Remote Sensing Hands-On Lesson (IDL) |
Table of ContentsRemote 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 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-spacecraft and Sub-solar Points (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 a Triaxial Ellipsoid (fovint) Task Statement Learning Goals Approach Solution Solution Meta-Kernel Solution Source Code Solution Sample Output Extra Credit Remote Sensing Hands-On Lesson (IDL)
Overview
Note About HTML Links
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. ReferencesTutorials
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 DataThese tutorials are available from the NAIF ftp server at JPL:
http://naif.jpl.nasa.gov/naif/tutorials.html Required Readings
Name Lesson steps/routines that it describes --------------- ----------------------------------------- time.req Time Conversion sclk.req SCLK Time Conversion spk.req Obtaining Ephemeris Data frames.req Using Reference Frames pck.req Obtaining Planetary Constants Data ck.req Obtaining Spacecraft Orientation Data naif_ids.req Determining Body ID Codes The Permuted Index
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. Icy API Documentation
For example, the document
icy/doc/html/icy/cspice_str2et.htmldescribes the cspice_str2et routine. Kernels Used
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.tiThese 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/ Icy Modules Used
CHAPTER EXERCISE FUNCTIONS NON-VOID KERNELS ------- --------- ------------- --------- ------- 1 convtm cspice_furnsh 1,2 cspice_prompt cspice_str2et cspice_etcal cspice_timout cspice_sce2s 2 getsta cspice_furnsh cspice_vnorm 1,3-6 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 1,3-6,8 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_et2lstRefer to the headers of the various procedures listed above, as detailed interface specifications are provided with the source code. Time Conversion (convtm)Task Statement
Learning Goals
Approach
When completing the ``calendar format'' step above, consider using one of two possible methods: cspice_etcal or cspice_timout. SolutionSolution Meta-Kernel
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/naif0012.tls', 'kernels/sclk/em16_tgo_step_20160414.tsc' ) \begintext Solution Source Code
PRO convtm ;; ;; Local Parameters ;; METAKR = "convtm.tm" SCLKID = -143 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 Solution Sample Output
Converting UTC Time: 2018 JUN 11 19:32:00 ET Seconds Past J2000: 582017589.185 Calendar ET (cspice_etcal): 2018 JUN 11 19:33:09.184 Calendar ET (cspice_timout): 2018-JUN-11T19:33:09 Spacecraft Clock Time: 1/0070841719.26698 Extra Credit
These ``extra credit'' tasks are provided as task statements, and unlike the regular tasks, no approach or solution source code is provided. In the next section, you will find the numeric solutions (when applicable) and answers to the questions asked in these tasks. Task statements and questions
Solutions and answers
Julian Date TDB: 2458281.3146896
Earliest UTC convertible to SCLK: 2016-03-13T21:34:13.194
UTC time from spacecraft clock: 2018-06-11T19:32:00.000 Obtaining Target States and Positions (getsta)Task Statement
Learning Goals
Approach
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. SolutionSolution Meta-Kernel
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/naif0012.tls', 'kernels/spk/de430.bsp', 'kernels/spk/mar085.bsp', 'kernels/spk/em16_tgo_mlt_20171205_20230115_v01.bsp', 'kernels/fk/em16_tgo_v07.tf' ) \begintext Solution Source Code
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. ;; ;; Since the SPICE body/ID mapping for TGO is not ;; yet included in the standard library, we will ;; need the frame kernel where the mapping is ;; defined. ;; cspice_furnsh, METAKR ;; ;; Prompt the user for the input time string. ;; read, utctim, PROMPT = 'Input UTC Time: ' print, 'Converting UTC Time: ', utctim ;; ;; Convert utctim to et. ;; cspice_str2et, utctim, et print, FORMAT='(A,F16.3)', ' ET seconds past J2000: ', et ;; ;; Compute the apparent state of Mars as seen from ;; ExoMars-16 TGO in the J2000 frame. All of the ephemeris ;; readers return states in units of kilometers and ;; kilometers per second. ;; cspice_spkezr, 'MARS', et , 'J2000', 'LT+S', $ 'TGO', state, ltime print, ' Apparent state of Mars as seen ' +$ 'from ExoMars-16 TGO in the J2000' print, ' frame (km, km/s): ' print, FORMAT='(A,F16.3)', ' X = ', state[0] print, FORMAT='(A,F16.3)', ' Y = ', state[1] print, FORMAT='(A,F16.3)', ' Z = ', state[2] print, FORMAT='(A,F16.3)', ' VX = ', state[3] print, FORMAT='(A,F16.3)', ' VY = ', state[4] print, FORMAT='(A,F16.3)', ' VZ = ', state[5] ;; ;; Compute the apparent position of Earth as seen from ;; ExoMars-16 TGO in the J2000 frame. Note: We could have ;; continued using cspice_spkezr and simply ignored the ;; velocity components. ;; cspice_spkpos, 'EARTH', et , 'J2000', 'LT+S', $ 'TGO', pos, ltime print, ' Apparent position of Earth as ' +$ 'seen from ExoMars-16 TGO in the' print, ' J2000 frame (km): ' print, FORMAT='(A,F16.3)', ' X = ', pos[0] print, FORMAT='(A,F16.3)', ' Y = ', pos[1] print, FORMAT='(A,F16.3)', ' Z = ', pos[2] ;; ;; We need only display LT, as it is precisely the ;; light time in which we are interested. ;; print, ' One way light time between ExoMars-16 ' +$ 'TGO and the apparent' print, FORMAT='(A,F16.3)', ' position of Earth ' +$ '(seconds): ', ltime ;; ;; Compute the apparent position of the Sun as seen ;; from Mars in the J2000 frame. ;; cspice_spkpos, 'SUN' , et , 'J2000', 'LT+S', $ 'MARS', pos, ltime print, ' Apparent position of Sun as seen ' +$ 'from Mars in the ' print, ' J2000 frame (km): ' print, FORMAT='(A,F16.3)', ' X = ', pos[0] print, FORMAT='(A,F16.3)', ' Y = ', pos[1] print, FORMAT='(A,F16.3)', ' Z = ', pos[2] ;; ;; Now we need to compute the actual distance between ;; the Sun and Mars. The above SPKPOS call gives us ;; the apparent distance, so we need to adjust our ;; aberration correction appropriately. ;; cspice_spkpos, 'SUN' , et , 'J2000', 'NONE', $ 'MARS', pos, ltime ;; ;; Compute the distance between the body centers in ;; kilometers. ;; dist = cspice_vnorm ( pos ) ;; ;; Convert this value to AU using cspice_convrt. ;; Recall, cspice_convrt cannot overwrite the ;; input with the output. Use 'dist_au' for the ;; output value. ;; cspice_convrt, dist, 'KM', 'AU', dist_au print, ' Actual distance between Sun and Mars ' + $ 'body centers:' print, FORMAT='(A,F16.3)', ' (AU): ', dist_au cspice_kclear END Solution Sample Output
Converting UTC Time: 2018 JUN 11 19:32:00 ET seconds past J2000: 582017589.185 Apparent state of Mars as seen from ExoMars-16 TGO in the J2000 frame (km, km/s): X = 2911.822 Y = -2033.802 Z = -1291.701 VX = 1.310 VY = -0.056 VZ = 3.104 Apparent position of Earth as seen from ExoMars-16 TGO in the J2000 frame (km): X = -49609884.080 Y = 57070665.862 Z = 30304236.930 One way light time between ExoMars-16 TGO and the apparent position of Earth (seconds): 271.738 Apparent position of Sun as seen from Mars in the J2000 frame (km): X = -24712734.289 Y = 194560532.943 Z = 89906636.789 Actual distance between Sun and Mars body centers: (AU): 1.442 Extra Credit
These ``extra credit'' tasks are provided as task statements, and unlike the regular tasks, no approach or solution source code is provided. In the next section, you will find the numeric solutions (when applicable) and answers to the questions asked in these tasks. Task statements and questions
Solutions and answers
cspice_spkezr, 'MARS', et , 'J2000', 'LT+S', $ '-143', state, ltime
http://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/satellites/
Actual position of Jupiter as seen from Mars in the J2000 frame (km): X = -536521483.296 Y = -384722940.462 Z = -145930841.439
Additional kernels used in this task: a. EDM lander FK: em16_emd_v00.tf b. EDM landing site SPK: em16_edm_sot_landing_site_20161020_21000101_v01.bsp c. Generic PCK, where the Mars orientation constants are provided: pck00010.tpc Apparent position of EDM Landing Site (EDM_LANDING_SITE, NAIF ID -117900) as seen from ExoMars-16 TGO in the J2000 frame (km): X = -131.716 Y = -2168.989 Z = 208.792
Actual (geometric) position of Sun as seen from Mars in the J2000 frame (km): X = -24730875.201 Y = 194558449.560 Z = 89906170.855 Light-time corrected position of Sun as seen from Mars in the J2000 frame (km): X = -24730866.489 Y = 194558445.246 Z = 89906168.754 Apparent position of Sun as seen from Mars in the J2000 frame (km): X = -24712734.289 Y = 194560532.943 Z = 89906636.789 Spacecraft Orientation and Reference Frames (xform)Task Statement
Learning Goals
Approach
You may find it useful to consult the permuted index, the headers of various source modules, and the following toolkit documentation:
SolutionSolution Meta-Kernel
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/naif0012.tls', 'kernels/sclk/em16_tgo_step_20160414.tsc', 'kernels/spk/de430.bsp', 'kernels/spk/mar085.bsp', 'kernels/spk/em16_tgo_mlt_20171205_20230115_v01.bsp', 'kernels/fk/em16_tgo_v07.tf', 'kernels/ck/em16_tgo_sc_slt_npo_20171205_20230115_s20160414_v01.bc', 'kernels/pck/pck00010.tpc' ) \begintext Solution Source Code
PRO xform ;; ;; Local Parameters ;; METAKR = 'xform.tm' STRLEN = 50 utctim = '' ;; ;; Load the kernels that this program requires. We ;; will need: ;; ;; A leapseconds kernel ;; A spacecraft clock kernel for ExoMars-16 TGO ;; The necessary ephemerides ;; A planetary constants file (PCK) ;; A spacecraft orientation kernel for ExoMars-16 TGO (CK) ;; A frame kernel (TF) ;; cspice_furnsh, METAKR ;; ;; Prompt the user for the input time string. ;; read, utctim, PROMPT = 'Input UTC Time: ' print, 'Converting UTC Time: ', utctim ;; ;; Convert utctim to et. ;; cspice_str2et, utctim, et print, FORMAT='(A,F16.3)', ' ET seconds past J2000: ', et ;; ;; Compute the apparent state of Mars as seen from ;; ExoMars-16 TGO in the J2000 frame. All of the ephemeris ;; readers return states in units of kilometers and ;; kilometers per second. ;; cspice_spkezr, 'MARS', et, 'J2000', 'LT+S', $ 'TGO', state, ltime ;; ;; Now obtain the transformation from the inertial ;; J2000 frame to the non-inertial body-fixed IAU_MARS ;; frame. Since we want the apparent state in the ;; IAU_MARS reference frame, we need to correct the ;; orientation and angular velocity of this frame ;; for one-way light time; hence we subtract ltime from ;; et in the call below. ;; cspice_sxform, 'J2000', 'IAU_MARS', et-ltime, sxfmat ;; ;; Now transform the apparent J2000 state into IAU_MARS ;; with the following matrix multiplication: ;; bfixst = transpose(sxfmat) # state ;; ;; Display the results. ;; print, ' Apparent state of Mars as seen ' +$ 'from ExoMars-16 TGO in the IAU_MARS' print, ' body-fixed frame (km, km/s):' print, FORMAT='(A,F19.6)', ' X = ', bfixst[0] print, FORMAT='(A,F19.6)', ' Y = ', bfixst[1] print, FORMAT='(A,F19.6)', ' Z = ', bfixst[2] print, FORMAT='(A,F19.6)', ' VX = ', bfixst[3] print, FORMAT='(A,F19.6)', ' VY = ', bfixst[4] print, FORMAT='(A,F19.6)', ' VZ = ', bfixst[5] ;; ;; It is worth pointing out, all of the above could ;; have been done with a single use of cspice_spkezr: ;; ;; cspice_spkezr, 'MARS', et, 'IAU_MARS', 'LT+S', $ 'TGO', state, ltime ;; ;; Display the results. ;; print, ' Apparent state of Mars as seen ' +$ 'from ExoMars-16 TGO in the IAU_MARS' print, ' body-fixed frame (km, km/s) ' +$ 'obtained using cspice_spkezr' print, ' directly:' print, FORMAT='(A,F19.6)', ' X = ', state[0] print, FORMAT='(A,F19.6)', ' Y = ', state[1] print, FORMAT='(A,F19.6)', ' Z = ', state[2] print, FORMAT='(A,F19.6)', ' VX = ', state[3] print, FORMAT='(A,F19.6)', ' VY = ', state[4] print, FORMAT='(A,F19.6)', ' VZ = ', state[5] ;; ;; Note that the velocity found by using cspice_spkezr ;; to compute the state in the IAU_MARS frame differs ;; at the few mm/second level from that found previously ;; by calling cspice_spkezr and then cspice_sxform. Computing ;; velocity via a single call to cspice_spkezr as we've ;; done immediately above is slightly more accurate because ;; it accounts for the effect of the rate of change of ;; light time on the apparent angular velocity of the ;; target's body-fixed reference frame. ;; ;; Now we are to compute the angular separation between ;; the apparent position of Mars as seen from the orbiter ;; and the nominal instrument view direction. First, ;; compute the apparent position of Mars as seen from ;; ExoMars-16 TGO in the J2000 frame. ;; cspice_spkpos, 'MARS', et, 'J2000', 'LT+S', $ 'TGO', pos, ltime ;; ;; Now compute the location of the nominal instrument view ;; direction. From reading the frame kernel we know that ;; the instrument view direction is nominally the -Y axis ;; of the TGO_SPACECRAFT frame defined there. ;; bsight = [ 0.D0, -1.D0, 0.D0] ;; ;; Now compute the rotation matrix from TGO_SPACECRAFT into ;; J2000. ;; cspice_pxform, 'TGO_SPACECRAFT', 'J2000', et, pform ;; ;; And multiply the result to obtain the nominal instrument ;; view direction in the J2000 reference frame. ;; cspice_mxv, pform, bsight, bsight ;; ;; Lastly compute the angular separation. ;; cspice_convrt, cspice_vsep(bsight, pos), 'RADIANS', $ 'DEGREES', sep print, ' Angular separation between the ' +$ 'apparent position of Mars and the' print, ' ExoMars-16 TGO nominal instrument ' +$ 'view direction (degrees):' print, FORMAT='(A,F16.3)', ' ', sep ;; ;; Or alternatively we can work in the spacecraft ;; frame directly. ;; cspice_spkpos, 'MARS', et, 'TGO_SPACECRAFT', 'LT+S', $ 'TGO', pos, ltime ;; ;; The nominal instrument view direction is the -Y-axis ;; in the TGO_SPACECRAFT frame. ;; bsight = [ 0.D0, -1.D0, 0.D0] ;; ;; Lastly compute the angular separation. ;; cspice_convrt, cspice_vsep(bsight, pos), 'RADIANS', $ 'DEGREES', sep print, ' Angular separation between the ' +$ 'apparent position of Mars and the' print, ' ExoMars-16 TGO nominal instrument ' +$ 'view direction computed' print, ' using vectors in the ' +$ 'TGO_SPACECRAFT frame (degrees):' print, FORMAT='(A,F16.3)', ' ', sep cspice_kclear END Solution Sample Output
Converting UTC Time: 2018 JUN 11 19:32:00 ET seconds past J2000: 582017589.185 Apparent state of Mars as seen from ExoMars-16 TGO in the IAU_MARS body-fixed frame (km, km/s): X = -2843.464125 Y = 2235.459544 Z = 1095.894969 VX = 0.311443 VY = -1.151929 VZ = 3.082123 Apparent state of Mars as seen from ExoMars-16 TGO in the IAU_MARS body-fixed frame (km, km/s) obtained using cspice_spkezr directly: X = -2843.464125 Y = 2235.459544 Z = 1095.894969 VX = 0.311443 VY = -1.151929 VZ = 3.082123 Angular separation between the apparent position of Mars and the ExoMars-16 TGO nominal instrument view direction (degrees): 5.438 Angular separation between the apparent position of Mars and the ExoMars-16 TGO nominal instrument view direction computed using vectors in the TGO_SPACECRAFT frame (degrees): 5.438 Extra Credit
These ``extra credit'' tasks are provided as task statements, and unlike the regular tasks, no approach or solution source code is provided. In the next section, you will find the numeric solutions (when applicable) and answers to the questions asked in these tasks. Task statements and questions
Solutions and answers
Segment No.: 1 Object: -143000 Interval Begin ET Interval End ET AV ------------------------ ------------------------ --- 2018-JUN-11 00:01:09.184 2018-JUN-12 06:28:03.102 Y 2018-JUN-12 06:58:03.102 2018-JUN-12 18:15:43.102 Y 2018-JUN-12 18:45:43.102 2018-JUN-13 04:03:23.102 Y 2018-JUN-13 04:33:23.102 2018-JUN-13 07:59:43.102 Y 2018-JUN-13 08:29:43.102 2018-JUN-13 12:01:09.184 Y
Object: -143000 Interval Begin ET Interval End ET AV ------------------------ ------------------------ --- 2018-JUN-11 00:01:09.184 2018-JUN-13 12:01:09.184 Y
Computing Sub-spacecraft and Sub-solar Points (subpts)Task Statement
Learning Goals
Approach
One point worth considering: Which method do you want to use to compute the sub-solar (or sub-observer) point? SolutionSolution Meta-Kernel
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/naif0012.tls', 'kernels/spk/de430.bsp', 'kernels/spk/mar085.bsp', 'kernels/spk/em16_tgo_mlt_20171205_20230115_v01.bsp', 'kernels/fk/em16_tgo_v07.tf', 'kernels/pck/pck00010.tpc' ) \begintext Solution Source Code
PRO subpts ;; ;; Local Parameters ;; METAKR = 'subpts.tm' STRLEN = 50 utctim = '' ;; ;; Load the kernels that this program requires. We ;; will need: ;; ;; A leapseconds kernel ;; The necessary ephemerides ;; A planetary constants file (PCK) ;; A frames kernel (TF) with the TGO ID/name mapping ;; 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 ExoMars-16 TGO ;; on Mars. ;; cspice_subpnt, 'NEAR POINT: ELLIPSOID', $ 'MARS', et, 'IAU_MARS', 'LT+S', $ 'TGO', spoint, trgepc, srfvec print, ' Apparent sub-observer point of ' +$ 'ExoMars-16 TGO on Mars in the' print, ' IAU_MARS frame (km):' print, FORMAT='(A,F16.3)', ' X = ', spoint[0] print, FORMAT='(A,F16.3)', ' Y = ', spoint[1] print, FORMAT='(A,F16.3)', ' Z = ', spoint[2] print, FORMAT='(A,F16.3)', ' ALT = ', cspice_vnorm(srfvec) ;; ;; Compute the apparent sub-solar point on Mars ;; as seen from ExoMars-16 TGO. ;; cspice_subslr, 'NEAR POINT: ELLIPSOID', $ 'MARS', et, 'IAU_MARS', 'LT+S', $ 'TGO', spoint, trgepc, srfvec print, ' Apparent sub-solar point on Mars ' +$ 'as seen from ExoMars-16 TGO in' print, ' the IAU_MARS frame (km):' print, FORMAT='(A,F16.3)', ' X = ', spoint[0] print, FORMAT='(A,F16.3)', ' Y = ', spoint[1] print, FORMAT='(A,F16.3)', ' Z = ', spoint[2] cspice_unload, METAKR END Solution Sample Output
Converting UTC Time: 2018 JUN 11 19:32:00 ET seconds past J2000: 582017589.185 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 Extra Credit
These ``extra credit'' tasks are provided as task statements, and unlike the regular tasks, no approach or solution source code is provided. In the next section, you will find the numeric solutions (when applicable) and answers to the questions asked in these tasks. Task statements and questions
Solutions and answers
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
Apparent 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 = -0.675
Planetocentric coordinates of the sub-spacecraft point on Phobos (degrees, km): LAT = -3.030 LON = 19.088 R = 12.779 Planetographic coordinates of the sub-spacecraft point on Phobos (degrees, km): LAT = -6.267 LON = 340.912 ALT = -0.202
Intersecting Vectors with a Triaxial Ellipsoid (fovint)Task Statement
At each point of intersection compute the following:
Use this program to compute values at the epoch:
Learning Goals
Approach
SolutionSolution Meta-Kernel
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/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' ) \begintext Solution Source Code
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', $ 'ExoMars-16 TGO NOMAD LNO Nadir Boresight' ] ;; ;; Load the kernels that this program requires. We ;; will need: ;; ;; A leapseconds kernel. ;; A SCLK kernel for ExoMars-16 TGO. ;; Any necessary ephemerides. ;; The ExoMars-16 TGO frame kernel. ;; An ExoMars-16 TGO C-kernel. ;; A PCK file with Mars constants. ;; The ExoMars-16 TGO NOMAD I-kernel. ;; 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 NOMAD LNO Nadir aperture. To do this we will need the ;; ID code for TGO_NOMAD_LNO_NAD. ;; cspice_bodn2c, 'TGO_NOMAD_LNO_NAD', lnonid, found ;; ;; Stop the program if the code was not found. ;; if ( ~ found ) then begin print, 'Unable to locate the ID code for ' +$ 'TGO_NOMAD_LNO_NAD' return endif ;; ;; Now retrieve the field of view parameters. ;; cspice_getfov, lnonid, BCVLEN, shape, insfrm, bsight, bounds ;; ;; Rather than treat 'bsight' as a separate vector, ;; copy it and 'bounds to 'scan_vecs'. ;; scan_vecs[ 0:11] = bounds[0:11] scan_vecs[12:14] = bsight[0:2] ;; ;; 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 Mars. ;; cspice_sincpt, 'Ellipsoid', 'MARS', et, $ 'IAU_MARS', 'LT+S', 'TGO', $ insfrm, scan_vecs[*,i], point, $ trgepc, srfvec, found ;; ;; Check the found flag. Display a message if ;; the point of intersection was not found, ;; otherwise continue with the calculations. ;; 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_MARS frame of the intersection. ;; print, ' Position vector of surface intercept ' +$ 'in the IAU_MARS frame (km):' print, FORMAT='(A,F16.3)', ' X = ', point[0] print, FORMAT='(A,F16.3)', ' Y = ', point[1] print, FORMAT='(A,F16.3)', ' Z = ', point[2] ;; ;; Display the planetocentric latitude and longitude ;; of the intercept. ;; cspice_reclat, point, radius, lon, lat print, ' Planetocentric coordinates of ' +$ 'the intercept (degrees):' print, FORMAT='(A,F16.3)', ' LAT = ', $ lat * cspice_dpr() print, FORMAT='(A,F16.3)', ' LON = ', $ lon * cspice_dpr() ;; ;; Compute the illumination angles at this ;; point. ;; cspice_ilumin, 'Ellipsoid', 'MARS', et, $ 'IAU_MARS', 'LT+S', 'TGO', $ 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 Mars ID. ;; cspice_bodn2c, 'MARS', marsid, found ;; ;; The ID code for MARS is built-in to the library. ;; However, it is good programming practice to get ;; in the habit of checking your found-flags. ;; if ( ~found ) then begin print, 'Unable to locate the ID code for Mars.' return endif ;; ;; Compute local solar time corresponding to ;; the TDB light time corrected epoch at the intercept. ;; cspice_et2lst, trgepc, marsid, lon, 'PLANETOCENTRIC', $ hr, min, sc, time, ampm print, ' 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 Solution Sample Output
Converting UTC Time: 2018 JUN 11 19:32:00 ET seconds past J2000: 582017589.185 Vector: Boundary Corner 1 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 Vector: Boundary Corner 2 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 Vector: Boundary Corner 3 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 Vector: Boundary Corner 4 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 Vector: ExoMars-16 TGO NOMAD LNO Nadir Boresight 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 Local Solar Time at boresight intercept (24 Hour Clock): 14:51:36 Extra Credit
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