loading and unloading 
      ------------------------- 
 
      DSK files providing data used by this routine are loaded by 
      calling furnsh_c and can be unloaded by calling unload_c or 
      kclear_c. See the documentation of furnsh_c for limits on numbers 
      of loaded DSK files. 
 
      For run-time efficiency, it's desirable to avoid frequent 
      loading and unloading of DSK files. When there is a reason to 
      use multiple versions of data for a given target body---for 
      example, if topographic data at varying resolutions are to be 
      used---the surface list can be used to select DSK data to be 
      used for a given computation. It is not necessary to unload 
      the data that are not to be used. This recommendation presumes 
      that DSKs containing different versions of surface data for a 
      given body have different surface ID codes. 
 
 
      DSK data priority 
      ----------------- 
 
      A DSK coverage overlap occurs when two segments in loaded DSK 
      files cover part or all of the same domain---for example, a 
      given longitude-latitude rectangle---and when the time 
      intervals of the segments overlap as well. 
 
      When DSK data selection is prioritized, in case of a coverage 
      overlap, if the two competing segments are in different DSK 
      files, the segment in the DSK file loaded last takes 
      precedence. If the two segments are in the same file, the 
      segment located closer to the end of the file takes 
      precedence. 
 
      When DSK data selection is unprioritized, data from competing 
      segments are combined. For example, if two competing segments 
      both represent a surface as sets of triangular plates, the 
      union of those sets of plates is considered to represent the 
      surface.  
 
      Currently only unprioritized data selection is supported. 
      Because prioritized data selection may be the default behavior 
      in a later version of the routine, the UNPRIORITIZED keyword is 
      required in the `method' argument. 
 
       
      Syntax of the `method' input argument 
      ----------------------------------- 
 
      The keywords and surface list in the `method' argument 
      are called "clauses." The clauses may appear in any 
      order, for example 
 
         "NADIR/DSK/UNPRIORITIZED/<surface list>"
         "DSK/NADIR/<surface list>/UNPRIORITIZED"
         "UNPRIORITIZED/<surface list>/DSK/NADIR"
 
      The simplest form of the `method' argument specifying use of 
      DSK data is one that lacks a surface list, for example: 
 
         "NADIR/DSK/UNPRIORITIZED" 
         "INTERCEPT/DSK/UNPRIORITIZED" 
 
      For applications in which all loaded DSK data for the target 
      body are for a single surface, and there are no competing 
      segments, the above strings suffice. This is expected to be 
      the usual case. 
 
      When, for the specified target body, there are loaded DSK 
      files providing data for multiple surfaces for that body, the 
      surfaces to be used by this routine for a given call must be 
      specified in a surface list, unless data from all of the 
      surfaces are to be used together. 
 
      The surface list consists of the string 
 
         "SURFACES = "
 
      followed by a comma-separated list of one or more surface 
      identifiers. The identifiers may be names or integer codes in 
      string format. For example, suppose we have the surface 
      names and corresponding ID codes shown below: 
 
         Surface Name                              ID code 
         ------------                              ------- 
         "Mars MEGDR 128 PIXEL/DEG"                1 
         "Mars MEGDR 64 PIXEL/DEG"                 2 
         "Mars_MRO_HIRISE"                         3 
 
      If data for all of the above surfaces are loaded, then 
      data for surface 1 can be specified by either 
 
         "SURFACES = 1" 
 
      or 
 
         "SURFACES = \"Mars MEGDR 128 PIXEL/DEG\"" 
 
      Escaped double quotes are used to delimit the surface name
      because it contains blank characters.
          
      To use data for surfaces 2 and 3 together, any 
      of the following surface lists could be used: 
 
         "SURFACES = 2, 3" 
 
         "SURFACES = \"Mars MEGDR  64 PIXEL/DEG\", 3" 
 
         "SURFACES = 2, Mars_MRO_HIRISE" 
 
         "SURFACES = \"Mars MEGDR 64 PIXEL/DEG\", Mars_MRO_HIRISE" 
        
      An example of a `method' argument that could be constructed 
      using one of the surface lists above is 
 
      "NADIR/DSK/UNPRIORITIZED/SURFACES= \"Mars MEGDR 64 PIXEL/DEG\",3" 
 
 
        
      Aberration corrections 
      ---------------------- 
 
      For irregularly shaped target bodies, the distance between the 
      observer and the nearest surface intercept need not be a 
      continuous function of time; hence the one-way light time 
      between the intercept and the observer may be discontinuous as 
      well. In such cases, the computed light time, which is found 
      using an iterative algorithm, may converge slowly or not at all. 
      In all cases, the light time computation will terminate, but 
      the result may be less accurate than expected. 
    
 

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) Find the sub-Earth point on Mars for a specified time. 
 
      Compute the sub-Earth points using both triaxial ellipsoid 
      and topographic surface models. Topography data are provided by 
      a DSK file. For the ellipsoid model, use both the "intercept" 
      and "near point" sub-observer point definitions; for the DSK 
      case, use both the "intercept" and "nadir" definitions. 
 
      Display the locations of both the Earth and the sub-Earth 
      point relative to the center of Mars, in the IAU_MARS 
      body-fixed reference frame, using both planetocentric and 
      planetographic coordinates. 
 
      The topographic model is based on data from the MGS MOLA DEM 
      megr90n000cb, which has a resolution of 4 pixels/degree. A 
      triangular plate model was produced by computing a 720 x 1440 
      grid of interpolated heights from this DEM, then tessellating 
      the height grid. The plate model is stored in a type 2 segment 
      in the referenced DSK file. 
 
      Use the meta-kernel shown below to load the required SPICE 
      kernels. 
  
 
         KPL/MK 
 
         File: subpnt_ex1.tm 
 
         This meta-kernel is intended to support operation of SPICE 
         example programs. The kernels shown here should not be 
         assumed to contain adequate or correct versions of data 
         required by SPICE-based user applications. 
 
         In order for an application to use this meta-kernel, the 
         kernels referenced here must be present in the user's 
         current working directory. 
 
         The names and contents of the kernels referenced 
         by this meta-kernel are as follows: 
 
            File name                        Contents 
            ---------                        -------- 
            de430.bsp                        Planetary ephemeris 
            mar097.bsp                       Mars satellite ephemeris 
            pck00010.tpc                     Planet orientation and 
                                             radii 
            naif0011.tls                     Leapseconds 
            megr90n000cb_plate.bds           Plate model based on 
                                             MEGDR DEM, resolution 
                                             4 pixels/degree. 
 
         \begindata 
 
            KERNELS_TO_LOAD = ( 'de430.bsp', 
                                'mar097.bsp', 
                                'pck00010.tpc', 
                                'naif0011.tls', 
                                'megr90n000cb_plate.bds' ) 
         \begintext 
 
 
 
     Example code begins here. 
 
       /.
          Program subpnt_ex1 
       ./
       #include <stdio.h>
       #include "SpiceUsr.h"

       int main()
       {    
          /.
          Local parameters
          ./
          #define META            "subpnt_ex1.tm"
          #define MTHLEN          81
          #define NMETH           4

          /.
          Local variables
          ./
          static SpiceChar      * method[NMETH] =
                                  {
                                     "Intercept/ellipsoid",
                                     "Near point/ellipsoid",
                                     "Intercept/DSK/Unprioritized",
                                     "Nadir/DSK/Unprioritized"
                                  };

          SpiceDouble             et;
          SpiceDouble             f;
          SpiceDouble             obspos [3];
          SpiceDouble             odist;
          SpiceDouble             opclat;
          SpiceDouble             opclon;
          SpiceDouble             opcrad;
          SpiceDouble             opgalt;
          SpiceDouble             opglat;
          SpiceDouble             opglon;
          SpiceDouble             radii  [3];
          SpiceDouble             re;
          SpiceDouble             rp;
          SpiceDouble             spclat;
          SpiceDouble             spclon;
          SpiceDouble             spcrad;
          SpiceDouble             spgalt;
          SpiceDouble             spglat;
          SpiceDouble             spglon;
          SpiceDouble             spoint [3];
          SpiceDouble             srfvec [3];
          SpiceDouble             trgepc;

          SpiceInt                i;
          SpiceInt                n;

          /.
          Load kernel files via the meta-kernel.
          ./
          furnsh_c ( META );

          /.
          Convert the UTC request time string to seconds past
          J2000, TDB.
          ./
          str2et_c ( "2008 aug 11 00:00:00", &et );

          /.
          Look up the target body's radii. We'll use these to
          convert Cartesian to planetographic coordinates. Use
          the radii to compute the flattening coefficient of
          the reference ellipsoid.
          ./
          bodvrd_c ( "MARS", "RADII", 3, &n, radii );

          /.
          Let `re and `rp' be, respectively, the equatorial and
          polar radii of the target.
          ./
          re = radii[0];
          rp = radii[2];

          f  = ( re - rp ) / re;

          /.
          Compute sub-observer point using light time and stellar
          aberration corrections. Use both ellipsoid and DSK 
          shape models, and use all of the "near point," 
          "intercept," and "nadir" sub-observer point definitions. 
          ./
          for ( i = 0;  i < NMETH;  i++ )
          {
             subpnt_c ( method[i],
                        "mars",  et,     "iau_mars", "cn+s",
                        "earth", spoint, &trgepc,    srfvec );
             /.
             Compute the observer's distance from `spoint'.
             ./
             odist = vnorm_c ( srfvec );

             /.
             Convert the sub-observer point's rectangular coordinates
             to planetographic longitude, latitude and altitude.
             Convert radians to degrees.
             ./
             recpgr_c ( "mars",  spoint,  re,     f,
                        &spglon, &spglat, &spgalt   );

             spglon *= dpr_c();
             spglat *= dpr_c();

             /.
             Convert sub-observer point's rectangular coordinates to
             planetocentric radius, longitude, and latitude. Convert
             radians to degrees.
             ./
             reclat_c ( spoint, &spcrad, &spclon, &spclat );

             spclon *= dpr_c();
             spclat *= dpr_c();

             /.
             Compute the observer's position relative to the center
             of the target, where the center's location has been
             adjusted using the aberration corrections applicable
             to the sub-point. Express the observer's location in
             planetographic coordinates.
             ./
             vsub_c ( spoint, srfvec, obspos );

             recpgr_c ( "mars",  obspos,  re,    f,
                        &opglon, &opglat, &opgalt   );

             opglon *= dpr_c ();
             opglat *= dpr_c ();

             /.
             Convert the observer's rectangular coordinates to
             planetocentric radius, longitude, and latitude.
             Convert radians to degrees.
             ./
             reclat_c ( obspos, &opcrad, &opclon, &opclat );

             opclon *= dpr_c();
             opclat *= dpr_c();

             /.
             Write the results.
             ./
             printf ( "\n"
                      " Computation method = %s\n\n"
                      "  Observer altitude relative to spheroid (km) = %21.9f\n"
                      "  Length of SRFVEC                       (km) = %21.9f\n"
                      "  Sub-observer point altitude            (km) = %21.9f\n"
                      "  Sub-observer planetographic longitude (deg) = %21.9f\n"
                      "  Observer planetographic longitude     (deg) = %21.9f\n"
                      "  Sub-observer planetographic latitude  (deg) = %21.9f\n"
                      "  Observer planetographic latitude      (deg) = %21.9f\n"
                      "  Sub-observer planetocentric longitude (deg) = %21.9f\n"
                      "  Observer planetocentric longitude     (deg) = %21.9f\n"
                      "  Sub-observer planetocentric latitude  (deg) = %21.9f\n"
                      "  Observer planetocentric latitude      (deg) = %21.9f\n"
                      "\n",
                      method[i], 
                      opgalt,
                      odist,
                      spgalt, 
                      spglon,
                      opglon, 
                      spglat, 
                      opglat, 
                      spclon, 
                      opclon,
                      spclat,
                      opclat      );
          }
          return ( 0 );
       }

 
 
   When this program was executed on a PC/Linux/gcc 64-bit 
   platform, the output was: 

      Computation method = Intercept/ellipsoid

       Observer altitude relative to spheroid (km) =   349199089.540947020
       Length of SRFVEC                       (km) =   349199089.577642679
       Sub-observer point altitude            (km) =           0.000000000
       Sub-observer planetographic longitude (deg) =         199.302305029
       Observer planetographic longitude     (deg) =         199.302305029
       Sub-observer planetographic latitude  (deg) =          26.262401237
       Observer planetographic latitude      (deg) =          25.994936751
       Sub-observer planetocentric longitude (deg) =         160.697694971
       Observer planetocentric longitude     (deg) =         160.697694971
       Sub-observer planetocentric latitude  (deg) =          25.994934171
       Observer planetocentric latitude      (deg) =          25.994934171


      Computation method = Near point/ellipsoid

       Observer altitude relative to spheroid (km) =   349199089.540938675
       Length of SRFVEC                       (km) =   349199089.540938675
       Sub-observer point altitude            (km) =          -0.000000000
       Sub-observer planetographic longitude (deg) =         199.302305029
       Observer planetographic longitude     (deg) =         199.302305029
       Sub-observer planetographic latitude  (deg) =          25.994936751
       Observer planetographic latitude      (deg) =          25.994936751
       Sub-observer planetocentric longitude (deg) =         160.697694971
       Observer planetocentric longitude     (deg) =         160.697694971
       Sub-observer planetocentric latitude  (deg) =          25.729407227
       Observer planetocentric latitude      (deg) =          25.994934171


      Computation method = Intercept/DSK/Unprioritized

       Observer altitude relative to spheroid (km) =   349199089.541018367
       Length of SRFVEC                       (km) =   349199091.809211493
       Sub-observer point altitude            (km) =          -2.231473229
       Sub-observer planetographic longitude (deg) =         199.302304998
       Observer planetographic longitude     (deg) =         199.302304998
       Sub-observer planetographic latitude  (deg) =          26.262578570
       Observer planetographic latitude      (deg) =          25.994936751
       Sub-observer planetocentric longitude (deg) =         160.697695002
       Observer planetocentric longitude     (deg) =         160.697695002
       Sub-observer planetocentric latitude  (deg) =          25.994934171
       Observer planetocentric latitude      (deg) =          25.994934171


      Computation method = Nadir/DSK/Unprioritized

       Observer altitude relative to spheroid (km) =   349199089.541007757
       Length of SRFVEC                       (km) =   349199091.709665895
       Sub-observer point altitude            (km) =          -2.168658141
       Sub-observer planetographic longitude (deg) =         199.302305000
       Observer planetographic longitude     (deg) =         199.302305000
       Sub-observer planetographic latitude  (deg) =          25.994936751
       Observer planetographic latitude      (deg) =          25.994936751
       Sub-observer planetocentric longitude (deg) =         160.697695000
       Observer planetocentric longitude     (deg) =         160.697695000
       Sub-observer planetocentric latitude  (deg) =          25.729237374
       Observer planetocentric latitude      (deg) =          25.994934171

 
 
   2) Use subpnt_c to find the sub-spacecraft point on Mars for the 
      Mars Reconnaissance Orbiter spacecraft (MRO) at a specified time,
      using both the 'Ellipsoid/Near point' computation method and an
      ellipsoidal target shape, and the "DSK/Unprioritized/Nadir"
      method and a DSK-based shape model.
 
      Use both LT+S and CN+S aberration corrections to illustrate 
      the differences. 
  
      Convert the spacecraft to sub-observer point vector obtained from
      subpnt_c into the MRO_HIRISE_LOOK_DIRECTION reference frame at
      the observation time. Perform a consistency check with this
      vector: compare the Mars surface intercept of the ray emanating
      from the spacecraft and pointed along this vector with the
      sub-observer point.
 
      Perform the sub-observer point and surface intercept computations
      using both triaxial ellipsoid and topographic surface models.
 
      For this example, the topographic model is based on the MGS MOLA
      DEM megr90n000eb, which has a resolution of 16 pixels/degree.
      Eight DSKs, each covering longitude and latitude ranges of 90
      degrees, were made from this data set. For the region covered by
      a given DSK, a grid of approximately 1500 x 1500 interpolated
      heights was produced, and this grid was tessellated using
      approximately 4.5 million triangular plates, giving a total plate
      count of about 36 million for the entire DSK set.
 
      All DSKs in the set use the surface ID code 499001, so there is
      no need to specify the surface ID in the `method' strings passed
      to sincpt_c and subpnt_c.
 
      Use the meta-kernel shown below to load the required SPICE
      kernels.
 
 
         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. 
 
         The names and contents of the kernels referenced 
         by this meta-kernel are as follows: 
 
            File name                        Contents 
            ---------                        -------- 
            de430.bsp                        Planetary ephemeris 
            mar097.bsp                       Mars satellite ephemeris 
            pck00010.tpc                     Planet orientation and 
                                             radii 
            naif0011.tls                     Leapseconds 
            mro_psp4_ssd_mro95a.bsp          MRO ephemeris 
            mro_v11.tf                       MRO frame specifications 
            mro_sclkscet_00022_65536.tsc     MRO SCLK coefficients 
                                             parameters 
            mro_sc_psp_070925_071001.bc      MRO attitude 
            megr90n000eb_*_plate.bds         Plate model DSKs based  
                                             on MEGDR DEM, resolution 
                                             16 pixels/degree. 
 
         \begindata 
 
            KERNELS_TO_LOAD = (  
 
               'de430.bsp', 
               'mar097.bsp', 
               'pck00010.tpc', 
               'naif0011.tls', 
               'mro_psp4_ssd_mro95a.bsp', 
               'mro_v11.tf', 
               'mro_sclkscet_00022_65536.tsc', 
               'mro_sc_psp_070925_071001.bc', 
               'megr90n000eb_LL000E00N_UR090E90N_plate.bds' 
               'megr90n000eb_LL000E90S_UR090E00S_plate.bds' 
               'megr90n000eb_LL090E00N_UR180E90N_plate.bds' 
               'megr90n000eb_LL090E90S_UR180E00S_plate.bds' 
               'megr90n000eb_LL180E00N_UR270E90N_plate.bds' 
               'megr90n000eb_LL180E90S_UR270E00S_plate.bds' 
               'megr90n000eb_LL270E00N_UR360E90N_plate.bds' 
               'megr90n000eb_LL270E90S_UR360E00S_plate.bds'  ) 
                                 
         \begintext  
 
 
 
     Example code begins here. 
         

        /.
        Program subpnt_ex2
        ./
        #include <stdio.h>
        #include "SpiceUsr.h"

        int main()
        {
           /.
           Local constants
           ./
           #define META            "subpnt_ex2.tm"
           #define NCORR           2
           #define NMETH           2

           /.    
           Local variables
           ./
           SpiceBoolean            found;

           static SpiceChar      * abcorr[NCORR] =
                                   {
                                      "LT+S", "CN+S"
                                   };

           static SpiceChar      * fixref = "IAU_MARS";

           static SpiceChar      * sinmth[NMETH] = 
                                   { 
                                      "Ellipsoid",
                                      "DSK/Unprioritized"
                                   };

           static SpiceChar      * submth[NMETH] = 
                                   { 
                                      "Ellipsoid/Near point",
                                      "DSK/Unprioritized/Nadir"
                                   };

           static SpiceChar      * hiref;

           SpiceDouble             alt;
           SpiceDouble             et;
           SpiceDouble             lat;
           SpiceDouble             lon;
           SpiceDouble             mrovec [3];
           SpiceDouble             radius;
           SpiceDouble             spoint [3];
           SpiceDouble             srfvec [3];
           SpiceDouble             trgepc;
           SpiceDouble             xepoch;
           SpiceDouble             xform  [3][3];
           SpiceDouble             xpoint [3];
           SpiceDouble             xvec   [3];

           SpiceInt                i;
           SpiceInt                j;

           /.
           Load kernel files via the meta-kernel.
           ./
           furnsh_c ( META );

           /.
           Convert the TDB request time string to seconds past
           J2000, TDB.
           ./
           str2et_c ( "2007 SEP 30 00:00:00 TDB", &et );

           /.
           Compute the sub-spacecraft point using each method. 
           Compute the results using both LT+S and CN+S aberration 
           corrections.
           ./
           for ( i = 0;  i < NMETH;  i++ )
           {

              printf ( "\nSub-observer point computation "
                       "method = %s\n",  submth[i] );

              for ( j = 0;  j < NCORR;  j++ )
              {
                 subpnt_c ( submth[i],                                      
                            "mars", et,     fixref,  abcorr[j], 
                            "mro",  spoint, &trgepc, srfvec    );

                 /.
                 Compute the observer's altitude above `spoint'.
                 ./
                 alt = vnorm_c ( srfvec );

                 /.
                 Express `srfvec' in the MRO_HIRISE_LOOK_DIRECTION
                 reference frame at epoch `et'. Since `srfvec' is expressed
                 relative to the IAU_MARS frame at `trgepc', we must
                 call pxfrm2_c to compute the position transformation matrix
                 from IAU_MARS at `trgepc' to the MRO_HIRISE_LOOK_DIRECTION
                 frame at time `et'.

                 To make code formatting a little easier, we'll store
                 the long MRO reference frame name in a variable:
                 ./
                 hiref = "MRO_HIRISE_LOOK_DIRECTION";

                 pxfrm2_c ( "iau_mars", hiref,  trgepc, et, xform );
                 mxv_c    (  xform,     srfvec, mrovec            );

                 /.
                 Convert sub-observer point rectangular coordinates to
                 planetocentric latitude and longitude. Convert radians to 
                 degrees.
                 ./
                 reclat_c ( spoint, &radius, &lon, &lat );

                 lon *= dpr_c();
                 lat *= dpr_c();

                 /.
                 Write the results.
                 ./
                 printf ( "\n"
                          "   Aberration correction = %s\n\n"
                          "      MRO-to-sub-observer vector in\n"
                          "      MRO HIRISE look direction frame\n"
                          "         X-component             (km) = %21.9f\n"
                          "         Y-component             (km) = %21.9f\n"
                          "         Z-component             (km) = %21.9f\n"
                          "      Sub-observer point radius  (km) = %21.9f\n"
                          "      Planetocentric latitude   (deg) = %21.9f\n"
                          "      Planetocentric longitude  (deg) = %21.9f\n"
                          "      Observer altitude          (km) = %21.9f\n",
                          abcorr[j],
                          mrovec[0],
                          mrovec[1],
                          mrovec[2],
                          radius,
                          lat,
                          lon,
                          alt         );

                 /.
                 Consistency check: find the surface intercept on
                 Mars of the ray emanating from the spacecraft and having
                 direction vector `mrovec' in the MRO HIRISE look direction
                 reference frame at `et'. Call the intercept point
                 `xpoint'. `xpoint' should coincide with `spoint', up to a
                 small round-off error.
                 ./
                 sincpt_c ( sinmth[i], "mars",  et,    "iau_mars",  
                            abcorr[j], "mro",   hiref, mrovec, 
                            xpoint,    &xepoch, xvec,  &found     );

                 if ( !found )
                 {
                    printf ( "Bug: no intercept\n" );        
                 }
                 else
                 {
                    /.
                    Report the distance between `xpoint' and `spoint'.
                    ./
                    printf ( "      Intercept comparison error (km) = "
                             "%21.9f\n\n",
                             vdist_c( xpoint, spoint )                  );
                 }
              }
           }
           return ( 0 );
        } 


   When this program was executed on a PC/Linux/gcc 64-bit  
   platform, the output was: 
          
      Sub-observer point computation method = Ellipsoid/Near point

         Aberration correction = LT+S

            MRO-to-sub-observer vector in
            MRO HIRISE look direction frame
               X-component             (km) =           0.286933229
               Y-component             (km) =          -0.260425939
               Z-component             (km) =         253.816326386
            Sub-observer point radius  (km) =        3388.299078378
            Planetocentric latitude   (deg) =         -38.799836378
            Planetocentric longitude  (deg) =        -114.995297227
            Observer altitude          (km) =         253.816622175
            Intercept comparison error (km) =           0.000002144


         Aberration correction = CN+S

            MRO-to-sub-observer vector in
            MRO HIRISE look direction frame
               X-component             (km) =           0.286933107
               Y-component             (km) =          -0.260426683
               Z-component             (km) =         253.816315915
            Sub-observer point radius  (km) =        3388.299078376
            Planetocentric latitude   (deg) =         -38.799836382
            Planetocentric longitude  (deg) =        -114.995297449
            Observer altitude          (km) =         253.816611705
            Intercept comparison error (km) =           0.000000001


      Sub-observer point computation method = DSK/Unprioritized/Nadir

         Aberration correction = LT+S

            MRO-to-sub-observer vector in
            MRO HIRISE look direction frame
               X-component             (km) =           0.282372596
               Y-component             (km) =          -0.256289313
               Z-component             (km) =         249.784871247
            Sub-observer point radius  (km) =        3392.330239436
            Planetocentric latitude   (deg) =         -38.800230156
            Planetocentric longitude  (deg) =        -114.995297338
            Observer altitude          (km) =         249.785162334
            Intercept comparison error (km) =           0.000002412


         Aberration correction = CN+S

            MRO-to-sub-observer vector in
            MRO HIRISE look direction frame
               X-component             (km) =           0.282372464
               Y-component             (km) =          -0.256290075
               Z-component             (km) =         249.784860121
            Sub-observer point radius  (km) =        3392.330239564
            Planetocentric latitude   (deg) =         -38.800230162
            Planetocentric longitude  (deg) =        -114.995297569
            Observer altitude          (km) =         249.785151209
            Intercept comparison error (km) =           0.000000001


 
 

Restrictions

 
  None. 
 

Literature_References

 
   None. 
 

Author_and_Institution

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

Version

 
   -CSPICE Version 2.0.0, 05-APR-2017 (NJB) 

       Fixed a few header comment typos.
      
    14-OCT-2015 (NJB) 

       Updated to support surfaces represented by DSK data.

   -CSPICE Version 1.0.2, 02-APR-2011 (NJB) (SCK)

       References to the new pxfrm2_c routine were added, which
       changed the Detailed Output section and the second example. 

       Miscellaneous, minor header comment corrections were made.

   -CSPICE Version 1.0.1, 06-FEB-2009 (NJB)
 
       Incorrect frame name fixfrm was changed to fixref in
       documentation.
 
       In the header examples, meta-kernel names were updated to use
       the suffix

          ".tm"

   -CSPICE Version 1.0.0, 02-MAR-2008 (NJB) 

Index_Entries

 
   find sub-observer point on target body 
   find sub-spacecraft point on target body 
   find nearest point to observer on target body 
 
Wed Apr  5 17:54:45 2017