N.B.: The Peer Review of the data set is not fully completed.The review is ongoing, but no major issues are expected. A data setversion 2.0 shall be released in the near future, as soon as the review processis complete. Data Set Overview: On January 14, 2005, the Huygens Probe, part of the jointNASA/ESA Cassini-Huygens mission to Saturn, entered the atmosphereof Titan, descended for 2.5 hours under a parachute andeventually 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 aureolecameras, two imagers covering the visible spectrum, two imagers in thenear infrared, two violet photometers, and a sun sensor. The data were taken from an altitude of approximately 145km down toTitan's surface. The data taking was optimized by altitude and spinrate to meet the science objectives. The DISR data are presented in the PDS archive as described below.  DATA Directory: The DATA directory contains all the data collected by DISR during theTitan descent arranged by detector system. Although the data aremanipulated using a variant of IDL (Interactive Data Language, aproduct of Research Systems, Inc. of Boulder, Colorado.), the data arehere presented in tabular or ASCII form for easy accessibility. The DARK datasets are the readout from covered columns of theCCD detector. Their values indicate the dark current beinggenerated by the chip during the other measurements. The DESCENT datasets record key parameters at the beginning of eachcycle of optimized data taking including the altitude and cycle type.There were about 110 Descent Cycles during the descent, and another50 or so on Titan's surface. The HKEEPING (Housekeeping) datasets record DISR temperatures,voltages, and software indices. The IMAGE directory contains tables of the detector readout values foreach pixel, after the image has been decompressed (lossy hardwarecompression). The IR datasets contain the per pixel (wavelength) readout from eachIR measurement. These readings have been summed into regions, relativeto the azimuth of the sun, to allow for accurate determination of thelight intensity in the directions of interest. LAMP datasets record when the calibration lamps and surface sciencelamp are powered, their applied voltage and current draw. The SOLAR directory contains the measurements of the light intensityfield around the sun (the solar aureole). The data are presented astables of pixel values. The STRIP datasets are summed columns on each side of the side lookingimager, used to determine the position of the sun as well as the tipof the probe. The SUN sensor records the time when the sun passes in front of theDISR instrument. It has a double V aperture slit, which allowsdetermination of the tip in the direction of the sun, by virtue of thecrossing times. The sun sensor data is used to 'time' the taking ofall other data sets relative to the sun (clocking to the sun azimuth).Its amplitude is an independent measurement of the solar absorption atits pass band (938nm). The data are presented as a table of the times(relative to DDB T0) that the sun passes in front of each of the 3slits, 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 datasets contain the reading (amplitude) of the violetphotometer. The VISIBLE directory contains the data from the Upward Looking andDownward Looking Visible spectrometers as a table of values. The rowsof the tables are the wavelength dimension, and the columns arespatial. In some cases the columns (spatial dimension) are summed toreduce noise. The VISIBLE_EXT datasets record the values of the column of pixels oneach side of the corresponding visible spectrometer. This data is usedto compensate for light bleeding through (scattered light) from theadjacent CCD instruments.  DERIVED DATA PRODUCTS: The following Derived Data Products have been included in the archive: DLIS/ULIS: A tabular presentation of the calculated light intensity ateach wavelength of the IR spectrometers averaged over the field ofview. DLV/ULV: Two sets of tables, one presenting the Net counts measuredduring the descent after the detector offset is removed. The otherpresents the average violet light intensity over the photometer's passband assuming a quadratic spectral shape (see Violet calibrationdocuments for details). DLVS/ULVS: Tables of the light intensity at each visible spectrometerwavelength, averaged over the field of view.   DOCUMENT Directory: This directory contains the documents which describe how the DISR wascalibrated, and how to convert the data into physical measurements. Italso contains information about the equipment used during calibrationand the method for compensating for the detectors' dark currentoffsets. The supporting documentation contains information about theinstrument design and science objectives. The DISR instrument calibration reports contain complete descriptionsof each instrument detector system, the calibration data, methods, andalgorithms for converting the instrument data numbers into physicalunits and intensities into data numbers. Reduced mean intensities over the field of view (FOV) are provided forthe spectrometers. However for the broad band instruments (imagers, SAcamera) the mean intensity over the FOV is not a useful number sincethe spectral variation is important, and the bandpass changessignificantly during the descent. It is felt that the best scientificapproach is to create models which reproduce data numbers rather thanmean intensities. Although some lines of code exist as examples in the calibrationreports, no generic calibration software is available. Interpretationof the DISR data is model dependent and selection of the modelparameters (i.e. atmospheric composition, intensity spectrum, surfacereflectance, variation over the field of view) is key in decipheringthe data. The scientist is encouraged to develop their own software toexplore 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: needed internally by archive Coordinate System: The DISR measurements were designed to be taken in the Titancoordinate system, relative to the Sun. Azimuths are relative to theSun, with Counter Clockwise rotation (to the left) taken to bepositive. Data Coverage and Quality: A good, although not entirely complete dataset was collected duringthe Titan descent. Most notably, only half of the DISR images takenwere transmitted back to the Earth. However, even with thislimitation, it has been possible to create a continuous view of theTitan descent, with no 'holes' in the construction. These assembleddatasets are available in the EXTRAS directory of the archive. However, a primary result is the extensive loss of ability to performstenographic analysis of the topography of Titan's surface. There we no incomplete or corrupted datasets. These would be removedby the error checking in the data link. Some datasets were lost afteran extended time on Titan's surface as the link margin degraded, butin general the link, and probe telecommunications worked amazinglywell. The signal to noise ratio in all of the data was better than targeted100/1. A good spectral dataset was collected from the near IR to the Violet,with matching spectral overlap and good spacial coverage. From thisdata, coupled with the Solar Aureole measurements, it has beenpossible to measure the atmosphere's optical depth, model Titan'saerosols, determine methane absorption coefficients, and determine theheating rates. Additionally, with the image measurement we have alsobeen able to calculate the winds and determine the reflectance spectraof the various materials that make up Titan's surface.  Limitations: Besides the problems mentioned above, there were other unexpectedlimitations. The probe swing rates underneath the parachute were about 3 timefaster than expected, especially high in the atmosphere. The result isthat the DISR sun sensor was not able to maintain sun lock throughoutthe descent, and consequently not all data were taken at the properazimuths relative to the sun. The sun sensor experience another problem in that its detector becametoo cold during the descent, such that its sensitivity was so low thatit failed to operate below 30 km altitude above Titan's surface. A compensating windfall was the realization that variations in the AGCsignal cause by the probe's rotation could be used to deduce theinstantaneous azimuth of the probe. This made reconstruction of theimage and spectral field possible. The reverse in spin direction of the probe also caused unforeseendifficulties with the placement of measurements, particularly the IRspectra and the Solar Aureole (SA) Camera Measurements. We obtained noSA data with the sun behind the shadow-baffle, and actually verylittle SA data near the sun at low altitudes. Another difficulty was that an anomaly of the radar altimeter causedthe loss of our coldest (lowest) calibration cycle, so the instrumentperformance had to be extrapolated over a fairly wide temperaturerange. Fortunately there was significant data taken over temperaturein the laboratory to make this possible.

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