NSSDCA ID: PSPA-00534
Availability: Archived at NSSDC, accessible from elsewhere
This description was generated automatically using input from the Planetary Data System.
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 Doppler Wind Experiment (DWE), which was designed to measure Titan's zonal winds by Doppler tracking Huygens from Cassini and from Earth [BIRDETAL2005].
In contrast to the other five experiments, DWE was not designed to collect data aboard Huygens. Instead, it was designed to measure the frequency of the Huygens carrier signal received aboard Cassini and on Earth. The radio link between Huygens and Cassini had two independent and partially redundant channels. One of them (channel A) was used for DWE Doppler tracking of Huygens. The accuracy desired by DWE required the use of special Ultra-Stable Oscillators (USO) for the channel A transmitter aboard Huygens and for the channel A receiver aboard Cassini. For the reception on Earth, a USO was not required, as the accuracy of the H-maser driven radio antennas is much higher than that of a USO. The data rate of the Huygens signal was 8 kHz. It was recorded, decoded and stored aboard Cassini in real-time for later transmission to Earth via the Cassini High Gain Antenna. The Huygens signal level was, however, to weak to be decoded on Earth. The minimum integration time for the detection of the Huygens signal on Earth was of the order of 1 sec.
Due to an error in the sequence, which was executed by Cassini during the Huygens mission, the receiver USO was not switched on for the mission. Consequently, the channel A receiver aboard Cassini could not lock onto the channel A signal from Huygens and all data transmitted via this channel including the DWE frequency measurement were lost. Due to this flaw, DWE relies now entirely on the data collected by Earth based telescopes.
The DWE data set consists of the sky frequencies measured at the Green Bank and Parkes telescopes (data from other stations may be included when they become available; see [BIRDETAL2005]), the retrieved zonal wind speed along the descent path, geometrical parameters used to separate the Huygens velocity components and documentation. It is organized in six directories, which we describe in the following.
The DATA directory contains the measured sky frequencies after signal processing in two files, one for Green Bank and one for Parkes. It is not intended to archive raw recordings of the signal. The signal processing method is described in [FOLKNERETAL2006]. Furthermore, it contains the retrieved zonal wind speed, the primary DWE result. The zonal wind speed was computed from each frequency sample, so that the total number of rows in the zonal wind table matches the sum of the frequency samples from Green Bank and Parkes.
The time tags for the frequencies is Earth Received Time (ERT), whereas the time tags for the zonal wind speed is Spacecraft Event Time (SCET). (By the way: Also START_TIME and STOP_TIME of this object, the DATA_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 the Huygens 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 data recordings. For the computation of the zonal wind speed, we used its precise value determined by our software, which made use of the SPICE toolkit (available from JPL). The precise value for the one-way-light-time can be reconstructed by subtracting our SCET time tags from the ERT time tags e.g. in the files HUYGENS_STATE.TAB and ANTENNA_STATE.TAB in the GEOMETRY directory (all data and geometry files have an identical number of rows that corresponds to the number of DWE sky frequency samples).
The data collected at Green Bank have a mean integration time of 2 sec. At Parkes, the mean integration time was 3 sec before landing and 5 sec thereafter. There is a 25-min gap between the last sample from Green Bank and the first sample from Parkes, which may possibly be filled by measurements of other VLBA stations in the future, albeit with a lower time resolution as the gain of those stations is significantly smaller than that of Green Bank or Parkes.
After launch, it was decided that in addition to the DWE Doppler tracking Huygens should also be tracked using a technique known as Very Long Baseline Interferometry (VLBI) [POGREBENKOETAL2004]. The VLBI group used a number of radio antennas, among them also Green Bank and Parkes, which were jointly used by the VLBI and DWE teams. The VLBI techique requires a continuous calibration with an extragalactic radio source, i.e. a quasar. As no quasar was in the beam when the antennas were pointed towards Huygens, they had to be nodded continuously. It was agreed among the teams that, in order to meet the requirements for the VLBI tracking, the antennas were pointed about 100 sec towards Huygens and 80 sec towards the VLBI calibration source in a 3-min cycle. These interruptions can consequently also be seen in the DWE data set.
This directory contains state vectors of Huygens and the receiving antennas and the resulting projection angles for the various velocity components. The time tags of the state vectors of Huygens and the projection angles are given as SCET, whereas the time tags for the state vectors of the receiving antennas is given as ERT to account for retardation. See the file GEOMINFO.TXT in the GEOMETRY directory for more information.
The altitude and meridional drift profiles of Huygens used for the generation of these files were provided by the Huygens Descent Trajectory Working Group (DTWG; see corresponding archive). The initial longitude (longitude at the time of the first DWE sky frequency sample) was taken from DTWG as well, whereas the subsequent longitude values were obtained by integration of the measured zonal wind speed. It is clear that any change of the DTWG profile will affect the retrieved zonal wind. For this version of the archive, we used the 4th DTWG delivery (DTWG#4), published in May 2006. Thus far, all DTWG deliveries including those which had not yet incorporated the image derived meridional drift, lead similar zonal wind profiles. There are no significant differences between the various zonal wind retrievals in the upper atmosphere. Near the surface, where the winds are generally weak, a slightly different descent speed or meridional drift profile could, however, lead to a significant change in the retrieved zonal wind speed.
The state vectors of the receiving antennas and Titan were computed using the 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 and the accompanying kernels are available at http://naif.jpl.nasa.gov/naif/
This directory contains the most important DWE documents and the DWE Health Report, which describes the performance of the DWE USOs during the Huygens mission. Although the paper [FOLKNERETAL2006] is considered an important document too, it was not included, because at the time of this writing, it was not yet published. More documentation is listed in the reference catalog in the CATALOG directory.
The CALIB directory contains a report that provides information on the DWE performance during pre-launch and in-flight tests and describes the final calibration of the frequency measurement.
This directory contains general information about the data set, such as involved personnel, instrument description, references, etc.
This directory is needed internally by the archive.
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).
These data are available on-line from the Planetary Data System (PDS) at:
Questions and comments about this data collection can be directed to: Dr. Edwin V. Bell, II
Name | Role | Original Affiliation | |
---|---|---|---|
Dr. Robindro Dutta-Roy | General Contact | Universitat Bonn |