subslr_c

Index Page subslr_c
A B C D E F G H I J K L M N O P Q R S T U V W X
Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries
#### Procedure
       void subslr_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-solar point on
       a target body at a specified epoch, optionally corrected for
       light time and stellar aberration.

       This routine supersedes subsol_c.
#### Required_Reading
       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 ephemeris seconds past J2000 TDB.
       fixref     I   Body-fixed, body-centered target body frame.
       abcorr     I   Aberration correction.
       obsrvr     I   Name of observing body.
       spoint     O   Sub-solar point on the target body.
       trgepc     O   Sub-solar point epoch.
       srfvec     O   Vector from observer to sub-solar point.
#### Detailed_Input
       method      is a short string providing parameters defining
                   the computation method to be used.

                   The supported values of `method' are listed below.
                   Please note that the colon is a required delimiter;
                   using a blank will not work.

                      "Near point: ellipsoid"   The sub-solar point
                                                computation uses a
                                                triaxial ellipsoid to
                                                model the surface of the
                                                target body. The
                                                sub-solar point is
                                                defined as the nearest
                                                point on the target
                                                relative to the Sun.

                      "Intercept: ellipsoid"    The sub-solar point
                                                computation uses a
                                                triaxial ellipsoid to
                                                model the surface of the
                                                target body. The
                                                sub-solar point is
                                                defined as the target
                                                surface intercept of the
                                                line containing the
                                                Sun and the
                                                target's center.

                   Neither case nor white space are significant in
                   `method'. For example, the string

                     " nearpoint:ELLIPSOID "

                   is valid.

       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 ephemeris 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 and the position of the Sun are computed.

                   When aberration corrections are used, `et' is the epoch
                   at which the observer's state relative to the solar
                   system barycenter is computed; in this case the position
                   and orientation of the target body are computed at
                   et-lt, where `lt' is the one-way light time between the
                   sub-solar point and the observer. See the description of
                   `abcorr' below for details.

       fixref      is the name of the body-fixed, body-centered reference
                   frame associated with the target body. The output
                   sub-solar point `spoint' will be expressed relative to
                   this reference frame. The string `fixref' is
                   case-insensitive, and leading and trailing blanks in
                   `fixref' are not significant.

       abcorr      indicates the aberration correction to be applied when
                   computing the target position and orientation, and the
                   position of the Sun.

                   For remote sensing applications, where the apparent
                   sub-solar 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-solar point on the
                                 target body.

                   Let `lt' represent the one-way light time between the
                   observer and the sub-solar 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-solar 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-solar
                                 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.

                                 The target position and orientation as
                                 seen by the observer are corrected for
                                 light time. The position of the Sun
                                 relative to the target is corrected for
                                 one-way light time between the Sun and
                                 target.

                      "LT+S"     Correct for one-way light time and stellar
                                 aberration using a Newtonian formulation.
                                 This option modifies the sub-solar 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-solar 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, and the position of the Sun,
                                 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.

                    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-solar point on 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 from the
                   Sun to the target's center; the input argument
                   `method' selects the 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-solar 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-solar point is computed.

                   The components of `spoint' have units of km.

       trgepc      is the "sub-solar point epoch." `trgepc' is defined
                   as follows: letting `lt' be the one-way light time
                   between the observer and the sub-solar point,
                   `trgepc' is either the epoch et-lt or `et' depending on
                   whether the requested aberration correction is,
                   respectively, for received 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' to a time-dependent
                   reference frame `ref' at `et', a sequence of two frame
                   transformations is required. For example, let `mfix'
                   and `mref' be 3x3 matrices respectively describing the
                   target body-fixed to J2000 frame transformation at
                   `trgepc' and the J2000 to (time-dependent frame) `ref'
                   transformation at `et', and let `xform' be the 3x3 matrix
                   representing the composition of `mref' with `mfix'. Then
                   `srfvec' can be transformed to the result `refvec' as
                   follows:

                       pxform_c ( fixref,  "j2000", trgepc, mfix   );
                       pxform_c ( "j2000", ref,     et,     mref   );
                       mxm_c    ( mref,    mfix,            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 relativistic or
           calls for stellar aberration but not light time correction,
           the error SPICE(NOTSUPPORTED) is signaled. If the specified
           aberration correction is any other unrecognized value, 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.

       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 subslr_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 observer,
           target, and the Sun 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 subslr_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: 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.

       11) If PCK data specifying the target body-fixed frame orientation
           have not been loaded prior to calling subslr_c, the error will
           be diagnosed and signaled by a routine in the call tree of
           this routine.

       12) The error SPICE(EMPTYSTRING) is signaled if any input string
           argument does not contain at least one character, since the
           input string cannot be converted to a Fortran-style string in
           this case.

       13) The error SPICE(NULLPOINTER) is signaled if any input
           string argument pointer is null.
#### 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, observer, and the Sun 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.

          - 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.

          - Further PCK data: rotation data for the target body must be
            loaded. These may be provided in a text or binary PCK file.

          - 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.

       In all cases, kernel data are normally loaded once per program
       run, NOT every time this routine is called.
#### Particulars
       There are two different popular ways to define the sub-solar point:
       "nearest point on the target to the Sun" or "target surface
       intercept of the line containing the Sun and the target." These
       coincide when the target is spherical and generally are distinct
       otherwise.

       This routine computes light time corrections using light time
       between the observer and the sub-solar 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-solar 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-solar point, in particular
       when the observer to sub-solar point distance is much less than the
       observer to target center distance.

       The definition of the aberration-corrected sub-solar point is
       implicit: `spoint' is defined by an equation of the general form

          spoint = f ( spoint )

       Because of the contraction properties of both light time and
       stellar aberration corrections---that is, the difference in the
       corrections for two vectors is much smaller than the difference
       between the vectors themselves---it's easy to solve this equation
       accurately and fairly quickly.

       When comparing sub-solar 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 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-solar point on Mars as seen from the Earth for a
          specified time. Perform the computation twice, using both the
          "intercept" and "near point" options. Display the locations of
          the Sun and the sub-solar point using both planetocentric
          and planetographic coordinates.

          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.

             \begindata

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

             \begintext

         Example code begins here.

          /.
          Program EX1
          ./

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

          int main()
          {
             /.
             Local parameters
             ./
             #define META            "example.tm"

             /.
             Local variables
             ./
             static SpiceChar      * method[2] =
                                     {
                                        "Intercept:  ellipsoid",
                                        "Near point: ellipsoid"
                                     };

             SpiceDouble             et;
             SpiceDouble             f;
             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             sunlt;
             SpiceDouble             sunpos [3];
             SpiceDouble             supcln;
             SpiceDouble             supclt;
             SpiceDouble             supcrd;
             SpiceDouble             supgal;
             SpiceDouble             supgln;
             SpiceDouble             supglt;
             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 the sub-solar point using light time and stellar
             aberration corrections. Use the "target surface intercept"
             definition of the sub-solar point on the first loop
             iteration, and use the "near point" definition on the
             second.
             ./

             for ( i = 0;  i < 2;  i++ )
             {
                subslr_c ( method[i],
                           "mars",  et,     "iau_mars", "lt+s",
                           "earth", spoint, &trgepc,    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 the sub-solar 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 Sun's apparent position relative to the
                center of the target at `trgepc'. Express the Sun's
                location in planetographic coordinates.
                ./
                spkpos_c ( "sun",   trgepc,  "iau_mars", "lt+s",
                           "mars",  sunpos,  &sunlt              );

                recpgr_c ( "mars",  sunpos,  re,     f,
                           &supgln, &supglt, &supgal    );

                supgln *= dpr_c ();
                supglt *= dpr_c ();

                /.
                Convert the Sun's rectangular coordinates to
                planetocentric radius, longitude, and latitude.
                Convert radians to degrees.
                ./
                reclat_c ( sunpos, &supcrd, &supcln, &supclt );

                supcln *= dpr_c();
                supclt *= dpr_c();

                /.
                Write the results.
                ./
                printf ( "\n"
                         " Computation method = %s\n\n"
                         "  Sub-solar point altitude            (km) = %21.9f\n"
                         "  Sub-solar planetographic longitude (deg) = %21.9f\n"
                         "  Sun's planetographic longitude     (deg) = %21.9f\n"
                         "  Sub-solar planetographic latitude  (deg) = %21.9f\n"
                         "  Sun's planetographic latitude      (deg) = %21.9f\n"
                         "  Sub-solar planetocentric longitude (deg) = %21.9f\n"
                         "  Sun's planetocentric longitude     (deg) = %21.9f\n"
                         "  Sub-solar planetocentric latitude  (deg) = %21.9f\n"
                         "  Sun's planetocentric latitude      (deg) = %21.9f\n"
                         "\n",
                         method[i],
                         spgalt,
                         spglon,
                         supgln,
                         spglat,
                         supglt,
                         spclon,
                         supcln,
                         spclat,
                         supclt      );
             }

             return ( 0 );
          }

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

          Computation method = Intercept:  ellipsoid

           Sub-solar point altitude            (km) =           0.000000000
           Sub-solar planetographic longitude (deg) =         175.810721566
           Sun's planetographic longitude     (deg) =         175.810721564
           Sub-solar planetographic latitude  (deg) =          23.668550265
           Sun's planetographic latitude      (deg) =          23.420823346
           Sub-solar planetocentric longitude (deg) =        -175.810721566
           Sun's planetocentric longitude     (deg) =        -175.810721564
           Sub-solar planetocentric latitude  (deg) =          23.420819920
           Sun's planetocentric latitude      (deg) =          23.420819920

          Computation method = Near point: ellipsoid

           Sub-solar point altitude            (km) =          -0.000000000
           Sub-solar planetographic longitude (deg) =         175.810721552
           Sun's planetographic longitude     (deg) =         175.810721550
           Sub-solar planetographic latitude  (deg) =          23.420823346
           Sun's planetographic latitude      (deg) =          23.420823346
           Sub-solar planetocentric longitude (deg) =        -175.810721552
           Sun's planetocentric longitude     (deg) =        -175.810721550
           Sub-solar planetocentric latitude  (deg) =          23.175085562
           Sun's planetocentric latitude      (deg) =          23.420819920
#### Restrictions
      None.
#### Literature_References
       None.
#### Author_and_Institution
       N.J. Bachman   (JPL)
#### Version
       -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-solar point on target body
       find nearest point to Sun on target body
#### Link to routine subslr_c source file subslr_c.c

    Thu Apr 16 08:59:20 2009