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Index Page subsol_c
Table of contents
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
       subsol_c ( Sub-solar point )

       void subsol_c ( ConstSpiceChar   * method,
                       ConstSpiceChar   * target,
                       SpiceDouble        et,
                       ConstSpiceChar   * abcorr,
                       ConstSpiceChar   * obsrvr,
                       SpiceDouble        spoint[3] )
#### Abstract
       Deprecated: This routine has been superseded by the CSPICE
       routine subslr_c. This routine is supported for purposes of
       backward compatibility only.

       Determine the coordinates of the sub-solar point on a target
       body as seen by a specified observer at a specified epoch,
       optionally corrected for planetary (light time) and stellar
       aberration.
#### Required_Reading
       FRAMES
       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 ephemeris seconds past J2000 TDB.
       abcorr     I   Aberration correction.
       obsrvr     I   Name of observing body.
       spoint     O   Sub-solar point on the target body.
#### Detailed_Input
       method      is a short string specifying the computation method
                   to be used. The choices are:

                      "Near point"       The sub-solar point is defined
                                         as the nearest point on the
                                         target to the sun.

                      "Intercept"        The sub-observer point is defined
                                         as the target surface intercept of
                                         the line containing the target's
                                         center and the sun's center.

                   In both cases, the intercept computation treats the
                   surface of the target body as a triaxial ellipsoid.
                   The ellipsoid's radii must be available in the kernel
                   pool.

                   Neither case nor white space are significant in
                   method. For example, the string " NEARPOINT" is
                   valid.

       target      is the name of the target body.  `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.

                   This routine assumes that the target body is modeled by
                   a tri-axial ellipsoid, and that a PCK file containing
                   its radii has been loaded into the kernel pool via
                   furnsh_c.

       et          is the epoch in ephemeris seconds past J2000 at which
                   the sub-solar point on the target body is to be
                   computed.

       abcorr      indicates the aberration corrections to be applied
                   when computing the observer-target state. abcorr
                   may be any of the following.

                      "NONE"     Apply no correction. Return the
                                 geometric sub-solar point on the target
                                 body.

                      "LT"       Correct for planetary (light time)
                                 aberration. Both the state and rotation
                                 of the target body are corrected for one
                                 way light time from target to observer.

                                 The state of the sun relative to the
                                 target is corrected for one way light
                                 from the sun to the target; this state
                                 is evaluated at the epoch obtained by
                                 retarding et by the one way light time
                                 from target to observer.

                      "LT+S"     Correct for planetary (light time) and
                                 stellar aberrations. Light time
                                 corrections are the same as in the "LT"
                                 case above. The target state is
                                 additionally corrected for stellar
                                 aberration as seen by the observer, and
                                 the sun state is corrected for stellar
                                 aberration as seen from the target.

                      "CN"       Converged Newtonian light time
                                 correction. In solving the light time
                                 equation, the "CN" correction iterates
                                 until the solution converges (three
                                 iterations on all supported platforms).
                                 Whether the "CN+S" solution is
                                 substantially more accurate than the
                                 "LT" solution depends on the geometry
                                 of the participating objects and on the
                                 accuracy of the input data. In all
                                 cases this routine will execute more
                                 slowly when a converged solution is
                                 computed. See the -Particulars section
                                 below for a discussion of precision of
                                 light time corrections.

                                 Light time corrections are applied as in
                                 the "LT" case.

                      "CN+S"     Converged Newtonian light time correction
                                 and stellar aberration correction.

                                 Light time and stellar aberration
                                 corrections are applied as in the "LT+S"
                                 case.

       obsrvr      is the name of the observing body. This is typically
                   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
                   "EARTH" and "399" are legitimate strings that indicate
                   the earth is the observer.
#### Detailed_Output
       spoint      is the sub-solar point on the target body at et,
                   expressed relative to the body-fixed frame of the
                   target body.

                   The sub-solar point is defined either as the point on
                   the target body that is closest to the sun, or the
                   target surface intercept of the line containing the sun's
                   center and the target's center; the input argument
                   method selects the definition to be used.

                   The body-fixed frame, which is time-dependent, is
                   evaluated at et if abcorr is "NONE"; otherwise the
                   frame is evaluated at et-lt, where lt is the one way
                   light time from target to observer.

                   The state of the target body is corrected for
                   aberration as specified by abcorr; the corrected
                   state is used in the geometric computation. As
                   indicated above, the rotation of the target is
                   retarded by one way light time if abcorr specifies
                   that light time correction is to be done.

                   The state of the sun as seen from the target body
                   body is also corrected for aberration as specified
                   by abcorr. The corrections, when selected, are
                   applied at the epoch et-lt, where lt is the one way
                   light time from target to observer.
#### Parameters
       None.
#### Exceptions
       If any of the listed errors occur, the output arguments are
       left unchanged.

       1)  If the input argument `method' is not recognized, the error
           SPICE(DUBIOUSMETHOD) is signaled by a routine in the call tree
           of this routine.

       2)  If either of the input body names `target' or `obsrvr' cannot be
           mapped to NAIF integer codes, the error SPICE(IDCODENOTFOUND)
           is signaled by a routine in the call tree of this routine.

       3)  If `obsrvr' and `target' map to the same NAIF integer ID codes,
           the error SPICE(BODIESNOTDISTINCT) is signaled by a routine in
           the call tree of this routine.

       4)  If frame definition data enabling the evaluation of the state
           of the target relative to the observer in target body-fixed
           coordinates have not been loaded prior to calling subsol_c, an
           error is signaled by a routine in the call tree of this
           routine.

       5)  If the specified aberration correction is not recognized, an
           error is signaled by a routine in the call tree of this
           routine.

       6)  If insufficient ephemeris data have been loaded prior to
           calling subsol_c, an error is signaled by a
           routine in the call tree of this routine.

       7)  If the triaxial radii of the target body have not been loaded
           into the kernel pool prior to calling subsol_c, an error is
           signaled by a routine in the call tree of this routine.

       8)  If the size of the `target' body radii kernel variable is not
           three, an error is signaled by a routine in the call tree of
           this routine.

       9)  If any of the three `target' body radii is less-than or equal to
           zero, an error is signaled by a routine in the call tree of
           this routine.

       10) If PCK data supplying a rotation model for the target body
           have not been loaded prior to calling subsol_c, an error is
           signaled by a routine in the call tree of this routine.

       11) If any of the `method', `target', `abcorr' or `obsrvr' input
           string pointers is null, the error SPICE(NULLPOINTER) is
           signaled.

       12) If any of the `method', `target', `abcorr' or `obsrvr' input
           strings has zero length, the error SPICE(EMPTYSTRING) is
           signaled.
#### Files
       Appropriate SPK, PCK, and frame data must be available to
       the calling program before this routine is called. Typically
       the data are made available by loading kernels; however the
       data may be supplied via subroutine interfaces if applicable.

       The following data are required:

       -  SPK data: ephemeris data for sun, target, and observer must be
          loaded. If aberration corrections are used, the states of sun,
          target, and observer relative to the solar system barycenter
          must be calculable from the available ephemeris data. Ephemeris
          data are made available by loading one or more SPK files via
          furnsh_c.

       -  PCK data: 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.

       -  Further PCK data:  a rotation model for the target body must be
          loaded. This may be provided in a text or binary PCK file
          which is loaded via furnsh_c.

       -  Frame data: if a frame definition is required to convert
          the sun, 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.

       In all cases, kernel data are normally loaded once per program
       run, NOT every time this routine is called.
#### Particulars
       subsol_c computes the sub-solar point on a target body, as seen by
       a specified observer.

       There are two different popular ways to define the sub-solar point:
       "nearest point on target to the sun" or "target surface intercept of
       line containing target and sun." These coincide when the target is
       spherical and generally are distinct otherwise.

       When comparing sub-point computations with results from sources
       other than SPICE, it's essential to make sure the same geometric
       definitions are used.
#### Examples
       The numerical results shown for this example may differ across
       platforms. The results depend on the SPICE kernels used as
       input, the compiler and supporting libraries, and the machine
       specific arithmetic implementation.

       1) Find the sub-solar point on Mars as seen from the Mars Global
          Surveyor (MGS) spacecraft for a specified time. Perform the
          computation twice, using both the "intercept" and "near point"
          options.

          Use the meta-kernel shown below to load the required SPICE
          kernels.

             KPL/MK

             File: subsol_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
                mgs_ext12_ipng_mgs95j.bsp        MGS ephemeris

             \begindata

                KERNELS_TO_LOAD = ( 'de430.bsp',
                                    'mar097.bsp',
                                    'pck00010.tpc',
                                    'naif0011.tls',
                                    'mgs_ext12_ipng_mgs95j.bsp')
             \begintext

             End of meta-kernel

          Example code begins here.

          /.
             Program subsol_ex1
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main( void )
          {
             #define METHLN          26

             SpiceChar               method [2][ METHLN ] =
                                     {
                                        "Intercept",
                                        "Near point"
                                     };

             SpiceDouble             et;
             SpiceDouble             lat;
             SpiceDouble             lon;
             SpiceDouble             radius;
             SpiceDouble             spoint[3];

             SpiceInt                i;

             /.
             Load kernel files.
             ./
             furnsh_c ( "subsol_ex1.tm" );

             /.
             Convert the UTC request time to ET (seconds past
             J2000, TDB).
             ./
             str2et_c ( "2003 OCT 13 06:00:00 UTC", &et );

             /.
             Compute sub-spacecraft point using light time and stellar
             aberration corrections.  Use the "target surface intercept"
             definition of sub-spacecraft point on the first loop
             iteration, and use the "near point" definition on the
             second.
             ./

             for ( i = 0;  i < 2;  i++ )
             {

                subsol_c ( method[i], "mars", et, "lt+s", "mgs", spoint );

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

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

                /.
                Write the results.
                ./

                printf ( "\n"
                         "Computation method:  %s\n"
                         "\n"
                         "  Radius                   (km)  = %f\n"
                         "  Planetocentric Latitude  (deg) = %f\n"
                         "  Planetocentric Longitude (deg) = %f\n"
                         "\n",

                         method[i], radius, lat, lon               );
             }

             return ( 0 );
          }

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

          Computation method:  Intercept

            Radius                   (km)  = 3392.623653
            Planetocentric Latitude  (deg) = -24.888050
            Planetocentric Longitude (deg) = 31.764895

          Computation method:  Near point

            Radius                   (km)  = 3392.692317
            Planetocentric Latitude  (deg) = -24.630785
            Planetocentric Longitude (deg) = 31.764895
#### Restrictions
       1)  The appropriate kernel data must have been loaded before this
           routine is called. See the -Files section above.
#### Literature_References
       None.
#### Author_and_Institution
       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
       J.E. McLean         (JPL)
       B.V. Semenov        (JPL)
#### Version
       -CSPICE Version 1.0.6, 01-NOV-2021 (JDR)

           Edited the header to comply with NAIF standard.
           Updated example to use a meta-kernel to load the required
           kernels. Added example's problem statement and solution.

       -CSPICE Version 1.0.5, 10-JUL-2014 (NJB)

           Discussion of light time corrections was updated. Assertions
           that converged light time corrections are unlikely to be
           useful were removed.

       -CSPICE Version 1.0.4, 19-MAY-2010 (BVS)

           Index line now states that this routine is deprecated.

       -CSPICE Version 1.0.3, 07-FEB-2008 (NJB)

           -Abstract now states that this routine is deprecated.

       -CSPICE Version 1.0.2, 22-JUL-2004 (NJB)

           Updated header to indicate that the `target' and `observer'
           input arguments can now contain string representations of
           integers. Deleted references to kernel-specific loaders.
           Made miscellaneous minor corrections to header comments.

       -CSPICE Version 1.0.1, 12-DEC-2002 (NJB)

           Corrected and updated code example in header.

       -CSPICE Version 1.0.0, 03-SEP-1999 (NJB) (JEM)
#### Index_Entries
       DEPRECATED sub-solar point
#### Link to routine subsol_c source file subsol_c.c

    Fri Dec 31 18:41:13 2021