Data Set Information
DATA_SET_NAME HUYGENS ENTRY, DESCENT AND SURFACE DATA
DATA_SET_ID HP-SSA-SSP-3/4-DESCENT-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview : This archive contains science data products from observations acquired bythe Huygens Probe Surface Science Package (SSP) instrument during theDescent of the Huygens Probe on 2005/01/15 as well as on the surface. SSP data products include raw data and calibration tables. This volumecontains both. Descent data can be found in the /DATA/DESCENT/RAW directory.All data are organised by sensor (housekeeping data being treated likeall other sensors), with the exception of the impact eventwhich, for reasons of preserving the original data structure, is in aseparate directory.SSP consists of nine sensors which are described in INST.CAT. In addition,housekeeping data and a special data packet on impact were created.The corresponding subdirectories are therefore ACCE ACCI APIS APIV DEN HK IMPACT PER REF THP TIL The /CALIB directory contains the necessary calibration information toconvert the raw data from Voltages or A/D counts respectively toscientifically meaningful quantities. Given the calibration informationalone, a conversion is possible. Should a clarification be required,please consult EAICD.DOC or SSP_SUM.DOC, both in the DOCUMENTS directory. DEN and PER (and, to some extent, also REF) have worked throughout the descent,but these sensors were not designed with the objective of atmosphericmeasurementsin mind. Therefore the related data may only be of limited use. TIMING SSP times are given with with reference to the DDB 'T0' event, based on theHuygens on-board clock (see INSTHOST.CAT). At the time of archiving thereappears to be an additional 250ms offset w.r.t. UTC pending verification. DATASET ORGANISATION Files on this volume are organized into a series of subdirectories belowthe top-level directory. The following table shows the structure andcontent of these directories. In the table, directory names are enclosed insquare brackets ([]). FILE CONTENTS Top-level directory | |- AAREADME.TXT The file you are reading. |- ERRATA.TXT Comments and errata on this volume. |- VOLDESC.CAT Description of the contents of this CD-ROM | volume in object format. |- [CALIB] Calibration directory. | |- SSP_CAL.LBL PDS label for SPSP_CAL.ASC. | |- SSP_CAL.ASC SSP calibration document. | |- [CATALOG] PDS catalog objects. | |- CATINFO.TXT Description of files in the CATALOG directory. | |- INST.CAT Description of the MOLA instrument. | |- INSTHOST.CAT Description of the MGS spacecraft. | |- MISSION.CAT Description of the MGS mission. | |- DATASET.CAT Description of SSP data set. | |- PERSON.CAT Description of personnel associated with | | production of this volume. | |- REF.CAT Reference objects mentioned in *.CAT files. | |- [DATA] | |- [DESCENT] SSP data directory, where nn represents a | |- [RAW] | |- [ACCE] Subdirectories of the sub-sensors of the | |- [ACCI] SSP. These contain the mission data. | |- [APIS] The impact event covers several of the sensors. | |- [APIV] Each directory contains one or several | |- [DEN] tables of sensor data and a detached label. | |- [HK] | |- [IMPACT] | |- [PER] | |- [REF] | |- [THP] | |- [TIL] | |- [DOCUMENT] Documentation files. | |- DOCINFO.TXT Description of files in DOCUMENT directory. | |- SSP_1177.ASC SSP article from ESA SP 1177, ASCII. | |- SSP_1177.LBL label for SSP_1177.ASC and SSP_1177.PDF | |- SSP_1177.PDF SSP article from ESA SP 1177, Adobe PDF format. | |- SSP_EAICD.DOC SSP EAICD. | |- SSP_EAICD.LBL PDS label for SSP_EAICD. | |- SSP_EAICD.PDF SSP EAICD in ASCII Format. | |- SSP_SUM.DOC SSP software user manual. | |- SSP_SUM.LBL PDS label for SSP_SUM.* | |- SSP_SUM.ASC SSP software user manual. | |- PHD_GHAFOOR.LBL PDS label for PHD_GHAFOOR.PDF. | |- PHD_GHAFOOR.PDF PHD_GHAFOOR.PDF. | |- PHD_LORENZ.LBL PDS label for PHD_LORENZ.PDF | |- PHD_LORENZ.PDF PHD_LORENZ.PDF | | | |- [INDEX] Index files. | |- INDXINFO.TXT Description of files in the INDEX directory. | |- INDEX.LBL PDS label for INDEX.TAB. | |- INDEX.TAB Index table of data products on this volume. OPEN ISSUES SSP is a 'Surface Science Package' and was designed to do what it says on thetin. Sensors were calibrated with a surface characterisation in mind. The dataprovided cover the entire descent. Some of the sensors not specificallydesigned for measurements throughout the descent can still revealinteresting information, but this is not always calibrated.A good example is DEN, an archimedes type float for a liquid landing,which does not show atmospheric density (of course) but behaves like anaccelerometer. Naturally, it was never calibrated to be used as such.We do not entirely understand why REF apparently overexposed throughout thedescent. Review: The Huygens data sets were reviewed in two steps: (1) a review of the Experimenter-to-archive-ICD, in 2004. (2) a review of the data sets, in 2006. Procedures and details of this process can be obtained from the ESA archive team: Olivier Witasse or Joe Zender ESA ESTEC Keplerlaan 1 2200 AG Noordwijk The Netherlands Olivier.Witasse@esa.int or Joe.Zender@esa.int This data set was reviewed by two external reviewers. Instrument Overview : The SSP experiment consists of nine separate sensors that are designedto measure a wide range of physical properties of Titan's lower atmosphere,surface, and sub-surface. By measuring a number of physical properties ofthe surface it is expected that the SSP will be able to constrain the inferredcomposition and structure of the Titan's near-surface environment.Although the SSP is primarily designed to sense properties of the surface,some of its sensors will also make measurements of the atmosphere alongthe probe's entry path and will complement the data gathered by otherexperiments on the Huygens probe. Science Objectives : Calibration Description :A detailed description of the conversion of raw data into meaningfulquantities is given in the software user manual SSP_SUM.Calibration tables can befound in the CALIB directory on this volume. Instrument Manufacturers :PSSRI, The Open University, Walton Hall, Milton Keynes (previouslyUniversity of Kent at Canterbury) is the lead Institute. Othercontributors include the Space Sciences Research Centre Warsaw,the Rutherford Appleton Laboratory. See SSP_1177.PDF for details. Sensors Overview : Accelerometer External (ACC-E) The accelerometer subsystem is designed to characterise the immediatesurface of the landing site by recording the dynamic response of twodevices mounted in different positions on the probe. One of thesensors is designed to sense the force exertedon a pylon that protrudes from the foredome aperture. The force issensed by a piezoelectric ceramic element that is mounted between ahemispherical titanium alloy head and the pylon shaft. If Huygens landson a relatively uniform surface the ACC-E penetrometer will be smoothlydriven into the surface material until the probe's fore-dome strikes thesurface, bringing it to a halt. During the impact process the ACC-E issampled at a rate of 10 kHz, giving it an effective depth resolution of1 mm for a nominal mission impact speed of 5 m s-1. Accelerometer Internal (ACC-I) A single commercially available accelerometer forms the second part ofthe ACC sensor. This device is mounted on a foot of the SSP electronicsbox, which is fixed to the upper experiment platform. The ACC-I providesinformation about the vertical non-static accelerations experienced bythe entire probe. Acoustic Properties Instrument - Sonar (API-S) Like the ACC subsystem, the API has two separate parts. The first ofthese is an active sonar system (API-Sonar) mounted on the front of theTop Hat cavity pointing downwards. This sensor will measure theeffective acoustic cross-section of the medium within its field of viewat a frequency of around 15kHz. Each echo is sampled at a rate of 1kHz,and during the final section of the probe's descent this sensor may beable to provide information about the topography of the landing sitewith a vertical precision of around 0.1 m. In the case of a liquidtouchdown the API-S may also be able to provide lower bounds to thedepth of the liquid in which it has landed.In the final few hundred metres of Huygens' trajectory the API-S will besufficiently close to the surface for it to detect the back-scatteredecho from the surface beneath it. Following the impact of the probe witha liquid body the API-S will act as a depth sounder, using informationgathered from the Acoustic Properties Instrument-Velocimeter (API-V) onthe speed of sound in the medium. In comparison to its atmosphericoperation the API-S operates with an increased efficiency when immersedsimply as a result of the medium's higher density and its betteracoustic coupling to the API-S. Whilst afloat the API-S should be ableto record the depth of the liquid beneath the probe (up to a maximumdepth of 1000m). Acoustic Properties Instrument - Velocimeter (API-V) The second portion of the API consists of a pair of piezoelectrictransducers mounted at the front surface of the Top Hat on either sideof the cavity. These sensors measure the speed of sound by transmitting,and subsequently receiving, a brief 1 MHz acoustic signal. The timeinterval between transmission and reception is measured with a precisionof 250 ns and the separation of 0.125 m gives a speed resolution of 8 cms-1 when operating in gas at Titan's surface. Throughout the descentthese sensors will be driven and subsequently sampled once a second,giving a detailed profile of the speed of sound along the probe'strajectory. At least three other sensors in the probe's payload cansense the atmospheric temperature, and thus the speed of sound willyield the ratio of gamma (the ratio of specific heats) to m (mean molecularmass). The next important contribution made by the API-V is at Titan'ssurface in the event of the probe landing in a liquid body. The speedof sound is measured to a precision of 8 m s-1, a fidelity thatcorresponds to a mixing ratio of 1.6 % for a methane / ethane ocean. Density Sensor (DEN) Upon landing in a liquid the density of any fluid that makes its wayinto the cavity of the SSP will be estimated by the DEN sensor. Thisinstrument measures the upthrust applied by a liquid to a small buoyantfloat which is attached to the SSP by a pair of epoxy beams that areequipped with strain gauges.In addition considerable scope remains for the detection of phenomenathat are secondary to the main role of the SSP. For example,immediately following the probe's impact with a liquid the DEN maydetect the periodic inflow and outflow of fluid from the SSP cavity.Measurements of the rate at which this bobbing motion decays will placeconstraints on the viscosity of the impacted liquid, a property that isnot directly measured by any sensor. Permittivity Sensor (PER) In the event of a liquid landing the SSP will also be able to determinea number of electrical properties of the fluid. The PER device consistsof 22 stacked parallel plates, the capacitance of which is measured at anumber of different frequencies. By briefly pulsing the sensor with DCvoltages the conductivity of the surrounding liquid may also beascertained, placing constraints on the population of dissolved ions (ifany) in the medium. The PER also carries a thermometer in the form of asilicon diode, which has a precision of better than 0.5 K. Although any probable Titan atmosphere has a relative permittivity thatis almost identical to 1, and therefore cannot be detected by PER, atthe tropopause (altitude 40 km) significant quantities ofmethane/nitrogen may condense temporarily on the PER sensor. Ifsufficient material collects on the PER some or all of the sensingplates may be bridged and the condensate may thus be detected. Refractive Index Sensor (REF) The REF sensor measures the refractive index of a liquid by using alinear critical angle refractometer, the method and design of which isdiscussed by Geake et al (1994).This device consists of a section of a cylindricalprism that can be illuminated by collimated sources (light guides fed bylight emitting diodes, LEDs, at 635 nm) that are both internal andexternal to the prism. When the REF is immersed in a medium of givenrefractive index light striking the interface between the prism and theliquid will experience a critical angle effect, in which case the lightis refracted or reflected. For both the internal and externalillumination only part of the beam is reflected or refracted onto thedetector, the remainder escaping or being reflected from the prism. A512 element linear photodiode array is attached to one face of the prismand this array is used to measure the resulting transition from light todark, the position of this transition, or cut-off, being linearlyrelated to the refractive index of the liquid. The sensor covers therefractive index range 1.250 to 1.450 with a discrimination of 0.001.The external light source is provided so that an estimate can be made ofthe opacity of the ambient liquid, from a comparison of the illuminationprofile received from the internal and external sources. Thermal Properties Sensor (THP) The main role of the THP is to measure the thermal conductivity anddiffusivity of the ambient medium in the SSP cavity. Along with theAcoustic Properties Instrument (API), the THP is designed to senseproperties of both liquid and gaseous media, using two separate sets ofredundant hot wire sensors enclosed in cylindrical shields. By applyinga known current for a fixed duration to the THP's sense wires in each ofthe four cylindrical canisters the wires are made to act as regulatedheat sources. This method is covered in detail by Healy et al. (1976). In the close confines of the wires' shields the transient heat pulsethus generated is lost by conduction to the medium surrounding the wiresat a rate that is determined by the thermal properties of the material. Measurements of the wires' resistance as a function of time before andafter the heating pulse reveal the initial temperature of the medium andits thermal properties. Two diameters of platinum wire are used in theTHP, the thinner wires (10 m diameter) are sized for the relatively lowthermal conductivity of the atmosphere, and the thicker 25 m diameterwires are only driven when the Huygens probe has reached the surface.A THP measurement is made every minute throughout the atmospheric phaseof the descent and will therefore provide a relatively fine record ofthe thermal properties of the atmosphere along Huygens' trajectory. Tiltmeter (TIL) One of the important analyses to be carried out after arrival at Titanis the reconstruction of the probe's motion, i.e. its trajectory,attitude, swing and spin, as it falls through the atmosphere and thensubsequently during any post-impact dynamics. Throughout Huygens'descent particular aspects of the probe's motion will be measured withvarying precision by three separate experiments, Doppler Wind Experiment,Huygens Atmospheric Structure Instrument (HASI), and SSP. Ofthese, TIL is the only device that provides unambiguous informationabout the Huygens probe's attitude with respect to the local verticalrather than its acceleration. Two inclinometers are arranged to form anorthogonal x-y pair inside the sensor housing which is attached to theSSP electronics box. During the probe's descent the TIL is sampled at arate of 1 Hz. Definition of SSP Operating Modes : The SSP operating modes are defined as follows:M0: Checkout / Diagnostic mode - not used during the real descentM1: upper atmosphere modeM2: mid atmosphere modeM3: lower atmosphere modeM4: proximity modeM5: surface modeM6: extended surface modeM7: Checkout / Diagnostic mode - not used during the real descentA functional breakdown of SSP's sensors is given in section 2.4of the software user manual SSP_SUM.
DATA_SET_RELEASE_DATE 2006-04-18T00:00:00.000Z
START_TIME 2005-01-14T09:10:21.000Z
STOP_TIME 2005-01-14T12:47:46.994Z
MISSION_NAME CASSINI-HUYGENS
MISSION_START_DATE 1997-10-15T12:00:00.000Z
MISSION_STOP_DATE N/A (ongoing)
TARGET_NAME TITAN
TARGET_TYPE SATELLITE
INSTRUMENT_HOST_ID HP
INSTRUMENT_NAME SURFACE SCIENCE PACKAGE
INSTRUMENT_ID SSP
INSTRUMENT_TYPE MATERIAL PROPERTY SENSOR
NODE_NAME Planetary Atmospheres
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE N/A
CITATION_DESCRIPTION Unknown
ABSTRACT_TEXT Unknown
PRODUCER_FULL_NAME AXEL HAGERMANN
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