PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 2005-04-22 INTERCHANGE_FORMAT = ASCII NOTE = "Errata for Cassini ISS EDR and Calibration Files Archive Volume DVDs." END_OBJECT = TEXT END COMMENTS ON THE CASSINI ISS EXPERIMENT DATA RECORDS AND CALIBRATION FILES ARCHIVE COLLECTION This document contains comments and errors concerning the Cassini ISS raw experiment data records and calibration files archive published on this collection of DVDs. This errata file is modified as problems/issues are encountered during the active mission lifetime, and result in changes or corrections, or notes worthy to the end user. For the most up-to-date version of this file look on the internet web site of the PDS Imaging Node: http://pds-imaging.jpl.nasa.gov/ ---------------------------------------------------------------------- ------------------ DATA Volume Errata ------------------ 1. PDS Standards Exceptions As part of the ISS index.tab files, the keyword DETECTOR_TEMPERATURE has been represented with measurements in units Celsius (C). Per the PDS Standards Reference Document, this keyword is typically found as units measured in Kelvin (K). See Appendix A, Cassini ISS Archive Volume SIS, JPL D-27548 and Cassini ISS Tour VICAR Image Data File and Detached PDS Label SIS JPL D-24724) for more details on this keyword. 2. Cassini ISS Ground Calibration Files The ground calibration image files acquired pre-launch from the Cassini ISS instrument were produced and retained on a collection of CD-ROMs intended to be PDS-compliant. The PDS Imaging Node converted the dataset from CD-ROMs to DVDs for inclusion in this ISS archive collection. They can be found on the first ten calibration volumes. 3. SPICE-generated Keywords Keywords calculated using SPICE may use a c-smithed c-kernel (as indicated in the SPICE_PRODUCT_ID keyword) in replacement of a reconstructed ACS c-kernel. The c-smithed c-kernel is produced by automated auto-navigation software on a subset of the images. In most cases it is more accurate than the ACS c-kernel. The kernel takes the naming convention "[image name].bc". For example: "N1363625260_2.IMG.bc. This c-kernel is delivered to NAIF some time afterwards in a concatenated file containing a larger time range of c-smithed c-kernels. This file takes the naming convention "[yy][doy]_[yy][doy]c[version]_ISS.bc". For example: "00275_00285ca_ISS.bc". To find the original c-smithed c-kernel, convert the image SCLK time to a day-of-year date to find the concatenated c-smithed kernel it is contained within. 4. TELEMETRY_FORMAT_ID keyword problem The label keyword value TELEMETRY_FORMAT_ID will be set to UNK for all images from Saturn Tour. This is due to a flight software upgrade in sequence C37 (starting at SCLK 1431917802) that routed the channelized engineering data to another partition, which isn't downlinked unless there is an emergency, so the ability to determine the telemetry format was lost. The INSTRUMENT_DATA_RATE keyword can be useful to help figure out how fast data was being sent to the CDS, and is somewhat an indicator of the telemetry mode, although it is not deterministic (i.e., some telemetry modes share the same instrument data rate.) 5. PDS Label pointer discrepancy There is a problem with the PDS label generation portion of the telemproc. The pointers were inadvertently hardcoded in the PDS labels. A fix will be issued in March of 2005 and efforts will be made to fix all affected labels previous to this date. 6. BIAS_STRIP_MEAN label value Before sequence C32 (before SCLK 1401927444), when FLIGHT_SOFTWARE_VERSION = 1.2, this uses one overclocked pixel value per line. This value should be ignored for LOSSY compressed data as it does not contain valid data. During sequence C32 (beginning at SCLK 1401927444), when FLIGHT_SOFTWARE_VERSION = 1.3, this uses six overclocked pixel values per line. This value should be ignored for LOSSY compressed data as it does not contain valid data. When FLIGHT_SOFTWARE_VERSION = 1.4, for Lossy compressed data, this value is the mean of the overclocked pixel sum returned in the last compression block of the image. 7. DARK_STRIP_MEAN label value For FLIGHT_SOFTWARE_VERSION_ID=1.2 or 1.3, this value should be ignored for LOSSY compressed data as it does not contain valid data. When the FLIGHT_SOFTWARE_VERSION_ID = 1.4, for Lossy compressed data this value is the mean of the extended pixel sum returned in the last compression block for the image. 8. Filename case consistency All filenames and directories on the DATA volume are in lower-case with the exception of the image vicar and label files in the data directory. There are possibly some inconsistencies in the documentation where it may reference the file by the wrong case. ---------------------------------------------------------------------- ------------------------- CALIBRATION Volume Errata ------------------------- Calibration activities for the ISS cameras are on-going, and they are incomplete at the present time. Some calibration algorithms have yet to be implemented or are only partially implemented. In addition, many future improvements to the current calibration algorithms have been identified. Some of these outstanding items are listed below. 1. PDS Standards Exceptions Many of the PDS labels for the data files on the calibration volume, as well as for the sample calibrated images, have yet to be finalized. We expect some errors to be present in these PDS labels, and will work to fix them as they are made known. 2. Dark current The current dark current modeling algorithm is divided into two parts: a code that examines a set of dark images and creates a parameter file from them, and another code that takes that parameter file and constructs a dark image appropriate for a specific set of camera parameters. Together, this dark model is referred to in the CISSCAL calibration software as the "interpolation model." The first code has been written by Bob West, but it is still being developed. Once complete, it will be included in future versions of the calibration archive volume. For now, the calibration volume contains only the parameter files produced by this code for NAC and WAC images. These can be found within the darkcurrent subdirectory of the CALIB directory. The second code that actually generates the dark current files is still being developed as well, but the current working version has already been integrated into CISSCAL. There is also an alternate dark subtraction method, which is referred to in CISSCAL as the "two-parameter model." This method is outdated and generally produces unsatisfactory results, but has been included for the time being for completeness while the interpolation model is being finalized. 3. Flat fields Each filter combination has a signature flat field pattern that needs to be divided out of each image during the calibration process. This pattern is captured in the "slope files" which were derived from ground calibration images, and they are included on the calibration volume. Unfortunately, the flatfield is not constant with time: its structure changes whenever a dust grain lands on one of the filters of the CCD window, or an already-present dust grain simply shifts position. Analysis is currently underway to determine the various flatfield changes that have occurred since launch, so that the appropriate corrections can be made to the flatfielding calibration step. So far only two of these corrections have been determined, both of which apply to the NAC only: a correction for a large dust ring discovered during Venus flyby, and another correction derived from Titan flyby data that removes a residual "mottle map." Both of these corrections are supplied with the calibration volume. Similar filter-specific and camera-specific flatfield corrections will be available in the future. The slope file subdirectory of the CALIB directory contains three database files which associate the various filter combinations with their appropriate slope files. The first of these, slope_db_1.tab, is the original slope file database. The third database file, slope_db_3.tab, has been altered to take into account new slope files generated for the NAC that have been corrected for anti-blooming pixel pairs. Technically this is the "correct" database file. Unfortunately, some of the slope files to which it refers have since been found to be unacceptably noisy. Thus, the database file that is used by CISSCAL and currently considered "best" is the second, slope_db_2.tab. 4. Absolute Calibration As detailed in [PORCOETAL2004], absolute calibration of the ISS cameras is currently incomplete. Quantum efficiency correction functions have been derived for both cameras, but filter-specific absolute correction factors have so far only been calculated for the NAC. Currently, absolute flux in the NAC is only known to within an error of about 10-20% depending on filter combination (better for single broad-band filters than for narrow-band filters or filter combinations), and 15-25% in the WAC. Also note that some filters, such as the polarized, methane, and continuous- band filters, require special attention for proper absolute calibration. This analysis is currently underway. 5. Temperature Dependency Many of the phenomena detailed during ground calibration testing were found to exhibit temperature dependency. Examples are the uneven bit-weighting, flatfield, and shutter offset corrections. Unfortunately, the algorithms currently implemented for these calibration steps do not give a realistic treatment of this temperature dependency. Attemps will be made to correct this shortfall in future versions. 6. Coherent Noise Sources Calibration algorithms have yet to be developed for some types of coherent noise found in the image data, such as the vertical banding.