PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2014-02-06, CSee, 2014-02-06: Added missing Imager Responsivity tables to archive. 2014-01-21: Revised to correspond with V1.1; Added change record to the end of this document." DD_VERSION_ID = PDSCAT1R90 RECORD_TYPE = STREAM OBJECT = DATA_SET DATA_SET_ID = "HP-SSA-DISR-2-3-DESCENT-V1.1" OBJECT = DATA_SET_HOST INSTRUMENT_HOST_ID = HP INSTRUMENT_ID = DISR END_OBJECT = DATA_SET_HOST OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = "HUYGENS PROBE DISR RESULTS V1.1" DATA_SET_COLLECTION_MEMBER_FLG = "N" DATA_OBJECT_TYPE = "TABLE" START_TIME = 2005-01-14T09:11:41 STOP_TIME = 2005-01-14T12:49:07 DATA_SET_RELEASE_DATE = 2006-07-15 PRODUCER_FULL_NAME = "CHARLES (CHUCK) SEE" ARCHIVE_STATUS = "ARCHIVED" CONFIDENCE_LEVEL_NOTE = "N/A" ABSTRACT_DESC = "N/A" CITATION_DESC = "N/A" DETAILED_CATALOG_FLAG = "N/A" DATA_SET_TERSE_DESC = "N/A" DATA_SET_DESC = " Data Set Overview ================= On January 14, 2005, the Huygens Probe, part of the joint NASA/ESA Cassini-Huygens mission to Saturn, entered the atmosphere of Titan, descended for 2.5 hours under a parachute and eventually landed softly on the surface of Titan [LEBRETONETAL2005]. Six experiments collected data during the descent and on ground. The data set, which this data set catalog belongs to, is the archive of the Huygens Descent Imager and Spectral Radiometer (DISR). The DISR consists of 14 instruments; Three imagers, four solar aureole cameras, two imagers covering the visible spectrum, two imagers in the near infrared, two violet photometers, and a sun sensor. The data were taken from an altitude of approximately 145km down to Titan's surface. The data taking was optimized by altitude and spin rate to meet the science objectives. The DISR data are presented in the PDS archive as described below. Data users are encouraged to also look at the DISR Data Users' Guide in: \DOCUMENT\DISR_SUPPORTING_DOCUMENTS\DISR_DATA_USERS_GUIDE DATA Directory ============== The DATA directory contains all the data collected by DISR during the Titan descent arranged by detector system. Although the data are manipulated using a variant of IDL (Interactive Data Language, a product of Research Systems, Inc. of Boulder, Colorado.), the data are here presented in tabular or ASCII form for easy accessibility. The CCD_DARK_DATA datasets are the readout from covered columns of the CCD detector. Their values indicate the column dependent dark current being generated by the chip during the other measurements. The DERIVED_DATA_PRODUCTS directory contains reduced spectrometer data products as described below. The DESCENT_CYCLES datasets record key parameters at the beginning of each cycle of optimized data taking including the altitude and cycle type. There were about 110 Descent Cycles during the descent, and another 50 or so on Titan's surface. The HIGHER_LEVEL_DATA directory contains mosaic views of Titan generated from the DISR images. The HOUSEKEEPING (Housekeeping) datasets record DISR temperatures, voltages, and software indices. The IMAGERS directory contains tables of the detector readout values for each image pixel, after the image has been decompressed (lossy hardware compression). The IR_SPECTROMETER datasets contain the per pixel (wavelength) readout from each IR Spectrometer measurement. These readings have been summed into regions, relative to the azimuth of the sun, to allow for accurate determination of the light intensity in the directions of interest. LAMP datasets record when the calibration lamps and surface science lamp are powered, their applied voltage and current draw. The SLI_STRIP datasets are summed columns on each side of the side looking imager, used to determine the position of the sun as well as the tip of the probe. The SOLAR_AUREOLE directory contains the measurements of the light intensity field around the sun (the solar aureole). The data is presented as tables of pixel values. The SUN_SENSOR records the time when the sun passes in front of the DISR instrument. It has a double V aperture slit, which allows determination of the tip in the direction of the sun, by virtue of the crossing times. The sun sensor data is used to 'time' the taking of all other data sets relative to the sun (clocking to the sun azimuth). It's amplitude is also an independent measurement of the solar absorption at it's pass band (938nm). The data is presented as a table of the times (relative to DDB T0) that the sun passes in front of each of the 3 slits, as well as the detector reading in DN. The TIME datasets record the DISR internal clock time, and DDB time, at each Broadcast Pulse. The VIOLET_PHOTOMETER datasets contain a reading (amplitude) of the violet photometer. The VISIBLE_EXTRA_COLUMNS datasets record the values of the column of pixels on each side of the corresponding visible spectrometer. This data is used to compensate for light bleeding through (scattered light) from the adjacent CCD instruments. The VISIBLE_SPECTROMETER directory contains the data from the Upward Looking and Downward Looking Visible spectrometers as a table of values. The rows of the tables are the wavelength dimension, and the columns are spacial. In some cases the columns (spacial dimension) are summed to reduce noise. DERIVED DATA PRODUCTS ===================== The following Derived Data Products have been included in the archive: DLIS/ULIS: A tabular presentation of the calculated light intensity at each wavelength of the IR spectrometers averaged over the field of view. DLV/ULV: Two sets of tables, one presenting the Net counts measured during the descent after the detector offset is removed. The other presents the average violet light intensity over the photometer's pass band assuming a quadratic spectral shape (see Violet calibration documents for details). DLVS/ULVS: Tables of the light intensity at each visible spectrometer wavelength, averaged over the field of view. DOCUMENT Directory ================== This directory contains the documents which describe how the DISR was calibrated, and how to convert the data into physical measurements. It also contains information about the equipment used during calibration and the method for compensating for the detectors' dark current offsets. The supporting documentation contains information about the instrument design and science objectives. The DISR instrument calibration reports contain complete descriptions of each instrument detector system, the calibration data, methods, and algorithms for converting the instrument data numbers into physical units and intensities into data numbers. Reduced mean intensities over the field of view (FOV) are provided for the spectrometers. However for the broad band instruments (imagers, SA camera) the mean intensity over the FOV is not a useful number since the spectral variation is important, and the bandpass changes significantly during the descent. It is felt that the best scientific approach is to create models which reproduce data numbers rather than mean intensities. Although some lines of code exist as examples in the calibration reports and EXTRAS directory, no generic calibration software is available. Interpretation of the DISR data is model dependent and selection of the model parameters (i.e. atmospheric composition, intensity spectrum, surface reflectance, variation over the field of view) is key in deciphering the data. The scientist is encouraged to develop their own software to explore the physical interpretations of the DISR data. CATALOG Directory ================= This directory contains general information about the data set, such as involved personnel, instrument description, references, etc. INDEX Directory =============== as needed internally by archive Coordinate System ================= The DISR measurements were designed to be taken in the Titan coordinate system, relative to the Sun. Azimuths are relative to the Sun, with Counter Clockwise rotation (to the left) taken to be positive. Data Coverage and Quality ========================= A good, although not entirely complete dataset was collected during the Titan descent. Most notably, only half of the DISR images taken were transmitted back to the Earth. However, even with this limitation, it has been possible to create a continuous view of the Titan descent, with no 'holes' in the construction. These assembled datasets are available in the EXTRA's directory of the archive. However, a primary result is the extensive loss of ability to perform stenographic analysis of the topography of Titan's surface. There we no incomplete or corrupted datasets. These would be removed by the error checking in the data link. Some datasets were lost after an extended time on Titan's surface as the link margin degraded, but in general the link, and probe telecommunications worked amazingly well. The signal to noise ratio in all of the data was better than targeted 100/1. A good spectral dataset was collected from the near IR to the Violet, with matching spectral overlap and good spacial coverage. From this data, coupled with the Solar Aureole measurements, it has been possible to measure the atmospheres's optical depth, model Titan's aerosols, determine methane absorption coefficients, and determine the heating rates. Additionally,with the image measurement we have also been able to calculate the winds and determine the reflectance spectra of the various materials that make up Titan's surface. Limitations =========== Besides the problems mentioned above, there were other unexpected limitations. The probe swing rates underneath the parachute were about 3 time faster than expected, especially high in the atmosphere. The results is that the DISR sun sensor was not able to maintain sun lock throughout the descent, and consequently not all data was taken at the proper azimuths relative to the sun. The sun sensor experience another problem in that it's detector became too cold during the descent, such that it's sensitivity was so low that it failed to operate below 30 km altitude above Titan's surface. A compensating windfall was the realization that variations in the AGC signal cause by the probes rotation could be used to deduce the instantaneous azimuth of the probe. This made reconstruction of the image and spectral field possible. The reverse in spin direction of the probe also caused unforeseen difficulties with the placement of measurements, particularly the IR spectra and the Solar Aureole (SA) Camera Measurements. We obtained no SA data with the sun behind the shadow-baffle, and actually very little SA data near the sun at low altitudes. Another difficulty was that an anomaly of the radar altimeter caused the loss of our coldest (lowest) calibration cycle, so the instrument performance had to be extrapolated over a fairly wide temperature range. Fortunately there was significant data taken over temperature in the laboratory to make this possible. _____________________________________________________________________________ Corrections Made to Earlier Datasets... ============================================= Revised Dataset: HP-SSA-DISR-2/3-EDR/RDR-V1.1 2014-02-06: Added missing Imager Responsivity tables under: \DOCUMENT\DISR_CALIBRATION_DOCUMENTS\IMAGERS\ABSOLUTE_RESPONSIVITY ============================================= Revised Dataset: HP-SSA-DISR-2/3-EDR/RDR-V1.0 Below is a list of changes made to archive version 1.0 to bring it up to version 1.1 (working toward version 2.0). Data Changes... No errors were found in the archived data; however a more photometrically accurate method of processing the Image data has been found, and the improved images have been added in the EXTRAS\IMAGE_ELEMENTS directory. Many improvements have been made to the supporting information contained in the label files. * A detailed list of the data changes... 1) Image elements with improved photometry have been added to the volume under: EXTRAS\IMAGE_ELEMENTS. Besides the raw imaged, improved flat fields, the dark current parameters, and the square-root compression tables have been added along with information on how to process the images. 2) Altitudes reported in the label files have been updated to reflect the newer Descent Trajectory Working Group (DTWG) release from June of 2011. 3) Label file values for Azimuth, Spin Rate, & E/W tilt are updated to be in line with the March 2013 Karkoschka analysis. 4) The probe Azimuth, Altitude & Tilt are reported at both the beginning and end of the observation (previously reported only at the beginning). 5) Added 'SPIN_RATE' keyword to most label files to allow the user to determine the direction of the azimuth progression. 6) The DISR temperature array has been expanded to include all available instrument measured temperatures (in Label Files), not just selected values as before, as some analyses have shown additional temperature dependencies. 7) Temperatures are reported at the mid-point (in time) of the measurement, rather than at the beginning. This allows more accuracy for long exposure measurements. 8) A better explanation of the azimuth definition is included in the DESCRIPTION elements. 9) The DESCRIPTION fields have been expanded to provide a more complete explanation of the dataset. 10) Where appropriate the 'EXPOSURE_TYPE' keyword was added to describe if the observation was automatically or manually exposed to help distinguish the calibration measurements. 11) The 'NULL_PIXEL' keywords were added to CCD measurements to allow determination of the dark-current offset. 12) A more complete data re-play that includes partial datasets was used. 13) Filenames and Product ID's have been revised to include more information. 14) Added reconstructed altitude, azimuth and spin information to DESCENT datasets to juxtapose real-time software values to the actuals. 15) Added XDR formatted images to the DATA/IMAGERS directory. 16) Replaced NASAView formatted (IMG) images with more generic TIFF images. 17) Added IMAGE_ID keyword to image labels to help user determine the format (MRI, SLI, HRI) of the dataset. 18) Added note to image labels pointing out the error in the on-board flat fields for the Medium Resolution Imagers (MRI). 19) Added 'MEASUREMENT_TYPE' keyword to IR spectrometer labels to help user determine the type of IR measurement (ULIS, DLIS, IR_COMB, IR,LONG, etc). 20) Added 'ROTATIONS' keyword to the IR spectrometer labels to help the user determine the spectrometer pointing history. 21) Added table column headers to make the data more understandable. 22) Added 'COLUMNS' keyword to the Visible Spectrometer label files to allow the user to determine the amount of column summing used. 23) Corrected many small errors including the syntax and offset for the pointers in the label files. * New Keywords Added... 1) SPACECRAFT_ALTITUDE_START = The reconstructed altitude of the probe at the start of the measurement. 2) SPACECRAFT_ALTITUDE_END = The reconstructed altitude of the probe at the end of the measurement. 3) PREDICTED_ALTITUDE = The real-time altitude (km) as predicted by the Huygens probe and relayed to the instrument via the Descent Data Broadcast (DDB). 4) AZIMUTH_START = The reconstructed pointing direction of the DISR instrument at the start of the observation. The angle is defined as being in a plane perpendicular to the Nadir vector (i.e. horizontal), measured in degrees Counterclockwise (CCW) from the vector to the Sun, as viewed from above. 5) AZIMUTH_END = The reconstructed pointing direction of the DISR instrument at the end of the observation. The angle is defined as being in a plane perpendicular to the Nadir vector (i.e. horizontal), measured in degrees Counterclockwise (CCW) from the vector to the Sun, as viewed from above. 6) AZIMUTH_NORTH_START = The same as AZIMUTH_START, except measured in degrees Clockwise (CW) from true North (Titan's spin vector) viewed from above, as one would for standard compass directions. 7) AZIMUTH_NORTH_START = The same as AZIMUTH_END, except measured in degrees Clockwise (CW) from true North (Titan's spin vector), viewed from above, as one would for standard compass directions. 8) SPIN_RATE = The approximate average spin rate (in RPM) of the Huygens probe during the measurement as determined by a polynomial fit to the local spin-rate observations. The sense is CCW positive in keeping with the original intended spin direction of the probe. 9) ROTATIONS = The number (or fraction) of spin revolutions the probe makes during the observation, CCW from above defined as positive. 10) SPIN_RATE_START = The instantaneous, re-constructed spin rate (in RPM) at the start of the observation, CCW from above defined as positive. 11) SPIN_RATE_END = The instantaneous, re-constructed spin rate (in RPM) at the end of the observation, CCW from above defined as positive. 12) HUYGENS:EW_TILT_ANGLE_START = The tilt of the Huygens probe spin axis at the start of the observation. The tilt is measured relative to the Zenith vector in the East/West direction. Positive tilt is defined as the spin vector being East of Zenith (i.e. the parachute being east of the probe). 13) HUYGENS:EW_TILT_ANGLE_END = The tilt of the Huygens probe spin axis at the end of the observation. The tilt is measured relative to the Zenith vector in the East/West direction. Positive tilt is defined as the spin vector being East of Zenith (i.e. the parachute being east of the probe). 14) DESCENT_CYCLE_NAME = The name of the descent cycle (Image, Non-Image, etc) that the observation was in. This can often effect data collection externalities such as azimuth timing, exposure time, column summing, etc. 15) NULL_PIXEL_2 & NULL_PIXEL_3 = Readout of covered pixels on the CCD chip which are needed to determine the dark current offset for the observation. 16) MEASUREMENT_TYPE = Distinguishes between sub-types within the DISR sub- instruments, such as Upward Looking vs. Downward Looking for the Visible Spectrometer. 17) IMAGE_ID = Distinguishes type of image dataset; Medium Resolution, High Resolution, or Side Looking. 18) EXPOSURE_TYPE = Distinguishes between Auto-exposed observations and pre-planned, fixed exposure observations. Can be used to identify calibration exposures. * Documentation Revisions... 1) The EAICD has been substantially revised. 2) Added the Visible Spectrometer Calibration Document (\DOCUMENT\ DISR_CALIBRATION_DOCUMENTS\VISIBLE_SPECTROMETERS\VISIBLE_SPECTROMETER_CALDOC), which contains all the details about how the Visible Spectrometers were calibrated. 3) Added the Infra-Red Spectrometer Calibration Document (\DOCUMENT\ DISR_CALIBRATION_DOCUMENTS\INFRARED_SPECTROMETERS\IR_SPECTROMETER_CAL_DOC), which contains all the details about how the Infra-Red Spectrometers were calibrated. 4) Added clarification to section 2.1 of VISIBLE_SPECTROMETER_CAL_NOTES, (\DOCUMENT\DISR_CALIBRATION_DOCUMENTS\VISIBLE_SPECTROMETERS\ VISIBLE_SPECTROMETER_CAL_NOTES). 5) Fixed an error in the IR spectrometer calibration notes (IR_SPECTROMETER_CAL_NOTES) section 2, equation f, and in section 3, the DLIS FWHM equation. 6) Corrected figure 8 of the SUN_SENSOR_CALIBRATION_DOC. 7) Incorporated DISR Archive Users' Guide in DOCUMENTS section of archive. " END_OBJECT = DATA_SET_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "TOMASKOETAL2005" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "LEBRETONETAL2005" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_TARGET TARGET_NAME = "TITAN" TARGET_TYPE = "SATELLITE" END_OBJECT = DATA_SET_TARGET END_OBJECT = DATA_SET END