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cspice_subpnt

Table of contents
Abstract
I/O
Parameters
Examples
Particulars
Exceptions
Files
Restrictions
Required_Reading
Literature_References
Author_and_Institution
Version
Index_Entries


Abstract


   CSPICE_SUBPNT computes 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 cspice_subpt.

I/O


   Given:

      method   a short string providing parameters defining the computation
               method to be used.

               help, method
                  STRING = Scalar

               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 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   the scalar string name of the target body.

               help, target
                  STRING = Scalar

               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       the scalar double precision epoch, expressed as seconds past
               J2000 TDB, of the observer: 'et' is the epoch at which the
               observer's state is computed.

               help, et
                  DOUBLE = Scalar

               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, '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 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   the name of a body-fixed reference frame centered on the target
               body.

               help, fixref
                  STRING = Scalar

               `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   the scalar string aberration correction to apply when computing
               the observer-target state and the orientation of the target
               body.

               help, abcorr
                  STRING = Scalar

               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
                                state obtained with the 'LT' option to
                                account for the observer's velocity
                                relative to the solar system
                                barycenter. The result is the apparent
                                sub-observer point as seen by the
                                observer.

                     'CN'       Converged Newtonian light time
                                correction. In solving the light time
                                equation, the 'CN' correction iterates
                                until the solution converges. 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.

      obsrvr   the scalar string name of the observing body.

               help, obsrvr
                  STRING = Scalar

               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 'obsrvr' are not
               significant. Optionally, you may trailing blanks in 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.

   the call:

      cspice_subpnt, method, target, et,     fixref, abcorr,                  $
                     obsrvr, spoint, trgepc, srfvec

   returns:

      spoint   a double precision 3-vector defining the sub-observer point on
               the target body.

               help, spoint
                  DOUBLE = Array[3]

               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   the scalar double precision "sub-observer point epoch."

               help, trgepc
                  DOUBLE = Scalar

               '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   a double precision 3-vector defining the position vector from
               the observer at 'et' to 'spoint'.

               help, srfvec
                  DOUBLE = Array[3]

               '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 function norm to obtain the
               distance between the observer and 'spoint':

                     dist = norm( 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 with:

                     obspos = spoint - srfvec

               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 'cspice_pxfrm2' 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:

                    cspice_pxfrm2, fixref, ref,    trgepc, et, xform
                    cspice_mxv,    xform,  srfvec, refvec

Parameters


   None.

Examples


   Any numerical results shown for these examples may differ between
   platforms as the results depend on the SPICE kernels used as input
   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

         End of meta-kernel


      Example code begins here.


      PRO subpnt_ex1

         ;;
         ;; Load kernel files via the meta-kernel.
         ;;
         cspice_furnsh, 'subpnt_ex1.tm'

         ;;
         ;; Convert the UTC request time to ET (seconds past
         ;; J2000, TDB).
         ;;
         cspice_str2et, '2008 aug 11 00:00:00', et

         ;;
         ;; Look up the target body's radii. We'll use these to
         ;; convert Cartesian to planetodetic coordinates. Use
         ;; the radii to compute the flattening coefficient of
         ;; the reference ellipsoid.
         ;;
         cspice_bodvrd, 'MARS', 'RADII', 3, 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.
         ;;

         method = [ 'Intercept: ellipsoid',        'Near point: ellipsoid',  $
                    'Intercept/DSK/Unprioritized', 'Nadir/DSK/Unprioritized' ]

         for i=0,3 do begin

            cspice_subpnt, method[i], 'MARS', et, 'IAU_MARS', 'LT+S', $
                                      'EARTH', spoint, trgepc, srfvec

            ;;
            ;; Compute the observer's distance from SPOINT.
            ;;
            odist = norm(srfvec);

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

            spglon = spglon * cspice_dpr();
            spglat = spglat * cspice_dpr();

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

            spclon = spclon * cspice_dpr()
            spclat = spclat * cspice_dpr()

            ;;
            ;; 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 geodetic coordinates.
            ;;
            obspos = spoint - srfvec;

            cspice_recpgr, 'mars', obspos, re, f, opglon, opglat, opgalt

            opglon = opglon * cspice_dpr()
            opglat = opglat * cspice_dpr()

            ;;
            ;; Convert the observer's rectangular coordinates to
            ;; planetocentric longitude, latitude and altitude. Convert
            ;; radians to degrees.
            ;;
            cspice_reclat, obspos, opcrad, opclon, opclat

            opclon = opclon * cspice_dpr()
            opclat = opclat * cspice_dpr()

            print, 'Computational Method = ', method[i]
            print, ' '
            print, FORMAT='(A,F21.9)',                                       $
                   '  Observer altitude                      (km) = ', opgalt
            print, FORMAT='(A,F21.9)',                                       $
                   '  Length of SRFVEC                       (km) = ', odist
            print, FORMAT='(A,F21.9)',                                       $
                   '  Sub-observer point altitude            (km) = ', spgalt
            print, FORMAT='(A,F21.9)',                                       $
                   '  Sub-observer planetographic longitude (deg) = ', spglon
            print, FORMAT='(A,F21.9)',                                       $
                   '  Observer planetographic longitude     (deg) = ', opglon
            print, FORMAT='(A,F21.9)',                                       $
                   '  Sub-observer planetographic latitude  (deg) = ', spglat
            print, FORMAT='(A,F21.9)',                                       $
                   '  Observer planetographic latitude      (deg) = ', opglat
            print, FORMAT='(A,F21.9)',                                       $
                   '  Sub-observer planetocentric longitude (deg) = ', spclon
            print, FORMAT='(A,F21.9)',                                       $
                   '  Observer planetocentric longitude     (deg) = ', opclon
            print, FORMAT='(A,F21.9)',                                       $
                   '  Sub-observer planetocentric latitude  (deg) = ', spclat
            print, FORMAT='(A,F21.9)',                                       $
                   '  Observer planetocentric latitude      (deg) = ', opclat
            print, ' '

         endfor

         ;;
         ;; It's always good form to unload kernels after use,
         ;; particularly in IDL due to data persistence.
         ;;
         cspice_kclear

      END


      When this program was executed on a Mac/Intel/IDL8.x/64-bit
      platform, the output was:


      Computational Method = Intercept: ellipsoid

        Observer altitude                      (km) =   349199089.540938914
        Length of SRFVEC                       (km) =   349199089.577634573
        Sub-observer point altitude            (km) =          -0.000000000
        Sub-observer planetographic longitude (deg) =         199.302305032
        Observer planetographic longitude     (deg) =         199.302305032
        Sub-observer planetographic latitude  (deg) =          26.262401237
        Observer planetographic latitude      (deg) =          25.994936751
        Sub-observer planetocentric longitude (deg) =         160.697694968
        Observer planetocentric longitude     (deg) =         160.697694968
        Sub-observer planetocentric latitude  (deg) =          25.994934171
        Observer planetocentric latitude      (deg) =          25.994934171

      Computational Method = Near point: ellipsoid

        Observer altitude                      (km) =   349199089.540930629
        Length of SRFVEC                       (km) =   349199089.540930629
        Sub-observer point altitude            (km) =          -0.000000000
        Sub-observer planetographic longitude (deg) =         199.302305032
        Observer planetographic longitude     (deg) =         199.302305032
        Sub-observer planetographic latitude  (deg) =          25.994936751
        Observer planetographic latitude      (deg) =          25.994936751
        Sub-observer planetocentric longitude (deg) =         160.697694968
        Observer planetocentric longitude     (deg) =         160.697694968
        Sub-observer planetocentric latitude  (deg) =          25.729407227
        Observer planetocentric latitude      (deg) =          25.994934171

      Computational Method = Intercept/DSK/Unprioritized

        Observer altitude                      (km) =   349199089.541009188
        Length of SRFVEC                       (km) =   349199091.785398543
        Sub-observer point altitude            (km) =          -2.207669751
        Sub-observer planetographic longitude (deg) =         199.302305002
        Observer planetographic longitude     (deg) =         199.302305002
        Sub-observer planetographic latitude  (deg) =          26.262576677
        Observer planetographic latitude      (deg) =          25.994936751
        Sub-observer planetocentric longitude (deg) =         160.697694998
        Observer planetocentric longitude     (deg) =         160.697694998
        Sub-observer planetocentric latitude  (deg) =          25.994934171
        Observer planetocentric latitude      (deg) =          25.994934171

      Computational Method = Nadir/DSK/Unprioritized

        Observer altitude                      (km) =   349199089.540999591
        Length of SRFVEC                       (km) =   349199091.707164228
        Sub-observer point altitude            (km) =          -2.166164622
        Sub-observer planetographic longitude (deg) =         199.302305004
        Observer planetographic longitude     (deg) =         199.302305003
        Sub-observer planetographic latitude  (deg) =          25.994936751
        Observer planetographic latitude      (deg) =          25.994936751
        Sub-observer planetocentric longitude (deg) =         160.697694996
        Observer planetocentric longitude     (deg) =         160.697694997
        Sub-observer planetocentric latitude  (deg) =          25.729237570
        Observer planetocentric latitude      (deg) =          25.994934171


   2) Use cspice_subpnt 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
      cspice_subpnt 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 cspice_sincpt and cspice_subpnt.

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


         KPL/MK

         File name: subpnt_ex2.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
            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

         End of meta-kernel


      Example code begins here.


      PRO subpnt_ex2

         ;;
         ;; Local constants
         ;;
         META   = 'subpnt_ex2.tm'
         NCORR  = 2
         NMETH  = 2

         abcorr = [ 'LT+S', 'CN+S' ]
         fixref = 'IAU_MARS'
         sinmth = [ 'Ellipsoid', 'DSK/Unprioritized' ]
         submth = [ 'Ellipsoid/Near point', 'DSK/Unprioritized/Nadir' ]

         ;;
         ;; Load kernel files via the meta-kernel.
         ;;
         cspice_furnsh, META

         ;;
         ;; Convert the TDB request time string to seconds past
         ;; J2000, TDB.
         ;;
         cspice_str2et, '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, NMETH - 1L do begin

            print, ''
            print, 'Sub-observer point computation method = ', submth(i)

            for j=0, NCORR - 1L do begin
               cspice_subpnt, submth(i), 'mars', et,     fixref, abcorr(j),  $
                              'mro',     spoint, trgepc, srfvec

               ;;
               ;; Compute the observer's altitude above `spoint'.
               ;;
               alt = cspice_vnorm( 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
               ;; cspice_pxfrm2 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'

               cspice_pxfrm2, 'iau_mars', hiref, trgepc, et, xform
               cspice_mxv, xform, srfvec, mrovec

               ;;
               ;; Convert sub-observer point rectangular coordinates to
               ;; planetocentric latitude and longitude. Convert radians to
               ;; degrees.
               ;;
               cspice_reclat, spoint, radius, lon, lat

               lon *= cspice_dpr( )
               lat *= cspice_dpr( )

               ;;
               ;; Write the results.
               ;;
               print, ''
               print, '   Aberration correction = ', abcorr(j)
               print, ''
               print, '      MRO-to-sub-observer vector in'
               print, '      MRO HIRISE look direction frame'
               print, format='(A,F22.9)', '         X-' +                    $
                                          'component             (km) =',    $
                                          mrovec[0]
               print, format='(A,F22.9)', '         Y-' +                    $
                                          'component             (km) =',    $
                                          mrovec[1]
               print, format='(A,F22.9)', '         Z-' +                    $
                                          'component             (km) =',    $
                                          mrovec[2]
               print, format='(A,F22.9)', '      Sub-observer point' +       $
                                          ' radius  (km) =', radius
               print, format='(A,F22.9)', '      Planetocentric' +           $
                                          ' latitude   (deg) =', lat
               print, format='(A,F22.9)', '      Planetocentric' +           $
                                          ' longitude  (deg) =', lon
               print, format='(A,F22.9)', '      Observer' +                 $
                                          ' altitude          (km) =', 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.
               ;;
               cspice_sincpt, sinmth(i), 'mars', et,    'iau_mars',          $
                              abcorr(j), 'mro',  hiref, mrovec,              $
                              xpoint,    xepoch, xvec,  found

               if (  not found ) then begin
                  print, 'Bug: no intercept'
               endif else begin

                  ;;
                  ;; Report the distance between `xpoint' and `spoint'.
                  ;;
                  print, format='(A,F22.9)', '      Intercept' +             $
                                             ' comparison error (km) =',     $
                                             cspice_vdist( xpoint, spoint )
                  print, ''
               endelse
            endfor
         endfor

         ;;
         ;; It's always good form to unload kernels after use,
         ;; particularly in IDL due to data persistence.
         ;;
         cspice_kclear

      END


      When this program was executed on a Mac/Intel/IDL8.x/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) =         253.816326385
            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


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 cspice_furnsh and can be unloaded by calling cspice_unload or
      cspice_kclear. See the documentation of cspice_furnsh 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"'

      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 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.

Exceptions


   1)  If the specified aberration correction is unrecognized, an
       error is 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 by a routine in the call
       tree of this routine.

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

   4)  If the input target body-fixed frame `fixref' is not recognized,
       the error SPICE(NOFRAME) is signaled by a routine in the call
       tree of this routine. 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 by a routine in the
       call tree of this routine.

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

   7)  If the sub-observer point type is not specified or is not
       recognized, the error SPICE(INVALIDSUBTYPE) is signaled by a
       routine in the call tree of this routine.

   8)  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 by a routine in the call tree
       of this routine.

   9)  If insufficient ephemeris data have been loaded prior to
       calling cspice_subpnt, an error is 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.

   10) 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 cspice_subpnt, an error
       is signaled by a routine in the call tree of this routine.

   11) 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 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.

   12) If PCK data specifying the target body-fixed frame orientation
       have not been loaded prior to calling cspice_subpnt, an 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 no DSK files are loaded for the specified
       target, an error is signaled by a routine in the call tree
       of this routine.

   14) 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) is signaled by a routine in the call
       tree of this routine.

   15) If the surface intercept on the target body's reference
       ellipsoid of the observer to target center vector cannot not
       be computed, the error SPICE(DEGENERATECASE) is signaled by a
       routine in the call tree of this routine. Note that this is a
       very rare case.

   16) If radii for `target' are not found in the kernel pool, an error
       is signaled by a routine in the call tree of this routine.

   17) 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.

   18) 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.

   19) If any of the input arguments, `method', `target', `et',
       `fixref', `abcorr' or `obsrvr', is undefined, an error is
       signaled by the IDL error handling system.

   20) If any of the input arguments, `method', `target', `et',
       `fixref', `abcorr' or `obsrvr', is not of the expected type,
       or it does not have the expected dimensions and size, an error
       is signaled by the Icy interface.

   21) If any of the output arguments, `spoint', `trgepc' or
       `srfvec', is not a named variable, an error is signaled by the
       Icy interface.

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 cspice_furnsh.

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

   -  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 cspice_furnsh.

            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 cspice_furnsh.

   -  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 an assignment is

         NAIF_SURFACE_NAME += 'Mars MEGDR 128 PIXEL/DEG'
         NAIF_SURFACE_CODE += 1
         NAIF_SURFACE_BODY += 499

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

Restrictions


   None.

Required_Reading


   ICY.REQ
   DSK.REQ
   NAIF_IDS.REQ
   PCK.REQ
   SPK.REQ
   TIME.REQ

Literature_References


   None.

Author_and_Institution


   N.J. Bachman        (JPL)
   J. Diaz del Rio     (ODC Space)
   S.C. Krening        (JPL)
   E.D. Wright         (JPL)

Version


   -Icy Version 2.0.1, 01-NOV-2021 (JDR)

       Edited the header to comply with NAIF standard. Added example's
       meta-kernel. Updated example #1 to use DSK data. Added second example.

       Added -Parameters, -Exceptions, -Files, -Restrictions,
       -Literature_References and -Author_and_Institution sections.

       Removed reference to the routine's corresponding CSPICE header from
       -Abstract section.

       Added arguments' type and size information in the -I/O section.

   -Icy Version 2.0.0, 04-APR-2017 (EDW) (NJB)

       Updated to support use of DSKs.

   -Icy Version 1.0.2, 15-NOV-2011 (SCK)

       References to the new cspice_pxfrm2 routine were added
       to the 'I/O returns' section. A problem description was
       added to the -Examples section.

   -Icy Version 1.0.1, 12-APR-2011 (EDW)

       Corrected typo in example program comments.

   -Icy Version 1.0.0, 01-FEB-2008 (EDW)

Index_Entries


   find sub-observer point on target body
   find sub-spacecraft point on target body
   find nearest point to observer on target body



Fri Dec 31 18:43:08 2021