Main Page Remote Sensing Hands-On Lesson (IDL)
## Table of Contents
       Remote Sensing Hands-On Lesson (IDL)
          Overview
          Note About HTML Links
          References
             Tutorials
             Required Readings
             The Permuted Index
             Icy API Documentation
          Kernels Used
          Icy Modules Used
       Time Conversion (convtm)
          Task Statement
          Learning Goals
          Approach
          Solution
             Solution Meta-Kernel
             Solution Source Code
             Solution Sample Output
       Obtaining Target States and Positions (getsta)
          Task Statement
          Learning Goals
          Approach
          Solution
             Solution Meta-Kernel
             Solution Source Code
             Solution Sample Output
       Spacecraft Orientation and Reference Frames (xform)
          Task Statement
          Learning Goals
          Approach
          Solution
             Solution Meta-Kernel
             Solution Source Code
             Solution Sample Output
       Computing Sub-spacecraft and Sub-solar Points (subpts)
          Task Statement
          Learning Goals
          Approach
          Solution
             Solution Meta-Kernel
             Solution Source Code
             Solution Sample Output
       Intersecting Vectors with a Triaxial Ellipsoid (fovint)
          Task Statement
          Learning Goals
          Approach
          Solution
             Solution Meta-Kernel
             Solution Source Code
             Solution Sample Output
Top # Remote Sensing Hands-On Lesson (IDL) February 8, 2010 Top ## Overview In this lesson you will develop a series of simple programs that demonstrate the usage of Icy to compute a variety of different geometric quantities applicable to experiments carried out by a remote sensing instrument flown on an interplanetary spacecraft. This particular lesson focuses on a framing camera flying on the Cassini spacecraft, but many of the concepts are easily extended and generalized to other scenarios. Top ## 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, 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. Top ## References Top ### Tutorials The following SPICE tutorials are referred to by the discussions in this lesson:
       Name             Lesson steps/routines it describes
       ---------------  -----------------------------------------
       Time             Time Conversion
       SCLK and LSK     Time Conversion
       SPK              Obtaining Ephemeris Data
       Frames           Reference Frames
       Using Frames     Reference Frames
       PCK              Planetary Constants Data
       CK               Spacecraft Orientation Data
These tutorials are available from the NAIF ftp server at JPL:
       http://naif.jpl.nasa.gov/naif/tutorials.html
Top ### Required Readings The Required Reading documents are provided with the Toolkit and are located under the ``icy/doc'' directory in the IDL installation tree.
       Name             Lesson steps/routines that it describes
       ---------------  -----------------------------------------
       time.req         Time Conversion
       sclk.req         SCLK Time Conversion
       spk.req          Obtaining Ephemeris Data
       frames.req       Using Reference Frames
       pck.req          Obtaining Planetary Constants Data
       ck.req           Obtaining Spacecraft Orientation Data
       naif_ids.req     Determining Body ID Codes
Top ### The Permuted Index Another useful document distributed with the Toolkit is the permuted index. This is located under the ``icy/doc'' directory in the IDL installation tree. This text document provides a simple mechanism to discover what Icy procedures perform a particular function of interest as well as the name of the source module that contains the procedure. Top ### Icy API Documentation An Icy routine's specification is found in the API documentation page located under ``icy/doc/html/icy''. For example, the document
       icy/doc/html/icy/cspice_str2et.html
describes the cspice_str2et routine. Top ## Kernels Used The following kernels are used in examples provided in this lesson:
       #  FILE NAME                 TYPE  DESCRIPTION
       -- ------------------------- ----  ------------------------
       1  naif0008.tls              LSK   Generic LSK
       2  cas00084.tsc              SCLK  Cassini SCLK
       3  981005_PLTEPH-DE405S.bsp  SPK   Solar System Ephemeris
       4  020514_SE_SAT105.bsp      SPK   Saturnian Satellite Ephemeris
       5  030201AP_SK_SM546_T45.bsp SPK   Cassini Spacecraft SPK
       6  cas_v37.tf                FK    Cassini FK
       7  04135_04171pc_psiv2.bc    CK    Cassini Spacecraft CK
       8  cpck05Mar2004.tpc         PCK   Cassini Project PCK
       9  cas_iss_v09.ti            IK    ISS Instrument Kernel
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/
Top ## Icy Modules Used This section provides a complete summary of the procedures, and the kernels that are suggested for usage in each of the exercises in this tutorial. (You may wish to not look at this list unless/until you ``get stuck'' while working on your own.)
       CHAPTER EXERCISE   FUNCTIONS      NON-VOID       KERNELS
       ------- ---------  -------------  ---------      -------
         1     convtm     cspice_furnsh                 1,2
                          cspice_prompt
                          cspice_str2et
                          cspice_etcal
                          cspice_timout
                          cspice_sce2c
                          cspice_sce2s

         2     getsta     cspice_furnsh  cspice_vnorm   1,3-5
                          cspice_prompt
                          cspice_str2et
                          cspice_spkezr
                          cspice_spkpos
                          cspice_convrt

         3     xform      cspice_furnsh  cspice_vsep    1-8
                          cspice_str2et
                          cspice_spkezr
                          cspice_sxform
                          cspice_mxvg
                          cspice_spkpos
                          cspice_pxform
                          cspice_mxv
                          cspice_convrt

         4     subpts     cspice_furnsh                 1,3-5,8
                          cspice_str2et
                          cspice_subpt
                          cspice_subsol

         5     fovint     cspice_furnsh  cspice_dpr     1-9
                          cspice_str2et
                          cspice_bodn2c
                          cspice_getfov
                          cspice_sincpt
                          cspice_reclat
                          cspice_ilumin
                          cspice_et2lst
Refer to the headers of the various procedures listed above, as detailed interface specifications are provided with the source code. Top # Time Conversion (convtm) Top ## Task Statement Write a program that prompts the user for an input UTC time string, converts it to the following time systems and output formats: `1.` Ephemeris Time (ET) in seconds past J2000 `2.` Calendar Ephemeris Time `3.` Spacecraft Clock Time and displays the results. Use the program to convert "2004 jun 11 19:32:00" UTC into these alternate systems. Top ## Learning Goals Familiarity with the various time conversion and parsing procedures available in the Toolkit. Exposure to source code headers and their usage in learning to call procedures. Top ## Approach The solution to the problem can be broken down into a series of simple steps: `--` Decide which SPICE kernels are necessary. Prepare a meta-kernel listing the kernels and load it into the program. `--` Prompt the user for an input UTC time string. `--` Convert the input time string into ephemeris time expressed as seconds past J2000 TDB. Display the result. `--` Convert ephemeris time into a calendar format. Display the result. `--` Convert ephemeris time into a spacecraft clock string. Display the result. You may find it useful to consult the permuted index, the headers of various source modules, and the ``Time Required Reading'' (time.req) and ``SCLK Required Reading'' (sclk.req) documents. When completing the ``calendar format'' step above, consider using one of two possible methods: cspice_etcal or cspice_timout. Top ## Solution Top ### 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.

          \begindata
          KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
                              'kernels/sclk/cas00084.tsc' )
          \begintext
Top ### Solution Source Code A sample solution to the problem follows:
       PRO convtm

          ;;
          ;; Local Parameters
          ;;

          METAKR = "convtm.tm"
          SCLKID = -82
          STRLEN = 50
          utctim = ''

          ;;
          ;; Load the kernels his program requires.
          ;; Both the spacecraft clock kernel and a
          ;; leapseconds kernel should be listed in
          ;; the meta-kernel.
          ;;
          cspice_furnsh, METAKR

          ;;
          ;; Prompt the user for the input time string.
          ;;
          read, utctim, PROMPT="Input UTC Time: "

          print, "Converting UTC Time: ", utctim

          ;;
          ;; Convert utctim to et.
          ;;
          cspice_str2et, utctim, et

          print, FORMAT="(A,F16.3)", "   ET Seconds Past J2000: ", et

          ;;
          ;; Now convert ET to a formal calendar time
          ;; string.  This can be accomplished in two
          ;; ways.
          ;;
          cspice_etcal, et, calet

          print, "   Calendar ET (cspice_etcal): ", calet

          ;;
          ;; Or use cspice_timout for finer control over the
          ;; output format.  The picture below was built
          ;; by examining the header of cspice_timout.
          ;;
          cspice_timout, et   , "YYYY-MON-DDTHR:MN:SC ::TDB", $
                        STRLEN, calet

          print, "   Calendar ET (cspice_timout): ", calet

          ;;
          ;; Convert ET to spacecraft clock time.
          ;;
          cspice_sce2s, SCLKID, et, sclkst

          print, "   Spacecraft Clock Time: ", sclkst

          cspice_unload, METAKR

       END
Top ### Solution Sample Output After compiling the program, execute it:
       Converting UTC Time: 2004 jun 11 19:32:00
          ET Seconds Past J2000:    140254384.185
          Calendar ET (cspice_etcal): 2004 JUN 11 19:33:04.184
          Calendar ET (cspice_timout): 2004-JUN-11T19:33:04
          Spacecraft Clock Time: 1/1465674964.105
Top # Obtaining Target States and Positions (getsta) Top ## Task Statement Write a program that prompts the user for an input UTC time string, computes the following quantities at that epoch: `1.` The apparent state of Phoebe as seen from CASSINI in the J2000 frame, in kilometers and kilometers/second. This vector itself is not of any particular interest, but it is a useful intermediate quantity in some geometry calculations. `2.` The apparent position of the Earth as seen from CASSINI in the J2000 frame, in kilometers. `3.` The one-way light time between CASSINI and the apparent position of Earth, in seconds. `4.` The apparent position of the Sun as seen from Phoebe in the J2000 frame (J2000), in kilometers. `5.` The actual (geometric) distance between the Sun and Phoebe, in astronomical units. and displays the results. Use the program to compute these quantities at "2004 jun 11 19:32:00" UTC. Top ## Learning Goals Understand the anatomy of an cspice_spkezr call. Discover the difference between cspice_spkezr and cspice_spkpos. Familiarity with the Toolkit utility ``brief''. Exposure to unit conversion with Icy. Top ## Approach The solution to the problem can be broken down into a series of simple steps: `--` Decide which SPICE kernels are necessary. Prepare a meta-kernel listing the kernels and load it into the program. `--` Prompt the user for an input time string. `--` Convert the input time string into ephemeris time expressed as seconds past J2000 TDB. `--` Compute the state of Phoebe relative to CASSINI in the J2000 reference frame, corrected for aberrations. `--` Compute the position of Earth relative to CASSINI in the J2000 reference frame, corrected for aberrations. (The procedure in the library that computes this also returns the one-way light time between CASSINI and Earth.) `--` Compute the position of the Sun relative to Phoebe in the J2000 reference frame, corrected for aberrations. `--` Compute the position of the Sun relative to Phoebe without correcting for aberration. `--` Compute the length of this vector. This provides the desired distance in kilometers. `--` Convert the distance in kilometers into AU. You may find it useful to consult the permuted index, the headers of various source modules, and the ``SPK Required Reading'' (spk.req) document. When deciding which SPK files to load, the Toolkit utility ``brief'' may be of some use. ``brief'' is located in the ``icy/exe'' directory for IDL toolkits. Consult its user's guide available in ``icy/doc/brief.ug'' for details. Top ## Solution Top ### 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.

          \begindata
          KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
                              'kernels/spk/981005_PLTEPH-DE405S.bsp',
                              'kernels/spk/020514_SE_SAT105.bsp',
                              'kernels/spk/030201AP_SK_SM546_T45.bsp' )
          \begintext
Top ### Solution Source Code A sample solution to the problem follows:
       PRO getsta

          ;;
          ;; Local Parameters
          ;;

          METAKR = 'getsta.tm'
          STRLEN = 50
          utctim = ''

          ;;
          ;; Load the kernels that this program requires.  We
          ;; will need a leapseconds kernel to convert input
          ;; UTC time strings into ET.  We also will need the
          ;; necessary SPK files with coverage for the bodies
          ;; in which we are interested.
          ;;
          cspice_furnsh, METAKR

          ;;
          ;; Prompt the user for the input time string.
          ;;
          read, utctim, PROMPT = 'Input UTC Time: '

          print, 'Converting UTC Time: ', utctim

          ;;
          ;; Convert utctim to et.
          ;;
          cspice_str2et, utctim, et

          print, FORMAT='(A,F16.3)', '   ET seconds past J2000: ', et

          ;;
          ;; Compute the apparent state of Phoebe as seen from
          ;; CASSINI in the J2000 frame.  All of the ephemeris
          ;; readers return states in units of kilometers and
          ;; kilometers per second.
          ;;
          cspice_spkezr, 'PHOEBE' , et   , 'J2000', 'LT+S', $
                         'CASSINI', state, ltime

          print, '   Apparent state of Phoebe as seen ' +$
                 'from CASSINI in the J2000 '
          print, '      frame (km, km/s): '
          print, FORMAT='(A,F16.3)', '      X = ', state[0]
          print, FORMAT='(A,F16.3)', '      Y = ', state[1]
          print, FORMAT='(A,F16.3)', '      Z = ', state[2]
          print, FORMAT='(A,F16.3)', '     VX = ', state[3]
          print, FORMAT='(A,F16.3)', '     VY = ', state[4]
          print, FORMAT='(A,F16.3)', '     VZ = ', state[5]

          ;;
          ;; Compute the apparent position of Earth as seen from
          ;; CASSINI in the J2000 frame.  Note: We could have
          ;; continued using cspice_spkezr and simply ignored the
          ;; velocity components.
          ;;
          cspice_spkpos, 'EARTH'  , et , 'J2000', 'LT+S', $
                         'CASSINI', pos, ltime

          print, '   Apparent position of Earth as '  +$
                      'seen from CASSINI in the J2000 '
          print, '      frame (km):  '
          print, FORMAT='(A,F16.3)', '      X = ', pos[0]
          print, FORMAT='(A,F16.3)', '      Y = ', pos[1]
          print, FORMAT='(A,F16.3)', '      Z = ', pos[2]

          ;;
          ;; We need only display LT, as it is precisely the
          ;; light time in which we are interested.
          ;;
          print, '   One way light time between CASSINI and ' +$
                 'the apparent position'
          print, FORMAT='(A,F16.3)', '      of Earth (seconds): ', $
                 ltime

          ;;
          ;; Compute the apparent position of the Sun as seen
          ;; from Phoebe in the J2000 frame.
          ;;
          cspice_spkpos, 'SUN'   , et , 'J2000', 'LT+S', $
                         'PHOEBE', pos, ltime

          print, '   Apparent position of Sun as seen ' +$
                      'from Phoebe in the '
          print, '      J2000 frame (km): '
          print, FORMAT='(A,F16.3)', '      X = ', pos[0]
          print, FORMAT='(A,F16.3)', '      Y = ', pos[1]
          print, FORMAT='(A,F16.3)', '      Z = ', pos[2]

          ;;
          ;; Now we need to compute the actual distance between
          ;; the Sun and Phoebe.  The above SPKPOS call gives us
          ;; the apparent distance, so we need to adjust our
          ;; aberration correction appropriately.
          ;;
          cspice_spkpos, 'SUN'   , et , 'J2000', 'NONE', $
                         'PHOEBE', pos, ltime

          ;;
          ;; Compute the distance between the body centers in
          ;; kilometers.
          ;;
          dist = cspice_vnorm ( pos )

          ;;
          ;; Convert this value to AU using cspice_convrt.
          ;; Recall, cspice_convrt cannot overwrite the
          ;; input with the output. Use 'dist_au' for the
          ;; output value.
          ;;
          cspice_convrt, dist, 'KM', 'AU', dist_au

          print, '   Actual distance between Sun and Phoebe ' + $
                 'body centers:'
          print, FORMAT='(A,F16.3)', '      (AU): ', dist_au

          cspice_kclear

       END
Top ### Solution Sample Output After compiling the program, execute it:
       Converting UTC Time: 2004 JUN 11 19:32:00
          ET seconds past J2000:    140254384.185
          Apparent state of Phoebe as seen from CASSINI in the J2000
             frame (km, km/s):
             X =         -119.921
             Y =         2194.139
             Z =          -57.639
            VX =           -5.980
            VY =           -2.119
            VZ =           -0.295
          Apparent position of Earth as seen from CASSINI in the J2000
             frame (km):
             X =    353019393.123
             Y =  -1328180352.140
             Z =   -568134171.697
          One way light time between CASSINI and the apparent position
             of Earth (seconds):         4960.427
          Apparent position of Sun as seen from Phoebe in the
             J2000 frame (km):
             X =    376551465.272
             Y =  -1190495630.303
             Z =   -508438699.110
          Actual distance between Sun and Phoebe body centers:
             (AU):            9.012
Top # Spacecraft Orientation and Reference Frames (xform) Top ## Task Statement Write a program that prompts the user for an input time string, computes and displays the following at the epoch of interest: `1.` The apparent state of Phoebe as seen from CASSINI in the IAU_PHOEBE body-fixed frame. This vector itself is not of any particular interest, but it is a useful intermediate quantity in some geometry calculations. `2.` The angular separation between the apparent position of Earth as seen from CASSINI and the nominal boresight of the CASSINI high gain antenna (HGA). `` The HGA boresight direction is provided by the kernel variable TKFRAME_-82101_BORESIGHT, which is defined in the Cassini frame kernel cited above in the section ``Kernels Used.'' In this kernel, the HGA boresight vector is expressed relative to the CASSINI_HGA reference frame. Use the program to compute these quantities at the epoch "2004 jun 11 19:32:00" UTC. Top ## Learning Goals Familiarity with the different types of kernels involved in chaining reference frames together, both inertial and non-inertial. Discover some of the matrix and vector math procedures. Understand the difference between cspice_pxform and cspice_sxform. Top ## Approach The solution to the problem can be broken down into a series of simple steps: `--` Decide which SPICE kernels are necessary. Prepare a meta-kernel listing the kernels and load it into the program. `--` Prompt the user for an input time string. `--` Convert the input time string into ephemeris time expressed as seconds past J2000 TDB. `--` Compute the state of Phoebe relative to CASSINI in the J2000 reference frame, corrected for aberrations. `--` Compute the state transformation matrix from J2000 to IAU_PHOEBE at the epoch, adjusted for light time. `--` Multiply the state of Phoebe relative to CASSINI in the J2000 reference frame by the state transformation matrix computed in the previous step. `--` Compute the position of Earth relative to CASSINI in the J2000 reference frame, corrected for aberrations. `--` Determine what the nominal boresight of the CASSINI high gain antenna is by examining the frame kernel's content. `--` Compute the rotation matrix from the CASSINI high gain antenna frame to J2000. `--` Multiply the nominal boresight expressed in the CASSINI high gain antenna frame by the rotation matrix from the previous step. `--` Compute the separation between the result of the previous step and the apparent position of the Earth relative to CASSINI in the J2000 frame. HINT: Several of the steps above may be compressed into a single using Icy procedures with which you are already familiar. The ``long-way'' presented above is intended to facilitate the introduction of the procedures cspice_pxform and cspice_sxform. You may find it useful to consult the permuted index, the headers of various source modules, and the following toolkit documentation: `1.` Frames Required Reading (frames.req) `2.` PCK Required Reading (pck.req) `3.` SPK Required Reading (spk.req) `4.` CK Required Reading (ck.req) This particular example makes use of many of the different types of SPICE kernels. You should spend a few moments thinking about which kernels you will need and what data they provide. Top ## Solution Top ### 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.

          \begindata
          KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
                              'kernels/sclk/cas00084.tsc',
                              'kernels/spk/981005_PLTEPH-DE405S.bsp',
                              'kernels/spk/020514_SE_SAT105.bsp',
                              'kernels/spk/030201AP_SK_SM546_T45.bsp',
                              'kernels/fk/cas_v37.tf',
                              'kernels/ck/04135_04171pc_psiv2.bc',
                              'kernels/pck/cpck05Mar2004.tpc' )
          \begintext
Top ### Solution Source Code A sample solution to the problem follows:
       PRO xform

          ;;
          ;; Local Parameters
          ;;

          METAKR = 'xform.tm'
          STRLEN = 50
          utctim = ''

          ;;
          ;; Load the kernels that this program requires. We
          ;; will need:
          ;;
          ;;    A leapseconds kernel
          ;;    A spacecraft clock kernel for CASSINI
          ;;    The necessary ephemerides
          ;;    A planetary constants file (PCK)
          ;;    A spacecraft orientation kernel for CASSINI (CK)
          ;;    A frame kernel (TF)
          ;;
          cspice_furnsh, METAKR

          ;;
          ;; Prompt the user for the input time string.
          ;;
          read, utctim, PROMPT = 'Input UTC Time: '

          print, 'Converting UTC Time: ', utctim

          ;;
          ;; Convert utctim to et.
          ;;
          cspice_str2et, utctim, et

          print, FORMAT='(A,F16.3)', '   ET seconds past J2000: ', et

          ;;
          ;; Compute the apparent state of Phoebe as seen from
          ;; CASSINI in the J2000 frame. All of the ephemeris
          ;; readers return states in units of kilometers and
          ;; kilometers per second.
          ;;
          cspice_spkezr, 'PHOEBE' , et   , 'J2000', 'LT+S', $
                         'CASSINI', state, ltime

          ;;
          ;; Now obtain the transformation from the inertial
          ;; J2000 frame to the non-inertial body-fixed IAU_PHOEBE
          ;; frame. Since we want the apparent state in the
          ;; IAU_PHOEBE reference frame, we need to correct the
          ;; orientation and angular velocity of this frame
          ;; for one-way light time; hence we subtract ltime from
          ;; et in the call below.
          ;;
          cspice_sxform, 'J2000', 'IAU_PHOEBE', et-ltime, sxfmat

          ;;
          ;; Now transform the apparent J2000 state into IAU_PHOEBE
          ;; with the following matrix multiplication:
          ;;
          bfixst = transpose(sxfmat) # state

          ;;
          ;; Display the results.
          ;;
          print, '   Apparent state of Phoebe as seen ' +$
                      'from CASSINI in the IAU_PHOEBE'
          print, '      body-fixed frame (km, km/s):'
          print, FORMAT='(A,F19.6)', '      X = ', bfixst[0]
          print, FORMAT='(A,F19.6)', '      Y = ', bfixst[1]
          print, FORMAT='(A,F19.6)', '      Z = ', bfixst[2]
          print, FORMAT='(A,F19.6)', '     VX = ', bfixst[3]
          print, FORMAT='(A,F19.6)', '     VY = ', bfixst[4]
          print, FORMAT='(A,F19.6)', '     VZ = ', bfixst[5]

          ;;
          ;; It is worth pointing out, all of the above could
          ;; have been done with a single use of cspice_spkezr:
          ;;
          ;;
          cspice_spkezr, 'PHOEBE' , et    , 'IAU_PHOEBE', 'LT+S', $
                         'CASSINI',  state, ltime

          ;;
          ;; Display the results.
          ;;
          print, '   Apparent state of Phoebe as seen ' +$
                      'from CASSINI in the IAU_PHOEBE'
          print, '       body-fixed frame (km, km/s) '  +$
                 'obtained using cspice_spkezr'
          print, '       directly:'
          print, FORMAT='(A,F19.6)', '      X = ', state[0]
          print, FORMAT='(A,F19.6)', '      Y = ', state[1]
          print, FORMAT='(A,F19.6)', '      Z = ', state[2]
          print, FORMAT='(A,F19.6)', '     VX = ', state[3]
          print, FORMAT='(A,F19.6)', '     VY = ', state[4]
          print, FORMAT='(A,F19.6)', '     VZ = ', state[5]

          ;;
          ;; Note that the velocity found by using cspice_spkezr
          ;; to compute the state in the IAU_PHOEBE frame differs
          ;; at the few mm/second level from that found previously
          ;; by calling cspice_spkezr and then cspice_sxform. Computing
          ;; velocity via a single call to cspice_spkezr as we've
          ;; done immediately above is slightly more accurate because
          ;; it accounts for the effect of the rate of change of
          ;; light time on the apparent angular velocity of the
          ;; target's body-fixed reference frame.
          ;;
          ;; Now we are to compute the angular separation between
          ;; the apparent position of the Earth as seen from the
          ;; orbiter and the nominal boresight of the high gain
          ;; antenna. First, compute the apparent position of
          ;; the Earth as seen from CASSINI in the J2000 frame.
          ;;
          cspice_spkpos, 'EARTH'  , et,  'J2000', 'LT+S', $
                         'CASSINI', pos,  ltime

          ;;
          ;; Now compute the location of the antenna boresight
          ;; at this same epoch. From reading the frame kernel
          ;; we know that the antenna boresight is nominally the
          ;; +Z axis of the CASSINI_HGA frame defined there.
          ;;
          bsight = [ 0.D0, 0.D0, 1.D0]

          ;;
          ;; Now compute the rotation matrix from CASSINI_HGA into
          ;; J2000.
          ;;
          cspice_pxform, 'CASSINI_HGA', 'J2000', et, pform

          ;;
          ;; And multiply the result to obtain the nominal
          ;; antenna boresight in the J2000 reference frame.
          ;;
          cspice_mxv, pform, bsight, bsight

          ;;
          ;; Lastly compute the angular separation.
          ;;
          cspice_convrt, cspice_vsep(bsight, pos), 'RADIANS', $
                        'DEGREES', sep

          print, '   Angular separation between the ' +$
                      'apparent position of'
          print, '      Earth and the CASSINI high '  +$
                      'gain antenna boresight (degrees):'
          print, FORMAT='(A,F16.3)', '      ', sep

          ;;
          ;; Or alternatively we can work in the antenna
          ;; frame directly.
          ;;
          cspice_spkpos, 'EARTH'  , et , 'CASSINI_HGA', 'LT+S', $
                         'CASSINI', pos, ltime

          ;;
          ;; The antenna boresight is the Z-axis in the
          ;; CASSINI_HGA frame.
          ;;
          bsight = [ 0.D0, 0.D0, 1.D0]

          ;;
          ;; Lastly compute the angular separation.
          ;;
          cspice_convrt, cspice_vsep(bsight, pos), 'RADIANS', $
                        'DEGREES', sep

          print, '   Angular separation between the '      +$
                      'apparent position of'
          print, '      Earth and the CASSINI high '       +$
                      'gain antenna boresight computed'
          print, '      using vectors in the CASSINI_HGA ' +$
                      'frame (degrees):'
          print, FORMAT='(A,F16.3)', '      ', sep

          cspice_kclear

       END
Top ### Solution Sample Output After compiling the program, execute it:
       Converting UTC Time: 2004 JUN 11 19:32:00
          ET seconds past J2000:    140254384.185
          Apparent state of Phoebe as seen from CASSINI in the IAU_PHOEBE
             body-fixed frame (km, km/s):
             X =        -1982.639762
             Y =         -934.530471
             Z =         -166.562595
            VX =            3.970833
            VY =           -3.812498
            VZ =           -2.371663
          Apparent state of Phoebe as seen from CASSINI in the IAU_PHOEBE
              body-fixed frame (km, km/s) obtained using cspice_spkezr
              directly:
             X =        -1982.639762
             Y =         -934.530471
             Z =         -166.562595
            VX =            3.970832
            VY =           -3.812496
            VZ =           -2.371663
          Angular separation between the apparent position of
             Earth and the CASSINI high gain antenna boresight (degrees):
                       71.924
          Angular separation between the apparent position of
             Earth and the CASSINI high gain antenna boresight computed
             using vectors in the CASSINI_HGA frame (degrees):
                       71.924
Top # Computing Sub-spacecraft and Sub-solar Points (subpts) Top ## Task Statement Write a program that prompts the user for an input UTC time string, computes the following quantities at that epoch: `1.` The apparent sub-observer point of CASSINI on Phoebe in the body fixed frame IAU_PHOEBE in kilometers. `2.` The apparent sub-solar point on Phoebe as seen from CASSINI in the body fixed frame IAU_PHOEBE in kilometers. and displays the results. Use the program to compute these quantities at "2004 jun 11 19:32:00" UTC. Top ## Learning Goals Discover higher level geometry calculation procedures in Icy and their usage as it relates to CASSINI. Top ## Approach This particular problem is more of an exercise in searching the permuted index to find the appropriate procedures and then reading their headers to understand how to call them. One point worth considering: Which method do you want to use to compute the sub-solar (or sub-observer) point? Top ## Solution Top ### 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-spacecraft and Sub-solar Points'' task
       in the Remote Sensing Hands On Lesson.

          \begindata
          KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
                              'kernels/spk/981005_PLTEPH-DE405S.bsp',
                              'kernels/spk/020514_SE_SAT105.bsp',
                              'kernels/spk/030201AP_SK_SM546_T45.bsp',
                              'kernels/pck/cpck05Mar2004.tpc' )
          \begintext
Top ### Solution Source Code A sample solution to the problem follows:
       PRO subpts

          ;;
          ;; Local Parameters
          ;;
          METAKR = 'subpts.tm'
          STRLEN = 50
          utctim = ''

          ;;
          ;; Load the kernels that this program requires.  We
          ;; will need:
          ;;
          ;;    A leapseconds kernel
          ;;    The necessary ephemerides
          ;;    A planetary constants file (PCK)
          ;;
          cspice_furnsh, METAKR

          ;;
          ;; Prompt the user for the input time string.
          ;;
          read, utctim, PROMPT = 'Input UTC Time: '

          print, 'Converting UTC Time: ', utctim

          ;;
          ;; Convert utctim to et.
          ;;
          cspice_str2et, utctim, et

          print, FORMAT='(A,F16.3)', '   ET seconds past J2000: ', et

          ;;
          ;; Compute the apparent sub-observer point of CASSINI
          ;; on Phoebe.
          ;;
          cspice_subpnt, 'NEAR POINT: ELLIPSOID',                 $
                         'PHOEBE',  et,     'IAU_PHOEBE', 'LT+S', $
                         'CASSINI', spoint, trgepc,       srfvec

          print, '   Apparent sub-observer point of CASSINI ' +$
                 'on Phoebe in the'
          print, '   IAU_PHOEBE frame (km):'
          print, FORMAT='(A,F16.3)', '      X = ', spoint[0]
          print, FORMAT='(A,F16.3)', '      Y = ', spoint[1]
          print, FORMAT='(A,F16.3)', '      Z = ', spoint[2]
          print, FORMAT='(A,F16.3)', '    ALT = ', cspice_vnorm(srfvec)

          ;;
          ;; Compute the apparent sub-solar point on Phoebe
          ;; as seen from CASSINI.
          ;;
          cspice_subslr, 'NEAR POINT: ELLIPSOID',                 $
                         'PHOEBE',  et,     'IAU_PHOEBE', 'LT+S', $
                         'CASSINI', spoint, trgepc,       srfvec

          print, '   Apparent sub-solar point on Phoebe ' +$
                 'as seen from CASSINI in'
          print, '   the IAU_PHOEBE frame (km):'
          print, FORMAT='(A,F16.3)', '      X = ', spoint[0]
          print, FORMAT='(A,F16.3)', '      Y = ', spoint[1]
          print, FORMAT='(A,F16.3)', '      Z = ', spoint[2]

          cspice_unload, METAKR

       END
Top ### Solution Sample Output After compiling the program, execute it:
       Converting UTC Time: 2004 JUN 11 19:32:00
          ET seconds past J2000:    140254384.185
          Apparent sub-observer point of CASSINI on Phoebe in the
          IAU_PHOEBE frame (km):
             X =          104.498
             Y =           45.269
             Z =            7.383
           ALT =         2084.116
          Apparent sub-solar point on Phoebe as seen from CASSINI in
          the IAU_PHOEBE frame (km):
             X =           78.681
             Y =           76.879
             Z =          -21.885
Top # Intersecting Vectors with a Triaxial Ellipsoid (fovint) Top ## Task Statement Write a program that prompts the user for an input UTC time string and, for that time, computes the intersection of the CASSINI ISS NAC camera boresight and field of view (FOV) boundary vectors with the surface of Phoebe. The program presents each point of intersection as `1.` A cartesian vector in the IAU_PHOEBE frame `2.` Planetocentric (latitudinal) coordinates in the IAU_PHOEBE frame. For each vector, if an intersection is found, the program displays the results of the above computations, otherwise it indicates no intersection has occurred. At each point of intersection compute the following: `3.` Phase angle `4.` Solar incidence angle `5.` Emission angle Additionally compute the local solar time at the intercept of the camera boresight with the surface of Phoebe. Use this program to compute values at the epoch: `` 2004 jun 11 19:32:00 UTC Top ## Learning Goals Understand how field of view parameters are retrieved from instrument kernels. Learn how various standard planetary constants are retrieved from text PCKs. Discover how to compute the intersection of field of view vectors with triaxial ellipsoidal target bodies. Discover another high level geometry procedure and another time conversion procedure in Icy. Top ## Approach This problem can be broken down into several simple, small steps: `--` Decide which SPICE kernels are necessary. Prepare a meta-kernel listing the kernels and load it into the program. Remember, you will need to find a kernel with information about the CASSINI NAC camera. `--` Prompt the user for an input time string. `--` Convert the input time string into ephemeris time expressed as seconds past J2000 TDB. `--` Retrieve the FOV (field of view) configuration for the CASSINI NAC camera. For each vector in the set of boundary corner vectors, and for the boresight vector, perform the following operations: `--` Compute the intercept of the vector with Phoebe. `--` If this intercept is found, 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 CASSINI ISS instrument kernel to determine the name of the NAC camera as well as its configuration. This exercise may make use of some of the concepts and (loosely) code from the ``Spacecraft Orientation and Reference Frames'' task. Top ## Solution Top ### 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 a Triaxial Ellipsoid'' task
       in the Remote Sensing Hands On Lesson.

          \begindata
          KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
                              'kernels/sclk/cas00084.tsc',
                              'kernels/spk/981005_PLTEPH-DE405S.bsp',
                              'kernels/spk/020514_SE_SAT105.bsp',
                              'kernels/spk/030201AP_SK_SM546_T45.bsp',
                              'kernels/fk/cas_v37.tf',
                              'kernels/ck/04135_04171pc_psiv2.bc',
                              'kernels/pck/cpck05Mar2004.tpc',
                              'kernels/ik/cas_iss_v09.ti' )
          \begintext
Top ### Solution Source Code A sample solution to the problem follows:
       PRO fovint

          ;;
          ;; Local Parameters
          ;;

          METAKR     = 'fovint.tm'
          STRLEN     = 50
          BCVLEN     = 5
          utctim     = ''
          scan_vecs = dblarr( 3, BCVLEN )

          vecnam  = ['Boundary Corner 1', $
                     'Boundary Corner 2', $
                     'Boundary Corner 3', $
                     'Boundary Corner 4', $
                     'Cassini NAC Boresight' ]

          ;;
          ;; Load the kernels that this program requires.  We
          ;; will need:
          ;;
          ;;    A leapseconds kernel.
          ;;    A SCLK kernel for CASSINI.
          ;;    Any necessary ephemerides.
          ;;    The CASSINI frame kernel.
          ;;    A CASSINI C-kernel.
          ;;    A PCK file with Phoebe constants.
          ;;    The CASSINI ISS I-kernel.
          ;;
          cspice_furnsh, METAKR

          ;;
          ;; Prompt the user for the input time string.
          ;;
          read, utctim, PROMPT = 'Input UTC Time: '

          print, 'Converting UTC Time: ', utctim

          ;;
          ;; Convert utctim to et.
          ;;
          cspice_str2et,  utctim, et

          print, FORMAT='(A,F16.3)', '  ET seconds past J2000: ', et

          ;;
          ;; Now we need to obtain the FOV configuration of
          ;; the ISS NAC camera.  To do this we will need the
          ;;ID code for CASSINI_ISS_NAC.
          ;;
          cspice_bodn2c, 'CASSINI_ISS_NAC', nacid, found

          ;;
          ;; Stop the program if the code was not found.
          ;;
          if ( ~ found ) then begin
             print, 'Unable to locate the ID code for ' +$
                    'CASSINI_ISS_NAC'
             return
          endif

          ;;
          ;; Now retrieve the field of view parameters.
          ;;
          cspice_getfov, nacid, BCVLEN, shape, insfrm, bsight, bounds

          ;;
          ;; Rather than treat 'bsight' as a separate vector,
          ;; copy it and 'bounds to 'scan_vecs'.
          ;;
          scan_vecs[ 0:11] = bounds[0:11]
          scan_vecs[12:14] = bsight[0:2]

          ;;
          ;; Now perform the same set of calculations for each
          ;; vector listed in the 'bounds' array.
          ;;
          for i=0, 4 do begin

             ;;
             ;; Call sincpt to determine coordinates of the
             ;; intersection of this vector with the surface
             ;; of Phoebe.
             ;;
             cspice_sincpt, 'Ellipsoid',  'PHOEBE',       et,        $
                            'IAU_PHOEBE', 'LT+S',         'CASSINI', $
                             insfrm,      scan_vecs[*,i], point,     $
                             trgepc,      srfvec,         found

             ;;
             ;; Check the found flag.  Display a message if
             ;; the point of intersection was not found,
             ;; otherwise continue with the calculations.
             ;;
             print, 'Vector: ', vecnam[i]

             if ( ~ found ) then begin
                print, 'No intersection point found at '  +$
                            'this epoch for this vector.'
             endif else begin

                ;;
                ;; Now, we have discovered a point of intersection.
                ;; Start by displaying the position vector in the
                ;; IAU_PHOEBE frame of the intersection.
                ;;
                print, '  Position vector of surface intercept ' +$
                       'in the IAU_PHOEBE frame (km):'
                print, FORMAT='(A,F16.3)', '     X   = ', point[0]
                print, FORMAT='(A,F16.3)', '     Y   = ', point[1]
                print, FORMAT='(A,F16.3)', '     Z   = ', point[2]

                ;;
                ;; Display the planetocentric latitude and longitude
                ;; of the intercept.
                ;;
                cspice_reclat, point, radius, lon, lat

                print, '  Planetocentric coordinates of ' +$
                       'the intercept (degrees):'
                print, FORMAT='(A,F16.3)', '     LAT = ', $
                       lat * cspice_dpr()
                print, FORMAT='(A,F16.3)', '     LON = ', $
                       lon * cspice_dpr()

                ;;
                ;; Compute the illumination angles at this
                ;; point.
                ;;
                cspice_ilumin, 'Ellipsoid',  'PHOEBE', et,        $
                               'IAU_PHOEBE', 'LT+S',   'CASSINI', $
                               point,        trgepc,   srfvec,    $
                               phase,        solar,    emissn

                print, FORMAT='(A,F16.3)', $
                       '  Phase angle (degrees):           ', $
                       phase * cspice_dpr()
                print, FORMAT='(A,F16.3)', $
                       '  Solar incidence angle (degrees): ', $
                       solar * cspice_dpr()
                print, FORMAT='(A,F16.3)', $
                       '  Emission angle (degrees):        ', $
                       emissn * cspice_dpr()
                print

             endelse

          endfor

          ;;
          ;; Lastly compute the local solar time at the boresight
          ;; intersection.
          ;;
          if  ( found )  then begin

             ;;
             ;; Get Phoebe ID.
             ;;
             cspice_bodn2c, 'PHOEBE', phoeid, found

             ;;
             ;; Return if the code was not found.
             ;;
             if ( ~found ) then begin
                print, 'Unable to locate the ID code for Phoebe.'
                return
             endif

             ;;
             ;; Compute local solar time corresponding to
             ;; the TDB light time corrected epoch at the intercept.
             ;;
             cspice_et2lst, trgepc, phoeid, lon, 'PLANETOCENTRIC', $
                            hr,     min,    sc,  time,  ampm

             print, '  Local Solar Time at boresight ' +$
                    'intercept (24 Hour Clock):'
             print, '     ' + time

          endif else begin

             print, '  No boresight intercept to compute ' +$
                    'local solar time.'
          endelse

          cspice_kclear

       END
Top ### Solution Sample Output After compiling the program, execute it:
       Converting UTC Time: 2004 JUN 11 19:32:00
         ET seconds past J2000:    140254384.185
       Vector: Boundary Corner 1
         Position vector of surface intercept in the IAU_PHOEBE frame (km):
            X   =           91.026
            Y   =           67.190
            Z   =            2.030
         Planetocentric coordinates of the intercept (degrees):
            LAT =            1.028
            LON =           36.433
         Phase angle (degrees):                     28.110
         Solar incidence angle (degrees):           16.121
         Emission angle (degrees):                  14.627

       Vector: Boundary Corner 2
         Position vector of surface intercept in the IAU_PHOEBE frame (km):
            X   =           89.991
            Y   =           66.726
            Z   =           14.733
         Planetocentric coordinates of the intercept (degrees):
            LAT =            7.492
            LON =           36.556
         Phase angle (degrees):                     27.894
         Solar incidence angle (degrees):           22.894
         Emission angle (degrees):                  14.988

       Vector: Boundary Corner 3
         Position vector of surface intercept in the IAU_PHOEBE frame (km):
            X   =           80.963
            Y   =           76.643
            Z   =           14.427
         Planetocentric coordinates of the intercept (degrees):
            LAT =            7.373
            LON =           43.430
         Phase angle (degrees):                     28.171
         Solar incidence angle (degrees):           21.315
         Emission angle (degrees):                  21.977

       Vector: Boundary Corner 4
         Position vector of surface intercept in the IAU_PHOEBE frame (km):
            X   =           81.997
            Y   =           77.106
            Z   =            1.699
         Planetocentric coordinates of the intercept (degrees):
            LAT =            0.865
            LON =           43.239
         Phase angle (degrees):                     28.385
         Solar incidence angle (degrees):           13.882
         Emission angle (degrees):                  21.763

       Vector: Cassini NAC Boresight
         Position vector of surface intercept in the IAU_PHOEBE frame (km):
            X   =           86.390
            Y   =           72.089
            Z   =            8.255
         Planetocentric coordinates of the intercept (degrees):
            LAT =            4.196
            LON =           39.844
         Phase angle (degrees):                     28.139
         Solar incidence angle (degrees):           18.247
         Emission angle (degrees):                  17.858

         Local Solar Time at boresight intercept (24 Hour Clock):
            11:31:50