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sincpt

Table of contents
Procedure
Abstract
Required_Reading
Keywords
Declarations
Brief_I/O
Detailed_Input
Detailed_Output
Parameters
Exceptions
Files
Particulars
Examples
Restrictions
Literature_References
Author_and_Institution
Version

Procedure

     SINCPT ( Surface intercept )

     SUBROUTINE SINCPT ( METHOD,  TARGET,  ET,      FIXREF,
    .                    ABCORR,  OBSRVR,  DREF,    DVEC,
    .                    SPOINT,  TRGEPC,  SRFVEC,  FOUND  )

Abstract

     Compute, for a given observer and a ray emanating from the
     observer, the surface intercept of the ray 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 SRFXPT.

Required_Reading

     CK
     DSK
     FRAMES
     NAIF_IDS
     PCK
     SCLK
     SPK
     TIME

Keywords

     GEOMETRY

Declarations

     IMPLICIT NONE

     INCLUDE               'dsk.inc'
     INCLUDE               'frmtyp.inc'
     INCLUDE               'gf.inc'
     INCLUDE               'zzabcorr.inc'
     INCLUDE               'zzctr.inc'
     INCLUDE               'zzdsk.inc'


     CHARACTER*(*)         METHOD
     CHARACTER*(*)         TARGET
     DOUBLE PRECISION      ET
     CHARACTER*(*)         FIXREF
     CHARACTER*(*)         ABCORR
     CHARACTER*(*)         OBSRVR
     CHARACTER*(*)         DREF
     DOUBLE PRECISION      DVEC   ( 3 )
     DOUBLE PRECISION      SPOINT ( 3 )
     DOUBLE PRECISION      TRGEPC
     DOUBLE PRECISION      SRFVEC ( 3 )
     LOGICAL               FOUND

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.
     DREF       I   Reference frame of ray's direction vector.
     DVEC       I   Ray's direction vector.
     SPOINT     O   Surface intercept point on the target body.
     TRGEPC     O   Intercept epoch.
     SRFVEC     O   Vector from observer to intercept point.
     FOUND      O   Flag indicating whether intercept was found.

Detailed_Input

     METHOD   is a short string providing parameters defining
              the computation method to be used. In the syntax
              descriptions below, items delimited by brackets
              are optional.

              METHOD may be assigned the following values:

                 'ELLIPSOID'

                    The intercept computation uses a triaxial
                    ellipsoid to model the surface of the target
                    body. The ellipsoid's radii must be available
                    in the kernel pool.


                 'DSK/UNPRIORITIZED[/SURFACES = <surface list>]'

                    The intercept computation uses topographic data
                    to model the surface of the target body. These
                    data must be provided by loaded DSK files.

                    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.


              Neither case nor white space are significant in
              METHOD, except within double-quoted strings. For
              example, the string ' eLLipsoid ' is valid.

              Within double-quoted strings, blank characters are
              significant, but multiple consecutive blanks are
              considered equivalent to a single blank. Case is
              not significant. So

                 "Mars MEGDR 128 PIXEL/DEG"

              is equivalent to

                 " mars megdr  128  pixel/deg "

              but not to

                 "MARS MEGDR128PIXEL/DEG"

     TARGET   is the name of the target body. 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 state and orientation of the
              target body are computed.

              When aberration corrections are used, the position
              and orientation of the target body are computed at
              ET-LT or ET+LT, where LT is the one-way light time
              between the intercept point and the observer, and the
              sign applied to LT depends on the selected
              correction. See the description of ABCORR below for
              details.

     FIXREF   is the name of a body-fixed reference frame centered
              on the target body. FIXREF may be any such frame
              supported by the SPICE system, including built-in
              frames (documented in the Frames Required Reading)
              and frames defined by a loaded frame kernel (FK). The
              string FIXREF is case-insensitive, and leading and
              trailing blanks in FIXREF are not significant.

              The output intercept point SPOINT and the observer-to-
              intercept vector SRFVEC will be expressed relative to
              this reference frame.

     ABCORR   indicates the aberration corrections to be applied
              when computing the target's position and orientation.

              For remote sensing applications, where the apparent
              surface intercept point seen by the observer is
              desired, normally the correction

                 'CN+S'

              should be used. This and the other supported options
              are described below. ABCORR may be any of the
              following:

                 'NONE'     Apply no correction. Return the
                            geometric surface intercept point on the
                            target body.

              Let LT represent the one-way light time between the
              observer and the surface intercept 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
              intercept 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 the surface
                            intercept 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
                            surface intercept obtained with the
                            'LT' option to account for the
                            observer's velocity relative to the
                            solar system barycenter. These
                            computations yield the apparent surface
                            intercept point.

                 'CN'       Converged Newtonian light time
                            correction. In solving the light time
                            equation, the 'CN' correction iterates
                            until the solution converges. Both the
                            position and rotation of the target
                            body are corrected for light time.

                 'CN+S'     Converged Newtonian light time and
                            stellar aberration corrections. This
                            option produces a solution that is at
                            least as accurate at that obtainable
                            with the 'LT+S' option. Whether the
                            'CN+S' solution is substantially more
                            accurate depends on the geometry of the
                            participating objects and on the
                            accuracy of the input data. In all
                            cases this routine will execute more
                            slowly when a converged solution is
                            computed.

                            For reception-case applications
                            involving intercepts near the target
                            body limb, this option should be used.

              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
              intercept 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
                            intercept 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
                            'XLT' 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
                            intercept 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. This option
                            produces a solution that is at least as
                            accurate at that obtainable with the
                            'XLT+S' option. Whether the 'XCN+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.

                            For transmission-case applications
                            involving intercepts near the target
                            body limb, this option should be used.

              Case and embedded blanks are not significant in
              ABCORR. For example, the string

                'Cn + s'

              is valid.

     OBSRVR   is the name of the observing body. This is typically
              a spacecraft, the earth, or a surface point on the
              earth or on another extended object.

              The observer must be outside the target body.

              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.

     DREF     is the name of the reference frame relative to which
              the ray's direction vector is expressed. This may be
              any 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 DREF is case-insensitive, and
              leading and trailing blanks in DREF are not
              significant.

              When DREF designates a non-inertial frame, the
              orientation of the frame is evaluated at an epoch
              dependent on the frame's center and, if the center is
              not the observer, on the selected aberration
              correction. See the description of the direction
              vector DVEC for details.

     DVEC     is a ray direction vector emanating from the observer.
              The intercept with the target body's surface of the ray
              defined by the observer and DVEC is sought.

              DVEC is specified relative to the reference frame
              designated by DREF.

              Non-inertial reference frames are treated as follows:
              if the center of the frame is at the observer's
              location, the frame is evaluated at ET. If the
              frame's center is located elsewhere, then letting
              LTCENT be the one-way light time between the observer
              and the central body associated with the frame, the
              orientation of the frame is evaluated at ET-LTCENT,
              ET+LTCENT, or ET depending on whether the requested
              aberration correction is, respectively, for received
              radiation, transmitted radiation, or is omitted.
              LTCENT is computed using the method indicated by
              ABCORR.

Detailed_Output

     SPOINT   is the surface intercept point on the target body of
              the ray defined by the observer and the direction
              vector. If the ray intersects the target body in
              multiple points, the selected intersection point is
              the one closest to the observer. The output argument
              FOUND (see below) indicates whether an intercept was
              found.

              SPOINT is expressed in Cartesian coordinates,
              relative to the target body-fixed frame designated by
              FIXREF. The body-fixed target frame is evaluated at
              the intercept 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 both
              light time and stellar aberration corrections are
              used, SPOINT is compute such that, when the vector
              from the observer to SPOINT is corrected for light
              time and stellar aberration, the resulting vector
              lies on the ray defined by the observer's location
              and DVEC.

              The components of SPOINT are given in units of km.

     TRGEPC   is the "intercept epoch." TRGEPC is defined as
              follows: letting LT be the one-way light time between
              the observer and the intercept point, TRGEPC is the
              epoch ET-LT, ET+LT, or ET depending on whether the
              requested aberration correction is, respectively, for
              received radiation, transmitted radiation, or
              omitted. LT is computed using the method indicated by
              ABCORR.

              TRGEPC is expressed as seconds past J2000 TDB.

     SRFVEC   is the vector from the observer's position at ET to
              the aberration-corrected (or optionally, geometric)
              position of SPOINT, where the aberration corrections
              are specified by ABCORR. SRFVEC is expressed in the
              target body-fixed reference frame designated by
              FIXREF, evaluated at TRGEPC.

              The components of SRFVEC are given in units of km.

              One can use the SPICELIB function VNORM to obtain the
              distance between the observer and SPOINT:

                 DIST = VNORM ( 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:

                 CALL VSUB ( SPOINT, SRFVEC, OBSPOS )

              To transform the vector SRFVEC from a reference frame
              FIXREF at time TRGEPC to a time-dependent reference
              frame REF at time ET, the routine PXFRM2 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:

                  CALL PXFRM2 ( FIXREF, REF,    TRGEPC, ET, XFORM )
                  CALL MXV    ( XFORM,  SRFVEC, REFVEC )

              The second example in the $Examples header section
              below presents a complete program that demonstrates
              this procedure.

     FOUND    is a logical flag indicating whether or not the ray
              intersects the target. If an intersection exists
              FOUND will be returned as .TRUE. If the ray misses
              the target, FOUND will be returned as .FALSE.

Parameters

     None.

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.

     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 cannot be parsed, an error
         is signaled by either this routine or a routine in the
         call tree of this routine.

     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 SINCPT, 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.

     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 SINCPT, an error
         is 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, an error is signaled by a
         routine 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 SINCPT, an error is
         signaled by a routine in the call tree of this routine.

     12) If the reference frame designated by DREF is not recognized
         by the SPICE frame subsystem, the error SPICE(NOFRAME)
         is signaled.

     13) If the direction vector DVEC is the zero vector, the error
         SPICE(ZEROVECTOR) is signaled.

     14) 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.

     15) If METHOD specifies that the target surface is represented
         by DSK data, and DSK data are not available for a portion of
         the target body's surface, an intercept might not be found.
         This routine does not revert to using an ellipsoidal surface
         in this case.

Files

     Appropriate kernels must be loaded by the calling program before
     this routine is called.

     The following data are required:

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

     -  PCK data: if the computation method is specified as
        "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.

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

     The following data may be required:

     -  DSK data: if METHOD indicates that DSK data are to be used,
        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.

     -  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

     -  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. Similarly, the frame definition required to map
        between the frame designated by DREF and the target
        body-fixed frame must be available. Typically the
        definitions of frames not already built-in to SPICE are
        supplied by loading a frame kernel.

     -  CK data: if the frame to which DREF refers is fixed to a
        spacecraft instrument or structure, at least one CK file
        will be needed to permit transformation of vectors between
        that frame and both the J2000 and the target body-fixed
        frames.

     -  SCLK data: if a CK file is needed, an associated SCLK
        kernel is required to enable conversion between encoded SCLK
        (used to time-tag CK data) and barycentric dynamical time
        (TDB).

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

Particulars

     Given a ray defined by a direction vector and the location of an
     observer, SINCPT computes the surface intercept point of the ray
     on a specified target body. SINCPT also determines the vector
     from the observer to the surface intercept point. If the ray
     intersects the target in multiple locations, the intercept
     closest to the observer is selected.

     When aberration corrections are used, this routine finds the
     value of SPOINT such that, if SPOINT is regarded as an ephemeris
     object, after the selected aberration corrections are applied to
     the vector from the observer to SPOINT, the resulting vector is
     parallel to the direction vector DVEC.

     This routine computes light time corrections using light time
     between the observer and the surface intercept 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
     intercept 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 intercept point, in particular when the
     observer-intercept point distance is much less than the
     observer-target center distance. It's also valid to use stellar
     aberration corrections even when the intercept point is near or
     on the limb (as may occur in occultation computations using a
     point target).

     When comparing surface intercept point computations with results
     from sources other than SPICE, it's essential to make sure the
     same geometric definitions are used.


     Using DSK data
     ==============

        DSK loading and unloading
        -------------------------

        DSK files providing data used by this routine are loaded by
        calling FURNSH and can be unloaded by calling UNLOAD or
        KCLEAR. See the documentation of 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

           DSK/<surface list>/UNPRIORITIZED
           DSK/UNPRIORITIZED/<surface list>
           UNPRIORITIZED/<surface list>/DSK

        The simplest form of the METHOD argument specifying use of
        DSK data is one that lacks a surface list, for example:

           '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 string suffices. 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

           'DSK/UNPRIORITIZED/SURFACES = "Mars MEGDR 64 PIXEL/DEG", 3'


        Round-off errors and mitigating algorithms
        ------------------------------------------

        When topographic data are used to represent the surface of a
        target body, round-off errors can produce some results that
        may seem surprising.

        Note that, since the surface in question might have mountains,
        valleys, and cliffs, the points of intersection found for
        nearly identical sets of inputs may be quite far apart from
        each other: for example, a ray that hits a mountain side in a
        nearly tangent fashion may, on a different host computer, be
        found to miss the mountain and hit a valley floor much farther
        from the observer, or even miss the target altogether.

        Round-off errors can affect segment selection: for example, a
        ray that is expected to intersect the target body's surface
        near the boundary between two segments might hit either
        segment, or neither of them; the result may be
        platform-dependent.

        A similar situation exists when a surface is modeled by a set
        of triangular plates, and the ray is expected to intersect the
        surface near a plate boundary.

        To avoid having the routine fail to find an intersection when
        one clearly should exist, this routine uses two "greedy"
        algorithms:

           1) If the ray passes sufficiently close to any of the
              boundary surfaces of a segment (for example, surfaces of
              maximum and minimum longitude or latitude), that segment
              is tested for an intersection of the ray with the
              surface represented by the segment's data.

              This choice prevents all of the segments from being
              missed when at least one should be hit, but it could, on
              rare occasions, cause an intersection to be found in a
              segment other than the one that would be found if higher
              precision arithmetic were used.

           2) For type 2 segments, which represent surfaces as
              sets of triangular plates, each plate is expanded very
              slightly before a ray-plate intersection test is
              performed. The default plate expansion factor is

                 1 + 1.E-10

              In other words, the sides of the plate are lengthened by
              1/10 of a micron per km. The expansion keeps the centroid
              of the plate fixed.

              Plate expansion prevents all plates from being missed
              in cases where clearly at least one should be hit.

              As with the greedy segment selection algorithm, plate
              expansion can occasionally cause an intercept to be
              found on a different plate than would be found if higher
              precision arithmetic were used. It also can occasionally
              cause an intersection to be found when the ray misses
              the target by a very small distance.


        Aberration corrections
        ----------------------

        For irregularly shaped target bodies, the distance between the
        observer and the nearest surface intercept need not be a
        continuous function of time; hence the one-way light time
        between the intercept and the observer may be discontinuous as
        well. In such cases, the computed light time, which is found
        using an iterative algorithm, may converge slowly or not at
        all. In all cases, the light time computation will terminate,
        but the result may be less accurate than expected.

Examples

     The numerical results shown for these examples may differ across
     platforms. The results depend on the SPICE kernels used as
     input, the compiler and supporting libraries, and the machine
     specific arithmetic implementation.


     1) The following program computes surface intercept points on Mars
        for the boresight and FOV boundary vectors of the MGS MOC
        narrow angle camera. The intercepts are computed for a single
        observation epoch. Light time and stellar aberration
        corrections are used. For simplicity, camera distortion is
        ignored.

        Intercepts are computed using both triaxial ellipsoid and
        topographic surface models.

        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: sincpt_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_moc_v20.ti                   MGS MOC instrument
                                               parameters
              mgs_sclkscet_00061.tsc           MGS SCLK coefficients
              mgs_sc_ext12.bc                  MGS s/c bus attitude
              mgs_ext12_ipng_mgs95j.bsp        MGS ephemeris
              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',
                                  'mgs_moc_v20.ti',
                                  'mgs_sclkscet_00061.tsc',
                                  'mgs_sc_ext12.bc',
                                  'mgs_ext12_ipng_mgs95j.bsp',
                                  'megr90n000cb_plate.bds'      )
           \begintext

           End of meta-kernel


        Example code begins here.


              PROGRAM SINCPT_EX1
              IMPLICIT NONE
        C
        C     SPICELIB functions
        C
              DOUBLE PRECISION      VNORM

        C
        C     Local parameters
        C
              CHARACTER*(*)         META
              PARAMETER           ( META   = 'sincpt_ex1.tm' )

              INTEGER               ABCLEN
              PARAMETER           ( ABCLEN = 20 )

              INTEGER               LNSIZE
              PARAMETER           ( LNSIZE = 78 )

              INTEGER               METLEN
              PARAMETER           ( METLEN = 40 )

              INTEGER               NAMLEN
              PARAMETER           ( NAMLEN = 32 )

              INTEGER               TIMLEN
              PARAMETER           ( TIMLEN = 50 )

              INTEGER               SHPLEN
              PARAMETER           ( SHPLEN = 80 )

              INTEGER               NCORNR
              PARAMETER           ( NCORNR = 4 )

              INTEGER               NMETH
              PARAMETER           ( NMETH  = 2 )

        C
        C     Local variables
        C
              CHARACTER*(ABCLEN)    ABCORR
              CHARACTER*(NAMLEN)    CAMERA
              CHARACTER*(NAMLEN)    DREF
              CHARACTER*(NAMLEN)    FIXREF
              CHARACTER*(METLEN)    METHDS ( NMETH )
              CHARACTER*(METLEN)    METHOD
              CHARACTER*(NAMLEN)    OBSRVR
              CHARACTER*(SHPLEN)    SHAPE
              CHARACTER*(NAMLEN)    SRFTYP ( NMETH )
              CHARACTER*(NAMLEN)    TARGET
              CHARACTER*(LNSIZE)    TITLE
              CHARACTER*(TIMLEN)    UTC

              DOUBLE PRECISION      BOUNDS ( 3, NCORNR )
              DOUBLE PRECISION      BSIGHT ( 3 )
              DOUBLE PRECISION      DIST
              DOUBLE PRECISION      DPR
              DOUBLE PRECISION      DVEC   ( 3 )
              DOUBLE PRECISION      ET
              DOUBLE PRECISION      LAT
              DOUBLE PRECISION      LON
              DOUBLE PRECISION      RADIUS
              DOUBLE PRECISION      SPOINT ( 3 )
              DOUBLE PRECISION      SRFVEC ( 3 )
              DOUBLE PRECISION      TRGEPC

              INTEGER               CAMID
              INTEGER               I
              INTEGER               J
              INTEGER               K
              INTEGER               N

              LOGICAL               FOUND

              DATA                  ABCORR / 'CN+S'              /
              DATA                  CAMERA / 'MGS_MOC_NA'        /
              DATA                  FIXREF / 'IAU_MARS'          /
              DATA                  METHDS / 'ELLIPSOID',
             .                               'DSK/UNPRIORITIZED' /
              DATA                  OBSRVR / 'MGS'               /
              DATA                  SRFTYP / 'Ellipsoid',
             .               'MGS/MOLA topography, 4 pixel/deg'  /
              DATA                  TARGET / 'Mars'              /
              DATA                  UTC    /
             .                        '2003 OCT 13 06:00:00 UTC' /

        C
        C     Load kernel files:
        C
              CALL FURNSH ( META )

        C
        C     Convert the UTC request time to ET (seconds past
        C     J2000, TDB).
        C
              CALL STR2ET ( UTC, ET )

        C
        C     Get the MGS MOC Narrow angle camera (MGS_MOC_NA)
        C     ID code. Then look up the field of view (FOV)
        C     parameters by calling GETFOV.
        C
              CALL BODN2C ( CAMERA, CAMID, FOUND )

              IF ( .NOT. FOUND ) THEN
                 CALL SETMSG ( 'Could not find ID code for ' //
             .                 'instrument #.'               )
                 CALL ERRCH  ( '#', CAMERA                   )
                 CALL SIGERR ( 'SPICE(NOTRANSLATION)'        )
              END IF

        C
        C     GETFOV will return the name of the camera-fixed frame
        C     in the string DREF, the camera boresight vector in
        C     the array BSIGHT, and the FOV corner vectors in the
        C     array BOUNDS.
        C
              CALL GETFOV ( CAMID,  NCORNR, SHAPE,  DREF,
             .              BSIGHT, N,      BOUNDS       )


              WRITE (*,*) ' '
              WRITE (*,*) 'Surface Intercept Locations for Camera'
              WRITE (*,*) 'FOV Boundary and Boresight Vectors'
              WRITE (*,*) ' '
              WRITE (*,*) '   Instrument:            ', CAMERA
              WRITE (*,*) '   Epoch:                 ', UTC
              WRITE (*,*) '   Aberration correction: ', ABCORR

        C
        C     Now compute and display the surface intercepts for the
        C     boresight and all of the FOV boundary vectors.
        C
              DO I = 1, NCORNR+1

                 IF ( I .LE. NCORNR ) THEN

                    TITLE = 'Corner vector #'
                    CALL REPMI ( TITLE, '#', I, TITLE )

                    CALL VEQU ( BOUNDS(1,I), DVEC )

                 ELSE

                    TITLE = 'Boresight vector'
                    CALL VEQU ( BSIGHT, DVEC )

                 END IF

                 WRITE (*,*) ' '
                 WRITE (*,*) TITLE

                 TITLE = '  Vector in # frame = '
                 CALL REPMC ( TITLE, '#', DREF, TITLE )

                 WRITE (*,*) ' '
                 WRITE (*,*) TITLE

                 IF ( I .LE. NCORNR ) THEN
                    WRITE (*, '(1X,3F20.14)') ( BOUNDS(J,I), J=1,3 )
                 ELSE
                    WRITE (*, '(1X,3F20.14)') BSIGHT
                 END IF

                 WRITE (*,*) ' '
                 WRITE (*,*) '  Intercept:'

        C
        C        Compute the surface intercept point using
        C        the specified aberration corrections. Loop
        C        over the set of computation methods.
        C
                 DO K = 1, NMETH

                    METHOD = METHDS(K)

                    CALL SINCPT ( METHOD, TARGET, ET,
             .                    FIXREF, ABCORR, OBSRVR,
             .                    DREF,   DVEC,   SPOINT,
             .                    TRGEPC, SRFVEC, FOUND   )

                    IF ( FOUND ) THEN
        C
        C              Compute range from observer to apparent
        C              intercept.
        C
                       DIST = VNORM ( SRFVEC )
        C
        C              Convert rectangular coordinates to
        C              planetocentric latitude and longitude.
        C              Convert radians to degrees.
        C
                       CALL RECLAT ( SPOINT, RADIUS, LON, LAT )

                       LON = LON * DPR ()
                       LAT = LAT * DPR ()
        C
        C              Display the results.
        C

                       WRITE (*,*) ' '
                       CALL TOSTDO ( '     Surface representation: '
             .         //            SRFTYP(K)                      )
                       WRITE (*,*) ' '
                       WRITE (*,*)
             .         '     Radius                   (km)  = ', RADIUS
                       WRITE (*,*)
             .         '     Planetocentric Latitude  (deg) = ', LAT
                       WRITE (*,*)
             .         '     Planetocentric Longitude (deg) = ', LON
                       WRITE (*,*)
             .         '     Range                    (km)  = ', DIST

                    ELSE

                       CALL TOSTDO ( '   Surface representation: '
             .         //            SRFTYP(K)                     )
                       WRITE (*,*) '     Intercept not found.'
                       WRITE (*,*) ' '

                    END IF

                 END DO

              END DO

              END


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


         Surface Intercept Locations for Camera
         FOV Boundary and Boresight Vectors

            Instrument:            MGS_MOC_NA
            Epoch:                 2003 OCT 13 06:00:00 UTC
            Aberration correction: CN+S

         Corner vector 1

           Vector in MGS_MOC_NA frame =
             0.00000185713838   -0.00380156226592    0.99999277403434

           Intercept:

             Surface representation: Ellipsoid

              Radius                   (km)  =    3384.9411357607282
              Planetocentric Latitude  (deg) =   -48.477482367206768
              Planetocentric Longitude (deg) =   -123.47407481971256
              Range                    (km)  =    388.98308225698986

             Surface representation: MGS/MOLA topography, 4 pixel/deg

              Radius                   (km)  =    3387.6408267726060
              Planetocentric Latitude  (deg) =   -48.492259559975267
              Planetocentric Longitude (deg) =   -123.47541193495911
              Range                    (km)  =    386.14510040407879

         Corner vector 2

           Vector in MGS_MOC_NA frame =
             0.00000185713838    0.00380156226592    0.99999277403434

           Intercept:

             Surface representation: Ellipsoid

              Radius                   (km)  =    3384.9396985743224
              Planetocentric Latitude  (deg) =   -48.481636778911913
              Planetocentric Longitude (deg) =   -123.39881874871132
              Range                    (km)  =    388.97510005267708

             Surface representation: MGS/MOLA topography, 4 pixel/deg

              Radius                   (km)  =    3387.6403704507966
              Planetocentric Latitude  (deg) =   -48.496386688872484
              Planetocentric Longitude (deg) =   -123.40074354811055
              Range                    (km)  =    386.13616443321536

         Corner vector 3

           Vector in MGS_MOC_NA frame =
            -0.00000185713838    0.00380156226592    0.99999277403434

           Intercept:

             Surface representation: Ellipsoid

              Radius                   (km)  =    3384.9396897286833
              Planetocentric Latitude  (deg) =   -48.481662348858336
              Planetocentric Longitude (deg) =   -123.39882195503854
              Range                    (km)  =    388.97464113550637

             Surface representation: MGS/MOLA topography, 4 pixel/deg

              Radius                   (km)  =    3387.6403603146168
              Planetocentric Latitude  (deg) =   -48.496412042429789
              Planetocentric Longitude (deg) =   -123.40074672915324
              Range                    (km)  =    386.13571069851986

         Corner vector 4

           Vector in MGS_MOC_NA frame =
            -0.00000185713838   -0.00380156226592    0.99999277403434

           Intercept:

             Surface representation: Ellipsoid

              Radius                   (km)  =    3384.9411269137695
              Planetocentric Latitude  (deg) =   -48.477507940479093
              Planetocentric Longitude (deg) =   -123.47407797517749
              Range                    (km)  =    388.98262331952731

             Surface representation: MGS/MOLA topography, 4 pixel/deg

              Radius                   (km)  =    3387.6408166344654
              Planetocentric Latitude  (deg) =   -48.492284916898356
              Planetocentric Longitude (deg) =   -123.47541506563023
              Range                    (km)  =    386.14464664863726

         Boresight vector

           Vector in MGS_MOC_NA frame =
             0.00000000000000    0.00000000000000    1.00000000000000

           Intercept:

             Surface representation: Ellipsoid

        [...]


        Warning: incomplete output. Only 100 out of 112 lines have been
        provided.


     2) Use SUBPNT to find the sub-spacecraft point on Mars for the
        Mars Reconnaissance Orbiter spacecraft (MRO) at a specified
        time, using the "near point: ellipsoid" computation method.
        Use both LT+S and CN+S aberration corrections to illustrate
        the differences.

        Convert the spacecraft to sub-observer point vector obtained
        from 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 SINCPT and SUBPNT.

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


           KPL/MK

           File: sincpt_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.


              PROGRAM SINCPT_EX2
              IMPLICIT NONE
        C
        C     SPICELIB functions
        C
              DOUBLE PRECISION      DPR
              DOUBLE PRECISION      VDIST
              DOUBLE PRECISION      VNORM

        C
        C     Local parameters
        C
              CHARACTER*(*)         META
              PARAMETER           ( META   = 'sincpt_ex2.tm' )

              CHARACTER*(*)         F1
              PARAMETER           ( F1     = '(A,F21.9)' )

              CHARACTER*(*)         F2
              PARAMETER           ( F2     = '(A)' )

              INTEGER               FRNMLN
              PARAMETER           ( FRNMLN = 32 )

              INTEGER               MTHLEN
              PARAMETER           ( MTHLEN = 50 )

              INTEGER               CORLEN
              PARAMETER           ( CORLEN = 5 )

              INTEGER               NCORR
              PARAMETER           ( NCORR  = 2 )

              INTEGER               NMETH
              PARAMETER           ( NMETH  = 2 )

        C
        C     Local variables
        C
              CHARACTER*(CORLEN)    ABCORR ( NCORR )
              CHARACTER*(FRNMLN)    FIXREF
              CHARACTER*(FRNMLN)    HIREF
              CHARACTER*(MTHLEN)    SINMTH ( NMETH )
              CHARACTER*(MTHLEN)    SUBMTH ( NMETH )

              DOUBLE PRECISION      ALT
              DOUBLE PRECISION      ET
              DOUBLE PRECISION      LAT
              DOUBLE PRECISION      LON
              DOUBLE PRECISION      MROVEC ( 3 )
              DOUBLE PRECISION      RADIUS
              DOUBLE PRECISION      SPOINT ( 3 )
              DOUBLE PRECISION      SRFVEC ( 3 )
              DOUBLE PRECISION      TRGEPC
              DOUBLE PRECISION      XFORM  ( 3, 3 )
              DOUBLE PRECISION      XEPOCH
              DOUBLE PRECISION      XPOINT ( 3 )
              DOUBLE PRECISION      XVEC   ( 3 )

              INTEGER               I
              INTEGER               J

              LOGICAL               FOUND

        C
        C     Initial values
        C
              DATA                  ABCORR / 'LT+S', 'CN+S'         /
              DATA                  FIXREF / 'IAU_MARS'             /
              DATA                  SINMTH / 'Ellipsoid',
             .                               'DSK/Unprioritized'    /
              DATA                  SUBMTH / 'Ellipsoid/Near point',
             .                            'DSK/Unprioritized/Nadir' /

        C
        C     Load kernel files via the meta-kernel.
        C
              CALL FURNSH ( META )

        C
        C     Convert the TDB request time string to seconds past
        C     J2000, TDB.
        C
              CALL STR2ET ( '2007 SEP 30 00:00:00 TDB', ET )

        C
        C     Compute the sub-spacecraft point using the
        C     "NEAR POINT: ELLIPSOID" definition.
        C     Compute the results using both LT+S and CN+S
        C     aberration corrections.
        C
        C     Repeat the computation for each method.
        C
        C
              DO I = 1, NMETH

                 WRITE(*,F2) ' '
                 WRITE(*,F2) 'Sub-observer point computation method = '
             .               // SUBMTH(I)

                 DO J = 1, NCORR

                    CALL SUBPNT ( SUBMTH(I),
             .                    'Mars', ET,     FIXREF, ABCORR(J),
             .                    'MRO',  SPOINT, TRGEPC, SRFVEC    )
        C
        C           Compute the observer's altitude above SPOINT.
        C
                    ALT = VNORM ( SRFVEC )
        C
        C           Express SRFVEC in the MRO_HIRISE_LOOK_DIRECTION
        C           reference frame at epoch ET. Since SRFVEC is
        C           expressed relative to the IAU_MARS frame at
        C           TRGEPC, we must call PXFRM2 to compute the position
        C           transformation matrix from IAU_MARS at TRGEPC to
        C           the MRO_HIRISE_LOOK_DIRECTION frame at time ET.
        C
        C           To make code formatting a little easier, we'll
        C           store the long MRO reference frame name in a
        C           variable:
        C
                    HIREF = 'MRO_HIRISE_LOOK_DIRECTION'

                    CALL PXFRM2 ( FIXREF, HIREF,  TRGEPC, ET, XFORM )
                    CALL MXV    ( XFORM,  SRFVEC, MROVEC )

        C
        C           Convert rectangular coordinates to planetocentric
        C           latitude and longitude. Convert radians to degrees.
        C
                    CALL RECLAT ( SPOINT, RADIUS, LON, LAT  )

                    LON = LON * DPR ()
                    LAT = LAT * DPR ()
        C
        C           Write the results.
        C
                    WRITE(*,F2) ' '
                    WRITE(*,F2) '   Aberration correction = '
             .                // ABCORR(J)
                    WRITE(*,F1) ' '
                    WRITE(*,F2) '      MRO-to-sub-observer vector in'
                    WRITE(*,F2) '      MRO HIRISE look direction frame'
                    WRITE(*,F1) '        X-component             '
             .               // '(km) = ', MROVEC(1)
                    WRITE(*,F1) '        Y-component             '
             .               // '(km) = ', MROVEC(2)
                    WRITE(*,F1) '        Z-component             '
             .               // '(km) = ', MROVEC(3)
                    WRITE(*,F1) '      Sub-observer point radius '
             .               // '(km) = ', RADIUS
                    WRITE(*,F1) '      Planetocentric latitude  '
             .               // '(deg) = ', LAT
                    WRITE(*,F1) '      Planetocentric longitude '
             .               // '(deg) = ', LON
                    WRITE(*,F1) '      Observer altitude         '
             .               // '(km) = ',  ALT

        C
        C           Consistency check: find the surface intercept on
        C           Mars of the ray emanating from the spacecraft and
        C           having direction vector MROVEC in the MRO HIRISE
        C           reference frame at ET. Call the intercept point
        C           XPOINT. XPOINT should coincide with SPOINT, up to
        C           a small round-off error.
        C
                    CALL SINCPT ( SINMTH(I), 'Mars', ET,    FIXREF,
             .                    ABCORR(J), 'MRO',  HIREF, MROVEC,
             .                    XPOINT,    XEPOCH, XVEC,  FOUND  )

                    IF ( .NOT. FOUND ) THEN
                       WRITE (*,F1) 'Bug: no intercept'
                    ELSE
        C
        C              Report the distance between XPOINT and SPOINT.
        C
                       WRITE (*,* ) ' '
                       WRITE (*,F1) '   Intercept comparison '
             .         //           'error (km) = ',
             .                      VDIST( XPOINT, SPOINT )
                    END IF

                 END DO

              END DO

              END


        When this program was executed on a Mac/Intel/gfortran/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

Restrictions

     1)  A cautionary note: if aberration corrections are used, and
         if DREF is the target body-fixed frame, the epoch at which
         that frame is evaluated is offset from ET by the light time
         between the observer and the *center* of the target body.
         This light time normally will differ from the light time
         between the observer and intercept point. Consequently the
         orientation of the target body-fixed frame at TRGEPC will
         not match that of the target body-fixed frame at the epoch
         associated with DREF. As a result, various derived quantities
         may not be as expected: for example, SRFVEC would not be
         parallel to DVEC.

         In many applications the errors arising from this frame
         discrepancy may be insignificant; however a safe approach is
         to always use as DREF a frame other than the target
         body-fixed frame.

     2)  This routine must not be used for cases where the observer
         is inside the target body. This routine does not attempt to
         detect this condition.

         If the observer is a point on a target surface described
         by DSK data, care must be taken to ensure the observer is
         sufficiently far outside the target. The routine should
         not be used for surfaces for which "outside" cannot be
         defined.

Literature_References

     None.

Author_and_Institution

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

Version

    SPICELIB Version 3.1.0, 26-OCT-2021 (JDR) (NJB)

        Bug fix: PRVCOR is no longer set to blank before
        ABCORR is parsed.

        ZZVALCOR is now used instead of ZZPRSCOR. This provides
        better error handling.

        Edits to $Examples section to comply with NAIF standard.

        The header's $Detailed_Input and $Restrictions sections
        were updated to state that the observer must be
        outside the target body.

    SPICELIB Version 3.0.0, 04-APR-2017 (NJB)

        01-FEB-2016 (NJB)

           Upgraded to support surfaces represented by DSKs.

           Updated kernels are used in header example programs.

    SPICELIB Version 2.0.0, 31-MAR-2014 (NJB) (SCK) (BVS)

        Bug fix: FIRST is now set to .FALSE. at the completion
        of a successful initialization pass. This does not affect
        the routine's outputs but improves efficiency.

        Bug fix: redundant call to SPKSSB was removed. This does not
        affect the routine's outputs but improves efficiency.

        References to the new PXFRM2 routine were added, which changed
        the Detailed Output section and the second example. Some header
        comment corrections were made.

        Upgrade: this routine now uses ZZVALCOR rather than
        ZZPRSCOR, simplifying the implementation.

        Upgrade: this routine now saves the input body names and
        ZZBODTRN state counters and does name-ID conversions only if
        the counters have changed.

        Upgrade: this routine now saves the input frame names and POOL
        state counters and does frame name-ID conversions only if the
        counters have changed.

    SPICELIB Version 1.2.0, 07-APR-2010 (NJB)

        Code style improvement: re-use of variables in
        FRINFO calls has been eliminated. There is no impact
        of the behavior of the routine.

    SPICELIB Version 1.1.0, 17-MAR-2009 (NJB) (EDW)

        Bug fix: quick test for non-intersection is
        no longer performed when observer-target distance
        is less than target's maximum radius.

        Typos in the Detailed Input section's description of DREF
        were corrected.

        In the header examples, meta-kernel names were updated to use
        the suffix

           ".tm"

        Incorrect frame name FIXFRM was changed to FIXREF in
        documentation.

        Typo correction in $Required_Reading, changed FRAME
        to FRAMES.

    SPICELIB Version 1.0.0, 02-MAR-2008 (NJB)
Fri Dec 31 18:36:48 2021