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 for2.5 hours under a parachute and eventually landed softly on the surface ofTitan [LEBRETONETAL2005]. Six experiments collected data during the descent and on ground. The dataset, which this data set catalog belongs to, is the archive of the HuygensDoppler Wind Experiment (DWE), which was designed to measure Titan's zonalwinds by Doppler tracking Huygens from Cassini and from Earth[BIRDETAL2005]. In contrast to the other five experiments, DWE was not designed to collectdata aboard Huygens. Instead, it was designed to measure the frequency ofthe Huygens carrier signal received aboard Cassini and on Earth. The radio link between Huygens and Cassini had two independent andpartially redundant channels. One of them (channel A) was used for DWEDoppler tracking of Huygens. The accuracy desired by DWE required the use of special Ultra-StableOscillators (USO) for the channel A transmitter aboard Huygens and for thechannel A receiver aboard Cassini. For the reception on Earth, a USO wasnot required, as the accuracy of the H-maser driven radio antennas is muchhigher than that of a USO. The data rate of the Huygens signal was 8 kHz. It was recorded, decoded andstored aboard Cassini in real-time for later transmission to Earth via theCassini High Gain Antenna. The Huygens signal level was, however, to weakto be decoded on Earth. The minimum integration time for the detection ofthe Huygens signal on Earth was of the order of 1 sec. Due to an error in the sequence, which was executed by Cassini during theHuygens mission, the receiver USO was not switched on for the mission.Consequently, the channel A receiver aboard Cassini could not lock onto thechannel A signal from Huygens and all data transmitted via this channelincluding the DWE frequency measurement were lost. Due to this flaw, DWErelies now entirely on the data collected by Earth based telescopes. The DWE data set consists of the sky frequencies measured at the Green Bankand Parkes telescopes (data from other stations may be included when theybecome available; see [BIRDETAL2005]), the retrieved zonal wind speed alongthe descent path, geometrical parameters used to separate the Huygensvelocity components and documentation. It is organized in six directories, which we describe in the following.  DATA Directory: The DATA directory contains the measured sky frequencies after signalprocessing in two files, one for Green Bank and one for Parkes. It is notintended to archive raw recordings of the signal. The signal processingmethod is described in [FOLKNERETAL2006]. Furthermore, it contains theretrieved zonal wind speed, the primary DWE result. The zonal wind speed was computed from each frequency sample, so that thetotal number of rows in the zonal wind table matches the sum of thefrequency samples from Green Bank and Parkes. The time tags for the frequencies is Earth Received Time (ERT), whereas thetime tags for the zonal wind speed is Spacecraft Event Time (SCET). (By theway: Also START_TIME and STOP_TIME of this object, theDATA_SET_INFORMATION, are given in ERT.) The latter is a derived quantity,computed by subtracting the one-way-light-time from ERT. The one-way-light-time between Titan and the receiving antenna during theHuygens mission was 67 min and 6.4 sec at the beginning of the mission,decreasing by approximately 0.1 sec over the duration of DWE datarecordings. For the computation of the zonal wind speed, we used itsprecise value determined by our software, which made use of the SPICEtoolkit (available from JPL). The precise value for the one-way-light-time can be reconstructed bysubtracting our SCET time tags from the ERT time tags e.g., in the filesHUYGENS_STATE.TAB and ANTENNA_STATE.TAB in the GEOMETRY directory (all dataand geometry files have an identical number of rows that corresponds to thenumber of DWE sky frequency samples). The data collected at Green Bank have a mean integration time of 2 sec. AtParkes, the mean integration time was 3 sec before landing and 5 secthereafter. There is a 25-min gap between the last sample from Green Bank and the firstsample from Parkes, which may possibly be filled by measurements of otherVLBA stations in the future, albeit with a lower time resolution as thegain of those stations is significantly smaller than that of Green Bank orParkes. After launch, it was decided that in addition to the DWE Doppler trackingHuygens should also be tracked using a technique known as Very LongBaseline Interferometry (VLBI) [POGREBENKOETAL2004]. The VLBI group used anumber of radio antennas, among them also Green Bank and Parkes, which werejointly used by the VLBI and DWE teams. The VLBI techique requires acontinuous calibration with an extragalactic radio source, i.e., a quasar.As no quasar was in the beam when the antennas were pointed towardsHuygens, they had to be nodded continuously. It was agreed among the teamsthat, in order to meet the requirements for the VLBI tracking, the antennaswere pointed about 100 sec towards Huygens and 80 sec towards the VLBIcalibration source in a 3-min cycle. These interruptions can consequentlyalso be seen in the DWE data set.  GEOMETRY Directory: This directory contains state vectors of Huygens and the receiving antennasand the resulting projection angles for the various velocity components. The time tags of the state vectors of Huygens and the projection angles aregiven as SCET, whereas the time tags for the state vectors of the receivingantennas is given as ERT to account for retardation. See the fileGEOMINFO.TXT in the GEOMETRY directory for more information. The altitude and meridional drift profiles of Huygens used for thegeneration of these files were provided by the Huygens Descent TrajectoryWorking Group (DTWG; see corresponding archive). The initial longitude(longitude at the time of the first DWE sky frequency sample) was takenfrom DTWG as well, whereas the subsequent longitude values were obtained byintegration of the measured zonal wind speed. It is clear that any change of the DTWG profile will affect the retrievedzonal wind. For this version of the archive, we used the 4th DTWG delivery(DTWG#4), published in May 2006. Thus far, all DTWG deliveries includingthose which had not yet incorporated the image derived meridional drift,lead similar zonal wind profiles. There are no significant differencesbetween the various zonal wind retrievals in the upper atmosphere. Near thesurface, where the winds are generally weak, a slightly different descentspeed or meridional drift profile could, however, lead to a significantchange in the retrieved zonal wind speed. The state vectors of the receiving antennas and Titan were computed usingthe software SPICE and the following kernels: vlba.bsp,earth_000101_050808_050517.bpc, earthstns_itrf93_040916.bsp,050214R_SCPSE_04336_05015.bsp, naif0007.tls, cpck19May2005.tpc SPICE andthe accompanying kernels are available at http://naif.jpl.nasa.gov/naif/  DOCUMENT Directory: This directory contains the most important DWE documents and the DWE HealthReport, which describes the performance of the DWE USOs during the Huygensmission. Although the paper [FOLKNERETAL2006] is considered an important documenttoo, it was not included, because at the time of this writing, it was notyet published. More documentation is listed in the reference catalog in the CATALOGdirectory.  CALIB Directory: The CALIB directory contains a report that provides information on the DWEperformance during pre-launch and in-flight tests and describes the finalcalibration of the frequency measurement.  CATALOG Directory: This directory contains general information about the data set, such asinvolved personnel, instrument description, references, etc.  INDEX Directory: needed internally by archive.  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  The Experimenter-to-archive-ICD was reviewed by V. Dehant (Royal Observatory of Brussels) and by D. Strobel (John Hopkins University). This data set was reviewed by D. Strobel and by R. Lorenz (University of Arizona).

N/A