 
Remote Sensing Hands-On Lesson, using TGO (IDL)
===========================================================================
 
   May 21, 2018
 
 
Overview
--------------------------------------------------------
 
   In this lesson you will develop a series of simple programs that
   demonstrate the usage of Icy 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 spectrometer flying on the ExoMars-16 TGO spacecraft, but
   many of the concepts are easily extended and generalized to other
   scenarios.
 
 
Note About HTML Links
--------------------------------------------------------
 
   The HTML version of this lesson contains links pointing to various HTML
   documents provided with the Toolkit. All of these links are relative
   and, in order to function, require this document to be in a certain
   location in the Toolkit HTML documentation directory tree.
 
   In order for the links to be resolved, if not done already by installing
   the lessons package under the Toolkit's ``doc/html'' directory, create a
   subdirectory called ``lessons'' under the ``doc/html'' directory of the
   ``icy/'' tree and copy this document to that subdirectory before loading
   it into a Web browser.
 
 
References
--------------------------------------------------------
 
   This section lists SPICE documents referred to in this lesson.
 
   Of these documents, the ``Tutorials'' contains the highest level
   descriptions with the least number of details while the ``Required
   Reading'' documents contain much more detailed specifications. The most
   complete specifications are provided in the ``API Documentation''.
 
   In some cases the lesson explanations also refer to the information
   provided in the meta-data area of the kernels used in the lesson
   examples. It is especially true in case of the FK and IK files, which
   often contain comprehensive descriptions of the frames, instrument FOVs,
   etc. Since both the FK and IK are text kernels, the information provided
   in them can be viewed using any text editor, while the meta information
   provided in binary kernels---SPKs and CKs---can be viewed using
   ``commnt'' or ``spacit'' utility programs located in ``icy/exe'' of
   Toolkit installation tree.
 
 
Tutorials
 
   The following SPICE tutorials serve as references for 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
      DSK               Detailed Target Shape (Topography) Data
 
   These tutorials are available from the NAIF ftp server at JPL:
 
      http://naif.jpl.nasa.gov/naif/tutorials.html
 
 
Required Readings
 
   The Required Reading documents are provided with the Toolkit and are
   located under the ``icy/doc'' directory in the Icy installation tree.
 
      Name             Lesson steps/routines that it describes
      ---------------  -----------------------------------------
      ck.req           Obtaining spacecraft orientation data
      dsk.req          Obtaining detailed body shape data
      frames.req       Using reference frames
      naif_ids.req     Determining body ID codes
      pck.req          Obtaining planetary constants data
      sclk.req         SCLK time conversion
      spk.req          Obtaining ephemeris data
      time.req         Time conversion
      icy.req          The Icy API
 
 
The Permuted Index
 
   Another useful document distributed with the Toolkit is the permuted
   index. This is located under the ``icy/doc'' directory in the IDL
   installation tree.
 
   This text document provides a simple mechanism by which users can
   discover which Icy procedures perform functions of interest, as well as
   the names of the source files that contain these procedures.
 
 
 
Icy API Documentation
 
   An Icy routine's specification is found in the HTML API documentation
   page located under ``icy/doc/html/icy''.
 
   For example, the document
 
      icy/doc/html/icy/cspice_str2et.html
 
   describes the cspice_str2et routine.
 
 
Kernels Used
--------------------------------------------------------
 
   The following kernels are used in examples provided in this lesson:
 
      1.  Generic LSK:
 
             naif0012.tls
 
      2.  ExoMars-16 TGO SCLK:
 
             em16_tgo_step_20160414.tsc
 
      3.  Solar System Ephemeris SPK, subsetted to cover only the time
          range of interest:
 
             de430.bsp
 
      4.  Martian Satellite Ephemeris SPK, subsetted to cover only the time
          range of interest:
 
             mar085.bsp
 
      5.  ExoMars-16 TGO Spacecraft Trajectory SPK, subsetted to cover only
          the time range of interest:
 
             em16_tgo_mlt_20171205_20230115_v01.bsp
 
      6.  Generic PCK:
 
             pck00010.tpc
 
      7.  ExoMars-16 TGO FK:
 
             em16_tgo_v07.tf
 
      8.  ExoMars-16 TGO Spacecraft CK, subsetted to cover only the time
          range of interest:
 
             em16_tgo_sc_slt_npo_20171205_20230115_s20160414_v01.bc
 
      9.  Low-resolution Mars DSK:
 
             mars_lowres.bds
 
      10. NOMAD IK:
 
             em16_tgo_nomad_v02.ti
 
 
   These SPICE kernels are included in the lesson package available from
   the NAIF server at JPL:
 
      ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Lessons/
 
   In addition to these kernels, the extra credit exercises require the
   following kernels:
 
      11. Generic Jovian Satellite Ephemeris SPK:
 
             jup310_2018.bsp
 
      12. EDM lander FK:
 
             em16_edm_v00.tf
 
      13. EDM landing site SPK:
 
             em16_edm_sot_landing_site_20161020_21000101_v01.bsp
 
 
   These SPICE kernels are available from the NAIF server at JPL:
 
      https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/
      https://naif.jpl.nasa.gov/pub/naif/EXOMARS2016/kernels/
 
 
Icy Modules Used
--------------------------------------------------------
 
   This section provides a complete list of the procedures and kernels that
   are suggested for usage in each of the exercises in this lesson. (You
   may wish to not look at this list unless/until you ``get stuck'' while
   working on your own.)
 
      CHAPTER EXERCISE   FUNCTIONS      NON-VOID       KERNELS
      ------- ---------  -------------  -------------  ----------
         1    convtm     cspice_furnsh                 1,2
                         cspice_str2et
                         cspice_etcal
                         cspice_timout
                         cspice_sce2s
                         cspice_unload
 
              extra (*)  cspice_sct2e   cspice_unitim  1,2
                         cspice_et2utc
                         cspice_scs2e
 
         2    getsta     cspice_furnsh  cspice_vnorm   1,3-5
                         cspice_str2et
                         cspice_spkezr
                         cspice_spkpos
                         cspice_convrt
                         cspice_kclear
 
              extra (*)  cspice_unload                 1,3-6,11-13
                         cspice_bodn2c
 
         3    xform      cspice_furnsh  cspice_vsep    1-8
                         cspice_str2et
                         cspice_spkezr
                         cspice_sxform
                         cspice_spkpos
                         cspice_pxform
                         cspice_mxv
                         cspice_convrt
                         cspice_kclear
 
              extra (*)  cspice_unload                 1-8
 
         4    subpts     cspice_furnsh  cspice_vnorm   1,3-6,9
                         cspice_str2et
                         cspice_subpnt
                         cspice_subslr
                         cspice_unload
 
              extra (*)  cspice_kclear  cspice_dpr     1,3-6
                         cspice_reclat
                         cspice_bodvrd
                         cspice_recpgr
 
         5    fovint     cspice_furnsh  cspice_dpr     1-10
                         cspice_str2et
                         cspice_bodn2c
                         cspice_getfov
                         cspice_sincpt
                         cspice_reclat
                         cspice_illumf
                         cspice_et2lst
                         cspice_kclear
 
 
         (*) Additional APIs and kernels used in Extra Credit tasks.
 
   Refer to the Icy HTML API documentation pages located under
   ``icy/doc/html/icy'' for detailed interface specifications of these
   procedures.
 
 
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 "2018 JUN 11
   19:32:00" UTC into these alternate systems.
 
 
Learning Goals
--------------------------------------------------------
 
   Familiarity with the various time conversion and parsing procedures
   available in the Toolkit. Exposure to source code headers and their
   usage in learning to call procedures.
 
 
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'' (time.req) and
   ``SCLK Required Reading'' (sclk.req) documents.
 
   When completing the ``calendar format'' step above, consider using one
   of two possible methods: cspice_etcal or cspice_timout.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
   The meta-kernel we created for the solution to this exercise is named
   'convtm.tm'. 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.
 
         The names and contents of the kernels referenced by this
         meta-kernel are as follows:
 
            1. Generic LSK:
 
                  naif0012.tls
 
            2. ExoMars-16 TGO SCLK:
 
                  em16_tgo_step_20160414.tsc
 
 
      \begindata
 
       KERNELS_TO_LOAD = (
 
       'kernels/lsk/naif0012.tls',
       'kernels/sclk/em16_tgo_step_20160414.tsc'
 
                         )
 
      \begintext
 
 
Solution Source Code
 
   A sample solution to the problem follows:
 
      PRO convtm
 
         ;;
         ;; Local Parameters
         ;;
 
         METAKR = "convtm.tm"
         SCLKID = -143
         STRLEN = 50
         utctim = ''
 
         ;;
         ;; Load the kernels this program requires.
         ;; Both the spacecraft clock kernel and a
         ;; leapseconds kernel should be listed in
         ;; the meta-kernel.
         ;;
         cspice_furnsh, METAKR
 
         ;;
         ;; Prompt the user for the input time string.
         ;;
         read, utctim, PROMPT="Input UTC Time: "
 
         print, "Converting UTC Time: ", utctim
 
         ;;
         ;; Convert utctim to et.
         ;;
         cspice_str2et, utctim, et
 
         print, FORMAT="(A,F16.3)", "   ET Seconds Past J2000: ", et
 
         ;;
         ;; Now convert ET to a formal calendar time
         ;; string.  This can be accomplished in two
         ;; ways.
         ;;
         cspice_etcal, et, calet
 
         print, "   Calendar ET (cspice_etcal): ", calet
 
 
         ;;
         ;; Or use cspice_timout for finer control over the
         ;; output format.  The picture below was built
         ;; by examining the header of cspice_timout.
         ;;
         cspice_timout, et   , "YYYY-MON-DDTHR:MN:SC ::TDB", $
                       STRLEN, calet
 
         print, "   Calendar ET (cspice_timout): ", calet
 
         ;;
         ;; Convert ET to spacecraft clock time.
         ;;
         cspice_sce2s, SCLKID, et, sclkst
 
         print, "   Spacecraft Clock Time: ", sclkst
 
         cspice_unload, METAKR
 
      END
 
 
Solution Sample Output
 
   After compiling the program, execute it:
 
      Input UTC Time: 2018 JUN 11 19:32:00
      Converting UTC Time: 2018 JUN 11 19:32:00
         ET Seconds Past J2000:    582017589.185
         Calendar ET (cspice_etcal): 2018 JUN 11 19:33:09.184
         Calendar ET (cspice_timout): 2018-JUN-11T19:33:09
         Spacecraft Clock Time: 1/0070841719.26698
 
 
Extra Credit
--------------------------------------------------------
 
   In this ``extra credit'' section you will be presented with more complex
   tasks, aimed at improving your understanding of time conversions, the
   Toolkit routines that deal with them, and some common errors that may
   happen during the execution of these conversions.
 
   These ``extra credit'' tasks are provided as task statements, and unlike
   the regular tasks, no approach or solution source code is provided. In
   the next section, you will find the numeric solutions (when applicable)
   and answers to the questions asked in these tasks.
 
 
Task statements and questions
 
       1.   Extend your program to convert the input UTC time string to TDB
            Julian Date. Convert "2018 JUN 11 19:32:00" UTC.
 
       2.   Remove the LSK from the original meta-kernel and run your
            program again, using the same inputs as before. Has anything
            changed? Why?
 
       3.   Remove the SCLK from the original meta-kernel and run your
            program again, using the same inputs as before. Has anything
            changed? Why?
 
       4.   Modify your program to perform conversion of UTC or ephemeris
            time, to a spacecraft clock string using the NAIF ID for the
            ExoMars-16 TGO NOMAD LNO Nadir aperture. Convert "2018 JUN 11
            19:32:00" UTC.
 
       5.   Find the earliest UTC time that can be converted to ExoMars-16
            TGO spacecraft clock.
 
       6.   Extend your program to convert the spacecraft clock time
            obtained in the regular task back to UTC Time and present it in
            ISO calendar date format, with a resolution of milliseconds.
 
       7.   Examine the contents of the generic LSK and the ExoMars-16 TGO
            SCLK kernels. Can you understand and explain what you see?
 
 
Solutions and answers
 
       1.   Two methods exist in order to convert ephemeris time to Julian
            Date: cspice_unitim and cspice_timout. The difference between
            them is the type of output produced by each method.
            cspice_unitim returns the double precision value of an input
            epoch, while cspice_timout returns the string representation of
            the ephemeris time in Julian Date format (when picture input is
            set to 'JULIAND.######### ::TDB'). Refer to the routine header
            for further details. The solution for the requested input UTC
            string is:
 
         Julian Date TDB:   2458281.3146896
 
       2.   When running the original program without the LSK kernel, an
            error is produced:
 
      % CSPICE_STR2ET: SPICE(NOLEAPSECONDS): [str2et_c->STR2ET->TTRANS]
                        The variable that points to the leapseconds
                        (DELTET/DELTA_AT) could not be located in the
                        kernel pool.  It is likely that the leapseconds
                        kernel has not been loaded via the routine
                        FURNSH.
      % Execution halted at: CONVTM             32
         /home/bsemenov/tmp/lessons_180518/lessons/remote_sensing_tgo/co
        nvtm.pro
      %                      $MAIN$
 
            This error is triggered by cspice_str2et because the variable
            that points to the leapseconds is not present in the kernel
            pool and therefore the program lacks data required to perform
            the requested UTC to ephemeris time conversion.
 
            By default, SPICE will report, as a minimum, a short
            descriptive message and a expanded form of this short message
            where more details about the error are provided. If this error
            message is not sufficient for you to understand what has
            happened, you could go to the ``Exceptions'' section in the
            SPICELIB or CSPICE headers of the routine that has triggered
            the error and find out more information about the possible
            causes.
 
       3.   When running the original program without the SCLK kernel, an
            error is produced:
 
      % CSPICE_SCE2S: SPICE(KERNELVARNOTFOUND):
                       [sce2s_c->SCE2S->SCE2T->SCTYPE->SCLI01]
                       SCLK_DATA_TYPE_143 not found. Did you load the
                       SCLK kernel?
      % Execution halted at: CONVTM             63
         /home/bsemenov/tmp/lessons_180518/lessons/remote_sensing_tgo/co
        nvtm.pro
      %                      $MAIN$
 
            This error is triggered by cspice_sce2s. In this case the error
            message may not give you enough information to understand what
            has actually happened. Nevertheless, the expanded form of this
            short message clearly indicates that the SCLK kernel for the
            spacecraft ID -143 has not been loaded.
 
            The UTC string to ephemeris time conversion and the conversion
            of ephemeris time into a calendar format worked normally as
            these conversions only require the LSK kernel to be loaded.
 
       4.   The first thing you need to do is to find out what the NAIF ID
            is for the NOMAD LNO Nadir aperture. In order to do so, examine
            the ExoMars-16 TGO frames definitions kernel listed above and
            look for the ``TGO NAIF ID Codes -- Summary Section'' or for
            the ``TGO NAIF ID Codes -- Definitions'' and there, for the
            NAIF ID given to TGO_NOMAD_LNO_NAD (which is -143311). Then
            replace in your code the SCLK ID -143 with -143311. After
            compiling and executing the program using the original
            meta-kernel, you will be getting the same error as in the
            previous task. Despite the error being exactly the same, this
            case is different. Generally, spacecraft clocks are associated
            with the spacecraft ID and not with its payload, sensors or
            structures IDs. Therefore, in order to do conversions from/to
            spacecraft clock for payload, sensors or spacecraft structures,
            the spacecraft ID must be used.
 
            Note that this does not need to be true for all missions or
            payloads, as SPICE does not restrict the SCLKs to spacecraft
            IDs only. Please refer to your mission's SCLK kernels for
            particulars.
 
       5.   Use cspice_sct2e with the encoding of the TGO spacecraft clock
            time set to 0.0 ticks and convert the resulting ephemeris time
            to UTC using either cspice_timout or cspice_et2utc. The
            solution for the requested SCLK string is:
 
         Earliest UTC convertible to SCLK: 2016-03-13T21:34:13.194
 
       6.   Use cspice_scs2e with the SCLK string obtained in the
            computations performed in the regular tasks and convert the
            resulting ephemeris time to UTC using either cspice_et2utc,
            with 'ISOC' format and 3 digits precision, or using
            cspice_timout using the time picture 'YYYY-MM-DDTHR:MN:SC.###
            ::RND'. The solution of the requested conversion is:
 
         Spacecraft Clock Time:          1/0070841719.26698
         UTC time from spacecraft clock: 2018-06-11T19:32:00.000
 
 
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 Mars as seen from ExoMars-16 TGO 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 ExoMars-16 TGO
            in the J2000 frame, in kilometers.
 
       3.   The one-way light time between ExoMars-16 TGO and the apparent
            position of Earth, in seconds.
 
       4.   The apparent position of the Sun as seen from Mars in the J2000
            frame (J2000), in kilometers.
 
       5.   The actual (geometric) distance between the Sun and Mars, in
            astronomical units.
 
   and displays the results. Use the program to compute these quantities at
   "2018 JUN 11 19:32:00" UTC.
 
 
Learning Goals
--------------------------------------------------------
 
   Understand the anatomy of an cspice_spkezr call. Discover the difference
   between cspice_spkezr and cspice_spkpos. Familiarity with the Toolkit
   utility ``brief''. Exposure to unit conversion with Icy.
 
 
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 Mars relative to ExoMars-16 TGO in the
            J2000 reference frame, corrected for aberrations.
 
       --   Compute the position of Earth relative to ExoMars-16 TGO in the
            J2000 reference frame, corrected for aberrations. (The
            procedure in the library that computes this also returns the
            one-way light time between ExoMars-16 TGO and Earth.)
 
       --   Compute the position of the Sun relative to Mars in the J2000
            reference frame, corrected for aberrations.
 
       --   Compute the position of the Sun relative to Mars 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'' (spk.req)
   document.
 
   When deciding which SPK files to load, the Toolkit utility ``brief'' may
   be of some use.
 
   ``brief'' is located in the ``icy/exe'' directory for IDL toolkits.
   Consult its user's guide available in ``icy/doc/brief.ug'' for details.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
   The meta-kernel we created for the solution to this exercise is named
   'getsta.tm'. 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.
 
         The names and contents of the kernels referenced by this
         meta-kernel are as follows:
 
            1. Generic LSK:
 
                  naif0012.tls
 
            2. Solar System Ephemeris SPK, subsetted to cover only
               the time range of interest:
 
                  de430.bsp
 
            3. Martian Satellite Ephemeris SPK, subsetted to cover
               only the time range of interest:
 
                  mar085.bsp
 
            4. ExoMars-16 TGO Spacecraft Trajectory SPK, subsetted
               to cover only the time range of interest:
 
                  em16_tgo_mlt_20171205_20230115_v01.bsp
 
 
      \begindata
 
       KERNELS_TO_LOAD = (
 
       'kernels/lsk/naif0012.tls',
       'kernels/spk/de430.bsp',
       'kernels/spk/mar085.bsp',
       'kernels/spk/em16_tgo_mlt_20171205_20230115_v01.bsp',
 
                           )
 
      \begintext
 
 
Solution Source Code
 
   A sample solution to the problem follows:
 
      PRO getsta
 
         ;;
         ;; Local Parameters
         ;;
 
         METAKR = 'getsta.tm'
         STRLEN = 50
         utctim = ''
 
         ;;
         ;; Load the kernels that this program requires.  We
         ;; will need a leapseconds kernel to convert input
         ;; UTC time strings into ET.  We also will need the
         ;; necessary SPK files with coverage for the bodies
         ;; in which we are interested.
         ;;
         cspice_furnsh, METAKR
 
         ;;
         ;; Prompt the user for the input time string.
         ;;
         read, utctim, PROMPT = 'Input UTC Time: '
 
         print, 'Converting UTC Time: ', utctim
 
         ;;
         ;; Convert utctim to et.
         ;;
         cspice_str2et, utctim, et
 
         print, FORMAT='(A,F16.3)', '   ET seconds past J2000: ', et
 
         ;;
         ;; Compute the apparent state of Mars as seen from
         ;; ExoMars-16 TGO in the J2000 frame.  All of the ephemeris
         ;; readers return states in units of kilometers and
         ;; kilometers per second.
         ;;
         cspice_spkezr, 'MARS', et   , 'J2000', 'LT+S', $
                        'TGO',  state, ltime
 
         print, '   Apparent state of Mars as seen ' +$
                'from ExoMars-16 TGO in the J2000'
         print, '      frame (km, km/s): '
         print, FORMAT='(A,F16.3)', '      X = ', state[0]
         print, FORMAT='(A,F16.3)', '      Y = ', state[1]
         print, FORMAT='(A,F16.3)', '      Z = ', state[2]
         print, FORMAT='(A,F16.3)', '     VX = ', state[3]
         print, FORMAT='(A,F16.3)', '     VY = ', state[4]
         print, FORMAT='(A,F16.3)', '     VZ = ', state[5]
 
 
         ;;
         ;; Compute the apparent position of Earth as seen from
         ;; ExoMars-16 TGO in the J2000 frame.  Note: We could have
         ;; continued using cspice_spkezr and simply ignored the
         ;; velocity components.
         ;;
         cspice_spkpos, 'EARTH', et , 'J2000', 'LT+S', $
                        'TGO',   pos, ltime
 
         print, '   Apparent position of Earth as '    +$
                     'seen from ExoMars-16 TGO in the'
         print, '      J2000 frame (km):  '
         print, FORMAT='(A,F16.3)', '      X = ', pos[0]
         print, FORMAT='(A,F16.3)', '      Y = ', pos[1]
         print, FORMAT='(A,F16.3)', '      Z = ', pos[2]
 
         ;;
         ;; We need only display LT, as it is precisely the
         ;; light time in which we are interested.
         ;;
         print, '   One way light time between ExoMars-16 ' +$
                'TGO and the apparent'
         print, FORMAT='(A,F16.3)', '      position of Earth ' +$
                                    '(seconds): ', ltime
 
         ;;
         ;; Compute the apparent position of the Sun as seen
         ;; from Mars in the J2000 frame.
         ;;
         cspice_spkpos, 'SUN' , et , 'J2000', 'LT+S', $
                        'MARS', pos, ltime
 
         print, '   Apparent position of Sun as seen ' +$
                'from Mars in the '
         print, '      J2000 frame (km): '
         print, FORMAT='(A,F16.3)', '      X = ', pos[0]
         print, FORMAT='(A,F16.3)', '      Y = ', pos[1]
         print, FORMAT='(A,F16.3)', '      Z = ', pos[2]
 
         ;;
         ;; Now we need to compute the actual distance between
         ;; the Sun and Mars.  The above SPKPOS call gives us
         ;; the apparent distance, so we need to adjust our
         ;; aberration correction appropriately.
         ;;
         cspice_spkpos, 'SUN' , et , 'J2000', 'NONE', $
                        'MARS', pos, ltime
 
         ;;
         ;; Compute the distance between the body centers in
         ;; kilometers.
         ;;
         dist = cspice_vnorm ( pos )
 
         ;;
         ;; Convert this value to AU using cspice_convrt.
         ;; Recall, cspice_convrt cannot overwrite the
         ;; input with the output. Use 'dist_au' for the
         ;; output value.
         ;;
         cspice_convrt, dist, 'KM', 'AU', dist_au
 
         print, '   Actual distance between Sun and Mars ' + $
                'body centers:'
         print, FORMAT='(A,F16.3)', '      (AU): ', dist_au
 
         cspice_kclear
 
      END
 
 
Solution Sample Output
 
   After compiling the program, execute it:
 
      Input UTC Time: 2018 JUN 11 19:32:00
      Converting UTC Time: 2018 JUN 11 19:32:00
         ET seconds past J2000:    582017589.185
         Apparent state of Mars as seen from ExoMars-16 TGO in the J2000
            frame (km, km/s):
            X =         2911.822
            Y =        -2033.802
            Z =        -1291.701
           VX =            1.310
           VY =           -0.056
           VZ =            3.104
         Apparent position of Earth as seen from ExoMars-16 TGO in the
            J2000 frame (km):
            X =    -49609884.080
            Y =     57070665.862
            Z =     30304236.930
         One way light time between ExoMars-16 TGO and the apparent
            position of Earth (seconds):          271.738
         Apparent position of Sun as seen from Mars in the
            J2000 frame (km):
            X =    -24712734.289
            Y =    194560532.943
            Z =     89906636.789
         Actual distance between Sun and Mars body centers:
            (AU):            1.442
 
 
Extra Credit
--------------------------------------------------------
 
   In this ``extra credit'' section you will be presented with more complex
   tasks, aimed at improving your understanding of state computations,
   particularly the application of the different light time and stellar
   aberration corrections available in the cspice_spkezr procedure, and
   some common errors that may happen when computing these states.
 
   These ``extra credit'' tasks are provided as task statements, and unlike
   the regular tasks, no approach or solution source code is provided. In
   the next section, you will find the numeric solutions (when applicable)
   and answers to the questions asked in these tasks.
 
 
Task statements and questions
 
       1.   Remove the Martian planetary ephemerides SPK from the original
            meta-kernel and run your program again, using the same inputs
            as before. Has anything changed? Why?
 
       2.   Extend your program to compute the geometric position of
            Jupiter as seen from Mars in the J2000 frame (J2000), in
            kilometers.
 
       3.   Extend your program to compute the apparent position of the
            Schiaparelli Entry, Descent and Landing Demonstrator Module
            (EDM) Landing Site as seen from the ExoMars-16 Trace Gas
            Orbiter (TGO) spacecraft in the J2000 frame (J2000), in
            kilometers.
 
       4.   Extend, or modify, your program to compute the position of the
            Sun as seen from Mars in the J2000 frame (J2000), in
            kilometers, using the following light time and aberration
            corrections: NONE, LT and LT+S. Explain the differences.
 
       5.   Examine the ExoMars-16 TGO frames definition kernel to find the
            SPICE ID/name definitions.
 
 
Solutions and answers
 
       1.   When running the original program without the Martian planetary
            ephemerides SPK, an error is produced by cspice_spkezr:
 
      % CSPICE_SPKEZR: SPICE(SPKINSUFFDATA):
                        [spkezr_c->SPKEZR->SPKEZ->SPKACS->SPKGEO]
                        Insufficient ephemeris data has been loaded to
                        compute the state of -143 (EXOMARS 2016 TGO)
                        relative to 0 (SOLAR SYSTEM BARYCENTER) at the
                        ephemeris epoch 2018 JUN 11 19:33:09.184.
      % Execution halted at: GETSTA             42
         /home/bsemenov/tmp/lessons_180518/lessons/remote_sensing_tgo/ge
        tsta.pro
      %                      $MAIN$
 
            This error is generated when trying to compute the apparent
            state of Mars as seen from ExoMars-16 TGO in the J2000 frame
            because despite the ExoMars-16 TGO ephemeris data being
            relative to Mars, the state of the spacecraft with respect to
            the solar system barycenter is required to compute the light
            time and stellar aberrations. The loaded SPK data are enough to
            compute geometric states of ExoMars-16 TGO with respect to Mars
            center, and geometric states of Mars barycenter with respect to
            the Solar System Barycenter, but insufficient to compute the
            state of the spacecraft relative to the Solar System Barycenter
            because the SPK data needed to compute geometric states of Mars
            center relative to its barycenter are no longer loaded. Run
            ``brief'' on the SPKs used in the original task to find out
            which ephemeris objects are available from those kernels. If
            you want to find out what is the 'center of motion' for the
            ephemeris object(s) included in an SPK, use the -c option when
            running ``brief'':
 
 
      BRIEF -- Version 4.0.0, September 8, 2010 -- Toolkit Version N0066
 
 
      Summary for: kernels/spk/de430.bsp
 
      Bodies: MERCURY BARYCENTER (1) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              VENUS BARYCENTER (2) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              EARTH BARYCENTER (3) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              MARS BARYCENTER (4) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              JUPITER BARYCENTER (5) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              SATURN BARYCENTER (6) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              URANUS BARYCENTER (7) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              NEPTUNE BARYCENTER (8) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              PLUTO BARYCENTER (9) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              SUN (10) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              MERCURY (199) w.r.t. MERCURY BARYCENTER (1)
              VENUS (299) w.r.t. VENUS BARYCENTER (2)
              MOON (301) w.r.t. EARTH BARYCENTER (3)
              EARTH (399) w.r.t. EARTH BARYCENTER (3)
              Start of Interval (UTC)             End of Interval (UTC)
              -----------------------------       -------------------------
      ----
              2018-JUN-09 23:30:00.000            2018-JUN-14 12:00:00.000
 
 
      Summary for: kernels/spk/mar085.bsp
 
      Bodies: EARTH BARYCENTER (3) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              MARS BARYCENTER (4) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              SUN (10) w.r.t. SOLAR SYSTEM BARYCENTER (0)
              EARTH (399) w.r.t. EARTH BARYCENTER (3)
              PHOBOS (401) w.r.t. MARS BARYCENTER (4)
              DEIMOS (402) w.r.t. MARS BARYCENTER (4)
              MARS (499) w.r.t. MARS BARYCENTER (4)
              Start of Interval (UTC)             End of Interval (UTC)
              -----------------------------       -------------------------
      ----
              2018-JUN-09 23:30:00.000            2018-JUN-14 12:00:00.000
 
 
      Summary for: kernels/spk/em16_tgo_mlt_20171205_20230115_v01.bsp
 
      Body: EXOMARS 2016 TGO (-143) w.r.t. MARS (499)
            Start of Interval (UTC)             End of Interval (UTC)
            -----------------------------       ---------------------------
      --
            2018-JUN-09 23:30:00.000            2018-JUN-14 12:00:00.000
 
 
 
       2.   If you run your extended program with the original meta-kernel,
            the SPICE(SPKINSUFFDATA) error should be produced by the
            cspice_spkpos procedure because you have not loaded enough
            ephemeris data to compute the position of Jupiter with respect
            to Mars. The loaded SPKs contain data for Mars relative to the
            Solar System Barycenter, and for the Jupiter System Barycenter
            relative to the Solar System Barycenter, but the data for
            Jupiter relative to the Jupiter System Barycenter are missing:
 
 
         Additional kernels required for this task:
 
            1. Generic Jovian Satellite Ephemeris SPK:
 
                  jup310_2018.bsp
 
         available in the NAIF server at:
 
      https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/
      satellites/a_old_versions
 
 
            Download the relevant SPK, add it to the meta-kernel and run
            again your extended program. The solution for the input UTC
            time "2018 JUN 11 19:32:00" when using the downloaded Jovian
            Satellite Ephemeris SPK:
 
         Actual position of Jupiter as seen from Mars in the
            J2000 frame (km):
            X =   -536521483.294
            Y =   -384722940.461
            Z =   -145930841.439
 
       3.   Once you have extended your program, download the required data
            from the official ExoMars-16 SPICE FTP site and update your
            meta-kernel:
 
 
         Additional kernels required for this task:
 
            1. EDM lander FK:
 
                  em16_edm_v00.tf
 
            2. EDM landing site SPK:
 
                  em16_edm_sot_landing_site_20161020_21000101_v01.bsp
 
            3. Generic PCK:
 
                  pck00010.tpc
 
         available in the NAIF server at:
 
            https://naif.jpl.nasa.gov/pub/naif/EXOMARS2016/kernels/
 
 
            This is the solution for the input UTC time "2018 JUN 11
            19:32:00" when using the following data for the EDM lander:
 
         EDM_LANDING_SITE NAIF ID:      -117900
         Apparent position of EDM Landing Site as seen from
            ExoMars-16 TGO in the J2000 frame (km):
            X =         -131.716
            Y =        -2168.989
            Z =          208.792
 
       4.   When using 'NONE' aberration corrections, cspice_spkpos returns
            the geometric position of the target body relative to the
            observer. If 'LT' is used, the returned vector corresponds to
            the position of the target at the moment it emitted photons
            arriving at the observer at `et'. If 'LT+S' is used instead,
            the returned vector takes into account the observer's velocity
            relative to the solar system barycenter. The solution for the
            input UTC time "2018 JUN 11 19:32:00" is:
 
 
         Actual (geometric) position of Sun as seen from Mars in the
            J2000 frame (km):
            X =    -24730874.909
            Y =    194558449.463
            Z =     89906170.829
         Light-time corrected position of Sun as seen from Mars in the
            J2000 frame (km):
            X =    -24730866.197
            Y =    194558445.149
            Z =     89906168.728
         Apparent position of Sun as seen from Mars in the
            J2000 frame (km):
            X =    -24712733.997
            Y =    194560532.846
            Z =     89906636.764
 
 
 
Spacecraft Orientation and Reference Frames (xform)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
   Write a program that prompts the user for an input time string, and
   computes and displays the following at the epoch of interest:
 
       1.   The apparent state of Mars as seen from ExoMars-16 TGO in the
            IAU_MARS 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 Mars as
            seen from ExoMars-16 TGO and the nominal instrument view
            direction.
 
            The nominal instrument view direction is not provided by any
            kernel variable, but it is indicated in the ExoMars-16 TGO
            frame kernel cited above in the section ``Kernels Used'' to be
            the -Y axis of the TGO_SPACECRAFT frame.
 
   Use the program to compute these quantities at the epoch "2018 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 procedures. Understand the difference
   between cspice_pxform and cspice_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 Mars relative to ExoMars-16 TGO in the
            J2000 reference frame, corrected for aberrations.
 
       --   Compute the state transformation matrix from J2000 to IAU_MARS
            at the epoch, adjusted for light time.
 
       --   Multiply the state of Mars relative to ExoMars-16 TGO in the
            J2000 reference frame by the state transformation matrix
            computed in the previous step.
 
       --   Compute the position of Mars relative to ExoMars-16 TGO in the
            J2000 reference frame, corrected for aberrations.
 
       --   Determine what the nominal instrument view direction of the
            ExoMars-16 TGO spacecraft is by examining the frame kernel's
            content.
 
       --   Compute the rotation matrix from the ExoMars-16 TGO spacecraft
            frame to J2000.
 
       --   Multiply the nominal instrument view direction expressed in the
            ExoMars-16 TGO spacecraft frame by the rotation matrix from the
            previous step.
 
       --   Compute the separation between the result of the previous step
            and the apparent position of Mars relative to ExoMars-16 TGO in
            the J2000 frame.
 
   HINT: Several of the steps above may be compressed into a single step
   using Icy procedures with which you are already familiar. The ``long
   way'' presented above is intended to facilitate the introduction of the
   procedures cspice_pxform and cspice_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 (frames.req)
 
       2.   PCK Required Reading (pck.req)
 
       3.   SPK Required Reading (spk.req)
 
       4.   CK Required Reading (ck.req)
 
   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.tm'. 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.
 
         The names and contents of the kernels referenced by this
         meta-kernel are as follows:
 
            1. Generic LSK:
 
                  naif0012.tls
 
            2. ExoMars-16 TGO SCLK:
 
                  em16_tgo_step_20160414.tsc
 
            3. Solar System Ephemeris SPK, subsetted to cover only
               the time range of interest:
 
                  de430.bsp
 
            4. Martian Satellite Ephemeris SPK, subsetted to cover
               only the time range of interest:
 
                  mar085.bsp
 
            5. ExoMars-16 TGO Spacecraft Trajectory SPK, subsetted
               to cover only the time range of interest:
 
                  em16_tgo_mlt_20171205_20230115_v01.bsp
 
            6. ExoMars-16 TGO FK:
 
                  em16_tgo_v07.tf
 
            7. ExoMars-16 TGO Spacecraft CK, subsetted to cover only
               the time range of interest:
 
                  em16_tgo_sc_slt_npo_20171205_20230115_s20160414_v01.bc
 
            8. Generic PCK:
 
                  pck00010.tpc
 
 
      \begindata
 
       KERNELS_TO_LOAD = (
 
       'kernels/lsk/naif0012.tls',
       'kernels/sclk/em16_tgo_step_20160414.tsc',
       'kernels/spk/de430.bsp',
       'kernels/spk/mar085.bsp',
       'kernels/spk/em16_tgo_mlt_20171205_20230115_v01.bsp',
       'kernels/fk/em16_tgo_v07.tf',
       'kernels/ck/em16_tgo_sc_slt_npo_20171205_20230115_s20160414_v01.bc',
       'kernels/pck/pck00010.tpc'
 
                          )
 
      \begintext
 
 
Solution Source Code
 
   A sample solution to the problem follows:
 
      PRO xform
 
         ;;
         ;; Local Parameters
         ;;
 
         METAKR = 'xform.tm'
         STRLEN = 50
         utctim = ''
 
         ;;
         ;; Load the kernels that this program requires. We
         ;; will need:
         ;;
         ;;    A leapseconds kernel
         ;;    A spacecraft clock kernel for ExoMars-16 TGO
         ;;    The necessary ephemerides
         ;;    A planetary constants file (PCK)
         ;;    A spacecraft orientation kernel for ExoMars-16 TGO (CK)
         ;;    A frame kernel (TF)
         ;;
         cspice_furnsh, METAKR
 
         ;;
         ;; Prompt the user for the input time string.
         ;;
         read, utctim, PROMPT = 'Input UTC Time: '
 
         print, 'Converting UTC Time: ', utctim
 
         ;;
         ;; Convert utctim to et.
         ;;
         cspice_str2et, utctim, et
 
         print, FORMAT='(A,F16.3)', '   ET seconds past J2000: ', et
 
         ;;
         ;; Compute the apparent state of Mars as seen from
         ;; ExoMars-16 TGO in the J2000 frame. All of the ephemeris
         ;; readers return states in units of kilometers and
         ;; kilometers per second.
         ;;
         cspice_spkezr, 'MARS', et,    'J2000', 'LT+S', $
                        'TGO',  state, ltime
 
         ;;
         ;; Now obtain the transformation from the inertial
         ;; J2000 frame to the non-inertial body-fixed IAU_MARS
         ;; frame. Since we want the apparent state in the
         ;; IAU_MARS reference frame, we need to correct the
         ;; orientation and angular velocity of this frame
         ;; for one-way light time; hence we subtract ltime from
         ;; et in the call below.
         ;;
         cspice_sxform, 'J2000', 'IAU_MARS', et-ltime, sxfmat
 
         ;;
         ;; Now transform the apparent J2000 state into IAU_MARS
         ;; with the following matrix multiplication:
         ;;
         bfixst = transpose(sxfmat) # state
 
         ;;
         ;; Display the results.
         ;;
         print, '   Apparent state of Mars as seen ' +$
                     'from ExoMars-16 TGO in the IAU_MARS'
         print, '      body-fixed frame (km, km/s):'
         print, FORMAT='(A,F19.6)', '      X = ', bfixst[0]
         print, FORMAT='(A,F19.6)', '      Y = ', bfixst[1]
         print, FORMAT='(A,F19.6)', '      Z = ', bfixst[2]
         print, FORMAT='(A,F19.6)', '     VX = ', bfixst[3]
         print, FORMAT='(A,F19.6)', '     VY = ', bfixst[4]
         print, FORMAT='(A,F19.6)', '     VZ = ', bfixst[5]
 
         ;;
         ;; It is worth pointing out, all of the above could
         ;; have been done with a single use of cspice_spkezr:
         ;;
         ;;
         cspice_spkezr, 'MARS', et,    'IAU_MARS', 'LT+S', $
                        'TGO',  state, ltime
 
         ;;
         ;; Display the results.
         ;;
         print, '   Apparent state of Mars as seen ' +$
                     'from ExoMars-16 TGO in the IAU_MARS'
         print, '      body-fixed frame (km, km/s) '  +$
                'obtained using cspice_spkezr'
         print, '      directly:'
         print, FORMAT='(A,F19.6)', '      X = ', state[0]
         print, FORMAT='(A,F19.6)', '      Y = ', state[1]
         print, FORMAT='(A,F19.6)', '      Z = ', state[2]
         print, FORMAT='(A,F19.6)', '     VX = ', state[3]
         print, FORMAT='(A,F19.6)', '     VY = ', state[4]
         print, FORMAT='(A,F19.6)', '     VZ = ', state[5]
 
         ;;
         ;; Note that the velocity found by using cspice_spkezr
         ;; to compute the state in the IAU_MARS frame differs
         ;; at the few mm/second level from that found previously
         ;; by calling cspice_spkezr and then cspice_sxform. Computing
         ;; velocity via a single call to cspice_spkezr as we've
         ;; done immediately above is slightly more accurate because
         ;; it accounts for the effect of the rate of change of
         ;; light time on the apparent angular velocity of the
         ;; target's body-fixed reference frame.
         ;;
         ;; Now we are to compute the angular separation between
         ;; the apparent position of Mars as seen from the orbiter
         ;; and the nominal instrument view direction.  First,
         ;; compute the apparent position of Mars as seen from
         ;; ExoMars-16 TGO in the J2000 frame.
         ;;
         cspice_spkpos, 'MARS', et,  'J2000', 'LT+S', $
                        'TGO',  pos,  ltime
 
         ;;
         ;; Now compute the location of the nominal instrument view
         ;; direction.  From reading the frame kernel we know that
         ;; the instrument view direction is nominally the -Y axis
         ;; of the TGO_SPACECRAFT frame defined there.
         ;;
         bsight = [ 0.D0, -1.D0, 0.D0]
 
         ;;
         ;; Now compute the rotation matrix from TGO_SPACECRAFT into
         ;; J2000.
         ;;
         cspice_pxform, 'TGO_SPACECRAFT', 'J2000', et, pform
 
         ;;
         ;; And multiply the result to obtain the nominal instrument
         ;; view direction in the J2000 reference frame.
         ;;
         cspice_mxv, pform, bsight, bsight
 
         ;;
         ;; Lastly compute the angular separation.
         ;;
         cspice_convrt, cspice_vsep(bsight, pos), 'RADIANS', $
                       'DEGREES', sep
 
         print, '   Angular separation between the '        +$
                     'apparent position of Mars and the'
         print, '      ExoMars-16 TGO nominal instrument '  +$
                     'view direction (degrees):'
         print, FORMAT='(A,F16.3)', '      ', sep
 
         ;;
         ;; Or alternatively we can work in the spacecraft
         ;; frame directly.
         ;;
         cspice_spkpos, 'MARS', et,  'TGO_SPACECRAFT', 'LT+S', $
                        'TGO',  pos, ltime
 
         ;;
         ;; The nominal instrument view direction is the -Y-axis
         ;; in the TGO_SPACECRAFT frame.
         ;;
         bsight = [ 0.D0, -1.D0, 0.D0]
 
         ;;
         ;; Lastly compute the angular separation.
         ;;
         cspice_convrt, cspice_vsep(bsight, pos), 'RADIANS', $
                       'DEGREES', sep
 
         print, '   Angular separation between the '        +$
                     'apparent position of Mars and the'
         print, '      ExoMars-16 TGO nominal instrument '  +$
                     'view direction computed'
         print, '      using vectors in the '               +$
                     'TGO_SPACECRAFT frame (degrees):'
         print, FORMAT='(A,F16.3)', '      ', sep
 
         cspice_kclear
 
      END
 
 
Solution Sample Output
 
   After compiling the program, execute it:
 
      Input UTC Time: 2018 JUN 11 19:32:00
      Converting UTC Time: 2018 JUN 11 19:32:00
         ET seconds past J2000:    582017589.185
         Apparent state of Mars as seen from ExoMars-16 TGO in the IAU_MARS
            body-fixed frame (km, km/s):
            X =        -2843.464125
            Y =         2235.459544
            Z =         1095.894969
           VX =            0.311443
           VY =           -1.151929
           VZ =            3.082123
         Apparent state of Mars as seen from ExoMars-16 TGO in the IAU_MARS
            body-fixed frame (km, km/s) obtained using cspice_spkezr
            directly:
            X =        -2843.464125
            Y =         2235.459544
            Z =         1095.894969
           VX =            0.311443
           VY =           -1.151929
           VZ =            3.082123
         Angular separation between the apparent position of Mars and the
            ExoMars-16 TGO nominal instrument view direction (degrees):
                       5.438
         Angular separation between the apparent position of Mars and the
            ExoMars-16 TGO nominal instrument view direction computed
            using vectors in the TGO_SPACECRAFT frame (degrees):
                       5.438
 
 
Extra Credit
--------------------------------------------------------
 
   In this ``extra credit'' section you will be presented with more complex
   tasks, aimed at improving your understanding of frame transformations,
   and some common errors that may happen when computing them.
 
   These ``extra credit'' tasks are provided as task statements, and unlike
   the regular tasks, no approach or solution source code is provided. In
   the next section, you will find the numeric solutions (when applicable)
   and answers to the questions asked in these tasks.
 
 
Task statements and questions
 
       1.   Run the original program using the input UTC time "2018 jun 12
            18:25:00". Explain what happens.
 
       2.   Compute the angular separation between the apparent position of
            the Sun as seen from ExoMars-16 TGO and the nominal instrument
            view direction. Is the science deck illuminated?
 
 
Solutions and answers
 
       1.   When running the original software using as input the UTC time
            string "2018 jun 12 18:25:00":
 
      % CSPICE_PXFORM: SPICE(NOFRAMECONNECT): [pxform_c->PXFORM->REFCHG]
                        At epoch 5.8209996918462E+08 TDB (2018 JUN 12
                        18:26:09.184 TDB), there is insufficient
                        information available to transform from
                        reference frame -143000 (TGO_SPACECRAFT) to
                        reference frame 1 (J2000). TGO_SPACECRAFT is a
                        CK frame; a CK file containing data for
                        instrument or structure -143000 at the epoch
                        shown above, as well as a corresponding SCLK
                        kernel, must be loaded in order to use this
                        frame. Failure to find required CK data could
                        be due to one or more CK files not having been
                        loaded, or to the epoch shown above lying
                        within a coverage gap or beyond the coverage
                        bounds of the loaded CK files. It is also
                        possible that no loaded CK file has required
                        angular velocity data for the input epoch, even
                        if a loaded CK does have attitude data for that
                        epoch. You can use CKBRIEF with the -dump
                        option to display coverage intervals of a CK
                        file.
      % Execution halted at: XFORM             134
         /home/bsemenov/tmp/lessons_180518/lessons/remote_sensing_tgo/xf
        orm.pro
      %                      $MAIN$
 
            cspice_pxform returns the SPICE(NOFRAMECONNECT) error, which
            indicates that there are not sufficient data to perform the
            transformation from the TGO_SPACECRAFT frame to J2000 at the
            requested epoch. If you summarize the ExoMars-16 TGO spacecraft
            CK using the ``ckbrief'' utility program with the -dump option
            (display interpolation intervals boundaries) you will find that
            the CK contains gaps within its segment:
 
 
      CKBRIEF -- Version 6.1.0, June 27, 2014 -- Toolkit Version N0066
 
 
      Summary for: kernels/ck/em16_tgo_sc_slt_npo_20171205_20230115_s201604
      14_v01.bc
 
      Segment No.: 1
 
      Object:  -143000
        Interval Begin UTC       Interval End UTC         AV
        ------------------------ ------------------------ ---
        2018-JUN-10 23:59:59.999 2018-JUN-12 06:26:53.918 Y
        2018-JUN-12 06:56:53.918 2018-JUN-12 18:14:33.918 Y
        2018-JUN-12 18:44:33.918 2018-JUN-13 04:02:13.918 Y
        2018-JUN-13 04:32:13.918 2018-JUN-13 07:58:33.918 Y
        2018-JUN-13 08:28:33.918 2018-JUN-13 11:59:59.999 Y
 
 
 
            whereas if you had used ckbrief without -dump you would have
            gotten the following information (only CK segment begin/end
            times):
 
 
      CKBRIEF -- Version 6.1.0, June 27, 2014 -- Toolkit Version N0066
 
 
      Summary for: kernels/ck/em16_tgo_sc_slt_npo_20171205_20230115_s201604
      14_v01.bc
 
      Object:  -143000
        Interval Begin UTC       Interval End UTC         AV
        ------------------------ ------------------------ ---
        2018-JUN-10 23:59:59.999 2018-JUN-13 11:59:59.999 Y
 
 
 
            which has insufficient detail to reveal the problem.
 
       2.   By computing the apparent position of the Sun as seen from
            ExoMars-16 TGO in the TGO_SPACECRAFT frame, and the angular
            separation between this vector and the nominal instrument view
            direction (-Y-axis of the TGO_SPACECRAFT frame), you will find
            whether the science deck is illuminated. The solution for the
            input UTC time "2018 JUN 11 19:32:00" is:
 
      Angular separation between the apparent position of the Sun and the
      ExoMars-16 TGO nominal instrument view direction (degrees):
          130.543
 
      Science Deck illumination:
         ExoMars-16 TGO Science Deck IS NOT illuminated.
 
            since the angular separation is greater than 90 degrees.
 
 
Computing Sub-s/c and Sub-solar Points on an Ellipsoid and a DSK (subpts)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
   Write a program that prompts the user for an input UTC time string and
   computes the following quantities at that epoch:
 
       1.   The apparent sub-observer point of ExoMars-16 TGO on Mars, in
            the body fixed frame IAU_MARS, in kilometers.
 
       2.   The apparent sub-solar point on Mars, as seen from ExoMars-16
            TGO in the body fixed frame IAU_MARS, in kilometers.
 
   The program computes each point twice: once using an ellipsoidal shape
   model and the
 
           near point/ellipsoid
 
   definition, and once using a DSK shape model and the
 
           nadir/dsk/unprioritized
 
   definition.
 
   The program displays the results. Use the program to compute these
   quantities at "2018 JUN 11 19:32:00" UTC.
 
 
Learning Goals
--------------------------------------------------------
 
   Discover higher level geometry calculation procedures in Icy and their
   usage as it relates to ExoMars-16 TGO.
 
 
Approach
--------------------------------------------------------
 
   This particular problem is more of an exercise in searching the permuted
   index to find the appropriate procedures and then reading their headers
   to understand how to call them.
 
   One point worth considering: how would the results change if the
   sub-solar and sub-observer points were computed using the
 
           intercept/ellipsoid
 
   and
 
           intercept/dsk/unprioritized
 
   definitions? Which definition is appropriate?
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
   The meta-kernel we created for the solution to this exercise is named
   'subpts.tm'. Its contents follow:
 
      KPL/MK
 
         This is the meta-kernel used in the solution of the
         ``Computing Sub-s/c and Sub-solar Points on an Ellipsoid
         and a DSK'' task in the Remote Sensing Hands On Lesson.
 
         The names and contents of the kernels referenced by this
         meta-kernel are as follows:
 
            1. Generic LSK:
 
                  naif0012.tls
 
            2. Solar System Ephemeris SPK, subsetted to cover only
               the time range of interest:
 
                  de430.bsp
 
            3. Martian Satellite Ephemeris SPK, subsetted to cover
               only the time range of interest:
 
                  mar085.bsp
 
            4. ExoMars-16 TGO Spacecraft Trajectory SPK, subsetted
               to cover only the time range of interest:
 
                  em16_tgo_mlt_20171205_20230115_v01.bsp
 
            5. Generic PCK:
 
                  pck00010.tpc
 
            6. Low-resolution Mars DSK:
 
                  mars_lowres.bds
 
      \begindata
 
       KERNELS_TO_LOAD = (
 
       'kernels/lsk/naif0012.tls',
       'kernels/spk/de430.bsp',
       'kernels/spk/mar085.bsp',
       'kernels/spk/em16_tgo_mlt_20171205_20230115_v01.bsp',
       'kernels/pck/pck00010.tpc'
       'kernels/dsk/mars_lowres.bds'
 
                         )
 
      \begintext
 
 
Solution Source Code
 
   A sample solution to the problem follows:
 
      PRO subpts
 
         ;;
         ;; Local Parameters
         ;;
         METAKR = 'subpts.tm'
         STRLEN = 50
         utctim = ''
 
         ;;
         ;; Load the kernels that this program requires.  We
         ;; will need:
         ;;
         ;;    A leapseconds kernel
         ;;    The necessary ephemerides
         ;;    A planetary constants file (PCK)
         ;;    A DSK file containing Mars shape data
         ;;
         cspice_furnsh, METAKR
 
         ;;
         ;; Prompt the user for the input time string.
         ;;
         read, utctim, PROMPT = 'Input UTC Time: '
 
         print, 'Converting UTC Time: ', utctim
 
         ;;
         ;; Convert utctim to et.
         ;;
         cspice_str2et, utctim, et
 
         print, FORMAT='(A,F16.3)', '   ET seconds past J2000: ', et
 
         for i = 0, 1  do begin
 
            if i eq 0 then begin
 
               ;;
               ;; Use the "near point" sub-point definition
               ;; and an ellipsoidal model.
               ;;
               method = 'NEAR POINT/Ellipsoid'
 
            endif else begin
 
               ;;
               ;; Use the "nadir" sub-point definition
               ;; and a DSK model.
               ;;
               method = 'NADIR/DSK/Unprioritized'
 
            endelse
 
            print
            print, FORMAT='(A)', ' Sub-point/target shape model: ' $
                   + method
            print
 
            ;;
            ;; Compute the apparent sub-observer point of ExoMars-16 TGO
            ;; on Mars.
            ;;
            cspice_subpnt, method,                                  $
                           'MARS',  et,     'IAU_MARS',   'LT+S',   $
                           'TGO',   spoint, trgepc,       srfvec
 
            print, '   Apparent sub-observer point of ExoMars-16 ' +$
                   'TGO on Mars'
            print, '   in the IAU_MARS frame (km):'
            print, FORMAT='(A,F16.3)', '      X = ', spoint[0]
            print, FORMAT='(A,F16.3)', '      Y = ', spoint[1]
            print, FORMAT='(A,F16.3)', '      Z = ', spoint[2]
            print, FORMAT='(A,F16.3)', '    ALT = ', cspice_vnorm(srfvec)
 
            ;;
            ;; Compute the apparent sub-solar point on Mars
            ;; as seen from ExoMars-16 TGO.
            ;;
            cspice_subslr, method,                                  $
                           'MARS',  et,     'IAU_MARS',   'LT+S',   $
                           'TGO',   spoint, trgepc,       srfvec
 
            print, '   Apparent sub-solar point on Mars ' +$
                   'as seen from ExoMars-16'
            print, '   TGO in the IAU_MARS frame (km):'
            print, FORMAT='(A,F16.3)', '      X = ', spoint[0]
            print, FORMAT='(A,F16.3)', '      Y = ', spoint[1]
            print, FORMAT='(A,F16.3)', '      Z = ', spoint[2]
 
         endfor
 
         print
 
         cspice_unload, METAKR
 
      END
 
 
Solution Sample Output
 
   After compiling the program, execute it:
 
      Input UTC Time: 2018 JUN 11 19:32:00
      Converting UTC Time: 2018 JUN 11 19:32:00
         ET seconds past J2000:    582017589.185
 
       Sub-point/target shape model: NEAR POINT/Ellipsoid
 
         Apparent sub-observer point of ExoMars-16 TGO on Mars
         in the IAU_MARS frame (km):
            X =         2554.165
            Y =        -2008.010
            Z =         -983.240
          ALT =          385.045
         Apparent sub-solar point on Mars as seen from ExoMars-16
         TGO in the IAU_MARS frame (km):
            X =          487.589
            Y =        -3348.610
            Z =         -286.697
 
       Sub-point/target shape model: NADIR/DSK/Unprioritized
 
         Apparent sub-observer point of ExoMars-16 TGO on Mars
         in the IAU_MARS frame (km):
            X =         2554.223
            Y =        -2008.057
            Z =         -983.263
          ALT =          384.967
         Apparent sub-solar point on Mars as seen from ExoMars-16
         TGO in the IAU_MARS frame (km):
            X =          488.096
            Y =        -3352.093
            Z =         -286.999
 
 
 
Extra Credit
--------------------------------------------------------
 
   In this ``extra credit'' section you will be presented with more complex
   tasks, aimed at improving your understanding of cspice_subpnt and
   cspice_subslr procedures.
 
   These ``extra credit'' tasks are provided as task statements, and unlike
   the regular tasks, no approach or solution source code is provided. In
   the next section, you will find the numeric solutions (when applicable)
   and answers to the questions asked in these tasks.
 
 
Task statements and questions
 
       1.   Recompute the apparent sub-solar point on Mars as seen from
            ExoMars-16 TGO in the body fixed frame IAU_MARS in kilometers
            using the 'Intercept/ellipsoid' method at "2018 JUN 11
            19:32:00". Explain the differences.
 
       2.   Compute the geometric sub-spacecraft point of ExoMars-16 TGO on
            Phobos in the body fixed frame IAU_PHOBOS in kilometers using
            the 'Near point/ellipsoid' method at "2018 JUN 11 19:32:00".
 
       3.   Transform the sub-spacecraft Cartesian coordinates obtained in
            the previous task to planetocentric and planetographic
            coordinates. When computing planetographic coordinates,
            retrieve Phobos' radii by calling cspice_bodvrd and use the
            first element of the returned radii values as Phobos'
            equatorial radius. Explain why planetocentric and
            planetographic latitudes and longitudes are different. Explain
            why the planetographic altitude for a point on the surface of
            Phobos is not zero and whether this is correct or not.
 
 
Solutions and answers
 
       1.   The differences observed are due to the computation method. The
            ``Intercept/ellipsoid'' method defines the sub-solar point as
            the target surface intercept of the line containing the Sun and
            the target's center, while the ``Near point/ellipsoid'' method
            defines the sub-solar point as the the nearest point on the
            target relative to the Sun. Since Mars is not spherical, these
            two points are not the same:
 
         Apparent sub-solar point on Mars as seen from ExoMars-16 TGO in
         the IAU_MARS frame using the 'Near Point: ellipsoid' method
         (km):
            X =          487.589
            Y =        -3348.610
            Z =         -286.697
 
         Apparent sub-solar point on Mars as seen from ExoMars-16 TGO in
         the IAU_MARS frame using the 'Intercept: ellipsoid' method
         (km):
            X =          487.547
            Y =        -3348.322
            Z =         -290.077
 
       2.   The geometric sub-spacecraft point of ExoMars-16 TGO on Phobos
            in the body fixed frame IAU_PHOBOS in kilometers at "2018 JUN
            11 19:32:00" UTC epoch is:
 
         Geometric sub-spacecraft point of ExoMars-16 TGO on Phobos in
         the IAU_PHOBOS frame using the 'Near Point: ellipsoid' method
         (km):
            X =           12.059
            Y =            4.173
            Z =           -0.676
 
       3.   The sub-spacecraft point of ExoMars-16 TGO on Phobos in
            planetocentric and planetographic coordinates at "2018 JUN 11
            19:32:00" UTC epoch is:
 
         Planetocentric coordinates of the ExoMars-16 TGO
         sub-spacecraft point on Phobos (degrees, km):
         LAT =           -3.030
         LON =           19.088
         R   =           12.779
 
         Planetographic coordinates of the ExoMars-16 TGO
         sub-spacecraft point on Phobos (degrees, km):
         LAT =           -6.268
         LON =          340.912
         ALT =           -0.202
 
            The planetocentric and planetographic longitudes are different
            (``graphic'' = 360 - ``centric'') because planetographic
            longitudes on Phobos are measured positive west as defined by
            the Phobos' rotation direction.
 
            The planetocentric and planetographic latitudes are different
            because the planetocentric latitude was computed as the angle
            between the direction from the center of the body to the point
            and the equatorial plane, while the planetographic latitude was
            computed as the angle between the surface normal at the point
            and the equatorial plane.
 
            The planetographic altitude is non zero because it was computed
            using a different and incorrect Phobos surface model: a
            spheroid with equal equatorial radii. The surface point
            computed by cspice_subpnt was computed by treating Phobos as a
            triaxial ellipsoid with different equatorial radii. The
            planetographic latitude is also incorrect because it is based
            on the normal to the surface of the spheroid rather than the
            ellipsoid, In general planetographic coordinates cannot be used
            for bodies with shapes modeled as triaxial ellipsoids.
 
 
Intersecting Vectors with an Ellipsoid and a DSK (fovint)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
   Write a program that prompts the user for an input UTC time string and,
   for that time, computes the intersection of the ExoMars-16 TGO NOMAD LNO
   Nadir aperture boresight and field of view (FOV) boundary vectors with
   the surface of Mars. Compute each intercept twice: once with Mars' shape
   modeled as an ellipsoid, and once with Mars' shape modeled by DSK data.
   The program presents each point of intersection as
 
       1.   A Cartesian vector in the IAU_MARS frame
 
       2.   Planetocentric (latitudinal) coordinates in the IAU_MARS frame.
 
   For each of the camera FOV boundary and boresight vectors, if an
   intersection is found, the program displays the results of the above
   computations, otherwise it indicates no intersection exists.
 
   At each point of intersection compute the following:
 
       3.   Phase angle
 
       4.   Solar incidence angle
 
       5.   Emission angle
 
   These angles should be computed using both ellipsoidal and DSK shape
   models.
 
   Additionally compute the local solar time at the intercept of the
   spectrometer aperture boresight with the surface of Mars, using both
   ellipsoidal and DSK shape models.
 
   Use this program to compute values at the UTC epoch:
 
            "2018 JUN 11 19:32:00"
 
 
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 target bodies whose shapes are modeled as ellipsoids
   or provided by DSKs. Discover another high level geometry procedure and
   another time conversion procedure in Icy.
 
 
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
            ExoMars-16 TGO NOMAD spectrometer.
 
       --   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
            ExoMars-16 TGO NOMAD LNO Nadir aperture.
 
   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 Mars modeled as an
            ellipsoid or using DSK data.
 
       --   If this intercept is found, convert the position vector of the
            intercept into planetocentric coordinates.
 
            Then compute the phase, solar incidence, and emission angles at
            the intercept. Otherwise indicate to the user no intercept was
            found for this vector.
 
       --   Compute the planetocentric longitude of the boresight
            intercept.
 
   Finally
 
       --   Compute the local solar time at the boresight intercept
            longitude on a 24-hour clock. The input time for this
            computation should be the TDB observation epoch minus one-way
            light time from the boresight intercept to the spacecraft.
 
   It may be useful to consult the ExoMars-16 TGO NOMAD instrument kernel
   to determine the name of the NOMAD LNO Nadir aperture 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.tm'. Its contents follow:
 
      KPL/MK
 
         This is the meta-kernel used in the solution of the
         ``Intersecting Vectors with an Ellipsoid and a DSK'' task
         in the Remote Sensing Hands On Lesson.
 
         The names and contents of the kernels referenced by this
         meta-kernel are as follows:
 
            1. Generic LSK:
 
                  naif0012.tls
 
            2. ExoMars-16 TGO SCLK:
 
                  em16_tgo_step_20160414.tsc
 
            3. Solar System Ephemeris SPK, subsetted to cover only
               the time range of interest:
 
                  de430.bsp
 
            4. Martian Satellite Ephemeris SPK, subsetted to cover
               only the time range of interest:
 
                  mar085.bsp
 
            5. ExoMars-16 TGO Spacecraft Trajectory SPK, subsetted
               to cover only the time range of interest:
 
                  em16_tgo_mlt_20171205_20230115_v01.bsp
 
            6. ExoMars-16 TGO FK:
 
                  em16_tgo_v07.tf
 
            7. ExoMars-16 TGO Spacecraft CK, subsetted to cover only
               the time range of interest:
 
                  em16_tgo_sc_slt_npo_20171205_20230115_s20160414_v01.bc
 
            8. Generic PCK:
 
                  pck00010.tpc
 
            9. NOMAD IK:
 
                  em16_tgo_nomad_v02.ti
 
           10. Low-resolution Mars DSK:
 
                  mars_lowres.bds
 
      \begindata
 
       KERNELS_TO_LOAD = (
 
       'kernels/lsk/naif0012.tls',
       'kernels/sclk/em16_tgo_step_20160414.tsc',
       'kernels/spk/de430.bsp',
       'kernels/spk/mar085.bsp',
       'kernels/spk/em16_tgo_mlt_20171205_20230115_v01.bsp',
       'kernels/fk/em16_tgo_v07.tf',
       'kernels/ck/em16_tgo_sc_slt_npo_20171205_20230115_s20160414_v01.bc',
       'kernels/pck/pck00010.tpc',
       'kernels/ik/em16_tgo_nomad_v02.ti'
       'kernels/dsk/mars_lowres.bds'
 
                         )
 
      \begintext
 
 
Solution Source Code
 
   A sample solution to the problem follows:
 
      PRO fovint
 
         ;;
         ;; Local Parameters
         ;;
 
         METAKR     = 'fovint.tm'
         STRLEN     = 50
         boolstr    = [ 'false', 'true' ]
 
         ;;
         ;; BCVLEN is the maximum number of boundary corner
         ;; vectors we can retrieve. We've extended this array by 1
         ;; element to make room for the boresight vector.
         ;;
         BCVLEN     = 5
         utctim     = ''
         scan_vecs = dblarr( 3, BCVLEN )
 
         vecnam  = ['Boundary Corner 1', $
                    'Boundary Corner 2', $
                    'Boundary Corner 3', $
                    'Boundary Corner 4', $
                    'TGO NOMAD LNO Nadir Boresight' ]
 
         ;;
         ;; Load the kernels that this program requires.  We
         ;; will need:
         ;;
         ;;    A leapseconds kernel.
         ;;    A SCLK kernel for ExoMars-16 TGO.
         ;;    Any necessary ephemerides.
         ;;    The ExoMars-16 TGO frame kernel.
         ;;    An ExoMars-16 TGO C-kernel.
         ;;    A PCK file with Mars constants.
         ;;    The ExoMars-16 TGO NOMAD I-kernel.
         ;;    A DSK file containing Mars shape data.
         ;;
         cspice_furnsh, METAKR
 
         ;;
         ;; Prompt the user for the input time string.
         ;;
         read, utctim, PROMPT = 'Input UTC Time: '
 
         print, 'Converting UTC Time: ', utctim
 
         ;;
         ;; Convert utctim to et.
         ;;
         cspice_str2et,  utctim, et
 
         print, FORMAT='(A,F16.3)', '  ET seconds past J2000: ', et
         print
 
         ;;
         ;; Now we need to obtain the FOV configuration of
         ;; the NOMAD LNO Nadir aperture.  To do this we will
         ;; need the ID code for TGO_NOMAD_LNO_NAD.
         ;;
         cspice_bodn2c, 'TGO_NOMAD_LNO_NAD', lnonid, found
 
         ;;
         ;; Stop the program if the code was not found.
         ;;
         if ( ~ found ) then begin
            print, 'Unable to locate the ID code for ' +$
                   'TGO_NOMAD_LNO_NAD'
            return
         endif
 
         ;;
         ;; Now retrieve the field of view parameters.
         ;;
         cspice_getfov, lnonid, BCVLEN, shape, insfrm, bsight, bounds
 
         ;;
         ;; Rather than treat 'bsight' as a separate vector,
         ;; copy it and 'bounds' to 'scan_vecs'.
         ;;
         scan_vecs[ 0:11] = bounds[0:11]
         scan_vecs[12:14] = bsight[0:2]
 
         ;;
         ;; Set values of "method" string that specify use of
         ;; ellipsoidal and DSK (topographic) shape models.
         ;;
         ;; In this case, we can use the same methods for calls to both
         ;; cspice_sincpt and cspice_ilumin. Note that some SPICE routines
         ;; require different "method" inputs from those shown here. See
         ;; the API documentation of each routine for details.
         ;;
         method = [ 'Ellipsoid', 'DSK/Unprioritized' ]
 
         ;;
         ;; Get Mars ID. We'll use this ID code later, when we
         ;; compute local solar time.
         ;;
         cspice_bodn2c, 'MARS', marsid, found
 
         ;;
         ;; The ID code for MARS is built in to the library.
         ;; However, it is good programming practice to get
         ;; in the habit of checking your found-flags.
         ;; Return if the code was not found.
         ;;
         if ( ~found ) then begin
            print, 'Unable to locate the ID code for Mars.'
            return
         endif
 
         ;;
         ;; Now perform the same set of calculations for each
         ;; vector listed in the 'bounds' array.
         ;;
         for i = 0, 4 do begin
 
            print, 'Vector: ', vecnam[i]
 
            for j = 0, 1 do begin
 
               print
               print, FORMAT = '(A)', ' Target shape model: ' + method[j]
               print
 
               ;;
               ;; Call sincpt to determine coordinates of the
               ;; intersection of this vector with the surface
               ;; of Mars.
               ;;
               cspice_sincpt, method[j],    'MARS',         et,        $
                              'IAU_MARS',   'LT+S',         'TGO',     $
                               insfrm,      scan_vecs[*,i], point,     $
                               trgepc,      srfvec,         found
               ;;
               ;; Check the found flag.  Display a message if
               ;; the point of intersection was not found,
               ;; otherwise continue with the calculations.
               ;;
 
               if ( ~ found ) then begin
 
                  print, 'No intersection point found at '  +$
                              'this epoch for this vector.'
 
               endif else begin
 
                  ;;
                  ;; Now, we have discovered a point of intersection.
                  ;; Start by displaying the position vector in the
                  ;; IAU_MARS frame of the intersection.
                  ;;
                  print, '  Position vector of surface intercept ' +$
                         'in the IAU_MARS frame (km):'
                  print, FORMAT='(A,F16.3)', '     X   = ', point[0]
                  print, FORMAT='(A,F16.3)', '     Y   = ', point[1]
                  print, FORMAT='(A,F16.3)', '     Z   = ', point[2]
 
                  ;;
                  ;; Display the planetocentric latitude and longitude
                  ;; of the intercept.
                  ;;
                  cspice_reclat, point, radius, lon, lat
 
                  print, '  Planetocentric coordinates of ' +$
                         'the intercept (degrees):'
                  print, FORMAT='(A,F16.3)', '     LAT = ', $
                         lat * cspice_dpr()
                  print, FORMAT='(A,F16.3)', '     LON = ', $
                         lon * cspice_dpr()
 
                  ;;
                  ;; Compute the illumination angles at this
                  ;; point.
                  ;;
                  cspice_illumf, method[j],  'MARS',       'SUN',  $
                                 et,         'IAU_MARS',   'LT+S', $
                                 'TGO',      point,        trgepc, $
                                 srfvec,     phase,        solar,  $
                                 emissn,     visibl,       lit
 
 
                  print, FORMAT='(A,F16.3)', $
                         '  Phase angle (degrees):           ', $
                         phase * cspice_dpr()
                  print, FORMAT='(A,F16.3)', $
                         '  Solar incidence angle (degrees): ', $
                         solar * cspice_dpr()
                  print, FORMAT='(A,F16.3)', $
                         '  Emission angle (degrees):        ', $
                         emissn * cspice_dpr()
                  print, FORMAT='(A)',                          $
                         '  Observer visible:  ' + boolstr[visibl]
                  print, FORMAT='(A)',                          $
                         '  Sun visible:       ' + boolstr[lit]
 
                  ;;
                  ;; Lastly compute the local solar time at the boresight
                  ;; intersection.
                  ;;
                  if  i eq 4  then begin
 
                     ;; Compute local solar time corresponding to
                     ;; the TDB light time corrected epoch at the
                     ;; intercept.
                     ;;
                     cspice_et2lst, trgepc, marsid, lon, 'PLANETOCENTRIC',$
                                    hr,     min,    sc,  time,  ampm
 
                     print
                     print, '  Local Solar Time at boresight ' +$
                            'intercept (24 Hour Clock):'
                     print, '     ' + time
 
                  endif
                  ;;
                  ;; End of LST computation block.
                  ;;
 
               endelse
               ;;
               ;; End of intercept processing block.
               ;;
 
            endfor
            ;;
            ;; End of shape model loop.
            ;;
 
            print
 
         endfor
         ;;
         ;; End of vector loop.
         ;;
 
 
         cspice_kclear
 
      END
 
 
Solution Sample Output
 
   After compiling the program, execute it:
 
      Input UTC Time: 2018 JUN 11 19:32:00
      Converting UTC Time: 2018 JUN 11 19:32:00
        ET seconds past J2000:    582017589.185
 
      Vector: Boundary Corner 1
 
       Target shape model: Ellipsoid
 
        Position vector of surface intercept in the IAU_MARS frame (km):
           X   =         2535.004
           Y   =        -2028.528
           Z   =         -990.594
        Planetocentric coordinates of the intercept (degrees):
           LAT =          -16.967
           LON =          -38.667
        Phase angle (degrees):                     48.207
        Solar incidence angle (degrees):           43.872
        Emission angle (degrees):                   4.798
        Observer visible:  true
        Sun visible:       true
 
       Target shape model: DSK/Unprioritized
 
        Position vector of surface intercept in the IAU_MARS frame (km):
           X   =         2535.278
           Y   =        -2028.712
           Z   =         -990.688
        Planetocentric coordinates of the intercept (degrees):
           LAT =          -16.967
           LON =          -38.667
        Phase angle (degrees):                     48.207
        Solar incidence angle (degrees):           44.387
        Emission angle (degrees):                   4.326
        Observer visible:  true
        Sun visible:       true
 
      Vector: Boundary Corner 2
 
       Target shape model: Ellipsoid
 
        Position vector of surface intercept in the IAU_MARS frame (km):
           X   =         2525.056
           Y   =        -2042.075
           Z   =         -988.196
        Planetocentric coordinates of the intercept (degrees):
           LAT =          -16.925
           LON =          -38.963
        Phase angle (degrees):                     50.707
        Solar incidence angle (degrees):           43.586
        Emission angle (degrees):                   7.432
        Observer visible:  true
        Sun visible:       true
 
       Target shape model: DSK/Unprioritized
 
        Position vector of surface intercept in the IAU_MARS frame (km):
           X   =         2525.359
           Y   =        -2042.259
           Z   =         -988.299
        Planetocentric coordinates of the intercept (degrees):
           LAT =          -16.925
           LON =          -38.962
        Phase angle (degrees):                     50.707
        Solar incidence angle (degrees):           42.457
        Emission angle (degrees):                   8.610
        Observer visible:  true
        Sun visible:       true
 
      Vector: Boundary Corner 3
 
       Target shape model: Ellipsoid
 
        Position vector of surface intercept in the IAU_MARS frame (km):
           X   =         2525.201
           Y   =        -2042.104
           Z   =         -987.770
        Planetocentric coordinates of the intercept (degrees):
           LAT =          -16.917
           LON =          -38.962
        Phase angle (degrees):                     50.708
        Solar incidence angle (degrees):           43.585
        Emission angle (degrees):                   7.413
        Observer visible:  true
        Sun visible:       true
 
       Target shape model: DSK/Unprioritized
 
        Position vector of surface intercept in the IAU_MARS frame (km):
           X   =         2525.506
           Y   =        -2042.289
           Z   =         -987.874
        Planetocentric coordinates of the intercept (degrees):
           LAT =          -16.917
           LON =          -38.961
        Phase angle (degrees):                     50.708
        Solar incidence angle (degrees):           42.457
        Emission angle (degrees):                   8.593
        Observer visible:  true
        Sun visible:       true
 
      Vector: Boundary Corner 4
 
       Target shape model: Ellipsoid
 
        Position vector of surface intercept in the IAU_MARS frame (km):
           X   =         2535.149
           Y   =        -2028.558
           Z   =         -990.170
        Planetocentric coordinates of the intercept (degrees):
           LAT =          -16.960
           LON =          -38.666
        Phase angle (degrees):                     48.208
        Solar incidence angle (degrees):           43.871
        Emission angle (degrees):                   4.769
        Observer visible:  true
        Sun visible:       true
 
       Target shape model: DSK/Unprioritized
 
        Position vector of surface intercept in the IAU_MARS frame (km):
           X   =         2535.423
           Y   =        -2028.741
           Z   =         -990.263
        Planetocentric coordinates of the intercept (degrees):
           LAT =          -16.960
           LON =          -38.665
        Phase angle (degrees):                     48.208
        Solar incidence angle (degrees):           44.387
        Emission angle (degrees):                   4.297
        Observer visible:  true
        Sun visible:       true
 
      Vector: TGO NOMAD LNO Nadir Boresight
 
       Target shape model: Ellipsoid
 
        Position vector of surface intercept in the IAU_MARS frame (km):
           X   =         2530.122
           Y   =        -2035.307
           Z   =         -989.188
        Planetocentric coordinates of the intercept (degrees):
           LAT =          -16.942
           LON =          -38.814
        Phase angle (degrees):                     49.457
        Solar incidence angle (degrees):           43.729
        Emission angle (degrees):                   6.086
        Observer visible:  true
        Sun visible:       true
 
        Local Solar Time at boresight intercept (24 Hour Clock):
           14:51:36
 
       Target shape model: DSK/Unprioritized
 
        Position vector of surface intercept in the IAU_MARS frame (km):
           X   =         2530.471
           Y   =        -2035.529
           Z   =         -989.307
        Planetocentric coordinates of the intercept (degrees):
           LAT =          -16.942
           LON =          -38.813
        Phase angle (degrees):                     49.457
        Solar incidence angle (degrees):           44.387
        Emission angle (degrees):                   5.462
        Observer visible:  true
        Sun visible:       true
 
        Local Solar Time at boresight intercept (24 Hour Clock):
           14:51:36
 
 
 
Extra Credit
--------------------------------------------------------
 
   There are no ``extra credit'' tasks for this step of the lesson.
 
