Geometric Event Finding Hands-On Lesson (MATLAB) |
Table of ContentsGeometric Event Finding Hands-On Lesson (MATLAB) Overview Note About HTML Links References Tutorials Required Readings The Permuted Index Mice API Documentation Kernels Used Mice Routines Used Find View Periods Task Statement Learning Goals Approach Solution steps Solution Solution Meta-Kernel Solution Code Solution Sample Output Find Times when Target is Visible Task Statement Learning Goals Approach Solution steps Solution Solution Code Solution Sample Output Geometric Event Finding Hands-On Lesson (MATLAB)
Overview
In this lesson the student is asked to construct a program that finds the time intervals, within a specified time range, when the Mars Express Orbiter (MEX) is visible from the DSN station DSS-14. Possible occultation of the spacecraft by Mars is to be considered. Note About HTML Links
In order for the links to be resolved, create a subdirectory called ``lessons'' under the ``doc/html'' directory of the ``mice/'' tree and copy this document to that subdirectory before loading it into a Web browser. ReferencesTutorials
Name Lesson steps/functions 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 Lunar-Earth PCK Lunar and Earth Orientation Data GF The SPICE Geometry Ginder (GF) subsystemThese tutorials are available from the NAIF ftp server at JPL:
http://naif.jpl.nasa.gov/naif/tutorials.html Required Readings
Name Lesson steps/functions that it describes --------------- ----------------------------------------- frames.req Using reference frames gf.req The SPICE geometry finder (GF) subsystem kernel.req Loading SPICE kernels naif_ids.req Body and reference frame names pck.req Obtaining planetary constants data spk.req Computing positions and velocities time.req UTC to ET time conversion windows.req The SPICE window data type mice.req The Mice API The Permuted Index
This text document provides a simple mechanism by which users can discover which Mice routines perform functions of interest, as well as the names of the source files that contain these routines. Mice API Documentation
For example, the document
mice/doc/html/mice/cspice_str2et.htmldescribes the cspice_str2et routine. Kernels Used
File Name Type Description ----------------------- ---- -------------------------- de405xs.bsp SPK Planetary ephemeris SPK, subsetted to cover only time range of interest earthstns_itrf93_050714.bsp SPK DSN station SPK earth_topo_050714.tf FK DSN station frame definitions earth_000101_060525_060303.bpc PCK Binary PCK for Earth ORMM__040501000000_00076XS.BSP SPK MEX Orbiter trajectory SPK, subsetted to cover only time range of interest naif0008.tls LSK Generic LSK pck00008.tpc PCK Generic PCKThese 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/ Mice Routines Used
Name Function that it performs -------------- --------------------------------------------------- cspice_furnsh Loads kernels, individually or listed in meta-kernel cspice_gfoclt Solve for times of occultation or transit cspice_gfposc Solve for times when a position vector coordinate constraint is met cspice_rpd Return number of radians per degree cspice_str2et Converts a time string to ET seconds past J2000 cspice_timout Format a time string for output cspice_wndifd Find the difference of two d.p. windows cspice_wnfetd Fetch a specified interval from a d.p. window cspice_wninsd Insert an interval into a d.p. windowRefer to the headers of the various functions listed above, as detailed interface specifications are provided with the source code. Find View PeriodsTask Statement
2004 MAY 2 TDB 2004 MAY 6 TDBwhen the Mars Express Orbiter (MEX) is visible from the DSN station DSS-14. These time intervals are frequently called ``view periods.'' The spacecraft is considered visible if its apparent position (that is, its position corrected for light time and stellar aberration) has elevation of at least 6 degrees in the topocentric reference frame DSS-14_TOPO. In this exercise, we ignore the possibility of occultation of the spacecraft by Mars. Use a search step size that ensures that no view periods of duration 5 minutes or longer will be missed by the search. Display the start and stop times of these intervals using TDB calendar dates and millisecond precision. Learning Goals
ApproachSolution steps
Preparation:
SolutionSolution Meta-Kernel
KPL/MK Example meta-kernel for geometric event finding hands-on coding lesson. Version 2.0.0 15-MAR-2008 (NJB) Identify names of kernels to load: \begindata KERNELS_TO_LOAD = ( 'kernels/spk/de405xs.bsp' 'kernels/spk/earthstns_itrf93_050714.bsp' 'kernels/fk/earth_topo_050714.tf' 'kernels/pck/earth_000101_060525_060303.bpc' 'kernels/lsk/naif0008.tls' 'kernels/spk/ORMM__040501000000_00076XS.BSP' 'kernels/pck/pck00008.tpc' ) \begintext Solution Code
% % Find and display the window of times when the MEX % spacecraft is above a specified elevation limit in the % topocentric reference frame of DSN station DSS-14. % function viewpr_m() % % The meta-kernel: % META = 'viewpr.tm'; % % Maximum number of intervals in any window: % MAXIVL = 1000; % % Time string length: % TIMLEN = 50; % % Format string for time output: % TDBFMT = 'YYYY MON DD HR:MN:SC.### (TDB) ::TDB'; % % Load the meta-kernel. % cspice_furnsh( META ); % % Assign the inputs for our search. % % Since we're interested in the apparent location of the % target, we use light time and stellar aberration % corrections. We use the "converged Newtonian" form % of the light time correction because this choice may % increase the accuracy of the occultation times we'll % compute using cspice_gfoclt. % srfpt = 'DSS-14'; obsfrm = 'DSS-14_TOPO'; target = 'MEX'; abcorr = 'CN+S'; start = '2004 MAY 2 TDB'; stop = '2004 MAY 6 TDB'; elvlim = 6.0; % % The elevation limit above has units of degrees; we convert % this value to radians for computation using SPICE routines. % We'll store the equivalent value in radians in REVLIM. % revlim = cspice_rpd() * elvlim; % % Since SPICE doesn't directly support the AZ/EL coordinate % system, we use the equivalent constraint % % latitude > REVLIM % % in the latitudinal coordinate system, where the reference % frame is topocentric and is centered at the viewing location. % crdsys = 'LATITUDINAL'; coord = 'LATITUDE'; relate = '>'; % % The adjustment value only applies to absolute extrema % searches; simply give it an initial value of zero % for this inequality search. % adjust = 0.0; % % STEPSZ is the step size, measured in seconds, used to search % for times bracketing a state transition. Since we don't expect % any events of interest to be shorter than five minutes, and % since the separation between events is well over 5 minutes, % we'll use this value as our step size. Units are seconds. % stepsz = 300.0; % % Display a banner for the output report: % disp( ' ' ); disp( 'Inputs for target visibility search:' ); disp( ' ' ); fprintf( ' Target = %s\n', target ); fprintf( ' Observation surface location = %s\n', srfpt ); fprintf( ' Observer''s reference frame = %s\n', obsfrm ); fprintf( ' Elevation limit (degrees) = %f\n', elvlim ); fprintf( ' Aberration correction = %s\n', abcorr ); fprintf( ' Step size (seconds) = %f\n', stepsz ); % % Convert the start and stop times to ET. % etbeg = cspice_str2et( start ); etend = cspice_str2et( stop ); % % Display the search interval start time and stop % times using the format shown below. % % 2004 MAY 06 20:15:00.000 (TDB) % timstr0 = cspice_timout( etbeg, TDBFMT ); timstr1 = cspice_timout( etend, TDBFMT ); fprintf( ' Start time = %s\n', timstr0 ); fprintf( ' Stop time = %s\n', timstr1 ); disp( ' ' ); % % Create the "confinement" window; store in this window % the interval over which we'll conduct the search. % cnfine = cspice_wninsd( etbeg, etend ); % % In the call below, the maximum number of window % intervals cspice_gfposc can store internally is % set to MAXIVL. We'll also use MAXIVL as the maximum % number of intervals in the result window. % % Now search for the time period, within our confinement % window, during which the apparent target has elevation % at least equal to the elevation limit. % riswin = cspice_gfposc ( target, obsfrm, abcorr, srfpt, ... crdsys, coord, relate, revlim, ... adjust, stepsz, MAXIVL, cnfine ); % % Let winsiz contain the number of intervals % in the SPICE window riswin. % winsiz = length( riswin )/2; % % Display the rise and set times. % if winsiz == 0 disp( 'No events were found.' ); else % % Display the visibility time periods. % fprintf( [ 'Visibility times of %s ' ... 'as seen from %s:\n\n' ], target, srfpt ); for i = 1:winsiz % % Fetch the Ith interval from the window. % [intbeg, intend] = cspice_wnfetd( riswin, i ); % % Convert the rise time to a TDB calendar string. % timstr = cspice_timout( intbeg, TDBFMT ); % % Write the string to standard output. % if i == 1 line = 'Visibility or window start time: '; else line = 'Visibility start time: '; end fprintf( '%s%s\n', line, timstr ); % % Convert the set time to a TDB calendar string. % timstr = cspice_timout( intend, TDBFMT ); % % Write the string to standard output. % if i == winsiz line = 'Visibility or window stop time: '; else line = 'Visibility stop time: '; end fprintf( '%s%s\n\n', line, timstr ); end end % % Unload kernels so they're not accidentally used by another % SPICE-based program during the current IDL session. % cspice_kclear; % % End of function viewpr % Solution Sample Output
Execute the program. The output is:
Inputs for target visibility search: Target = MEX Observation surface location = DSS-14 Observer's reference frame = DSS-14_TOPO Elevation limit (degrees) = 6.000000 Aberration correction = CN+S Step size (seconds) = 300.000000 Start time = 2004 MAY 02 00:00:00.000 (TDB) Stop time = 2004 MAY 06 00:00:00.000 (TDB) Visibility times of MEX as seen from DSS-14: Visibility or window start time: 2004 MAY 02 00:00:00.000 (TDB) Visibility stop time: 2004 MAY 02 05:35:03.096 (TDB) Visibility start time: 2004 MAY 02 16:09:14.078 (TDB) Visibility stop time: 2004 MAY 03 05:33:57.257 (TDB) Visibility start time: 2004 MAY 03 16:08:02.279 (TDB) Visibility stop time: 2004 MAY 04 05:32:50.765 (TDB) Visibility start time: 2004 MAY 04 16:06:51.259 (TDB) Visibility stop time: 2004 MAY 05 05:31:43.600 (TDB) Visibility start time: 2004 MAY 05 16:05:40.994 (TDB) Visibility or window stop time: 2004 MAY 06 00:00:00.000 (TDB) Find Times when Target is VisibleTask Statement
Display each of the intervals in the result window as a pair of start and stop times. Express each time as a TDB calendar date using the same format as in the previous program. Learning Goals
ApproachSolution steps
SolutionSolution Code
% % Find and display the window of times when the MEX % spacecraft is above a specified elevation limit in the % topocentric reference frame of DSN station DSS-14 % and is not occulted by Mars. % function visibl_m() % % The meta-kernel: % META = 'viewpr.tm'; % % Maximum number of intervals in any window: % MAXIVL = 1000; % % Time string length: % TIMLEN = 50; % % Format string for time output: % TDBFMT = 'YYYY MON DD HR:MN:SC.### (TDB) ::TDB'; % % Load the meta-kernel. % cspice_furnsh( META ); % % Assign the inputs for our search. % % Since we're interested in the apparent location of the % target, we use light time and stellar aberration % corrections. We use the "converged Newtonian" form % of the light time correction because this choice may % increase the accuracy of the occultation times we'll % compute using cspice_gfoclt. % srfpt = 'DSS-14'; obsfrm = 'DSS-14_TOPO'; target = 'MEX'; abcorr = 'CN+S'; start = '2004 MAY 2 TDB'; stop = '2004 MAY 6 TDB'; elvlim = 6.0; % % The elevation limit above has units of degrees; we convert % this value to radians for computation using SPICE routines. % We'll store the equivalent value in radians in REVLIM. % revlim = cspice_rpd() * elvlim; % % We model the target shape as a point and the blocking body's % shape as an ellipsoid. No body-fixed reference frame is % required for the target since its orientation is not used. % back = target; bshape = 'POINT'; bframe = ' '; front = 'MARS'; fshape = 'ELLIPSOID'; fframe = 'IAU_MARS'; % % The occultation type should be set to 'ANY' for a point % target. % occtyp = 'any'; % % Since SPICE doesn't directly support the AZ/EL coordinate % system, we use the equivalent constraint % % latitude > REVLIM % % in the latitudinal coordinate system, where the reference % frame is topocentric and is centered at the viewing location. % crdsys = 'LATITUDINAL'; coord = 'LATITUDE'; relate = '>'; % % The adjustment value only applies to absolute extrema % searches; simply give it an initial value of zero % for this inequality search. % adjust = 0.0; % % STEPSZ is the step size, measured in seconds, used to search % for times bracketing a state transition. Since we don't expect % any events of interest to be shorter than five minutes, and % since the separation between events is well over 5 minutes, % we'll use this value as our step size. Units are seconds. % stepsz = 300.0; % % Display a banner for the output report: % disp( ' ' ); disp( 'Inputs for target visibility search:' ); disp( ' ' ); fprintf( ' Target = %s\n', target ); fprintf( ' Observation surface location = %s\n', srfpt ); fprintf( ' Observer''s reference frame = %s\n', obsfrm ); fprintf( ' Blocking body = %s\n', front ); fprintf( ' Blocker''s reference frame = %s\n', fframe ); fprintf( ' Elevation limit (degrees) = %f\n', elvlim ); fprintf( ' Aberration correction = %s\n', abcorr ); fprintf( ' Step size (seconds) = %f\n', stepsz ); % % Convert the start and stop times to ET. % etbeg = cspice_str2et( start ); etend = cspice_str2et( stop ); % % Display the search interval start time and stop % times using the format shown below. % % 2004 MAY 06 20:15:00.000 (TDB) % timstr0 = cspice_timout( etbeg, TDBFMT ); timstr1 = cspice_timout( etend, TDBFMT ); fprintf( ' Start time = %s\n', timstr0 ); fprintf( ' Stop time = %s\n', timstr1 ); disp( ' ' ); % % Create the "confinement" window; store in this window % the interval over which we'll conduct the search. % cnfine = cspice_wninsd( etbeg, etend ); % % In the call below, the maximum number of window % intervals cspice_gfposc can store internally is % set to MAXIVL. We'll also use MAXIVL as the maximum % number of intervals in the result window. % % Now search for the time period, within our confinement % window, during which the apparent target has elevation % at least equal to the elevation limit. % riswin = cspice_gfposc ( target, obsfrm, abcorr, srfpt, ... crdsys, coord, relate, revlim, ... adjust, stepsz, MAXIVL, cnfine ); % % Now find the times when the apparent target is above % the elevation limit and is not occulted by the % blocking body (Mars). We'll find the window of times when % the target is above the elevation limit and *is* occulted, % then subtract that window from the view period window % riswin found above. % % For this occultation search, we can use riswin as % the confinement window because we're not interested in % occultations that occur when the target is below the % elevation limit. % % Find occultations within the view period window. % occwin = cspice_gfoclt( occtyp, front, fshape, fframe, ... back, bshape, bframe, abcorr, ... srfpt, stepsz, riswin, MAXIVL ); % % Subtract the occultation window from the view period % window: this yields the time periods when the target % is visible. % viswin = cspice_wndifd( riswin, occwin ); % % Let winsiz contain the number of intervals % in the SPICE window riswin. % winsiz = length( viswin )/2; % % Display the rise and set times. % if winsiz == 0 disp( 'No events were found.' ); else % % Display the visibility time periods. % fprintf( [ 'Visibility times of %s ' ... 'as seen from %s:\n\n' ], target, srfpt ); for i = 1:winsiz % % Fetch the Ith interval from the window. % [intbeg, intend] = cspice_wnfetd( viswin, i ); % % Convert the rise time to a TDB calendar string. % timstr = cspice_timout( intbeg, TDBFMT ); % % Write the string to standard output. % if i == 1 line = 'Visibility or window start time: '; else line = 'Visibility start time: '; end fprintf( '%s%s\n', line, timstr ); % % Convert the set time to a TDB calendar string. % timstr = cspice_timout( intend, TDBFMT ); % % Write the string to standard output. % if i == winsiz line = 'Visibility or window stop time: '; else line = 'Visibility stop time: '; end fprintf( '%s%s\n\n', line, timstr ); end end % % Unload kernels so they're not accidentally used by another % SPICE-based program during the current MATLAB session. % cspice_kclear; % % End of function visibl % Solution Sample Output
Execute the program. The output is:
Inputs for target visibility search: Target = MEX Observation surface location = DSS-14 Observer's reference frame = DSS-14_TOPO Blocking body = MARS Blocker's reference frame = IAU_MARS Elevation limit (degrees) = 6.000000 Aberration correction = CN+S Step size (seconds) = 300.000000 Start time = 2004 MAY 02 00:00:00.000 (TDB) Stop time = 2004 MAY 06 00:00:00.000 (TDB) Visibility times of MEX as seen from DSS-14: Visibility or window start time: 2004 MAY 02 00:00:00.000 (TDB) Visibility stop time: 2004 MAY 02 04:49:30.827 (TDB) Visibility start time: 2004 MAY 02 16:09:14.078 (TDB) Visibility stop time: 2004 MAY 02 20:00:22.514 (TDB) Visibility start time: 2004 MAY 02 21:01:38.222 (TDB) Visibility stop time: 2004 MAY 03 03:35:42.256 (TDB) Visibility start time: 2004 MAY 03 04:36:42.484 (TDB) Visibility stop time: 2004 MAY 03 05:33:57.257 (TDB) Visibility start time: 2004 MAY 03 16:08:02.279 (TDB) Visibility stop time: 2004 MAY 03 18:46:26.013 (TDB) Visibility start time: 2004 MAY 03 19:46:54.618 (TDB) Visibility stop time: 2004 MAY 04 02:21:44.562 (TDB) Visibility start time: 2004 MAY 04 03:21:56.347 (TDB) Visibility stop time: 2004 MAY 04 05:32:50.765 (TDB) Visibility start time: 2004 MAY 04 16:06:51.259 (TDB) Visibility stop time: 2004 MAY 04 17:32:25.809 (TDB) Visibility start time: 2004 MAY 04 18:32:05.975 (TDB) Visibility stop time: 2004 MAY 05 01:07:48.264 (TDB) Visibility start time: 2004 MAY 05 02:07:11.601 (TDB) Visibility stop time: 2004 MAY 05 05:31:43.600 (TDB) Visibility start time: 2004 MAY 05 16:05:40.994 (TDB) Visibility stop time: 2004 MAY 05 16:18:35.560 (TDB) Visibility start time: 2004 MAY 05 17:17:27.717 (TDB) Visibility or window stop time: 2004 MAY 05 23:54:04.672 (TDB) |