 
Remote Sensing Hands-On Lesson (FORTRAN)
===========================================================================
 
     October 14, 2004
 
 
Overview
--------------------------------------------------------
 
     In this lesson you will develop a series of simple programs that
     demonstrate the usage of SPICE to compute a variety of different
     geometric quantities applicable to experiments carried out by a remote
     sensing instrument flown on an interplanetary spacecraft. This
     particular lesson focuses on a framing camera flying on the Cassini
     spacecraft, but many of the concepts are easily extended and
     generalized to other scenarios.
 
 
References
--------------------------------------------------------
 
 
Tutorials
 
     The following SPICE tutorials are referred to by the discussions in
     this lesson:
 
        Name             Lesson steps/routines it describes
        ---------------  -----------------------------------------
        Time             Time Conversion
        SCLK and LSK     Time Conversion
        SPK              Obtaining Ephemeris Data
        Frames           Reference Frames
        Using Frames     Reference Frames
        PCK              Planetary Constants Data
        CK               Spacecraft Orientation Data
 
     These tutorials are available from the NAIF ftp server at JPL:
 
        ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials
 
 
Required Readings
 
     The Required Reading documents are provided with the Toolkit and are
     located under the ``toolkit/doc'' directory in the FORTRAN
     installation tree.
 
        Name             Lesson steps/routines that it describes
        ---------------  -----------------------------------------
        time.req         Time Conversion
        sclk.req         SCLK Time Conversion
        spk.req          Obtaining Ephemeris Data
        frames.req       Using Reference Frames
        pck.req          Obtaining Planetary Constants Data
        ck.req           Obtaining Spacecraft Orientation Data
        naif_ids.req     Determining Body ID Codes
 
 
The Permuted Index
 
     Another useful document distributed with the Toolkit is the permuted
     index. This is located under the ``toolkit/doc'' directory in the
     FORTRAN installation tree. This text document provides a simple
     mechanism to discover what SPICE routines perform a particular
     function of interest as well as the name of the source module that
     contains the routine. This is particularly useful for FORTRAN
     programmers because some of the routines are entry points and,
     therefore, the name does not translate directly into the name of the
     source module that contains them.
 
 
Source Code Headers
 
     The most detailed specification of a given SPICE routine is contained
     in the header section of its source code. The source code is
     distributed with the Toolkit and is located under
 
     ``toolkit/src/spicelib'' in the FORTRAN versions. For example the
     header of STR2ET is contained in the file:
 
        toolkit/src/spicelib/str2et.for
 
            or ...
 
        toolkit/src/spicelib/str2et.f
 
     Some of the FORTRAN routines are entry points -- these are part of a
     source module that has a different name. The aforementioned permuted
     index is helpful in locating the files that contain the entry point
     headers.
 
 
Kernels Used
--------------------------------------------------------
 
     The programs that are produced in the course of this lesson will
     compute geometry for the Cassini orbiter. The following CASSINI SPICE
     kernels will be used:
 
        #  FILE NAME                 TYPE  DESCRIPTION
        -- ------------------------- ----  ------------------------
        1  naif0007.tls              LSK   Generic LSK
        2  cas00084.tsc              SCLK  Cassini SCLK
        3  sat128.bsp                SPK   Saturnian Satellite Ephemeris
        4  981005_PLTEPH-DE405S.bsp  SPK   Solar System Ephemeris
        5  020514_SE_SAT105.bsp      SPK   Saturnian Satellite Ephemeris
        6  030201AP_SK_SM546_T45.bsp SPK   Cassini Spacecraft SPK
        7  cas_v37.tf                FK    Cassini FK
        8  04135_04171pc_psiv2.bc    CK    Cassini Spacecraft CK
        9  cpck05Mar2004.tpc         PCK   Cassini Project PCK
        10 cas_iss_v09.ti            IK    ISS Instrument Kernel
 
 
SPICE Modules Used
--------------------------------------------------------
 
     This section provides a complete summary of the routines, and the
     kernels that are suggested for usage in each of the exercises in this
     tutorial. (You may wish to not look at this list unless/until you
     ``get stuck'' while working on your own.)
 
        CHAPTER EXERCISE   ROUTINES   FUNCTIONS  KERNELS
        ------- ---------  ---------  ---------  ---------
          1     convtm     FURNSH                1,2
                           PROMPT
                           STR2ET
                           ETCAL
                           TIMOUT
                           SCE2C
                           SCE2S
 
          2     getsta     FURNSH     VNORM      1,3-6
                           PROMPT
                           STR2ET
                           SPKEZR
                           SPKPOS
                           CONVRT
 
          3     xform      FURNSH     VSEP       1-9
                           PROMPT
                           STR2ET
                           SPKEZR
                           SXFORM
                           MXVG
                           SPKPOS
                           PXFORM
                           MXV
                           CONVRT
 
          4     subpts     FURNSH                1,3-6,9
                           PROMPT
                           STR2ET
                           SUBPT
                           SUBSOL
 
          5     fovint     FURNSH     DPR        1-10
                           PROMPT
                           STR2ET
                           BODN2C
                           BYEBYE
                           GETFOV
                           SRFXPT
                           RECLAT
 
          6     angles     FURNSH     DPR        1-10
                           PROMPT
                           STR2ET
                           BODN2C
                           BYEBYE
                           GETFOV
                           SRFXPT
                           RECLAT
                           ILLUM
                           ET2LST
 
     Refer to the headers of the various routines listed above, as detailed
     interface specifications are provided with the source code.
 
 
Time Conversion (convtm)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
     Write a program that prompts the user for an input UTC time string,
     converts it to the following time systems and output formats:
 
         1.   Ephemeris Time (ET) in seconds past J2000
 
         2.   Calendar Ephemeris Time
 
         3.   Spacecraft Clock Time
 
     and displays the results. Use the program to convert "2004 jun 11
     19:32:00" UTC into these alternate systems.
 
 
Learning Goals
--------------------------------------------------------
 
     Familiarity with the various time conversion and parsing routines
     available in the Toolkit. Exposure to source code headers and their
     usage in learning to call routines.
 
 
Approach
--------------------------------------------------------
 
     The solution to the problem can be broken down into a series of simple
     steps:
 
         --   Decide which SPICE kernels are necessary. Prepare a
              meta-kernel listing the kernels and load it into the program.
 
         --   Prompt the user for an input UTC time string.
 
         --   Convert the input time string into ephemeris time expressed
              as seconds past J2000 TDB. Display the result.
 
         --   Convert ephemeris time into a calendar format. Display the
              result.
 
         --   Convert ephemeris time into a spacecraft clock string.
              Display the result.
 
     You may find it useful to consult the permuted index, the headers of
     various source modules, and the ``Time Required Reading'' and ``SCLK
     Required Reading'' documents.
 
     When completing the ``calendar format'' step above, consider using one
     of two possible methods: ETCAL or TIMOUT.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
     The meta-kernel we created for the solution to this exercise is named
     'convtm.mk'. Its contents follow:
 
        KPL/MK
 
        This is the meta-kernel used in the solution of the ``Time
        Conversion'' task in the Remote Sensing Hands On Lesson.
 
           \begindata
           KERNELS_TO_LOAD = ( 'kernels/lsk/naif0007.tls',
                               'kernels/sclk/cas00084.tsc' )
           \begintext
 
 
 
 
Solution Source Code
 
     A sample solution to the problem follows:
 
              PROGRAM CONVTM
 
              IMPLICIT NONE
 
        C
        C     Local Parameters
        C
        C     The name of the meta-kernel that lists the kernels
        C     to load into the program.
        C
              CHARACTER*(*)         METAKR
              PARAMETER           ( METAKR = 'convtm.mk' )
 
        C
        C     The spacecraft clock ID code for CASSINI.
        C
              INTEGER               SCLKID
              PARAMETER           ( SCLKID = -82 )
 
        C
        C     The length of various string variables.
        C
              INTEGER               STRLEN
              PARAMETER           ( STRLEN = 50 )
 
        C
        C     Local Variables
        C
              CHARACTER*(STRLEN)    CALET
              CHARACTER*(STRLEN)    SCLKST
              CHARACTER*(STRLEN)    UTCTIM
 
              DOUBLE PRECISION      ET
 
        C
        C     Load the kernels this program requires.
        C     Both the spacecraft clock kernel and a
        C     leapseconds kernel should be listed
        C     in the meta-kernel.
        C
              CALL FURNSH ( METAKR )
 
        C
        C     Prompt the user for the input time string.
        C
              CALL PROMPT ( 'Input UTC Time: ', UTCTIM )
 
              WRITE (*,*) 'Converting UTC Time: ', UTCTIM
 
        C
        C     Convert UTCTIM to ET.
        C
              CALL STR2ET ( UTCTIM, ET )
 
              WRITE (*,'(A,F16.3)') '   ET Seconds Past 2000: ', ET
 
        C
        C     Now convert ET to a formal calendar time
        C     string.  This can be accomplished in two
        C     ways.
        C
              CALL ETCAL ( ET, CALET )
 
              WRITE (*,*) '   Calendar ET (ETCAL): ', CALET
 
        C
        C     Or use TIMOUT for finer control over the
        C     output format.  The picture below was built
        C     by examining the header of TIMOUT.
        C
              CALL TIMOUT ( ET, 'YYYY-MON-DDTHR:MN:SC ::TDB', CALET )
 
              WRITE (*,*) '   Calendar ET (TIMOUT): ', CALET
 
        C
        C     Convert ET to spacecraft clock time.
        C
              CALL SCE2S ( SCLKID, ET, SCLKST )
 
              WRITE (*,*) '   Spacecraft Clock Time: ', SCLKST
 
              END
 
 
Solution Sample Output
 
     After compiling the program, execute it:
 
        Converting UTC Time: 2004 jun 11 19:32:00
           ET Seconds Past 2000:    140254384.185
            Calendar ET (ETCAL): 2004 JUN 11 19:33:04.184
            Calendar ET (TIMOUT): 2004-JUN-11T19:33:04
            Spacecraft Clock Time: 1/1465674964.105
 
 
Obtaining Target States and Positions (getsta)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
     Write a program that prompts the user for an input UTC time string,
     computes the following quantities at that epoch:
 
         1.   The apparent state of Phoebe as seen from CASSINI in the
              J2000 frame, in kilometers and kilometers/second. This vector
              itself is not of any particular interest, but it is a useful
              intermediate quantity in some geometry calculations.
 
         2.   The apparent position of the Earth as seen from CASSINI in
              the J2000 frame, in kilometers.
 
         3.   The one-way light time between CASSINI and the apparent
              position of Earth, in seconds.
 
         4.   The apparent position of the Sun as seen from Phoebe in the
              J2000 frame (J2000), in kilometers.
 
         5.   The actual (geometric) distance between the Sun and Phoebe,
              in astronomical units.
 
     and displays the results. Use the program to compute these quantities
     at "2004 jun 11 19:32:00" UTC.
 
 
Learning Goals
--------------------------------------------------------
 
     Understand the anatomy of an SPKEZR call. Discover the difference
     between SPKEZR and SPKPOS. Familiarity with the Toolkit utility
     ``brief''. Exposure to unit conversion with SPICE.
 
 
Approach
--------------------------------------------------------
 
     The solution to the problem can be broken down into a series of simple
     steps:
 
         --   Decide which SPICE kernels are necessary. Prepare a
              meta-kernel listing the kernels and load it into the program.
 
         --   Prompt the user for an input time string.
 
         --   Convert the input time string into ephemeris time expressed
              as seconds past J2000 TDB.
 
         --   Compute the state of Phoebe relative to CASSINI in the J2000
              reference frame, corrected for aberrations.
 
         --   Compute the position of Earth relative to CASSINI in the
              J2000 reference frame, corrected for aberrations. (The
              routine in the library that computes this also returns the
              one-way light time between CASSINI and Earth.)
 
         --   Compute the position of the Sun relative to Phoebe in the
              J2000 reference frame, corrected for aberrations.
 
         --   Compute the position of the Sun relative to Phoebe without
              correcting for aberration.
 
         --   Compute the length of this vector. This provides the desired
              distance in kilometers.
 
         --   Convert the distance in kilometers into AU.
 
     You may find it useful to consult the permuted index, the headers of
     various source modules, and the ``SPK Required Reading'' document.
 
     When deciding which SPK files to load, the Toolkit utility ``brief''
     may be of some use.
 
     ``brief'' is located in the ``toolkit/exe'' directory for FORTRAN
     toolkits. Consult its user's guide available in
     ``toolkit/doc/brief.ug'' for details.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
     The meta-kernel we created for the solution to this exercise is named
     'getsta.mk'. Its contents follow:
 
        KPL/MK
 
        This is the meta-kernel used in the solution of the
        ``Obtaining Target States and Positions'' task in the
        Remote Sensing Hands On Lesson.
 
           \begindata
           KERNELS_TO_LOAD = ( 'kernels/lsk/naif0007.tls',
                               'kernels/spk/sat128.bsp'
                               'kernels/spk/981005_PLTEPH-DE405S.bsp',
                               'kernels/spk/020514_SE_SAT105.bsp',
                               'kernels/spk/030201AP_SK_SM546_T45.bsp' )
           \begintext
 
 
Solution Source Code
 
     A sample solution to the problem follows:
 
              PROGRAM GETSTA
 
              IMPLICIT NONE
 
        C
        C     SPICELIB Functions
        C
              DOUBLE PRECISION      VNORM
 
        C
        C     Local Parameters
        C
        C
        C     The name of the meta-kernel that lists the kernels
        C     to load into the program.
        C
              CHARACTER*(*)         METAKR
              PARAMETER           ( METAKR = 'getsta.mk' )
 
        C
        C     The length of various string variables.
        C
              INTEGER               STRLEN
              PARAMETER           ( STRLEN = 50 )
 
        C
        C     Local Variables
        C
              CHARACTER*(STRLEN)    UTCTIM
 
              DOUBLE PRECISION      DIST
              DOUBLE PRECISION      ET
              DOUBLE PRECISION      LTIME
              DOUBLE PRECISION      POS   ( 3 )
              DOUBLE PRECISION      STATE ( 6 )
 
        C
        C     Load the kernels that this program requires.  We
        C     will need a leapseconds kernel to convert input
        C     UTC time strings into ET.  We also will need the
        C     necessary SPK files with coverage for the bodies
        C     in which we are interested.
        C
              CALL FURNSH ( METAKR )
 
        C
        C     Prompt the user for the input time string.
        C
              CALL PROMPT ( 'Input UTC Time: ', UTCTIM )
 
              WRITE (*,*) 'Converting UTC Time: ', UTCTIM
 
        C
        C     Convert UTCTIM to ET.
        C
              CALL STR2ET ( UTCTIM, ET )
 
              WRITE (*,'(A,F16.3)') '   ET Seconds Past 2000: ', ET
 
        C
        C     Compute the apparent state of Phoebe as seen from
        C     CASSINI in the J2000 frame.  All of the ephemeris
        C     readers return states in units of kilometers and
        C     kilometers per second.
        C
              CALL SPKEZR ( 'PHOEBE', ET,    'J2000', 'LT+S',
             .              'CASSINI',  STATE, LTIME               )
 
              WRITE (*,*) '   Apparent State of Phoebe as seen from '
             .//          'CASSINI in the J2000 frame'
              WRITE (*,*) '      (kilometers and kilometers per '
             .//          'second):'
 
              WRITE (*,'(A,F16.3)') '      X = ', STATE(1)
              WRITE (*,'(A,F16.3)') '      Y = ', STATE(2)
              WRITE (*,'(A,F16.3)') '      Z = ', STATE(3)
              WRITE (*,'(A,F16.3)') '     VX = ', STATE(4)
              WRITE (*,'(A,F16.3)') '     VY = ', STATE(5)
              WRITE (*,'(A,F16.3)') '     VZ = ', STATE(6)
 
        C
        C     Compute the apparent position of Earth as seen from
        C     CASSINI in the J2000 frame.  Note: We could have continued
        C     using SPKEZR and simply ignored the velocity components.
        C
              CALL SPKPOS ( 'EARTH', ET,  'J2000', 'LT+S',
             .              'CASSINI',   POS, LTIME               )
 
              WRITE (*,*) '   Apparent Position of Earth as seen from '
             .//          'CASSINI in the J2000'
              WRITE (*,*) '      frame (kilometers):'
 
              WRITE (*,'(A,F16.3)') '      X = ', POS(1)
              WRITE (*,'(A,F16.3)') '      Y = ', POS(2)
              WRITE (*,'(A,F16.3)') '      Z = ', POS(3)
 
        C
        C     We need only display LTIME, as it is precisely the light
        C     time in which we are interested.
        C
              WRITE (*,*) '   One way light time between CASSINI and the '
             .//          'apparent position'
              WRITE (*,'(A,F16.3)') '      of Earth (seconds): ', LTIME
 
        C
        C     Compute the apparent position of the Sun as seen from
        C     Phoebe in the J2000 frame.
        C
              CALL SPKPOS ( 'SUN',  ET,  'J2000', 'LT+S',
             .              'PHOEBE', POS, LTIME                    )
 
              WRITE (*,*) '   Apparent position of Sun as seen from '
             .//          'Phoebe in the'
              WRITE (*,*) '      J2000 frame (kilometers):'
 
              WRITE (*,'(A,F16.3)') '      X = ', POS(1)
              WRITE (*,'(A,F16.3)') '      Y = ', POS(2)
              WRITE (*,'(A,F16.3)') '      Z = ', POS(3)
 
        C
        C     Now we need to compute the actual distance between the Sun
        C     and Phoebe.  The above SPKPOS call gives us the apparent
        C     distance, so we need to adjust our aberration correction
        C     appropriately.
        C
              CALL SPKPOS ( 'SUN',  ET,  'J2000', 'NONE',
             .              'PHOEBE', POS, LTIME                  )
 
        C
        C     Compute the distance between the body centers in
        C     kilometers.
        C
              DIST = VNORM(POS)
 
        C
        C     Convert this value to AU using CONVRT.
        C
              CALL CONVRT ( DIST, 'KM', 'AU', DIST )
 
              WRITE (*,*) '   Actual distance between Sun and Phoebe body '
             .//          'centers: '
              WRITE (*,'(A,F16.3)') '      (AU):', DIST
 
              END
 
 
Solution Sample Output
 
     After compiling the program, execute it:
 
        Converting UTC Time: 2004 jun 11 19:32:00
           ET Seconds Past 2000:    140254384.185
            Apparent State of Phoebe as seen from CASSINI in the J2000 fram
               (kilometers and kilometers per second):
              X =         -119.921
              Y =         2194.139
              Z =          -57.639
             VX =           -5.980
             VY =           -2.119
             VZ =           -0.295
            Apparent Position of Earth as seen from CASSINI in the J2000
               frame (kilometers):
              X =    353019393.123
              Y =  -1328180352.140
              Z =   -568134171.697
            One way light time between CASSINI and the apparent position
              of Earth (seconds):         4960.427
            Apparent position of Sun as seen from Phoebe in the
               J2000 frame (kilometers):
              X =    376551465.272
              Y =  -1190495630.303
              Z =   -508438699.110
            Actual distance between Sun and Phoebe body centers:
              (AU):           9.012
 
 
Spacecraft Orientation and Reference Frames (xform)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
     Write a program that prompts the user for an input time string,
     computes the following at the epoch of interest:
 
         1.   The apparent state of Phoebe as seen from CASSINI in the
              IAU_PHOEBE body-fixed frame. This vector itself is not of any
              particular interest, but it is a useful intermediate quantity
              in some geometry calculations.
 
         2.   The angular separation between the apparent position of Earth
              as seen from CASSINI and the nominal boresight of the CASSINI
              high gain antenna.
 
     and displays the results. Use the program to compute these quantities
     at the epoch "2004 jun 11 19:32:00" UTC.
 
 
Learning Goals
--------------------------------------------------------
 
     Familiarity with the different types of kernels involved in chaining
     reference frames together, both inertial and non-inertial. Discover
     some of the matrix and vector math routines. Understand the difference
     between PXFORM and SXFORM.
 
 
Approach
--------------------------------------------------------
 
     The solution to the problem can be broken down into a series of simple
     steps:
 
         --   Decide which SPICE kernels are necessary. Prepare a
              meta-kernel listing the kernels and load it into the program.
 
         --   Prompt the user for an input time string.
 
         --   Convert the input time string into ephemeris time expressed
              as seconds past J2000 TDB.
 
         --   Compute the state of Phoebe relative to CASSINI in the J2000
              reference frame, corrected for aberrations.
 
         --   Compute the state transformation matrix from J2000 to
              IAU_PHOEBE at the epoch, adjusted for light time.
 
         --   Multiply the state of Phoebe relative to CASSINI in the J2000
              reference frame by the state transformation matrix computed
              in the previous step.
 
         --   Compute the position of Earth relative to CASSINI in the
              J2000 reference frame, corrected for aberrations.
 
         --   Determine what the nominal boresight of the CASSINI high gain
              antenna is by examining the frame kernel's content.
 
         --   Compute the rotation matrix from the CASSINI high gain
              antenna frame to J2000.
 
         --   Multiply the nominal boresight expressed in the CASSINI high
              gain antenna frame by the rotation matrix from the previous
              step.
 
         --   Compute the separation between the result of the previous
              step and the apparent position of the Earth relative to
              CASSINI in the J2000 frame.
 
     HINT: Several of the steps above may be compressed into a single using
     SPICE routines with which you are already familiar. The ``long-way''
     presented above is intended to facilitate the introduction of the
     routines PXFORM and SXFORM.
 
     You may find it useful to consult the permuted index, the headers of
     various source modules, and the following toolkit documentation:
 
         1.   Frames Required Reading
 
         2.   PCK Required Reading
 
         3.   SPK Required Reading
 
         4.   CK Required Reading
 
     This particular example makes use of many of the different types of
     SPICE kernels. You should spend a few moments thinking about which
     kernels you will need and what data they provide.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
     The meta-kernel we created for the solution to this exercise is named
     'xform.mk'. Its contents follow:
 
        KPL/MK
 
        This is the meta-kernel used in the solution of the ``Spacecraft
        Orientation and Reference Frames'' task in the Remote Sensing
        Hands On Lesson.
 
           \begindata
           KERNELS_TO_LOAD = ( 'kernels/lsk/naif0007.tls',
                               'kernels/sclk/cas00084.tsc',
                               'kernels/spk/sat128.bsp'
                               'kernels/spk/981005_PLTEPH-DE405S.bsp',
                               'kernels/spk/020514_SE_SAT105.bsp',
                               'kernels/spk/030201AP_SK_SM546_T45.bsp',
                               'kernels/fk/cas_v37.tf',
                               'kernels/ck/04135_04171pc_psiv2.bc',
                               'kernels/pck/cpck05Mar2004.tpc' )
           \begintext
 
 
Solution Source Code
 
     A sample solution to the problem follows:
 
              PROGRAM XFORM
 
              IMPLICIT NONE
 
        C
        C     SPICELIB Functions
        C
              DOUBLE PRECISION      VSEP
 
        C
        C     Local Parameters
        C
        C
        C     The name of the meta-kernel that lists the kernels
        C     to load into the program.
        C
              CHARACTER*(*)         METAKR
              PARAMETER           ( METAKR = 'xform.mk' )
 
        C
        C     The length of various string variables.
        C
              INTEGER               STRLEN
              PARAMETER           ( STRLEN = 50 )
 
        C
        C     Local Variables
        C
              CHARACTER*(STRLEN)    UTCTIM
 
              DOUBLE PRECISION      ET
              DOUBLE PRECISION      LTIME
              DOUBLE PRECISION      STATE  ( 6 )
              DOUBLE PRECISION      BFIXST ( 6 )
              DOUBLE PRECISION      POS    ( 3 )
              DOUBLE PRECISION      SFORM  ( 6, 6 )
              DOUBLE PRECISION      PFORM  ( 3, 3 )
              DOUBLE PRECISION      BSIGHT ( 3 )
              DOUBLE PRECISION      SEP
 
        C
        C     Load the kernels that this program requires.  We
        C     will need:
        C
        C        A leapseconds kernel
        C        A spacecraft clock kernel for CASSINI
        C        The necessary ephemerides
        C        A planetary constants file (PCK)
        C        A spacecraft orientation kernel for CASSINI (CK)
        C        A frame kernel (TF)
        C
              CALL FURNSH ( METAKR )
 
        C
        C     Prompt the user for the input time string.
        C
              CALL PROMPT ( 'Input UTC Time: ', UTCTIM )
 
              WRITE (*,*) 'Converting UTC Time: ', UTCTIM
 
        C
        C     Convert UTCTIM to ET.
        C
              CALL STR2ET ( UTCTIM, ET )
 
              WRITE (*,'(A,F16.3)') '   ET Seconds Past 2000: ', ET
 
        C
        C     Compute the apparent state of Phoebe as seen from CASSINI
        C     in the J2000 reference frame.
        C
              CALL SPKEZR ( 'PHOEBE', ET,    'J2000', 'LT+S',
             .              'CASSINI',  STATE, LTIME               )
 
        C
        C     Now obtain the transformation from the inertial
        C     J2000 frame to the non-inertial body-fixed IAU_PHOEBE
        C     frame.  Since we want the apparent position, we need to
        C     subtract LTIME from ET.
        C
              CALL SXFORM ( 'J2000', 'IAU_PHOEBE', ET-LTIME, SFORM )
 
        C
        C     Now rotate the apparent J200) state into IAU_PHOEBE
        C     with the following matrix multiplication:
        C
              CALL MXVG ( SFORM, STATE, 6, 6, BFIXST )
 
        C
        C     Display the results.
        C
              WRITE (*,*) '   Apparent state of Phoebe as seen from '
             .//          'CASSINI in the IAU_PHOEBE'
              WRITE (*,*) '      body-fixed frame (kilometers and '
             .//          'kilometers per'
              WRITE (*,*) '      second):'
              WRITE (*,'(A,F19.6)') '      X = ', BFIXST(1)
              WRITE (*,'(A,F19.6)') '      Y = ', BFIXST(2)
              WRITE (*,'(A,F19.6)') '      Z = ', BFIXST(3)
              WRITE (*,'(A,F19.6)') '     VX = ', BFIXST(4)
              WRITE (*,'(A,F19.6)') '     VY = ', BFIXST(5)
              WRITE (*,'(A,F19.6)') '     VZ = ', BFIXST(6)
 
        C
        C     It is worth pointing out, all of the above could have
        C     been done with a single call to SPKEZR:
        C
              CALL SPKEZR ( 'PHOEBE', ET,    'IAU_PHOEBE', 'LT+S',
             .              'CASSINI',  STATE, LTIME               )
 
        C
        C     Display the results.
        C
              WRITE (*,*) '   Apparent state of Phoebe as seen from CASSINI
             .//          'in the IAU_PHOEBE'
              WRITE (*,*) '      body-fixed frame (kilometers and '
             .//          'kilometers per'
              WRITE (*,*) '      second) obtained using SPKEZR '
             .//          'directly:'
              WRITE (*,'(A,F19.6)') '      X = ', STATE(1)
              WRITE (*,'(A,F19.6)') '      Y = ', STATE(2)
              WRITE (*,'(A,F19.6)') '      Z = ', STATE(3)
              WRITE (*,'(A,F19.6)') '     VX = ', STATE(4)
              WRITE (*,'(A,F19.6)') '     VY = ', STATE(5)
              WRITE (*,'(A,F19.6)') '     VZ = ', STATE(6)
 
        C
        C     Now we are to compute the angular separation between
        C     the apparent position of the Earth as seen from the
        C     orbiter and the nominal boresight of the high gain
        C     antenna.  First, compute the apparent position of
        C     the Earth as seen from CASSINI in the J2000 frame.
        C
              CALL SPKPOS ( 'EARTH', ET,  'J2000', 'LT+S',
             .              'CASSINI',   POS, LTIME               )
 
        C
        C     Now compute the location of the antenna boresight
        C     at this same epoch.  From reading the frame kernel
        C     we know that the antenna boresight is nominally the
        C     +Z axis of the CASSINI_HGA frame defined there.
        C
              BSIGHT(1) = 0.0D0
              BSIGHT(2) = 0.0D0
              BSIGHT(3) = 1.0D0
 
        C
        C     Now compute the rotation matrix from CASSINI_HGA into
        C     J2000.
        C
              CALL PXFORM ( 'CASSINI_HGA', 'J2000', ET, PFORM )
 
        C
        C     And multiply the result to obtain the nominal antenna
        C     boresight in the J2000 reference frame.
        C
              CALL MXV ( PFORM, BSIGHT, BSIGHT )
 
        C
        C     Lastly compute the angular separation.
        C
              CALL CONVRT ( VSEP(BSIGHT, POS), 'RADIANS',
             .              'DEGREES',         SEP        )
 
              WRITE (*,*) '   Angular separation between the '
             .//          'apparent position of'
              WRITE (*,*) '      Earth and the CASSINI high '
             .//          'gain antenna boresight (degrees): '
              WRITE (*,'(A,F19.3)') '      ', SEP
 
        C
        C     Or, alternately we can work in the antenna
        C     frame directly.
        C
              CALL SPKPOS ( 'EARTH', ET,  'CASSINI_HGA', 'LT+S',
             .              'CASSINI',   POS, LTIME              )
 
        C
        C     The antenna boresight is the Z-axis in the
        C     CASSINI_HGA frame.
        C
              BSIGHT(1) = 0.0D0
              BSIGHT(2) = 0.0D0
              BSIGHT(3) = 1.0D0
 
        C
        C     Lastly compute the angular separation.
        C
              CALL CONVRT ( VSEP(BSIGHT, POS), 'RADIANS',
             .              'DEGREES',         SEP        )
 
              WRITE (*,*) '   Angular separation between the '
             .//          'apparent position of'
              WRITE (*,*) '      Earth and the CASSINI high gain '
             .//          'antenna boresight computed '
              WRITE (*,*) '      using vectors in the CASSINI_HGA '
             .//          'frame (degrees): '
              WRITE (*,'(A,F19.3)') '      ', SEP
 
              END
 
 
Solution Sample Output
 
     After compiling the program, execute it:
 
        Converting UTC Time: 2004 jun 11 19:32:00
           ET Seconds Past 2000:    140254384.185
            Apparent state of Phoebe as seen from CASSINI in the IAU_PHOEBE
               body-fixed frame (kilometers and kilometers per
               second):
              X =        -1982.639762
              Y =         -934.530471
              Z =         -166.562595
             VX =            3.970729
             VY =           -3.812531
             VZ =           -2.371665
            Apparent state of Phoebe as seen from CASSINI in the IAU_PHOEBE
               body-fixed frame (kilometers and kilometers per
               second) obtained using SPKEZR directly:
              X =        -1982.639762
              Y =         -934.530471
              Z =         -166.562595
             VX =            3.970729
             VY =           -3.812531
             VZ =           -2.371665
            Angular separation between the apparent position of
               Earth and the CASSINI high gain antenna boresight (degrees):
                           71.924
            Angular separation between the apparent position of
               Earth and the CASSINI high gain antenna boresight computed
               using vectors in the CASSINI_HGA frame (degrees):
                           71.924
 
 
Computing Sub-spacecraft and Sub-solar Points (subpts)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
     Write a program that prompts the user for an input UTC time string,
     computes the following quantities at that epoch:
 
         1.   The apparent sub-observer point of CASSINI on Phoebe in the
              body fixed frame IAU_PHOEBE in kilometers.
 
         2.   The apparent sub-solar point on Phoebe as seen from CASSINI
              in the body fixed frame IAU_PHOEBE in kilometers.
 
     and displays the results. Use the program to compute these quantities
     at "2004 jun 11 19:32:00" UTC.
 
 
Learning Goals
--------------------------------------------------------
 
     Discover higher level geometry calculation routines in SPICE and their
     usage as it relates to CASSINI.
 
 
Approach
--------------------------------------------------------
 
     This particular problem is more of an exercise in searching the
     permuted index to find the appropriate routines and then reading their
     headers to understand how to call them.
 
     One point worth considering: Which method do you want to use to
     compute the sub-solar (or sub-observer) point?
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
     The meta-kernel we created for the solution to this exercise is named
     'subpts.mk'. Its contents follow:
 
        KPL/MK
 
        This is the meta-kernel used in the solution of the
        ``Computing Sub-spacecraft and Sub-solar Points'' task
        in the Remote Sensing Hands On Lesson.
 
           \begindata
           KERNELS_TO_LOAD = ( 'kernels/lsk/naif0007.tls',
                               'kernels/spk/sat128.bsp'
                               'kernels/spk/981005_PLTEPH-DE405S.bsp',
                               'kernels/spk/020514_SE_SAT105.bsp',
                               'kernels/spk/030201AP_SK_SM546_T45.bsp',
                               'kernels/pck/cpck05Mar2004.tpc' )
           \begintext
 
 
Solution Source Code
 
     A sample solution to the problem follows:
 
              PROGRAM SUBPTS
 
              IMPLICIT NONE
        C
        C     Local Parameters
        C
        C
        C     The name of the meta-kernel that lists the kernels
        C     to load into the program.
        C
              CHARACTER*(*)         METAKR
              PARAMETER           ( METAKR = 'subpts.mk' )
 
        C
        C     The length of various string variables.
        C
              INTEGER               STRLEN
              PARAMETER           ( STRLEN = 50 )
 
        C
        C     Local Variables
        C
              CHARACTER*(STRLEN)    UTCTIM
 
              DOUBLE PRECISION      ALT
              DOUBLE PRECISION      ET
              DOUBLE PRECISION      SPOINT ( 3 )
 
        C
        C     Load the kernels that this program requires.  We
        C     will need:
        C
        C        A leapseconds kernel
        C        The necessary ephemerides
        C        A planetary constants file (PCK)
        C
              CALL FURNSH ( METAKR )
 
        C
        C     Prompt the user for the input time string.
        C
              CALL PROMPT ( 'Input UTC Time: ', UTCTIM )
 
              WRITE (*,*) 'Converting UTC Time: ', UTCTIM
 
        C
        C     Convert UTCTIM to ET.
        C
              CALL STR2ET ( UTCTIM, ET )
 
              WRITE (*,'(A,F16.3)') '   ET Seconds Past 2000: ', ET
 
        C
        C     Compute the apparent sub-observer point of CASSINI on Phoebe.
        C
              CALL SUBPT ( 'NEAR POINT', 'PHOEBE', ET,  'LT+S',
             .             'CASSINI',        SPOINT, ALT          )
 
              WRITE (*,*) '   Apparent Sub-Observer point of CASSINI '
             .//          'on Phoebe in IAU_PHOEBE'
              WRITE (*,*) '      (kilometers):'
              WRITE (*,'(A,F16.3)') '      X = ', SPOINT(1)
              WRITE (*,'(A,F16.3)') '      Y = ', SPOINT(2)
              WRITE (*,'(A,F16.3)') '      Z = ', SPOINT(3)
              WRITE (*,'(A,F16.3)') '    ALT = ', ALT
 
        C
        C     Compute the apparent sub-solar point on Phoebe as seen
        C     from CASSINI.
        C
              CALL SUBSOL ( 'NEAR POINT', 'PHOEBE', ET, 'LT+S',
             .              'CASSINI', SPOINT                     )
 
              WRITE (*,*) '   Apparent Sub-Solar point on Phoebe as '
             .//          'seen from CASSINI in IAU_PHOEBE'
              WRITE (*,*) '      (kilometers):'
              WRITE (*,'(A,F16.3)') '      X = ', SPOINT(1)
              WRITE (*,'(A,F16.3)') '      Y = ', SPOINT(2)
              WRITE (*,'(A,F16.3)') '      Z = ', SPOINT(3)
 
              END
 
 
Solution Sample Output
 
     After compiling the program, execute it:
 
        Converting UTC Time: 2004 jun 11 19:32:00
           ET Seconds Past 2000:    140254384.185
            Apparent Sub-Observer point of CASSINI on Phoebe in IAU_PHOEBE
               (kilometers):
              X =          104.498
              Y =           45.269
              Z =            7.383
            ALT =         2084.116
            Apparent Sub-Solar point on Phoebe as seen from CASSINI in IAU_
               (kilometers):
              X =           78.681
              Y =           76.879
              Z =          -21.885
 
 
Intersecting Vectors with a Triaxial Ellipsoid (fovint)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
     Write a program that prompts the user for an input UTC time string and
     computes the intersection of the CASSINI ISS NAC camera boresight with
     the surface of Phoebe and presents it in the following coordinates:
 
         1.   Cartesian vector in the IAU_PHOEBE frame
 
         2.   Planetocentric (latitudinal)
 
     If this intersection is found, the program displays the results of the
     above computations, otherwise it indicates no intersection has
     occurred. Use this program to compute values at the following epochs:
 
         1.   2004 jun 11 19:32:00 UTC
 
 
Learning Goals
--------------------------------------------------------
 
     Understand how field of view parameters are retrieved from instrument
     kernels. Learn how various standard planetary constants are retrieved
     from text PCKs. Discover how to compute the intersection of field of
     view vectors with triaxial ellipsoidal target bodies.
 
 
Approach
--------------------------------------------------------
 
     This problem can be broken down into several simple, small steps:
 
         --   Decide which SPICE kernels are necessary. Prepare a
              meta-kernel listing the kernels and load it into the program.
              Remember, you will need to find a kernel with information
              about the CASSINI NAC camera.
 
         --   Prompt the user for an input time string.
 
         --   Convert the input time string into ephemeris time expressed
              as seconds past J2000 TDB.
 
         --   Retrieve the field of view configuration for the CASSINI ISS
              NAC camera.
 
         --   Determine if an intercept of the camera boresight and Phoebe
              exists.
 
         --   Convert the position vector of the intercept into
              planetocentric coordinates.
 
     It may be useful to consult the CASSINI ISS instrument kernel to
     determine the name of the NAC camera as well as its configuration.
     This exercise may make use of some of the concepts and (loosely) code
     from the ``Spacecraft Orientation and Reference Frames'' task.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
     The meta-kernel we created for the solution to this exercise is named
     'fovint.mk'. Its contents follow:
 
        KPL/MK
 
        This is the meta-kernel used in the solution of the
        ``Intersecting Vectors with a Triaxial Ellipsoid'' task
        in the Remote Sensing Hands On Lesson.
 
           \begindata
           KERNELS_TO_LOAD = ( 'kernels/lsk/naif0007.tls',
                               'kernels/sclk/cas00084.tsc',
                               'kernels/spk/sat128.bsp'
                               'kernels/spk/981005_PLTEPH-DE405S.bsp',
                               'kernels/spk/020514_SE_SAT105.bsp',
                               'kernels/spk/030201AP_SK_SM546_T45.bsp',
                               'kernels/fk/cas_v37.tf',
                               'kernels/ck/04135_04171pc_psiv2.bc',
                               'kernels/pck/cpck05Mar2004.tpc',
                               'kernels/ik/cas_iss_v09.ti' )
           \begintext
 
 
Solution Source Code
 
     A sample solution to the problem follows:
 
              PROGRAM FOVINT
 
              IMPLICIT NONE
 
        C
        C     Local Parameters
        C
        C
        C     The name of the meta-kernel that lists the kernels
        C     to load into the program.
        C
              CHARACTER*(*)         METAKR
              PARAMETER           ( METAKR = 'fovint.mk' )
 
        C
        C     The length of various string variables.
        C
              INTEGER               STRLEN
              PARAMETER           ( STRLEN = 50 )
 
        C
        C     The maximum number of boundary corner vectors
        C     we can retrieve.
        C
              INTEGER               BCVLEN
              PARAMETER           ( BCVLEN = 4 )
 
        C
        C     Local Variables
        C
              CHARACTER*(STRLEN)    FRAME
              CHARACTER*(STRLEN)    SHAPE
              CHARACTER*(STRLEN)    UTCTIM
 
              DOUBLE PRECISION      BOUNDS ( 3, BCVLEN )
              DOUBLE PRECISION      BSIGHT ( 3 )
              DOUBLE PRECISION      DIST
              DOUBLE PRECISION      ET
              DOUBLE PRECISION      LAT
              DOUBLE PRECISION      LON
              DOUBLE PRECISION      OBSPOS ( 3 )
              DOUBLE PRECISION      POINT  ( 3 )
              DOUBLE PRECISION      RADIUS
              DOUBLE PRECISION      TRGEPC
 
              INTEGER               N
              INTEGER               NACID
 
              LOGICAL               FOUND
 
        C
        C     SPICELIB functions
        C
              DOUBLE PRECISION      DPR
 
        C
        C     Load the kernels that this program requires.  We
        C     will need:
        C
        C        A leapseconds kernel.
        C        A SCLK kernel for CASSINI.
        C        Any necessary ephemerides.
        C        The CASSINI frame kernel.
        C        A CASSINI C-kernel.
        C        A PCK file with Phoebe constants.
        C        The CASSINI ISS I-kernel.
        C
              CALL FURNSH ( METAKR )
 
        C
        C     Prompt the user for the input time string.
        C
              CALL PROMPT ( 'Input UTC Time: ', UTCTIM )
 
              WRITE (*,*) 'Converting UTC Time: ', UTCTIM
 
        C
        C     Convert UTCTIM to ET.
        C
              CALL STR2ET ( UTCTIM, ET )
 
              WRITE (*,'(A,F16.3)') '   ET Seconds Past 2000: ', ET
 
        C
        C     Now we need to obtain the FOV configuration of the NAC
        C     camera. To do this we will need the ID code for
        C     CASSINI_ISS_NAC.
        C
              CALL BODN2C ( 'CASSINI_ISS_NAC', NACID, FOUND )
 
        C
        C     Stop the program if the code was not found.
        C
              IF ( .NOT. FOUND ) THEN
                 WRITE (*,*) 'Unable to locate the ID code for '
             .   //          'CASSINI_ISS_NAC'
                 CALL BYEBYE ( 'FAILURE' )
              END IF
 
        C
        C     Now retrieve the field of view parameters.
        C
              CALL GETFOV ( NACID,  BCVLEN, SHAPE, FRAME,
             .              BSIGHT, N,      BOUNDS        )
 
        C
        C     Call SRFXPT to determine coordinates of the
        C     intersection of the NAC boresight with the surface
        C     of Phoebe.
        C
              CALL SRFXPT ( 'Ellipsoid', 'PHOEBE', ET, 'LT+S',
             .              'CASSINI', FRAME, BSIGHT, POINT,
             .              DIST, TRGEPC, OBSPOS, FOUND )
 
        C
        C     Check the found flag.  Display a message if the point
        C     of intersection was not found and stop.
        C
              IF ( .NOT. FOUND ) THEN
                  WRITE (*,*) 'No intersection point found at this '
             .    //          'epoch.'
                  CALL BYEBYE ( 'SUCCESS' )
              END IF
 
        C
        C     Now, we have discovered a point of intersection.
        C     Start by displaying the position vector in the
        C     IAU_PHOEBE frame of the intersection.
        C
              WRITE (*,*) '   Position vector of CASSINI NA camera '
             .//          'boresight surface intercept '
              WRITE (*,'(A,F16.3)') '      in the IAU_PHOEBE frame (km):'
              WRITE (*,'(A,F16.3)') '      X = ', POINT(1)
              WRITE (*,'(A,F16.3)') '      Y = ', POINT(2)
              WRITE (*,'(A,F16.3)') '      Z = ', POINT(3)
 
        C
        C     Now express the coordinates of this point in
        C     planetocentric latitude and longitude.
        C
              CALL RECLAT ( POINT, RADIUS, LON, LAT )
 
        C
        C     Convert the angles to degrees for displaying.
        C
              WRITE (*,*) '   Planetocentric coordinates of the '
             .//          'intercept (degrees):'
              WRITE (*,'(A,F16.3)') '    LAT = ', LAT * DPR()
              WRITE (*,'(A,F16.3)') '    LON = ', LON * DPR()
 
 
              END
 
 
Solution Sample Output
 
     After compiling the program, execute it:
 
        Converting UTC Time: 2004 jun 11 19:32:00
           ET Seconds Past 2000:    140254384.185
            Position vector of CASSINI NA camera boresight surface intercep
              in the IAU_PHOEBE frame (km):
              X =           86.390
              Y =           72.089
              Z =            8.255
            Planetocentric coordinates of the intercept (degrees):
            LAT =            4.196
            LON =           39.844
 
 
Computing Illumination Angles and Local Time (angles)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
     Write a program that prompts the user for an input time string and
     computes the intersection of the CASSINI NAC camera boresight and
     field of view boundary vectors with the surface of Phoebe. At these
     points of intersection, if they exist, compute the following:
 
         1.   Phase angle
 
         2.   Solar incidence angle
 
         3.   Emission angle
 
     Additionally compute the local solar time at the intercept of the
     camera boresight with the surface of Phoebe.
 
     Display the results of the above computations if an intersection
     occurs, otherwise indicate the absence of an intersection. Use this
     program to compute values at the epoch "2004-01-12T4:15.24.000" UTC.
 
 
Learning Goals
--------------------------------------------------------
 
     Discover another high level geometry routine and another time
     conversion routine in SPICE. Reinforce the concepts introduced in the
     previous task.
 
 
Approach
--------------------------------------------------------
 
     Making use of the code you wrote for the previous task is probably the
     fastest means to an end. A significant percentage of the task is
     devoted to similar computations.
 
     This problem can be broken down into several steps:
 
         --   Decide which SPICE kernels are necessary. Prepare a
              meta-kernel listing these kernels and load it into the
              program.
 
         --   Prompt the user for an input time string.
 
         --   Convert the input time string into ephemeris time expressed
              as seconds past J2000 TDB.
 
         --   Retrieve the FOV (field of view) configuration for the
              CASSINI NAC camera.
 
     For each vector in the set of boundary corner vectors, and for the
     boresight vector, perform the following operations:
 
         --   Compute the intercept of the vector with Phoebe.
 
         --   If this intercept is found, then compute the phase, solar
              incidence, and emission angles. Otherwise indicate to the
              user no intercept was found for this vector.
 
     At this point, if a boresight intercept was located, then proceed.
 
         --   Compute the planetocentric longitude of the boresight
              intercept.
 
         --   Compute the local solar time at this longitude on a 24-hour
              clock.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
     The meta-kernel we created for the solution to this exercise is named
     'angles.mk'. Its contents follow:
 
        KPL/MK
        This is the meta-kernel used in the solution of the
        ``Computing Illumination Angles and Local Time'' task
        in the Remote Sensing Hands On Lesson.
 
           \begindata
           KERNELS_TO_LOAD = ( 'kernels/lsk/naif0007.tls',
                               'kernels/sclk/cas00084.tsc',
                               'kernels/spk/sat128.bsp'
                               'kernels/spk/981005_PLTEPH-DE405S.bsp',
                               'kernels/spk/020514_SE_SAT105.bsp',
                               'kernels/spk/030201AP_SK_SM546_T45.bsp',
                               'kernels/fk/cas_v37.tf',
                               'kernels/ck/04135_04171pc_psiv2.bc',
                               'kernels/pck/cpck05Mar2004.tpc',
                               'kernels/ik/cas_iss_v09.ti' )
           \begintext
 
 
Solution Source Code
 
     A sample solution to the problem follows:
 
              PROGRAM ANGLES
 
              IMPLICIT NONE
 
        C
        C     Local Parameters
        C
        C
        C     The name of the meta-kernel that lists the kernels
        C     to load into the program.
        C
              CHARACTER*(*)         METAKR
              PARAMETER           ( METAKR = 'angles.mk' )
 
        C
        C     The length of various string variables.
        C
              INTEGER               STRLEN
              PARAMETER           ( STRLEN = 50 )
 
        C
        C     The maximum number of boundary corner vectors
        C     we can retrieve.
        C
              INTEGER               BCVLEN
              PARAMETER           ( BCVLEN = 5 )
 
        C
        C     Local Variables
        C
              CHARACTER*(STRLEN)    AMPM
              CHARACTER*(STRLEN)    FRAME
              CHARACTER*(STRLEN)    SHAPE
              CHARACTER*(STRLEN)    TIME
              CHARACTER*(STRLEN)    UTCTIM
              CHARACTER*(STRLEN)    VECNAM ( BCVLEN )
 
              DOUBLE PRECISION      BOUNDS ( 3, BCVLEN )
              DOUBLE PRECISION      BSIGHT ( 3 )
              DOUBLE PRECISION      DIST
              DOUBLE PRECISION      EMISSN
              DOUBLE PRECISION      ET
              DOUBLE PRECISION      LAT
              DOUBLE PRECISION      LON
              DOUBLE PRECISION      OBSPOS ( 3 )
              DOUBLE PRECISION      PHASE
              DOUBLE PRECISION      POINT  ( 3 )
              DOUBLE PRECISION      RADIUS
              DOUBLE PRECISION      SOLAR
              DOUBLE PRECISION      TRGEPC
 
              INTEGER               HR
              INTEGER               I
              INTEGER               PHOEID
              INTEGER               MN
              INTEGER               N
              INTEGER               SC
              INTEGER               NACID
 
              LOGICAL               FOUND
 
        C
        C     SPICELIB functions
        C
              DOUBLE PRECISION      DPR
 
        C
        C     Load the kernels that this program requires.  We
        C     will need:
        C
        C        A leapseconds kernel.
        C        A SCLK kernel for CASSINI.
        C        Any necessary ephemerides.
        C        The CASSINI frame kernel.
        C        A CASSINI C-kernel.
        C        A PCK file with Phoebe constants.
        C        The CASSINI ISS I-kernel.
        C
              CALL FURNSH ( METAKR )
 
        C
        C     Prompt the user for the input time string.
        C
              CALL PROMPT ( 'Input UTC Time: ', UTCTIM )
 
              WRITE (*,*) 'Converting UTC Time: ', UTCTIM
 
        C
        C     Convert UTCTIM to ET.
        C
              CALL STR2ET ( UTCTIM, ET )
 
              WRITE (*,'(A,F16.3)') '   ET Seconds Past 2000: ', ET
 
        C
        C     Now we need to obtain the FOV configuration of the NAC
        C     camera. To do this we will need the ID code for
        C     CASSINI_ISS_NAC.
        C
              CALL BODN2C ( 'CASSINI_ISS_NAC', NACID, FOUND )
 
        C
        C     Stop the program if the code was not found.
        C
              IF ( .NOT. FOUND ) THEN
                 WRITE (*,*) 'Unable to locate the ID code for '
             .   //          'CASSINI_ISS_NAC'
                 CALL BYEBYE ( 'FAILURE' )
              END IF
 
        C
        C     Now retrieve the field of view parameters.
        C
              CALL GETFOV ( NACID,  BCVLEN, SHAPE, FRAME,
             .              BSIGHT, N,      BOUNDS        )
 
        C
        C     Rather than treat BSIGHT as a separate vector,
        C     copy it into the last slot of BOUNDS.
        C
              CALL MOVED ( BSIGHT, 3, BOUNDS(1,5) )
 
        C
        C     Define names for each of the vectors for display
        C     purposes.
        C
              VECNAM (1) = 'Boundary Corner 1'
              VECNAM (2) = 'Boundary Corner 2'
              VECNAM (3) = 'Boundary Corner 3'
              VECNAM (4) = 'Boundary Corner 4'
              VECNAM (5) = 'Boresight'
 
        C
        C     Now perform the same set of calculations for each
        C     vector listed in the BOUNDS array.
        C
              DO I = 1, 5
 
        C
        C        Call SRFXPT to determine coordinates of the
        C        intersection of this vector with the surface
        C        of Phoebe.
        C
                 CALL SRFXPT ( 'Ellipsoid', 'PHOEBE', ET, 'LT+S',
             .                 'CASSINI', FRAME, BOUNDS(1,I), POINT,
             .                 DIST, TRGEPC, OBSPOS, FOUND )
 
        C
        C        Check the found flag.  Display a message if the point
        C        of intersection was not found, otherwise continue with
        C        the calculations.
        C
                 WRITE (*,*) 'Vector: ', VECNAM(I)
 
                 IF ( .NOT. FOUND ) THEN
 
                    WRITE (*,*) 'No intersection point found at '
             .      //          'this epoch for this vector.'
 
                 ELSE
 
        C
        C           Display the planetocentric latitude and longitude
        C           of the intercept.
        C
                    CALL RECLAT ( POINT, RADIUS, LON, LAT )
 
                    WRITE (*,*) '   Planetocentric coordinates of the '
             .      //          'intercept (degrees):'
                    WRITE (*,'(A,F16.3)') '    LAT = ', LAT * DPR()
                    WRITE (*,'(A,F16.3)') '    LON = ', LON * DPR()
 
        C
        C           Compute the illumination angles at this
        C           point.
        C
                    CALL ILLUM ( 'PHOEBE', ET,    'LT+S', 'CASSINI',
             .                   POINT,  PHASE, SOLAR,  EMISSN  )
 
                    WRITE (*,'(A,F16.3)') '   Phase angle (degrees):'
             .      //                    '           ', PHASE * DPR()
                    WRITE (*,'(A,F16.3)') '   Solar incidence angle '
             .      //                    '(degrees): ', SOLAR * DPR()
                    WRITE (*,'(A,F16.3)') '   Emission angle (degree'
             .      //                    's):        ', EMISSN* DPR()
 
                 END IF
 
                 WRITE (*,*) ' '
 
              END DO
 
        C
        C     Lastly compute the local solar time at the boresight
        C     intersection.
        C
              IF ( FOUND ) THEN
 
        C
        C        Get Phoebe ID.
        C
                 CALL BODN2C ( 'PHOEBE', PHOEID, FOUND )
 
        C
        C        Stop the program if the code was not found.
        C
                 IF ( .NOT. FOUND ) THEN
                    WRITE (*,*) 'Unable to locate the ID code for '
             .   //             'PHOEBE'
                    CALL BYEBYE ( 'FAILURE' )
                 END IF
 
        C
        C        Compute local time.
        C
                 CALL ET2LST ( ET,
             .                 PHOEID,
             .                 LON,
             .                 'PLANETOCENTRIC',
             .                 HR,
             .                 MN,
             .                 SC,
             .                 TIME,
             .                 AMPM              )
 
                 WRITE (*,*) '   Local Solar Time at boresight '
             .   //          'intercept (24 Hour Clock): '
                 WRITE (*,*) '      ', TIME
 
              ELSE
 
                 WRITE (*,*) '   No boresight intercept to compute '
             .   //          'local solar time.'
 
              END IF
 
              END
 
 
Solution Sample Output
 
     After compiling the program, execute it:
 
        Converting UTC Time: 2004 jun 11 19:32:00
           ET Seconds Past 2000:    140254384.185
         Vector: Boundary Corner 1
            Planetocentric coordinates of the intercept (degrees):
            LAT =            1.028
            LON =           36.433
           Phase angle (degrees):                     28.110
           Solar incidence angle (degrees):           16.121
           Emission angle (degrees):                  14.627
 
         Vector: Boundary Corner 2
            Planetocentric coordinates of the intercept (degrees):
            LAT =            7.492
            LON =           36.556
           Phase angle (degrees):                     27.894
           Solar incidence angle (degrees):           22.894
           Emission angle (degrees):                  14.988
 
         Vector: Boundary Corner 3
            Planetocentric coordinates of the intercept (degrees):
            LAT =            7.373
            LON =           43.430
           Phase angle (degrees):                     28.171
           Solar incidence angle (degrees):           21.315
           Emission angle (degrees):                  21.977
 
         Vector: Boundary Corner 4
            Planetocentric coordinates of the intercept (degrees):
            LAT =            0.865
            LON =           43.239
           Phase angle (degrees):                     28.385
           Solar incidence angle (degrees):           13.882
           Emission angle (degrees):                  21.763
 
         Vector: Boresight
            Planetocentric coordinates of the intercept (degrees):
            LAT =            4.196
            LON =           39.844
           Phase angle (degrees):                     28.140
           Solar incidence angle (degrees):           18.247
           Emission angle (degrees):                  17.858
 
            Local Solar Time at boresight intercept (24 Hour Clock):
               11:31:50
 
