USGS

ISIS 2 Documentation


Isis Mars Odyssey THEMIS Geometry Processing With ISIS

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Written by Jim Torson
August 29, 2003
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  1. Introduction
  2. Documentation
  3. Summary of Primary Programs
  4. Summary of Additional Programs
  5. Map Projections and OCENTRIC/OGRAPHIC Latitudes
  6. Notes On Geometry Values
  7. Using Alternative Mars Target Definition Files
  8. Reducing Unnecessary Processing
  9. Timing Uncertainties
  10. Times Recorded in File Labels
  11. Notes on SPICE Files
  12. Dealing with Missing Camera Pointing
  13. Adding New SPICE Files
  14. Concurrent Processing in One Directory
  15. Error Reporting
  16. Making Mosaics Near Longitude = 0, 180
  17. Known Problems (As of July 23, 2003 ISIS Update)

1. INTRODUCTION

This document describes ISIS geometry processing for data from the Mars Odyssey THEMIS instrument. This describes the software as of the August 28, 2003 Release #20 of the THEMIS geometry software.

The input to the processing consists of PDS QUB files that are produced by software running at Arizona State University (ASU). The PDS QUB files are first translated to ISIS cube format, and then all the geometry processing described below is run within the ISIS environment.

Note that the ISIS geometry software is designed to process THEMIS IR/VIS EDR and RDR QUBE files. However, this sofware is NOT intended to process the THEMIS BTR, ABR or browse image files.

2. DOCUMENTATION

The software consists of a number of ISIS executable programs and four TAE procedure PDFs that execute a sequence of ISIS programs.

If you are not familiar with TAE, look at the ISIS web-based documentation, e.g.:

    netscape $ISISHTML/documentation.html

The document Overview of ISIS Architecture contains a description of how to use TAE. In particular, this describes how to obtain documentation on a particular program or procedure PDF.

You can also use the "isisdoc" program to produce a nicely-formatted plain-text file that contains the documentation for a particular program or procedure PDF. You can run "isisdoc" in TAE or just run it at the operating system command prompt. E.g., for the "thmvismc" procedure PDF that is in the standard ISIS system, type the following at the operating system prompt:

    isisdoc $ISISEXE/thmvismc

If you have a THEMIS geometry software update installed in the ISIS development area, the following command should be used:

    isisdoc $DEVISISEXE/thmvismc

This will produce a file called thmvismc.doc. This is the most complex of the four procedure PDFs because it deals with the VIS framelets.

3. SUMMARY OF PRIMARY PROGRAMS

All of the geometry software and related programs can be executed by using TAE. All of the software can also be executed without the use of TAE. The ISIS executable programs can be run at the operating system command prompt. The procedure PDFs cannot be run without TAE. However, they call Perl scripts to do all the work, and the Perl scripts can be executed without the use of TAE. The documentation in the procedure PDFs describes how to call the Perl scripts.

The following are the four primary programs you need to be aware of. Each of these is a procedure PDF and associated Perl scripts that run a sequence of executable programs. Each is able to process a single input data file or set of input data files that are listed in a plain-text file.

Note that "thmirmc" and "thmvismc" include SAMPINC/LINEINC parameters that control the sampling increment for determining the lat/lon range for the output map projection file. The default value for these parameters is 10, which is adequate for most cases. However, there might occasionally be a file in which this fails to accurately determine the lat/lon range. This would result in an output file that includes a spatial area slightly smaller than needed to include all the input data. This problem can be eliminated by setting SAMPINC/LINEINC to smaller values, e.g., 1. However, note that setting these values to 1 can result in the total projection process taking significantly longer to run, e.g., as much as five times longer.

Note also that "thmirmc" and "thmvismc" can optionally use the MOLA Mars DEM data for projecting to map coordinates. The default is to use the MOLA DEM data. This can increase the processing time by about 20%, but this results in better band-to-band registration. ("thmiric" is not able to use the MOLA DEM. Thus, the band registration in the IRIC files will not be as good as in the IRMC files.)

4. SUMMARY OF ADDITIONAL PROGRAMS

There are several additional ISIS executable programs that provide related geometry functions:

5. MAP PROJECTIONS AND OCENTRIC/OGRAPHIC LATITUDES

The output map projection is specified by the MAPPARS parameter to "thmirmc" and "thmvismc". The following is the description of this parameter for these programs:
    MAPPARS allows the user to specify the desired output
    map projection and its associated parameters that
    will be used by the "lev1tolev2" program to create the
    output map-projected file. When this is specified as
    the TAE Null parameter value (--), the MAPPARS value
    that will be used for calling "lev1tolev2" will be
    MAPPARS="SINU:clon,plattyp", which will create a
    Sinusoidal Equal-Area map with center longitude (clon)
    equal to the average of the longitude range of the
    output map. The projection latitude type (plattyp)
    will be the latitude system that is recorded in the
    LATITUDE_SYSTEM keyword in the ISIS_TARGET keyword
    Group in the input file. The value of this keyword
    is normally set by the LATSYS parameter when running
    "thm2isis" to translate the PDS QUBE file to ISIS
    format.

    If a non-null value is specified for MAPPARS, then
    this value will be used as the value of the MAPPARS
    parameter passed to "lev1tolev2". An extensive
    explanation of this parameter and its various options
    and defaults can be obtained by tutoring "mappars" in
    TAE and typing "help *".

    Note that when SINU or SIMP is specified as the
    projection name in the MAPPARS value, the default
    latitude type OGRAPHIC will be used if the projection
    latitude type is not specified, i.e., the standard
    MAPPARS default will be used rather than the latitude
    system that is specified in the input file
    LATITUDE_SYSTEM label keyword. If OCENTRIC latitudes
    (the usual standard for THEMIS) are desired, then
    OCENTRIC must be specified in the MAPPARS value.

The following discussion clarifies the OCENTRIC/OGRAPHIC handling:

    ISIS handles the difference between OGRAPHIC/OCENTRIC
    latitudes in two totally separate and parallel ways:

    1) The latsys chosen when running "levinit" (which is run
    by "thm2isis") governs the way that latitudes will be
      (a) reported to the user and
      (b) interpreted when input by the user
    So if you choose planetographic then for example "qview"
    will display the planetographic latitude of the cursor,
    and if you next ask for a map with lat=(15,30) it will be
    from 15 degrees planetographic latitude to 30 deg
    planetographic.  The latsys specified when running "levinit"
    is recorded in the LATITUDE_SYSTEM label keyword in the
    ISIS_TARGET keyword Group.

    2) Entirely separate from this, we now have two types of
    sinusoidal projection and two types of simple cylindrical
    projection.  One has line number equally spaced in ographic
    latitude and the other equally spaced in ocentric latitude.
    Which one you get is controlled by the plattyp (projection
    latitude type - OGRAPHIC or OCENTRIC) in the MAPPARS string.
    In a Level 2 projected file, the projection latitude type
    is recorded in the PROJECTION_LATITUDE_TYPE label keyword
    in the IMAGE_MAP_PROJECTION keyword Group.  Note that this
    keyword Group also includes the KEYWORD_LATITUDE_TYPE keyword.
    This value comes from the LATITUDE_SYSTEM keyword in the
    ISIS_TARGET Group, and it specifies the latitude system
    used for the values in the MINIMUM_LATITUDE and MAXIMUM_LATITUDE
    keywords in the IMAGE_MAP_PROJECTION Group.  Thus, it is
    possible to have a file that is projected to one type
    (PLANETOGRAPHIC or PLANETOCENTRIC) but which has the
    minimum/maximum latitudes recorded in the other type.

    Yes, this is confusing.  Why would anyone want this
    distinction?  Well, it lets you put appropriate labels on,
    say, a MOLA product (cylindrical projection equisampled in
    ocentric latitude) and then work with it in ographic latitudes,
    e.g., ask for a Mercator map from -60 to +60 deg ographic.
    Or update labels on an old USGS archived product (sinusoidal
    projection equisampled in ographic latitude) to the new
    standard and then reproject to, say, 15 to 30 deg planetocentric
    latitude and a cylindrical-planetocentric projection, which
    would line up with a chunk of the MOLA archive.  So there is
    a reason for this confusing behavior.

6. NOTES ON GEOMETRY VALUES

It is important to remember that different bands observe the same point on the ground at different times. Also, for UNPROJECTED files, the different bands are not spatially registered. This applies to both the IR and the VIS.

Of course the computed latitude and longitude values on the ground do not depend upon time. However, the other geometry values are a function of time and spacecraft position.

When "levpt", "levgeoplane", and "lev1stats" are run on an UNPROJECTED IR or VIS file, the geometry values are calculated for the FIRST band in the file. Thus, none of the values will be exactly correct for other bands in the file.

When "levpt", "levgeoplane", and "lev1stats" are run on an IRIC file, the geometry values are computed for the REFERENCE band that was used for producing the file. Thus, the lat/lon values should be correct for all bands in the file. However, even though the bands are spatially registered, the other geometry values will not be exactly correct for the other bands (because they observe a given point on the ground at different times).

When "levpt", "levgeoplane", and "lev1stats" are run on an UNPROJECTED file or an IRIC file, all of the geometry values can be computed for all spatial points within the file.

When "levpt" and "levgeoplane" are run on an IRMC or VISMC file, the geometry values are computed for the MIDDLE band in the file. (For a file with an even number of bands, the "middle" band is the number of bands divided by two.) In this case, lat/lon values are computed for all spatial points within the file. However, the other geometry values can only be computed for spatial points corresponding to actual image data for the middle band in the file.

A further complication arises with the VIS data. In general, any single band of an UNPROJECTED VIS file contains multiple "framelets" stacked vertically through the spatial area of the band image. Each framelet was observed at one instant of time, but different framelets were observed at different times. Also, in general there is some overlap in the area of ground covered by adjacent framelets. Thus, a given lat/lon point on the ground can correspond to two points in the UNPROJECTED VIS file.

When "levpt", "levgeoplane" and "lev1stats" are run on an UNPROJECTED VIS file, the geometry values are computed for the appropriate time and spacecraft position within each framelet for the "reference" band, i.e., first band in the file. This results in discontinuities for all geometry values (including lat/lon) at the boundaries between framelets.

When "levpt" and "levgeoplane" are run on a VISMC file, the lat/lon values are continuous and cover the entire spatial area of the file, including locations where the "reference" band (middle file band) do not contain image data from any framelets. The other geometry values can be computed only at spatial locations which contain image data from at least one framelet in the "reference" band. For spatial locations that correspond to image data from a single framelet, the geometry values are computed using the time and spacecraft position for the framelet. For spatial locations that correspond to image data from two framelets, the geometry values are computed using the time and spacecraft position for the EARLIER of the two framelets. Note that using TOP=NO (which is NOT the default parameter value) when running "thmvismc" will result in image data for the reference band that corresponds to the computed geometry values in the areas where framelets overlap.

There are two ways to obtain geometry values based on a band that is different from the "reference" band described above (first file band for an UNPROJECTED file, reference band for an IRIC file, middle file band for an IRMC or VISMC file). First, the "dsk2dsk" program can be run using the SFROM parameter to extract just the band of interest into a separate file. Then, compute the geometry values using this single-band file. Second, the "labels" program can be used to set the REFERENCE_INSTRUMENT_BAND keyword in the VOLATILE_INFO keyword Group to the desired band number. In this case, the value should be set to the desired INSTRUMENT band number (not the file band number). Then, compute the geometry values using this updated IRMC or VISMC file.

7. USING ALTERNATE MARS TARGET DEFINITION FILES

There are several Mars target definition files contained in $ISISDATA/targets:

IMPORTANT NOTE #1: The Mars target definition file to be used is determined when you run "thm2isis" to translate the .QUB file into an ISIS format file. The "thm2isis" TARGDEF parameter specifies the target definition file to be used. The default value is the Null TAE parameter value, which will result in using the latest version of mars.def.N in $ISISDATA/targets/. When the mars.def.4 file is used, you will get coordinates consistent with the 2000 IAU report. Also, note that this will produce longitudes that increase to the EAST. The previous ".3" version of this file produced longitudes that increase to the WEST. You can specify a specific target definition file to be used by specifying it's name and location in the "thm2isis" TARGDEF parameter. For example, TARGDEF="$ISISDATA/targets/MDIM1.0.west.def" will give you coordinates that are consistent with MDIM 1.0.

The TARGDEF keyword in the ISIS_TARGET keyword Group in the ISIS file records the name of the target definition file that is being used.

IMPORTANT NOTE #2: When the software automatically uses the latest version of a file (e.g., thm2isis_translation.def.N or mars.def.N), it uses the following algorithm to find the "latest version". It first looks for a ".1" version. Then it looks for a ".2" version. If ".2" does not exist, it assumes ".1" is the latest version and uses it. Etc. This means that correctly finding the "latest version" depends upon all the previous versions also existing in the directory being searched.

8. REDUCING UNNECESSARY PROCESSING

Each band is geometrically processed independently. Thus, you can reduce processing time by processing only the desired bands. For example, suppose you have a complete 10-band IR observation but are only interested in having two of the bands projected to map coordinates so that you can compute a ratio image. You can use the "dsk2dsk" program to process the output of "thm2isis" to produce a sub-cube that contains only the two bands of interest. Running "thmirmc" on this 2-band subcube will take approximately 20% as much time as processing the complete 10-band cube.

9. TIMING UNCERTAINTIES

The software assumes that the SCLK (spacecraft clock) recorded for an IR image is the time at which the first detector frame readout occurs. Apparently, the actual first readout can be delayed from this by the following amounts:

Currently, the software ignores these three because the 8 microseconds is a negligible fraction of the 1/30 second line rate and the other two are (apparently) variable and unknowable. However, this can result in an incorrect absolute position of as much as four pixels.

Similar timing uncertainties probably are applicable to the VIS images.

The effect of these timing uncertainties is primarily to potentially introduce errors in absolute coordinates in the along-track direction. This in turn can result in misalignment when mosaicking multiple observations together.

The TIME parameter on "thm2isis" allows the user to apply a correction for this time delay, which can thus improve absolute coordinate accuracy and improve mosaic alignments.

10. TIMES RECORDED IN FILE LABELS

The input QUBE files and the output ISIS files contain several keywords that record important time information. The input QUBE files contain TWO keywords that record spacecraft clock times:
UNCORRECTED_SCLK_START_COUNT (in the SPECTRAL_QUBE object) -
  The commanded time for the start of the observation.

SPACECRAFT_CLOCK_START_COUNT (not in any Group or Object) -
  For IR, this is the time corresponding to the first line of
  data for Filter number 1 (even if Filter number 1 data are
  not contained in the file).  For IR, it is always the
  same as UNCORRECTED_SCLK_START_COUNT.
  For VIS, this is the time corresponding to the first framelet
  of data for Filter number 1 (even if Filter number 1 data are
  not contained in the file).  If Filter number 1 data are
  included in the file, then this will be the same value as
  UNCORRECTED_SCLK_START_COUNT.  If Filter number 1 data are
  not contained in the file, then this could differ from
  UNCORRECTED_SCLK_START_COUNT by as much as four times the
  INTERFRAME_DELAY (usually 1.0 seconds).
The output ISIS files contain THREE keywords that record spacecraft clock times:
UNCORRECTED_SCLK_START_COUNT (in the THEMIS_KEYWORDS Group
    within the QUBE Object) -
  A copy of the UNCORRECTED_SCLK_START_COUNT from the input file.

SPACECRAFT_CLOCK_START_COUNT (in the THEMIS_KEYWORDS Group
    within the QUBE Object) -
  A copy of the SPACECRAFT_CLOCK_START_COUNT from the input file.

ORIGINAL_SPACECRAFT_CLOCK_COUNT (in the ISIS_INSTRUMENT Group
    within the QUBE Object) -
  Derived from the SPACECRAFT_CLOCK_START_COUNT in the input file
  by adding the time adjustment specified by the TIME or TIMELIST
  parameters when running "thm2isis".  (This allows the user to
  apply corrections for the time delay between the commanded
  start of observation and the actual start of observation.)
  This is the time that is used by the ISIS geometry software.
  (It is confusing that this adjusted time is recorded in a keyword
  whose name starts with "ORIGINAL_".  However, this keyword name
  is used to be consistent with the corresponding keyword used
  in previous missions for recording the observation time.)

11. NOTES ON SPICE FILES

The file $ISISM01DATA/thm_kernels.def.N (where N is a version number) lists the SPICE files to be used by the geometry software. (By default, the software will use the file with the highest version number. For this to work correctly, all lower version numbers for the file must also exist in the directory.)

The Frames Kernel file that should be used is the latest that is available from the NAIF ftp site, which as of the time of writing this document is m01_v26.tf. This includes the latest values for the yaw rotation of the IR detector (-0.672 degrees) and the yaw rotation of the VIS detector (-0.25 degrees). (Previously, a USGS-modified version of m01_v23.tf needed to be used because the latest alignment angles had not yet been incorporated into the NAIF-supplied file.)

If you are maintaining your own thm_kernels.def.N file, you should make sure your file lists m01_v26.tf rather than some other version of this file.

The $ISISM01DATA directory contains the mars_iau2000_v0.tpc file, which contains Mars planetary constants for use by THEMIS. However, the ISIS software does not use this file. Instead, it uses the $ISISDATA/targets/mars.def.N file to obtain constants for Mars. The mars.def.4 file contains constants that are identical to the values in mars_iau2000_v0.tpc.

12. DEALING WITH MISSING CAMERA POINTING

By default, "thm2isis" will initialize the output ISIS cube file to use the SPICE files listed in $ISISM01DATA/thm_kernels.def.N. This file lists the CK camera pointing files (file extension .bc) that contain the "real" camera pointing data. Unfortunately, there are gaps in these data that prevent ISIS from geometrically processing some of the THEMIS images. For these, will there will usually be an error when running "thm2isis", or perhaps when running "thmirmc" or "thmvismc".

To allow dealing with these missing camera pointing data, the NAIF team has prepared special CK files that contain "fake" nadir pointing data. Images that have missing "real" camera pointing data can be projected using the "fake" nadir camera pointing data. The THEMIS IR and VIS instruments are usually pointing close to nadir, so using the nadir pointing data will produce projected images that are close to being "correct." However, there will probably be some errors in absolute coordinates because the true pointing is not exactly nadir.

There are two ways to use the "fake" nadir camera pointing data. First, the "thm2isis" KERNLST parameter can be set to $ISISM01DATA/thm_kernels_nadir.def.N rather than its default value. This kernel list includes only the "fake" nadir data and none of the "real" camera pointing data. This will thus force the use of the nadir data, even if "real" pointing data exists for the image.

In some cases, it might be useful to use the $ISISM01DATA/thm_kernels_both.def.N for the KERNLST. This file lists both the "real" camera pointing kernel files and the "fake" nadir pointing kernel files. In most cases, if "real" camera pointing is available then it will be used, and the nadir pointing will automatically be used if "real" pointing is not available. However, there might be some images for which "thm2isis" succeeds with finding "real" pointing but "thmirmc" or "thmvismc" fails because there is "real" pointing for only part of the image. These images will have to be processed using thm_kernels_nadir.def.N to force use of the nadir pointing. (For efficiency, "levinit" (called by "thm2isis" to select a single camera pointing file) does a partial check for pointing data and might choose a CK file that contains pointing for only part of the image.)

Note that the KERNLST keyword in the ISIS_GEOMETRY Group in the ISIS cube file records which kernel list was used. Also, the CAMERA_KERNELS keyword in this Group records the name of the CK file that was used, which will indicate whether "real" pointing or "fake" nadir pointing was used. The "fake" nadir files have "nadir" in the filename, e.g., m01_sc_map2_rec_nadir.bc.

13. ADDING NEW SPICE FILES

If you want to use additional SPICE files, you should edit $ISISM01DATA/thm_kernels.def.N to list the additional files and put the additional files into $ISISM01DATA.

The THEMIS software does not yet use the latest version of the NAIF routines, so you will need to use SPICE files that are in the appropriate format (Sun or PC) for the computer architecture.

The THEMIS SPICE files can be obtained at the NAIF ftp site:

Note that in addition to adding recent spacecraft (SPK) and camera (CK) SPICE kernel files, it is important to use the latest file that defines the mapping from Spacecraft Clock (SCLK) values into "real" time. These files have names of the form ORB1_SCLKSCET.nnnnn.tsc, where "nnnnn" is a version number. These are in the sclk subdirectory of the NAIF ftp site. While the mission is ongoing, a new version of this file is made available every 2-4 weeks. If you are not using a recent enough version of this file for the observation being processed, then an extrapolation will be used for the time mapping. This can lead to time errors and thus errors in absolute coordinates for the projected images.

14. CONCURRENT PROCESSING IN ONE DIRECTORY

The "thmiric", "thmirmc", and "thmvismc" processing Perl scripts make use of temporary disk files. If "thmiric" and "thmirmc" are run concurrently in the same directory to process the same input IR file, then file name conflicts for the temporary files can cause the processing to fail.

The "thmiric", "thmirmc", and "thmvismc" scripts can be used to concurrently process different input IR and/or VIS files in the same directory. However, whenever concurrent ISIS processing is done in one directory, you should prevent conflicts with access to the ISIS Session Log File. By default, a session log of ISIS processing is written to the print.prt file. The ISIS_LOG_FILE environment variable can be set to the name of another file to be used for the session log file. This should be used to prevent multiple concurrent processes from attempting to write to the same session log file at the same time.

15. ERROR REPORTING

The processing scripts use the ISIS "errors" program to examine the current session log file for error reports. Note that the entire log file is examined. Thus, "old" errors that occurred prior to the current processing run will be included in the count of errors encountered. To get an accurate count of errors, you should start with a fresh log file. To do this, delete (or rename) the print.prt file or the file named by the ISIS_LOG_FILE environment variable.

16. MAKING MOSAICS NEAR LONGITUDE = 0, 180

Projected ISIS files can record longitudes in either the 0 to 360 degree system or the -180 to +180 degree system. By default, when your run "thm2isis" the output file is initialized to use the 0-360 longitude system. Most of the time, when you run "thmirmc" you will thus get an output file that is in the 0-360 system. However, by default, if an image crosses the 0/360 boundary, "thmirmc" or "thmvismc" will switch to the -180 to +180 system. (If it didn't, you would get a big file with half your data on the left and half on the right with a big blank area in the middle.) So, if you want to mosaic several images that are in the vicinity of 0/360 longitude, some of the images might be in the 0-360 system and some might be in the -180 to +180 system. Mosaicking images that use different longitude systems will not work correctly.

So, for mosaicking images near longitude=0, you need to get all you images to use the -180 to +180 system. You could do this by specifying LONSYS=180 when you run "thm2isis" on all your images. You could also just run "levinit" on the output of "thm2isis" to set the longitude system to -180 to +180 for each of the images. You could also just run the "labels" program to set the LONGITUDE_SYSTEM keyword in the ISIS_TARGET keyword Group to the integer value 180. (Running "labels" would be more efficient than re-running "levinit".)

In a similar manner, when an image is initialized for the +/- 180 system, by default it will be switched to the 360 system if it crosses longitude=180.

This automatic switching of the longitude system can be turned off by setting AUTOLON=NO when running "thmirmc" or "thmvismc". When doing this, the LATRANGE and LONRANGE parameters can also be set to specify the desired latitude/longitude range for the projected image. This can be useful for doing things such as producing images for mosaics of the polar regions in four separate quadrants using a consistent longitude system.

17. KNOWN PROBLEMS (AS OF THE AUGUST 28, 2003 THEMIS GEOMETRY SOFTWARE UPDATE #20)

  1. For producing IRMC files, an empirical correction for optical distortions is being applied. This is probably not exactly correct, but the residual misregistration of each band relative to band 5 should usually be on the order of 0.1 pixel or less. (Band 10 mostly sees only the atmosphere. An approximate optical distortion correction is applied to Band 10 that is an extrapolation of the empirical corrections for the other bands.)

  2. Because the IR optical distortion correction is an empirical model that assumes that band 5 is "correct," there might be errors in absolute coordinates for projected IR files.

  3. The "thmiric" software currently cannot use the MOLA DEM for producing IRIC files. Thus, the band registration for IRIC files will not be as good as for IRMC files that were computed by "thmirmc" using the MOLA DEM.

    In a similar manner, "levpt", "lev1stats", and "levgeoplane" are not able to use the MOLA DEM. This can result in small errors in the geometry values that these programs calculate.

  4. We are currently assuming that the IR "exposure time" is 1/60 second. It might be more correct to assume an "exposure time" of essentially zero for the IR detector array. If this is correct, then our incorrect assumption would result in a 1/2 pixel error in the absolute locations.

  5. As described above, IR timing uncertainties can result in along-track errors in absolute coordinates of as much as four pixels. Similar potential errors probably also are applicable to VIS images.

    The TIME parameter on "thm2isis" allows the user to apply a correction for this uncertain time delay.

  6. The "thmiric" program (procedure PDF) creates an output file in which the reference band is unchanged except for adding padding of NULL pixel values around the original image data. This is intended to allow the other bands to be registered to the reference band without losing data. The default value is to add ten lines/columns of padding around the reference band. This might not be enough to prevent losing some data from the other bands. The padding parameters can be increased on a particular run of "thmiric". Or, the thmiric.pdf file can be edited to change the default values.

  7. For producing VISMC files, an empirical correction for optical distortions is being applied. The distortion corrections were derived from the IR empirical corrections, which assume that IR Band 5 is "correct." This might result in errors in absolute coordinates for the projected VIS files.

  8. It appears that frequent pointing corrections for the high gain antenna are usually being done during VIS and IR observations. This causes small variations in the THEMIS camera pointing that are too frequent to be tracked by the camera pointing in the SPICE data, which is recorded at intervals of one to six seconds. For projected VIS files, this can result in band-to-band registration errors relative to VIS Band 3 of as much as one or two pixels. The registration errors can be different for each VIS framelet.

    These pointing variations can also cause errors in projected IR images, but it is less of a problem with IR due to the lower spatial resolution and the Time Delay Integration (TDI), which effectively averages 16 observations taken over approximately 1/2 second.

  9. The software currently assumes that the time of observation for the first framelet of a VIS observation is one-half of the VIS exposure duration earlier than the time recorded in the label (which might have been adjusted by the user with the "thm2isis" TIME parameter). It is not yet clear whether or not this is a correct model of how the instrument works. If this timing assumption is incorrect, then there will be corresponding errors in absolute coordinates.

  10. The "levpt" and "levgeoplane" programs do not yet calculate the limb angle. This will be implemented at a later time.

  11. The "levpt" program sometimes does not compute the Sun Azimuth value for pixels in the corners of images. This will be corrected in a future release. Also, note that the Sum Azimuth value is relative to the horizontal axis of the image file and thus the value for a given point on the planet can be different for different files, e.g., unprojected Level 1 vs. map projection Level 2 files. (The sun azimuth relative to North, which depends only upon the time and the point on the planet, could be computed by the user from the North Azimuth and Sun Azimuth values.)

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Last updated: Oct 7 2004