KPL/IK THEMIS Instrument kernel =========================================================================== This instrument kernel (I-kernel) contains the Mars Odyssey 2001 Thermal Emission Imaging System (THEMIS) optics, detector and filter parameters. Important Note --------------------------------------------------------------------------- The instrument optical and other geometric parameters provided in this version of the IK file are based on the data from PRE-LAUNCH calibration reports and/or NOMINAL instrument design documents. While the PRE-LAUNCH and NOMINAL values give a good approximation of the instrument properties, they should be replaced with the more accurate IN-FLIGHT calibrated values to achieve correct pixel view direction computations. Unfortunately, the complete set of IN-FLIGHT calibrated geometric instrument models, including optical distortion model, was not available at the time when this IK was released. As soon as such data become available, the IK will be updated to incorporate it. Version and Date --------------------------------------------------------------------------- Version 2.0 -- September 26, 2002 -- Boris Semenov, NAIF/JPL Corrected IR filter numbering. Added section containing IR and VIS timing data. Version 1.2 -- January 7, 2002 -- Boris Semenov, NAIF/JPL Corrected VIS filter band center/band width values and added VIS band ID parameter based on the information provided by Greg Mehall on January 3, 2002. Version 1.1 -- August 1, 2001 -- Boris Semenov, NAIF/JPL Updated the active IR detector pixel size with values provided by Greg Mehall. Version 1.0 -- July 30, 2001 -- Boris Semenov, NAIF/JPL Included description and parameter values provided by Greg Mehall. Version 0.1 -- March 12, 2001 -- Boris Semenov, NAIF/JPL Pre-release. Added IR and VIS FOV definitions based on nominal FOV angular sizes. Version 0.0 -- March 5, 2001 -- Boris Semenov, NAIF/JPL Pre-release. References --------------------------------------------------------------------------- 1. ``Kernel Pool Required Reading'' 2. ``C-kernel Required Reading'' 3. ``THEMIS ICD'', MSP01-97-0008, June 10, 1999 4. M'01 Frames Definition Kernel (FK), latest version. 5. THEMIS Instrument Characteristics, ASU THEMIS Team Web Site, http://emma.la.asu.edu/THEMIS/inst.html, March 2001. 6. THEMIS Information Sheet, JPL M'01 Odyssey Web Site, http://mars.jpl.nasa.gov/2001/instruments/images/themis1.jpg, March 2001. 7. Corrections and additions by Greg Mehall, July 2001. 8. E-mail communications between Jim Thornton, USGS and Greg Mehall, January-June 2002. Implementation Notes -------------------------------------------------------- User programs that need I-kernel data must `load' the I-kernel file, normally during program initialization. Loading the kernel using SPICELIB routine LDPOOL causes the data items and their associated values present in the kernel to become associated with a data structure called the ``kernel pool''. Then a user's program can obtain the value(s) for any data item using the SPICELIB routines GDPOOL, GIPOOL and GCPOOL. See [1] for details. This file was created and can be updated with a text editor or word processor. Naming Conventions -------------------------------------------------------- All names referencing values in this I-kernel start with the characters `INS' followed by the NAIF M'01 spacecraft ID number (-53) followed by the NAIF three digit THEMIS IR instrument reference number (031) or THEMIS VIS instrument reference number (032). The remainder of the name is an underscore character followed by the unique name of the data item. For example, the focal length of the THEMIS IR camera is specified by INS-53031_FOCAL_LENGTH The upper bound on the length of the name of any data item is 32 characters. If the same item is included in more then one file, or if the same item appears more than once within a single file, the latest value supersedes any earlier values. THEMIS Overview -------------------------------------------------------- From [5] & [7]: "Infrared Imaging by THEMIS -------------------------- The Thermal Emission Imaging System (THEMIS) will determine the mineralogy and petrology of the Martian surface using a multi-spectral, thermal infrared images with 10 spectral filters filters between 6.6 and 15.0 micrometers. The 9 infrared spectral filters were selected in part based on the mineral results from the Mars Global Surveyor Thermal Emission Spectrometer (TES). The THEMIS infrared camera utilizes an uncooled 240 x 320 pixel microbolometer detector array. The array readout rate is 30 frames per second. The detector array is divided into ten 24 x 320 pixel "data" bands. The infrared camera also uses "time delayed integration" (TDI) to increase the signal to noise for each band. This is accomplished by summing the data from 16 of each 24 active rows in each data band. Any combination of the 10 data bands can be selected and downlinked for each infrared image. The resultant downlinked data is a 1 x 320 pixel line at a rate of 30 lines per second for each selected data band. The TDI can be disabled but will result in the reduction of the signal to noise by a factor of four. When TDI is disabled, only the data from 1 of each 24 active rows in each data band is downlinked The 9 spectral optical filters are attached to the front of the detector array. The 6.6 microns spectral filter covers 2 data bands (48 x 320 pixels) and the remaining spectral filters cover 1 data band (24 x 320 pixels). Each spectral filter has a full-width half-maximum bandwidth of approximately 1 microns. The instantaneous field of view of each infrared pixel is 250 microrad. The resulting cross-track field of view is 80 mrad. The image length is selectable in increments of 64 mrad allowing the along-track field of view to vary from 68 to 16324 mrad. From a 400 km mapping orbit, the entire planet will be mapped in the infrared with a ground sampling distance of 100 meters. At this altitude, the image cross-track width will be 32 km and the image along-track length can vary from 27 to 6530 km. Visible Imaging by THEMIS ------------------------- The THEMIS will study small-scale geologic processes on the Martian surface using a multi-spectral, visible images with 5 spectral filters between 420 and 870 nanometers. The THEMIS visible camera utilizes a 1024 x 1024 pixel CCD. The array integration time is selectable with a nominal setting of 6 milliseconds for Martian mapping operations. The array inter-frame time is also selectable with a nominal setting of 1 second. The detector array is divided into five 1024 x 192 pixel data bands. Any combination of these 5 bands can be selected and downlinked for each visible image. The resultant downlinked data is a 1024 x 192 pixel frame at a rate of one frame per second for each selected band. The visible camera also allows for 2x and 4x pixel spatial summing to reduce data volume. The 5 spectral optical filters are attached to the front of the CCD. Each filter covers approximately 1024 x 204 pixels on the CCD and includes the 1024 x 192 pixels of the data band. Each spectral filter has a full-width half-maximum bandwidth of approximately 50 nanometers. The instantaneous field of view of each visible pixel is 45 microrad. The resulting cross-track field of view is 46.08 mrad. The image length is selectable in increments of 192 line frames. Based on the nominal interframe time of 1 sec, the corresponding incremental along-track field of view is 7.5 mrad. The data volume for each visible image is also limited to the 3,915,776 byte internal buffer in the visible camera electronics. From a 400 km mapping orbit, the planet will be mapped in the visible with a ground sampling distance of 18 meters. At this altitude, the image cross-track width will be 18.4 km and the image along-track length can vary from 3.5 to 9 km for 5 band images and from 3.5 to 57 km for single band images. Use of spatial summing will allow longer image lengths with reduced spatial resolution. The THEMIS instrument was designed and built by Arizona State University, Tempe, Arizona, and Raytheon, Santa Barbara Remote Sensing, Goleta California. The visible camera sub-system and flight software development were provided by Malin Space Science Systems, San Diego, California. The THEMIS will be controlled and operated from the mission operations facility at Arizona State University, Tempe, Arizona." THEMIS Mounting Alignment -------------------------------------------------------- Refer to the latest version of Mars Odyssey 2001 Frames Definition Kernel ([4]) for the THEMIS frame definitions and mounting alignment information. THEMIS Optics Parameters -------------------------------------------------------- The following THEMIS IR and VIS first order optical parameters are included in the data section below ([6]&[7]): ----------------------------------------------------------------- parameter IR VIS ----------------------------------------------------------------- Focal Length, mm 200.0 200.0 f/ratio f/1.66 f/1.66 IFOV, rad/pixel 0.000250 0.000045 Field of view (rad) Cross-track 0.080 0.04608 Along-track 0.060 0.04608 ----------------------------------------------------------------- The values in the keywords are given in the same units as in the table. \begindata INS-53031_FOCAL_LENGTH = ( 200.0 ) INS-53031_F/RATIO = ( 1.667 ) INS-53031_FOV_ANGULAR_SIZE = ( 0.080, 0.060 ) INS-53031_IFOV = ( 0.000250 ) INS-53032_FOCAL_LENGTH = ( 200.0 ) INS-53032_F/RATIO = ( 1.667 ) INS-53032_FOV_ANGULAR_SIZE = ( 0.04608, 0.04608 ) INS-53032_IFOV = ( 0.000045 ) \begintext THEMIS FOV Definitions --------------------------------------------------------------------------- Both FOVs defined in this section are based on the nominal THEMIS detector FOV angular sizes provided in the Table in "THEMIS Optics Parameters" section above. The set of keywords in the data section below defines nominal THEMIS IR FOV with respect to the M01_THEMIS_IR frame to be a rectangle with angular sizes of 0.080 radians in M01_THEMIS_IR XZ-plane (plane parallel to the IR CCD lines and, nominally, normal to the mapping orbit plane) and 0.060 radians in M01_THEMIS_IR YZ-plane (normal to the IR CCD columns and, nominally, parallel to the mapping orbit plane) with the boresight along +Z axis of M01_THEMIS_IR frame. \begindata INS-53031_FOV_FRAME = 'M01_THEMIS_IR' INS-53031_FOV_SHAPE = 'RECTANGLE' INS-53031_BORESIGHT = ( 0.0000000000000000 0.0000000000000000 +1.0000000000000000 ) INS-53031_FOV_BOUNDARY_CORNERS = ( +0.0399713690821580 +0.0299715285563850 +0.9987512188376500 -0.0399713690821580 +0.0299715285563850 +0.9987512188376500 -0.0399713690821580 -0.0299715285563850 +0.9987512188376500 +0.0399713690821580 -0.0299715285563850 +0.9987512188376500 ) \begintext The set of keywords in the data section below defines nominal THEMIS VIS FOV with respect to the M01_THEMIS_VIS frame to be a rectangle with angular sizes of 0.04608 radians in M01_THEMIS_VIS XZ-plane (plane parallel to the VIS CCD lines and, nominally, normal to the mapping orbit plane) and 0.04608 radians in M01_THEMIS_VIS YZ-plane (normal to the VIS CCD columns and, nominally, parallel to the mapping orbit plane) with the boresight along +Z axis of M01_THEMIS_VIS frame. \begindata INS-53032_FOV_FRAME = 'M01_THEMIS_VIS' INS-53032_FOV_SHAPE = 'RECTANGLE' INS-53032_BORESIGHT = ( 0.0000000000000000 0.0000000000000000 +1.0000000000000000 ) INS-53032_FOV_BOUNDARY_CORNERS = ( +0.0230318503829290 +0.0230318503829290 +0.9994693930961000 -0.0230318503829290 +0.0230318503829290 +0.9994693930961000 -0.0230318503829290 -0.0230318503829290 +0.9994693930961000 +0.0230318503829290 -0.0230318503829290 +0.9994693930961000 ) \begintext THEMIS Optical Distortion Specification -------------------------------------------------------- The optical distortion for IR for VIS camera was not available at the time when this version of the IK was released. THEMIS Detector Parameters -------------------------------------------------------- The following THEMIS IR and VIS detector geometry parameters are included in the data section below ([6]&[7]): ----------------------------------------------------------------- parameter IR VIS ----------------------------------------------------------------- Pixel Size, microns Physical Cross-track 50 9.0 Along-track 50 9.0 Active Cross-track 45.4 2.6 Along-track 24.6 6.2 Detector Array Size (physical) Cross-track 320 1024 Along-track 240 1024 Detector Array Center (physical) Cross-track 160.5 512.5 Along-track 120.5 512.5 Detector Array Size (Active) Cross-track 320 1008 Along-track 240 1018 Detector Array Center (Active) Cross-track 160.5 504.5 Along-track 120.5 509.5 ----------------------------------------------------------------- The values in the keywords are given in the same units as in the table above. \begindata INS-53031_PIXEL_SIZE = ( 50, 50 ) INS-53031_PIXEL_SAMPLES = ( 320 ) INS-53031_PIXEL_LINES = ( 240 ) INS-53031_CENTER = ( 160.5, 120.5 ) INS-53032_PIXEL_SIZE = ( 9, 9 ) INS-53032_PIXEL_SAMPLES = ( 1008 ) INS-53032_PIXEL_LINES = ( 1018 ) INS-53032_CENTER = ( 504.5, 509.5 ) \begintext Relationship Between THEMIS Frames and Detector Line/Sample Coordinates --------------------------------------------------------------------------- This section provides the location of the pixel (1,1) of the IR and VIS cameras with respect to the corresponding camera frames. IR Pixel (1,1) Direction The first pixel in the first row of the IR images is in the fore-track S/C +Y direction and in the cross-track S/C +Z direction or in the +X/+Y quadrant of the M01_THEMIS_IR frame: ^ +Yir -- s/c velocity/along track | Pixel | (1,1)-------|---------+ | | | | | | | | | | | | <-------------x | 240 rows +Xir | +Zir | cross-track | boresight | | (into page) | | | +-------------------+ 320 samples/row VIS Pixel (1,1) Direction The first pixel in the first row of the VIS images is in the fore-track S/C +Y direction and in the cross-track S/C +Z direction or in the +X/+Y quadrant of the M01_THEMIS_VIS frame: ^ +Yvis -- s/c velocity/along track | Pixel | (1,1)-------|---------+ | | | | | | | | | | | | <-------------x | 1018 rows +Xvis | +Zvis | cross-track | boresight | | (into page) | | | +-------------------+ 1008 samples/row Filters --------------------------------------------------------------------------- IR Filters The AR coated ZeSe window and multi-layer thin-film Ge filters comprise the spectral separation for the nine THEMIS infrared filters. Filters cover all 320 pixels in the cross-track S/C z-axis direction. Filter 1 (6.6 microns band) is in the fore-track S/C +y direction. Each filter covers approximately 20 rows on the array but only 16 rows of optically clear pixels are used. Dark mirror coating covers the rows between each filter. ----------------------------------------------------------------- Filter Band Center, Bandwidth, Start End # microns microns Row Row ----------------------------------------------------------------- 1 6.62 1.01 1 16 1 6.62 1.01 17 32 3 7.88 1.09 43 58 4 8.56 1.18 69 84 5 9.30 1.18 95 110 6 10.11 1.10 121 136 7 11.03 1.19 147 162 8 11.78 1.07 173 188 9 12.58 0.81 198 213 10 14.96 0.86 224 239 ----------------------------------------------------------------- The IR filter layout summarized in the table looks like this: ^ Row Index | s/c velocity --------- | 1 (*)----------------------------------------------+ | IR Filter 1 | 16 rows 16/17|-----------------------------------------------| | IR Filter 2 | 16 rows 32 +-----------------------------------------------+ 43 +---------------------------------(10 dark rows)+ | IR Filter 3 | 16 rows 58 +-----------------------------------------------+ 69 +---------------------------------(10 dark rows)+ 2 | IR Filter 4 | 16 rows 4 84 +-------------- +Yir ----------------+ 0 95 +-------------- ^ --(10 dark rows)+ | IR Filter 5 | | 16 rows r 110 +-------------- | ----------------+ o 121 +-------------- <---- x --(10 dark rows)+ w | IR Filter 6 +Xir +Zir | 16 rows s 136 +-------------- (into ----------------+ 147 +-------------- page) --(10 dark rows)+ | IR Filter 7 | 16 rows 162 +-----------------------------------------------+ 173 +---------------------------------(10 dark rows)+ | IR Filter 8 | 16 rows 188 +-----------------------------------------------+ 198 +----------------------------------(9 dark rows)+ | IR Filter 9 | 16 rows 213 +-----------------------------------------------+ 224 +---------------------------------(10 dark rows)+ | IR Filter 10 | 16 rows 239 +-----------------------------------------------+ (1 dark row) 320 samples The set of keywords below contains wavelength center, bandwidth and CCD row ranges for each IR filter. \begindata INS-53031_FILTER_BANDCENTER = ( 6.62, 6.62, 7.88, 8.56, 9.30, 10.11, 11.03, 11.78, 12.58, 14.96 ) INS-53031_FILTER_BANDWIDTH = ( 1.01, 1.01, 1.09, 1.18, 1.18, 1.10, 1.19, 1.07, 0.81, 0.86 ) INS-53031_FILTER_FIRST_ROW = ( 1, 17. 43, 69, 95, 121, 147, 173, 198, 224 ) INS-53031_FILTER_LAST_ROW = ( 16, 32, 58, 84, 110, 136, 162, 188, 213, 239 ) \begintext VIS Filters Five color filters are cemented onto the visible detector surface with the gaps between filters aligned with the detector rows. For non-calibration images, the camera firmware skips the dark rows and the gaps between filters, producing 192 lines per filter in the 1x1 summing mode. A special calibration image capability can be used to read out all rows, 1x1 summing only. Filters cover all 1024 pixels in the cross track S/C z-axis direction. Filter 1 (0.870 microns band) is in the fore-track S/C +y direction. Each filter covers approximately 205 rows on the array but only the 192 rows of optically clear pixels are used. There are no dark mirror coatings over the rows between each filter. ----------------------------------------------------------------- Filter Band Band Center, Bandwidth, Start End # # (*) microns microns Row Row ----------------------------------------------------------------- 1 5 0.860 0.045 4 195 2 1 0.425 0.049 203 394 3 3 0.654 0.053 404 595 4 4 0.749 0.053 612 803 5 2 0.540 0.051 814 1005 ----------------------------------------------------------------- (*) To be consistent with the IR, the "filters" are numbered in physical order on the array and "bands" are numbered in chromatic order. Filter 1/Band 5 is the first filter to reach the scene. The VIS filter layout summarized in the table looks like this: ^ Row Index | s/c velocity --------- | (3 dark rows) 4 (*)----------------------------------------------+ | | | VIS Filter 1 | 192 rows | | 195 +-----------------------------------------------+ 203 +----------------------------------(7 dark rows)+ | | | VIS Filter 2 | 192 rows | +Yvis | 1 394 +--------------- ^ ----------------+ 0 404 +--------------- | ---(9 dark rows)+ 1 | | | 8 | VIS Filter 3 <----x | 192 rows | +Xvis +Zvis | r 595 +-------------- ----------------+ o 612 +-------------- --(16 dark rows)+ w | | s | VIS Filter 4 | 192 rows | | 803 +-----------------------------------------------+ 814 +---------------------------------(10 dark rows)+ | | | VIS Filter 5 | 192 rows | | 1005 +-----------------------------------------------+ (13 dark rows) 1008 samples The set of keywords below contains wavelength center, bandwidth and CCD row ranges for each VIS filter. \begindata INS-53032_FILTER_BANDID = ( 5, 1, 3, 4, 2 ) INS-53032_FILTER_BANDCENTER = ( 0.860, 0.425, 0.654, 0.749, 0.540 ) INS-53032_FILTER_BANDWIDTH = ( 0.045, 0.049, 0.053, 0.053, 0.051 ) INS-53032_FILTER_FIRST_ROW = ( 4, 203, 404, 612, 814 ) INS-53032_FILTER_LAST_ROW = ( 195, 394, 595, 803, 1005 ) \begintext Timing Offset --------------------------------------------------------------------------- This section provides information required compute the time associated with a particular line of an IR or VIS image using the time tag provided in the image header. IR Timing Most of the images taken by THEMIS IR detector during operations are taken in summing mode when the readouts of from all lines of a particular band taken over the same area on the surface are added up to produce a single line per band. In this mode the IR exposure time, fixed in firmware, is 1/60 second. Every other frame is thrown away to give to 1/30 sec line rate. During 1/30 of a second the spacecraft moves one pixel spacing on the ground from a 400 km orbit (250 urad IFOV), making sure that 16 lines of each band accumulate light coming from the same area. The following description by Jim Thornton (from [8]) presents the on-board IR image acquisition process in greater detail: "... Detector row number 1 is the first to see an area on the ground. In frame readout number 2, detector row number 2 sees the same area on the ground as row number 1 saw in frame number 1. Thus, the TDI to produce line 1 of band 1 (using detector rows 1-16) of the image file consists of combining detector row 1 from frame 1 with detector row 2 from frame 2, etc. The TDI for line 1 of band 2 (detector rows 17-32) begins with frame 17, when detector row 17 sees the same area on the ground as was seen for line 1 band 1 of the image file. Thus, the observation time for line 1 band 2 is (17-1)/30=16/30 seconds later than for line 1 band 1. The TDI for line 1 band 3 (detector rows 43-58) begins with frame 43, when detector row 43 sees the same area of the ground as was seen for line 1 band 1. Thus, the observation time for line 1 band 3 in the image file is (43-1)/30=42/30 seconds later than the observation time for line 1 band 1. In a similar manner, the observation time for line 1 band 9 (detector rows 198-213) is (198-1)/30=197/30 seconds later than the observation time for line 1 band 1. Thus, for line 1 of the image file, the design is an attempt to have all the different bands for line 1 looking at the same area on the ground. This will not be exactly achieved for two reasons. First, each band in line 1 is observed at a different time and the planet rotates a small amount during the time interval, resulting in a left-right misalignment of the different bands in the image file. Second, the 1/30 second interval between frame readouts might not exactly compensate for the ground motion (or spacecraft slew for the test images), resulting in an up-down misalignment of the different bands." Based on this description the following simple formula can be used to compute the time when the N-th line of the IR CCD "observed" the M-th line in a particular band of an assembled IR image: T = SCLK + ( M - 1 )/30 + ( N - 1 )/30 - 1/120 where: SCLK -- is time from the image header, which nominally corresponds to the time at which the first detector frame readout occurs; M -- is the image line number in a particular band image; N -- is the physical CCD line number belonging to the range of lines for that band; first and last line index for each band are provided in the FILTER_FIRST_ROW/FILTER_LAST_ROW keywords above; 2 -- must be subtracted to compensate for the fact that numbering starts with "1" in both image and CCD lines arrays; 30 -- is the fixed IR line rate; 1/120 -- is a half exposure time (1/2 of 1/60 of a second). Using the "( N - 1 )/30 - 1/120" member of the formula the following fixed IR timing offsets can be computed: -- offset between the image time and the time when the first line of a particular band "observes" the surface area that appears in first line of the assembled image for that band (column A); -- offset between the image time and the time when the middle -- imaginary line between the 8th and 9th physical line in a band -- of a particular band "observes" the surface area that appears in first line of the assembled image for that band (column B); -- offset between the image time and the time when the last line of a particular band "observes" the surface area that appears in first line of the assembled image for that band (column C); which are summarized in this table (offsets are given in seconds): Filter # A B C ---------- ---------- ---------- ---------- 1 - 0.008333 + 0.241667 + 0.491667 2 + 0.525000 + 0.775000 + 1.025000 3 + 1.391667 + 1.641667 + 1.891667 4 + 2.258333 + 2.508333 + 2.758333 5 + 3.125000 + 3.375000 + 3.625000 6 + 3.991667 + 4.241667 + 4.491667 7 + 4.858333 + 5.108333 + 5.358333 8 + 5.725000 + 5.975000 + 6.225000 9 + 6.558333 + 6.808333 + 7.058333 10 + 7.425000 + 7.675000 + 7.925000 (When added to "SCLK + ( M - 1 ) / 30" term the offset will result in correct time for M-th image line taken by the first, middle or last line of the image band.) The keywords in this data section provide the lane rate and offsets for the band middle points: \begindata INS-53031_LINE_RATE = 0.03333333333333 INS-53031_FILTER_TIME_OFFSET = ( 0.241667 0.775000 1.641667 2.508333 3.375000 4.241667 5.108333 5.975000 6.808333 7.675000 ) \begintext The following are the delays that should be added "on top" the the SCLK recorded for an image, which nominally corresponds to the time at which the first detector frame readout occurs: 0.000008 seconds -- PACI write delay 0 to 1/30 seconds -- delay of the image acquisition after receipt of the command 0 to 0.10 seconds -- delay for the command sending by the HPP code so that it is aligned on a 0.1 second boundary Since the 8 microseconds is a negligible fraction of the 1/30 second line rate it can be ignored. The other two delays are (apparently) variable and unknowable and are a potential source of error that could result in projected coordinates being in error by as much as four (4) pixels long track affecting all band acquisitions equally. The overall uncertainty of the image time tag is specified in the keyword below in seconds. \begindata INS-53031_TIME_UNCERTAINTY = 0.13 \begintext VIS Timing Each of the images taken by THEMIS VIS detector during mapping operations consists of the consequent framelets -- mini-images taken by a particular VIS filter/band at time instants that are nominally 1 second apart -- "glued" together in one image strip. The following description by Greg Mehall (from [8]) presents the on-board VIS image acquisition process: ``The time given [in the image label] is the time that the first framelet of the first selected filter is exposed. This is the case no matter which is the first selected filter. ... The SCLK that is included is the time when the VIS data acquisition is initiated by the flight software. At this time the entire array is "exposed" with the exposure that was specified (typically 3-6 msec). Only the data from the first selected filter's framelet is sent to the downlink queue at this point. All other filter's "framelets" are discarded. [Knowing the filter, BVS] ... the offset from the optical axis for this filter [can be computed] to get the correct projection on the surface. On the next interframe time (typically 1 sec later) the entire array is "exposed" again. At this time the second framelet from the first filter and the first framelet from the second selected filter (if selected and adjacent) is sent to the downlink queue. All other filter's framelets are discarded. This process is repeated until you acquire the specified number of frames for all selected filters. In essence, a ramp up and ramp down of the images occur as the array is exposed on every interframe time. The time that is included with the VIS files is the SCLK (± flight software uncertainties TBD) of the first selected filter's first framelet that is acquired. An example of a five frame, filter 2 (band 1=423nm) and filter 4 (band 4=721nm) image, that is commanded to start at SCLK=v, with interframe time = 1 sec: Time Filter Data Saved and Downlinked v F2 v+1 sec F2 v+2 sec F2, F4 v+3 sec F2, F4 v+4 sec F2, F4 v+5 sec F4 v+6 sec F4 In this example, the center of the first filter 2 framelet (that is collected at time=v) is foretrack of the THEMIS optical axis by ~300 lines. At 45 urad per line this equates to a foretrack projection of this framelet relative to the THEMIS optical axis of 13.5 mrad. From 420 km altitude this would represent a foretrack shift from the sub-Nadir point of 5.4 km. ... There are some small timing errors in the VIS flight software that are comparable to the IR numbers. The following information is based on discussions ... with Mike Caplinger at MSSS. The SCLK time reported is the time that the PACI write for the command that starts the image acquisition is issued. There should be no delays in the PACI write since it is issued from the High Precision Processing (HPP) part of the flight software, which is effectively running with interrupts disabled. The PACI write itself will take 64 microseconds plus bus overhead. The VIS imaging starts about 50 milliseconds after the receipt of the command. It should also be noted that although you can specify the start time to, and it is reported to, a precision of 1/256 of a second, the HPP code only executes it at times aligned on 0.1 second boundaries." The current version of the IK does not attempt to provide any of the timing parameters mentioned in this description as data values retrievable by SPICE Toolkit. Platform ID --------------------------------------------------------------------------- This number is the NAIF instrument ID of the platform on which the cameras are mounted. For both THEMIS detectors such platform is the spacecraft itself. \begindata INS-53031_PLATFORM_ID = ( -53000 ) INS-53032_PLATFORM_ID = ( -53000 ) \begintext