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Remote Sensing Hands-On Lesson, using TGO (FORTRAN)

Table of Contents 

   Remote Sensing Hands-On Lesson, using TGO (FORTRAN)
      Overview
      Note About HTML Links
      References
         Tutorials
         Required Readings
         The Permuted Index
         API Documentation
      Kernels Used
      SPICE Modules Used
   Time Conversion (convtm)
      Task Statement
      Learning Goals
      Approach
      Solution
         Solution Meta-Kernel
         Solution Source Code
         Solution Sample Output
      Extra Credit
         Task statements and questions
         Solutions and answers
   Obtaining Target States and Positions (getsta)
      Task Statement
      Learning Goals
      Approach
      Solution
         Solution Meta-Kernel
         Solution Source Code
         Solution Sample Output
      Extra Credit
         Task statements and questions
         Solutions and answers
   Spacecraft Orientation and Reference Frames (xform)
      Task Statement
      Learning Goals
      Approach
      Solution
         Solution Meta-Kernel
         Solution Source Code
         Solution Sample Output
      Extra Credit
         Task statements and questions
         Solutions and answers
   Computing Sub-s/c and Sub-solar Points on an Ellipsoid and a DSK (subpts)
      Task Statement
      Learning Goals
      Approach
      Solution
         Solution Meta-Kernel
         Solution Source Code
         Solution Sample Output
      Extra Credit
         Task statements and questions
         Solutions and answers
   Intersecting Vectors with an Ellipsoid and a DSK (fovint)
      Task Statement
      Learning Goals
      Approach
      Solution
         Solution Meta-Kernel
         Solution Source Code
         Solution Sample Output
      Extra Credit


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Remote Sensing Hands-On Lesson, using TGO (FORTRAN)




May 21, 2018



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Overview



In this lesson you will develop a series of simple programs that demonstrate the usage of SPICE to compute a variety of different geometric quantities applicable to experiments carried out by a remote sensing instrument flown on an interplanetary spacecraft. This particular lesson focuses on a spectrometer flying on the ExoMars-16 TGO spacecraft, but many of the concepts are easily extended and generalized to other scenarios.



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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 ``toolkit/'' tree and copy this document to that subdirectory before loading it into a Web browser.



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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 ``toolkit/exe'' of Toolkit installation tree.



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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


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Required Readings



The Required Reading documents are provided with the Toolkit and are located under the ``toolkit/doc'' directory in the SPICE Toolkit 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


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The Permuted Index



Another useful document distributed with the Toolkit is the permuted index. This is located under the ``toolkit/doc'' directory in the FORTRAN installation tree.

This text document provides a simple mechanism by which users can discover which SPICE routines perform functions of interest, as well as the names of the source files that contain these routines. This is particularly useful for FORTRAN programmers because some of the routines are entry points; the names of these routines do not translate directly into the name of the respective source files that contain them.



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API Documentation



The most detailed specification of a given SPICE FORTRAN routine is contained in the header section of its source code. The source code is distributed with the Toolkit and is located under the ``toolkit/src/spicelib'' path.

For example the path of the source code of the STR2ET routine is 

   toolkit/src/spicelib/str2et.for
Since some of the FORTRAN routines are entry points they may be part of a source file that has different name. The ``Permuted Index'' document mentioned above can be used to locate the name of their source file.



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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/


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SPICE Modules Used



This section provides a complete list of the routines 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   ROUTINES   FUNCTIONS  KERNELS
   ------- ---------  ---------  ---------  ----------
      1    convtm     FURNSH                1,2
                      PROMPT
                      STR2ET
                      ETCAL
                      TIMOUT
                      SCE2S
 
           extra (*)  UNLOAD     UNITIM     1,2
                      SCT2E
                      ET2UTC
                      SCS2E
 
      2    getsta     FURNSH     VNORM      1,3-5
                      PROMPT
                      STR2ET
                      SPKEZR
                      SPKPOS
                      CONVRT
 
           extra (*)  KCLEAR                1,3-6,11-13
                      UNLOAD
                      BODN2C
 
      3    xform      FURNSH     VSEP       1-8
                      PROMPT
                      STR2ET
                      SPKEZR
                      SXFORM
                      MXVG
                      SPKPOS
                      PXFORM
                      MXV
                      CONVRT
 
           extra (*)  KCLEAR                1-8
                      UNLOAD
 
      4    subpts     FURNSH     VNORM      1,3-6,9
                      PROMPT
                      STR2ET
                      SUBPNT
                      SUBSLR
 
           extra (*)  KCLEAR     DPR        1,3-6
                      RECLAT
                      BODVRD
                      RECPGR
 
      5    fovint     FURNSH     DPR        1-10
                      PROMPT
                      STR2ET
                      BODN2C
                      BYEBYE
                      GETFOV
                      MOVED
                      SINCPT
                      RECLAT
                      ILLUMF
                      ET2LST
 
 
      (*) Additional APIs and kernels used in Extra Credit tasks.
Refer to the headers of the various routines listed above, as detailed interface specifications are provided with the source code.



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Time Conversion (convtm)






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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.



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Learning Goals



Familiarity with the various time conversion and parsing routines available in the Toolkit. Exposure to source code headers and their usage in learning to call routines.



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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: ETCAL or TIMOUT.



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Solution





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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


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Solution Source Code



A sample solution to the problem follows: 

         PROGRAM CONVTM
 
         IMPLICIT NONE
 
   C
   C     Local Parameters
   C
   C     The name of the meta-kernel that lists the kernels
   C     to load into the program.
   C
         CHARACTER*(*)         METAKR
         PARAMETER           ( METAKR = 'convtm.tm' )
 
   C
   C     The spacecraft clock ID code for ExoMars-16 TGO.
   C
         INTEGER               SCLKID
         PARAMETER           ( SCLKID = -143 )
 
   C
   C     The length of various string variables.
   C
         INTEGER               STRLEN
         PARAMETER           ( STRLEN = 50 )
 
   C
   C     Local Variables
   C
         CHARACTER*(STRLEN)    CALET
         CHARACTER*(STRLEN)    SCLKST
         CHARACTER*(STRLEN)    UTCTIM
 
         DOUBLE PRECISION      ET
 
   C
   C     Load the kernels this program requires.
   C     Both the spacecraft clock kernel and a
   C     leapseconds kernel should be listed
   C     in the meta-kernel.
   C
         CALL FURNSH ( METAKR )
 
   C
   C     Prompt the user for the input time string.
   C
         CALL PROMPT ( 'Input UTC Time: ', UTCTIM )
 
         WRITE (*,*) 'Converting UTC Time: ', UTCTIM
 
   C
   C     Convert UTCTIM to ET.
   C
         CALL STR2ET ( UTCTIM, ET )
 
         WRITE (*,'(A,F16.3)') '   ET Seconds Past J2000: ', ET
 
   C
   C     Now convert ET to a formal calendar time
   C     string.  This can be accomplished in two
   C     ways.
   C
         CALL ETCAL ( ET, CALET )
 
         WRITE (*,*) '   Calendar ET (ETCAL): ', CALET
 
   C
   C     Or use TIMOUT for finer control over the
   C     output format.  The picture below was built
   C     by examining the header of TIMOUT.
   C
         CALL TIMOUT ( ET, 'YYYY-MON-DDTHR:MN:SC ::TDB', CALET )
 
         WRITE (*,*) '   Calendar ET (TIMOUT): ', CALET
 
   C
   C     Convert ET to spacecraft clock time.
   C
         CALL SCE2S ( SCLKID, ET, SCLKST )
 
         WRITE (*,*) '   Spacecraft Clock Time: ', SCLKST
 
         END


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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 (ETCAL): 2018 JUN 11 19:33:09.184
       Calendar ET (TIMOUT): 2018-JUN-11T19:33:09
       Spacecraft Clock Time: 1/0070841719.26698


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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.



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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?



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Solutions and answers



    1. Two methods exist in order to convert ephemeris time to Julian Date: UNITIM and TIMOUT. The difference between them is the type of output produced by each method. UNITIM returns the double precision value of an input epoch, while 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: 

 
   =====================================================================
   ===========
 
   Toolkit version: N0066
 
   SPICE(NOLEAPSECONDS) --
 
   The variable that points to the leapseconds (DELTET/DELTA_AT) could n
   ot be
   located in the kernel pool. It is likely that the leapseconds kernel
   has not
   been loaded via the routine FURNSH.
 
   A traceback follows.  The name of the highest level module is first.
   STR2ET --> TTRANS
 
   Oh, by the way:  The SPICELIB error handling actions are USER-TAILORA
   BLE.  You
   can choose whether the Toolkit aborts or continues when errors occur,
    which
   error messages to output, and where to send the output.  Please read
   the ERROR
   "Required Reading" file, or see the routines ERRACT, ERRDEV, and ERRP
   RT.
 
   =====================================================================
   ===========
    This error is triggered by 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: 

 
   =====================================================================
   ===========
 
   Toolkit version: N0066
 
   SPICE(KERNELVARNOTFOUND) -- The Variable Was not Found in the Kernel
   Pool.
 
   SCLK_DATA_TYPE_143 not found. Did you load the SCLK kernel?
 
   A traceback follows.  The name of the highest level module is first.
   SCE2S --> SCE2T --> SCTYPE --> SCLI01
 
   Oh, by the way:  The SPICELIB error handling actions are USER-TAILORA
   BLE.  You
   can choose whether the Toolkit aborts or continues when errors occur,
    which
   error messages to output, and where to send the output.  Please read
   the ERROR
   "Required Reading" file, or see the routines ERRACT, ERRDEV, and ERRP
   RT.
 
   =====================================================================
   ===========
    This error is triggered by 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 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 TIMOUT or ET2UTC. The solution for the requested SCLK string is: 

      Earliest UTC convertible to SCLK: 2016-03-13T21:34:13.194
    6. Use SCS2E with the SCLK string obtained in the computations performed in the regular tasks and convert the resulting ephemeris time to UTC using either ET2UTC, with 'ISOC' format and 3 digits precision, or using 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


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Obtaining Target States and Positions (getsta)






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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.



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Learning Goals



Understand the anatomy of an SPKEZR call. Discover the difference between SPKEZR and SPKPOS. Familiarity with the Toolkit utility ``brief''. Exposure to unit conversion with SPICE.



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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 routine 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 ``toolkit/exe'' directory for FORTRAN toolkits. Consult its user's guide available in ``toolkit/doc/brief.ug'' for details.



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Solution





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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


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Solution Source Code



A sample solution to the problem follows: 

         PROGRAM GETSTA
 
         IMPLICIT NONE
 
   C
   C     SPICELIB Functions
   C
         DOUBLE PRECISION      VNORM
 
   C
   C     Local Parameters
   C
   C
   C     The name of the meta-kernel that lists the kernels
   C     to load into the program.
   C
         CHARACTER*(*)         METAKR
         PARAMETER           ( METAKR = 'getsta.tm' )
 
   C
   C     The length of various string variables.
   C
         INTEGER               STRLEN
         PARAMETER           ( STRLEN = 50 )
 
   C
   C     Local Variables
   C
         CHARACTER*(STRLEN)    UTCTIM
 
         DOUBLE PRECISION      DIST
         DOUBLE PRECISION      ET
         DOUBLE PRECISION      LTIME
         DOUBLE PRECISION      POS   ( 3 )
         DOUBLE PRECISION      STATE ( 6 )
 
   C
   C     Load the kernels that this program requires.  We
   C     will need a leapseconds kernel to convert input
   C     UTC time strings into ET.  We also will need the
   C     necessary SPK files with coverage for the bodies
   C     in which we are interested.
   C
         CALL FURNSH ( METAKR )
 
   C
   C     Prompt the user for the input time string.
   C
         CALL PROMPT ( 'Input UTC Time: ', UTCTIM )
 
         WRITE (*,*) 'Converting UTC Time: ', UTCTIM
 
   C
   C     Convert UTCTIM to ET.
   C
         CALL STR2ET ( UTCTIM, ET )
 
         WRITE (*,'(A,F16.3)') '   ET seconds past J2000: ', ET
 
   C
   C     Compute the apparent state of Mars as seen from
   C     ExoMars-16 TGO in the J2000 frame.  All of the ephemeris
   C     readers return states in units of kilometers and
   C     kilometers per second.
   C
         CALL SPKEZR ( 'MARS', ET,    'J2000', 'LT+S',
        .              'TGO',  STATE, LTIME               )
 
         WRITE (*,*) '   Apparent state of Mars as seen from '
        .//          'ExoMars-16 TGO in the J2000'
         WRITE (*,*) '      frame (km, km/s):'
 
         WRITE (*,'(A,F16.3)') '      X = ', STATE(1)
         WRITE (*,'(A,F16.3)') '      Y = ', STATE(2)
         WRITE (*,'(A,F16.3)') '      Z = ', STATE(3)
         WRITE (*,'(A,F16.3)') '     VX = ', STATE(4)
         WRITE (*,'(A,F16.3)') '     VY = ', STATE(5)
         WRITE (*,'(A,F16.3)') '     VZ = ', STATE(6)
 
   C
   C     Compute the apparent position of Earth as seen from
   C     ExoMars-16 TGO in the J2000 frame.  Note: We could have
   C     continued using SPKEZR and simply ignored the velocity
   C     components.
   C
         CALL SPKPOS ( 'EARTH', ET,  'J2000', 'LT+S',
        .              'TGO',   POS, LTIME               )
 
         WRITE (*,*) '   Apparent position of Earth as seen from '
        .//          'ExoMars-16 TGO in the'
         WRITE (*,*) '      J2000 frame (km):'
 
         WRITE (*,'(A,F16.3)') '      X = ', POS(1)
         WRITE (*,'(A,F16.3)') '      Y = ', POS(2)
         WRITE (*,'(A,F16.3)') '      Z = ', POS(3)
 
   C
   C     We need only display LTIME, as it is precisely the light
   C     time in which we are interested.
   C
         WRITE (*,*) '   One way light time between ExoMars-16 TGO '
        .//          'and the apparent'
         WRITE (*,'(A,F16.3)') '      position of Earth '
        .//          '(seconds): ', LTIME
 
   C
   C     Compute the apparent position of the Sun as seen from
   C     Mars in the J2000 frame.
   C
         CALL SPKPOS ( 'SUN',  ET,  'J2000', 'LT+S',
        .              'MARS', POS, LTIME                    )
 
         WRITE (*,*) '   Apparent position of Sun as seen from '
        .//          'Mars in the'
         WRITE (*,*) '      J2000 frame (km):'
 
         WRITE (*,'(A,F16.3)') '      X = ', POS(1)
         WRITE (*,'(A,F16.3)') '      Y = ', POS(2)
         WRITE (*,'(A,F16.3)') '      Z = ', POS(3)
 
   C
   C     Now we need to compute the actual distance between the Sun
   C     and Mars.  The above SPKPOS call gives us the apparent
   C     distance, so we need to adjust our aberration correction
   C     appropriately.
   C
         CALL SPKPOS ( 'SUN',  ET,  'J2000', 'NONE',
        .              'MARS', POS, LTIME                  )
 
   C
   C     Compute the distance between the body centers in
   C     kilometers.
   C
         DIST = VNORM(POS)
 
   C
   C     Convert this value to AU using CONVRT.
   C
         CALL CONVRT ( DIST, 'KM', 'AU', DIST )
 
         WRITE (*,*) '   Actual distance between Sun and Mars body '
        .//          'centers: '
         WRITE (*,'(A,F16.3)') '      (AU):', DIST
 
         END


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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


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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 SPKEZR routine, 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.



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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.



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Solutions and answers



    1. When running the original program without the Martian planetary ephemerides SPK, an error is produced by SPKEZR: 

 
   =====================================================================
   ===========
 
   Toolkit version: N0066
 
   SPICE(SPKINSUFFDATA) --
 
   Insufficient ephemeris data has been loaded to compute the state of -
   143
   (EXOMARS 2016 TGO) relative to 0 (SOLAR SYSTEM BARYCENTER) at the eph
   emeris
   epoch 2018 JUN 11 19:33:09.184.
 
   A traceback follows.  The name of the highest level module is first.
   SPKEZR --> SPKEZ --> SPKACS --> SPKGEO
 
   Oh, by the way:  The SPICELIB error handling actions are USER-TAILORA
   BLE.  You
   can choose whether the Toolkit aborts or continues when errors occur,
    which
   error messages to output, and where to send the output.  Please read
   the ERROR
   "Required Reading" file, or see the routines ERRACT, ERRDEV, and ERRP
   RT.
 
   =====================================================================
   ===========
    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 SPKPOS routine 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, 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


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Spacecraft Orientation and Reference Frames (xform)






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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.



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Learning Goals



Familiarity with the different types of kernels involved in chaining reference frames together, both inertial and non-inertial. Discover some of the matrix and vector math routines. Understand the difference between PXFORM and SXFORM.



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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 SPICE routines with which you are already familiar. The ``long way'' presented above is intended to facilitate the introduction of the routines PXFORM and SXFORM.

You may find it useful to consult the permuted index, the headers of various source modules, and the following toolkit documentation:

    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.



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Solution





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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


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Solution Source Code



A sample solution to the problem follows: 

         PROGRAM XFORM
 
         IMPLICIT NONE
 
   C
   C     SPICELIB Functions
   C
         DOUBLE PRECISION      VSEP
 
   C
   C     Local Parameters
   C
   C
   C     The name of the meta-kernel that lists the kernels
   C     to load into the program.
   C
         CHARACTER*(*)         METAKR
         PARAMETER           ( METAKR = 'xform.tm' )
 
   C
   C     The length of various string variables.
   C
         INTEGER               STRLEN
         PARAMETER           ( STRLEN = 50 )
 
   C
   C     Local Variables
   C
         CHARACTER*(STRLEN)    UTCTIM
 
         DOUBLE PRECISION      ET
         DOUBLE PRECISION      LTIME
         DOUBLE PRECISION      STATE  ( 6 )
         DOUBLE PRECISION      BFIXST ( 6 )
         DOUBLE PRECISION      POS    ( 3 )
         DOUBLE PRECISION      SXFMAT ( 6, 6 )
         DOUBLE PRECISION      PFORM  ( 3, 3 )
         DOUBLE PRECISION      BSIGHT ( 3 )
         DOUBLE PRECISION      SEP
 
   C
   C     Load the kernels that this program requires.  We
   C     will need:
   C
   C        A leapseconds kernel
   C        A spacecraft clock kernel for ExoMars-16 TGO
   C        The necessary ephemerides
   C        A planetary constants file (PCK)
   C        A spacecraft orientation kernel for ExoMars-16 TGO (CK)
   C        A frame kernel (TF)
   C
         CALL FURNSH ( METAKR )
 
   C
   C     Prompt the user for the input time string.
   C
         CALL PROMPT ( 'Input UTC Time: ', UTCTIM )
 
         WRITE (*,'(2A)') 'Converting UTC Time: ', UTCTIM
 
   C
   C     Convert UTCTIM to ET.
   C
         CALL STR2ET ( UTCTIM, ET )
 
         WRITE (*,'(A,F16.3)') '   ET seconds past J2000: ', ET
 
   C
   C     Compute the apparent state of Mars as seen from ExoMars-16
   C     TGO in the J2000 reference frame.
   C
         CALL SPKEZR ( 'MARS', ET,    'J2000', 'LT+S',
        .              'TGO',  STATE, LTIME           )
 
   C
   C     Now obtain the transformation from the inertial
   C     J2000 frame to the non-inertial, body-fixed IAU_MARS
   C     frame. Since we'll use this transformation to produce
   C     the apparent state in the IAU_MARS reference frame,
   C     we need to correct the orientation of this frame for
   C     one-way light time; hence we subtract LTIME from ET
   C     in the call below.
   C
         CALL SXFORM ( 'J2000', 'IAU_MARS', ET-LTIME, SXFMAT )
 
   C
   C     Now transform the apparent J2000 state into IAU_MARS
   C     with the following matrix multiplication:
   C
         CALL MXVG ( SXFMAT, STATE, 6, 6, BFIXST )
 
   C
   C     Display the results.
   C
         WRITE (*,'(A)') '   Apparent state of Mars as seen from '
        .//          'ExoMars-16 TGO in the IAU_MARS'
         WRITE (*,*) '      body-fixed frame (km, km/s):'
         WRITE (*,'(A,F19.6)') '      X = ', BFIXST(1)
         WRITE (*,'(A,F19.6)') '      Y = ', BFIXST(2)
         WRITE (*,'(A,F19.6)') '      Z = ', BFIXST(3)
         WRITE (*,'(A,F19.6)') '     VX = ', BFIXST(4)
         WRITE (*,'(A,F19.6)') '     VY = ', BFIXST(5)
         WRITE (*,'(A,F19.6)') '     VZ = ', BFIXST(6)
 
   C
   C     It is worth pointing out, all of the above could have
   C     been done with a single call to SPKEZR:
   C
         CALL SPKEZR ( 'MARS', ET,    'IAU_MARS', 'LT+S',
        .              'TGO',  STATE, LTIME               )
 
   C
   C     Display the results.
   C
         WRITE (*,'(A)') '   Apparent state of Mars as seen from '
        .//          'ExoMars-16 TGO in the IAU_MARS'
         WRITE (*,'(A)') '      body-fixed frame (km, km/s) '
        .//          'obtained using SPKEZR directly:'
         WRITE (*,'(A,F19.6)') '      X = ', STATE(1)
         WRITE (*,'(A,F19.6)') '      Y = ', STATE(2)
         WRITE (*,'(A,F19.6)') '      Z = ', STATE(3)
         WRITE (*,'(A,F19.6)') '     VX = ', STATE(4)
         WRITE (*,'(A,F19.6)') '     VY = ', STATE(5)
         WRITE (*,'(A,F19.6)') '     VZ = ', STATE(6)
 
   C
   C     Note that the velocity found by using SPKEZR
   C     to compute the state in the IAU_MARS frame differs
   C     at the few mm/second level from that found previously
   C     by calling SPKEZR and then SXFORM. Computing velocity
   C     via a single call to SPKEZR as we've done immediately
   C     above is slightly more accurate because it accounts for
   C     the effect of the rate of change of light time on the
   C     apparent angular velocity of the target's body-fixed
   C     reference frame.
   C
   C     Now we are to compute the angular separation between
   C     the apparent position of Mars as seen from the orbiter
   C     and the nominal instrument view direction.  First,
   C     compute the apparent position of Mars as seen from
   C     ExoMars-16 TGO in the J2000 frame.
   C
         CALL SPKPOS ( 'MARS', ET,  'J2000', 'LT+S',
        .              'TGO',  POS, LTIME               )
 
   C
   C     Now compute the location of the nominal instrument view
   C     direction.  From reading the frame kernel we know that
   C     the instrument view direction is nominally the -Y axis
   C     of the TGO_SPACECRAFT frame defined there.
   C
         BSIGHT(1) =  0.0D0
         BSIGHT(2) = -1.0D0
         BSIGHT(3) =  0.0D0
 
   C
   C     Now compute the rotation matrix from TGO_SPACECRAFT into
   C     J2000.
   C
         CALL PXFORM ( 'TGO_SPACECRAFT', 'J2000', ET, PFORM )
 
   C
   C     And multiply the result to obtain the nominal instrument
   C     view direction in the J2000 reference frame.
   C
         CALL MXV ( PFORM, BSIGHT, BSIGHT )
 
   C
   C     Lastly compute the angular separation.
   C
         CALL CONVRT ( VSEP(BSIGHT, POS), 'RADIANS',
        .              'DEGREES',         SEP        )
 
         WRITE (*,'(A)') '   Angular separation between the '
        .//          'apparent position of Mars and the'
         WRITE (*,'(A)') '      ExoMars-16 TGO nominal '
        .//          'instrument view direction (degrees):'
         WRITE (*,'(A,F19.3)') '      ', SEP
 
   C
   C     Or, alternately we can work in the spacecraft
   C     frame directly.
   C
         CALL SPKPOS ( 'MARS', ET,  'TGO_SPACECRAFT', 'LT+S',
        .              'TGO',  POS, LTIME                    )
 
   C
   C     The nominal instrument view direction is the -Y-axis
   C     in the TGO_SPACECRAFT frame.
   C
         BSIGHT(1) =  0.0D0
         BSIGHT(2) = -1.0D0
         BSIGHT(3) =  0.0D0
 
   C
   C     Lastly compute the angular separation.
   C
         CALL CONVRT ( VSEP(BSIGHT, POS), 'RADIANS',
        .              'DEGREES',         SEP        )
 
         WRITE (*,'(A)') '   Angular separation between the '
        .//          'apparent position of Mars and the'
         WRITE (*,'(A)') '      ExoMars-16 TGO nominal '
        .//          'instrument view direction at computed'
         WRITE (*,'(A)') '      using vectors in the '
        .//          'TGO_SPACECRAFT frame (degrees): '
         WRITE (*,'(A,F19.3)') '      ', SEP
 
         END


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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 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 at computed
         using vectors in the TGO_SPACECRAFT frame (degrees):
                       5.438


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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.



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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?



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Solutions and answers



    1. When running the original software using as input the UTC time string "2018 jun 12 18:25:00": 

 
   =====================================================================
   ===========
 
   Toolkit version: N0066
 
   SPICE(NOFRAMECONNECT) --
 
   At epoch 5.8209996918462E+08 TDB (2018 JUN 12 18:26:09.184 TDB), ther
   e 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 ep
   och shown
   above, as well as a corresponding SCLK kernel, must be loaded in orde
   r to use
   this frame. Failure to find required CK data could be due to one or m
   ore CK
   files not having been loaded, or to the epoch shown above lying withi
   n 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 fo
   r the
   input epoch, even if a loaded CK does have attitude data for that epo
   ch. You
   can use CKBRIEF with the -dump option to display coverage intervals o
   f a CK
   file.
 
   A traceback follows.  The name of the highest level module is first.
   PXFORM --> REFCHG
 
   Oh, by the way:  The SPICELIB error handling actions are USER-TAILORA
   BLE.  You
   can choose whether the Toolkit aborts or continues when errors occur,
    which
   error messages to output, and where to send the output.  Please read
   the ERROR
   "Required Reading" file, or see the routines ERRACT, ERRDEV, and ERRP
   RT.
 
   =====================================================================
   ===========
    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.



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Computing Sub-s/c and Sub-solar Points on an Ellipsoid and a DSK (subpts)






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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.



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Learning Goals



Discover higher level geometry calculation routines in SPICE and their usage as it relates to ExoMars-16 TGO.



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Approach



This particular problem is more of an exercise in searching the permuted index to find the appropriate routines and then reading their headers to understand how to call them.

One point worth considering: 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?



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Solution





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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


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Solution Source Code



A sample solution to the problem follows: 

         PROGRAM SUBPTS
 
         IMPLICIT NONE
   C
   C     SPICELIB functions
   C
         DOUBLE PRECISION      VNORM
 
   C
   C     Local Parameters
   C
   C
   C     The name of the meta-kernel that lists the kernels
   C     to load into the program.
   C
         CHARACTER*(*)         METAKR
         PARAMETER           ( METAKR = 'subpts.tm' )
 
   C
   C     The length of various string variables.
   C
         INTEGER               STRLEN
         PARAMETER           ( STRLEN = 50 )
 
   C
   C     Local Variables
   C
         CHARACTER*(STRLEN)    METHOD
         CHARACTER*(STRLEN)    UTCTIM
 
         DOUBLE PRECISION      ET
         DOUBLE PRECISION      SPOINT ( 3 )
         DOUBLE PRECISION      SRFVEC ( 3 )
         DOUBLE PRECISION      TRGEPC
 
         INTEGER               I
 
   C
   C     Load the kernels that this program requires.  We
   C     will need:
   C
   C        A leapseconds kernel
   C        The necessary ephemerides
   C        A planetary constants file (PCK)
   C        A DSK file containing Mars shape data
   C
         CALL FURNSH ( METAKR )
 
   C
   C     Prompt the user for the input time string.
   C
         CALL PROMPT ( 'Input UTC Time: ', UTCTIM )
 
         WRITE (*,*) 'Converting UTC Time: ', UTCTIM
 
   C
   C     Convert UTCTIM to ET.
   C
         CALL STR2ET ( UTCTIM, ET )
 
         WRITE (*,'(A,F16.3)') '   ET seconds past J2000: ', ET
 
 
         DO I = 1, 2
 
            IF ( I .EQ. 1 ) THEN
   C
   C           Use the "near point" sub-point definition
   C           and an ellipsoidal model.
   C
               METHOD = 'NEAR POINT/Ellipsoid'
 
            ELSE
   C
   C           Use the "nadir" sub-point definition and a
   C           DSK model.
   C
               METHOD = 'NADIR/DSK/Unprioritized'
 
            END IF
 
            WRITE (*,*) ' '
            WRITE (*,*) 'Sub-point/target shape model: '//METHOD
            WRITE (*,*) ' '
 
 
   C
   C        Compute the apparent sub-observer point of ExoMars-16 TGO
   C        on Mars.
   C
            CALL SUBPNT ( METHOD,
        .                 'MARS',  ET,     'IAU_MARS',   'LT+S',
        .                 'TGO',   SPOINT, TRGEPC,       SRFVEC )
 
            WRITE (*,*) '   Apparent sub-observer point of '
        .   //          'ExoMars-16 TGO on Mars '
            WRITE (*,*) '   in the IAU_MARS frame (km):'
            WRITE (*,'(A,F16.3)') '      X = ', SPOINT(1)
            WRITE (*,'(A,F16.3)') '      Y = ', SPOINT(2)
            WRITE (*,'(A,F16.3)') '      Z = ', SPOINT(3)
            WRITE (*,'(A,F16.3)') '    ALT = ', VNORM(SRFVEC)
 
   C
   C        Compute the apparent sub-solar point on Mars as seen
   C        from ExoMars-16 TGO.
   C
            CALL SUBSLR ( METHOD,
        .                 'MARS',  ET,     'IAU_MARS',   'LT+S',
        .                 'TGO',   SPOINT, TRGEPC,       SRFVEC )
 
            WRITE (*,*) '   Apparent sub-solar point on Mars as '
        .   //          'seen from ExoMars-16'
            WRITE (*,*) '   TGO in the IAU_MARS frame (km):'
            WRITE (*,'(A,F16.3)') '      X = ', SPOINT(1)
            WRITE (*,'(A,F16.3)') '      Y = ', SPOINT(2)
            WRITE (*,'(A,F16.3)') '      Z = ', SPOINT(3)
 
         END DO
 
         END


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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


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Extra Credit



In this ``extra credit'' section you will be presented with more complex tasks, aimed at improving your understanding of SUBPNT and SUBSLR routines.

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.



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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 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.



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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 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.



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Intersecting Vectors with an Ellipsoid and a DSK (fovint)






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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"



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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 routine and another time conversion routine in SPICE.



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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.



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Solution





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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


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Solution Source Code



A sample solution to the problem follows: 

         PROGRAM FOVINT
         IMPLICIT NONE
 
   C
   C     SPICELIB functions
   C
         DOUBLE PRECISION      DPR
   C
   C     Local Parameters
   C
   C     The name of the meta-kernel that lists the kernels
   C     to load into the program.
   C
         CHARACTER*(*)         METAKR
         PARAMETER           ( METAKR = 'fovint.tm' )
 
   C
   C     The length of various string variables.
   C
         INTEGER               STRLEN
         PARAMETER           ( STRLEN = 50 )
 
   C
   C     The maximum number of boundary corner vectors
   C     we can retrieve. We've extended this array by 1
   C     element to make room for the boresight vector.
   C
         INTEGER               BCVLEN
         PARAMETER           ( BCVLEN = 5 )
 
   C
   C     Local Variables
   C
         CHARACTER*(STRLEN)    AMPM
         CHARACTER*(STRLEN)    INSFRM
         CHARACTER*(STRLEN)    METHOD ( 2 )
         CHARACTER*(STRLEN)    SHAPE
         CHARACTER*(STRLEN)    TIME
         CHARACTER*(STRLEN)    UTCTIM
         CHARACTER*(STRLEN)    VECNAM ( BCVLEN )
 
         DOUBLE PRECISION      BOUNDS ( 3, BCVLEN )
         DOUBLE PRECISION      BSIGHT ( 3 )
         DOUBLE PRECISION      EMISSN
         DOUBLE PRECISION      ET
         DOUBLE PRECISION      LAT
         DOUBLE PRECISION      LON
         DOUBLE PRECISION      PHASE
         DOUBLE PRECISION      POINT  ( 3 )
         DOUBLE PRECISION      RADIUS
         DOUBLE PRECISION      SOLAR
         DOUBLE PRECISION      SRFVEC ( 3 )
         DOUBLE PRECISION      TRGEPC
 
         INTEGER               HR
         INTEGER               I
         INTEGER               J
         INTEGER               MN
         INTEGER               N
         INTEGER               LNONID
         INTEGER               MARSID
         INTEGER               SC
 
         LOGICAL               FOUND
         LOGICAL               LIT
         LOGICAL               VISIBL
 
 
   C
   C     Load the kernels that this program requires. We
   C     will need:
   C
   C        A leapseconds kernel.
   C        A SCLK kernel for ExoMars-16 TGO.
   C        Any necessary ephemerides.
   C        The ExoMars-16 TGO frame kernel.
   C        An ExoMars-16 TGO C-kernel.
   C        A PCK file with Mars constants.
   C        The ExoMars-16 TGO NOMAD I-kernel.
   C        A DSK file containing Mars shape data.
   C
         CALL FURNSH ( METAKR )
 
   C
   C     Prompt the user for the input time string.
   C
         CALL PROMPT ( 'Input UTC Time: ', UTCTIM )
 
         WRITE (*,*) 'Converting UTC Time: ', UTCTIM
 
   C
   C     Convert UTCTIM to ET.
   C
         CALL STR2ET ( UTCTIM, ET )
 
         WRITE (*,'(A,F16.3)') '   ET seconds past J2000: ', ET
 
   C
   C     Now we need to obtain the FOV configuration of the
   C     NOMAD LNO Nadir aperture. To do this we will need the ID
   C     code for TGO_NOMAD_LNO_NAD.
   C
         CALL BODN2C ( 'TGO_NOMAD_LNO_NAD', LNONID, FOUND )
 
   C
   C     Stop the program if the code was not found.
   C
         IF ( .NOT. FOUND ) THEN
            WRITE (*,*) 'Unable to locate the ID code for '
        .   //          'TGO_NOMAD_LNO_NAD'
            CALL BYEBYE ( 'FAILURE' )
         END IF
 
   C
   C     Now retrieve the field of view parameters.
   C
         CALL GETFOV ( LNONID, BCVLEN, SHAPE, INSFRM,
        .              BSIGHT, N,      BOUNDS        )
 
   C
   C     Rather than treat BSIGHT as a separate vector,
   C     copy it into the last slot of BOUNDS.
   C
         CALL MOVED ( BSIGHT, 3, BOUNDS(1,5) )
 
   C
   C     Define names for each of the vectors for display
   C     purposes.
   C
         VECNAM (1) = 'Boundary Corner 1'
         VECNAM (2) = 'Boundary Corner 2'
         VECNAM (3) = 'Boundary Corner 3'
         VECNAM (4) = 'Boundary Corner 4'
         VECNAM (5) = 'TGO NOMAD LNO Nadir Boresight'
 
   C
   C     Set values of "method" string that specify use of
   C     ellipsoidal and DSK (topographic) shape models.
   C
   C     In this case, we can use the same methods for calls to both
   C     SINCPT and ILUMIN. Note that some SPICE routines require
   C     different "method" inputs from those shown here. See the
   C     API documentation of each routine for details.
   C
         METHOD(1) = 'Ellipsoid'
         METHOD(2) = 'DSK/Unprioritized'
 
   C
   C     Get Mars ID. We'll use this ID code later, when we
   C     compute local solar time.
   C
         CALL BODN2C ( 'MARS', MARSID, FOUND )
   C
   C     Stop the program if the code was not found.
   C
         IF ( .NOT. FOUND ) THEN
            WRITE (*,*) 'Unable to locate the ID code for MARS'
            CALL BYEBYE ( 'FAILURE' )
         END IF
 
   C
   C     Now perform the same set of calculations for each
   C     vector listed in the BOUNDS array. Use both
   C     ellipsoidal and detailed (DSK) shape models.
   C
         DO I = 1, 5
 
            WRITE (*,*) ' '
            WRITE (*,*) 'Vector: ', VECNAM(I)
            WRITE (*,*) ' '
 
            DO J = 1, 2
 
               WRITE (*,*) ' Target shape model: '//METHOD(J)
               WRITE (*,*) ' '
   C
   C           Call SINCPT to determine coordinates of the
   C           intersection of this vector with the surface
   C           of Mars.
   C
               CALL SINCPT ( METHOD(J),    'MARS',      ET,
        .                    'IAU_MARS',   'LT+S',      'TGO',
        .                    INSFRM,       BOUNDS(1,I), POINT,
        .                    TRGEPC,       SRFVEC,      FOUND    )
   C
   C           Check the found flag. Display a message if the point
   C           of intersection was not found, otherwise continue with
   C           the calculations.
   C
               IF ( .NOT. FOUND ) THEN
 
                  WRITE (*,*) 'No intersection point found at '
        .         //          'this epoch for this vector.'
 
               ELSE
   C
   C              Now, we have discovered a point of intersection.
   C              Start by displaying the position vector in the
   C              IAU_MARS frame of the intersection.
   C
                  WRITE (*,*) '  Position vector of '
        .         //          'surface intercept in '
        .         //          'the IAU_MARS frame (km):'
                  WRITE (*,'(A,F16.3)') '      X   = ', POINT(1)
                  WRITE (*,'(A,F16.3)') '      Y   = ', POINT(2)
                  WRITE (*,'(A,F16.3)') '      Z   = ', POINT(3)
 
   C
   C              Display the planetocentric latitude and longitude
   C              of the intercept.
   C
                  CALL RECLAT ( POINT, RADIUS, LON, LAT )
 
                  WRITE (*,*) '  Planetocentric coordinates of the '
        .         //          'intercept (degrees):'
                  WRITE (*,'(A,F16.3)') '      LAT = ', LAT * DPR()
                  WRITE (*,'(A,F16.3)') '      LON = ', LON * DPR()
 
   C
   C              Compute the illumination angles at this
   C              point.
   C
                  CALL ILLUMF ( METHOD(J),  'MARS',       'SUN',
        .                       ET,         'IAU_MARS',   'LT+S',
        .                       'TGO',      POINT,        TRGEPC,
        .                       SRFVEC,     PHASE,        SOLAR,
        .                       EMISSN,     VISIBL,       LIT    )
 
                  WRITE (*,'(A,F16.3)') '   Phase angle (degrees):'
        .         //                    '           ', PHASE * DPR()
                  WRITE (*,'(A,F16.3)') '   Solar incidence angle '
        .         //                    '(degrees): ', SOLAR * DPR()
                  WRITE (*,'(A,F16.3)') '   Emission angle (degree'
        .         //                    's):        ', EMISSN* DPR()
                  WRITE (*,'(A,L2)'   ) '   Observer visible: ', VISIBL
                  WRITE (*,'(A,L2)'   ) '   Sun visible:      ', LIT
 
                  IF ( I .EQ. 5 ) THEN
   C
   C                 Compute local time corresponding to the TDB
   C                 light time corrected epoch at the boresight
   C                 intercept.
   C
                     CALL ET2LST ( TRGEPC,
        .                          MARSID,
        .                          LON,
        .                          'PLANETOCENTRIC',
        .                          HR,
        .                          MN,
        .                          SC,
        .                          TIME,
        .                          AMPM              )
 
                     WRITE (*,*) ' '
                     WRITE (*,*) '  Local Solar Time at boresight '
        .            //          'intercept (24 Hour Clock): '
                     WRITE (*,*) '     ', TIME
 
                  END IF
 
               END IF
 
               WRITE (*,*) ' '
 
            END DO
   C
   C        End of shape model loop.
   C
         END DO
   C
   C     End of vector loop.
   C
         END


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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:  T
      Sun visible:       T
 
     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:  T
      Sun visible:       T
 
 
    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:  T
      Sun visible:       T
 
     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:  T
      Sun visible:       T
 
 
    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:  T
      Sun visible:       T
 
     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:  T
      Sun visible:       T
 
 
    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:  T
      Sun visible:       T
 
     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:  T
      Sun visible:       T
 
 
    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:  T
      Sun visible:       T
 
      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:  T
      Sun visible:       T
 
      Local Solar Time at boresight intercept (24 Hour Clock):
         14:51:36


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Extra Credit



There are no ``extra credit'' tasks for this step of the lesson.