NSSDCA ID: PSPA-00532
Availability: Archived at NSSDC, accessible from elsewhere
This description was generated automatically using input from the Planetary Data System.
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 set version 2.0 shall be released in the near future, as soon as the review process is complete.
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.
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 DARK datasets are the readout from covered columns of the CCD detector. Their values indicate the dark current being generated by the chip during the other measurements.
The DESCENT 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 HKEEPING (Housekeeping) datasets record DISR temperatures, voltages, and software indices.
The IMAGE directory contains tables of the detector readout values for each pixel, after the image has been decompressed (lossy hardware compression).
The IR datasets contain the per pixel (wavelength) readout from each IR 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 SOLAR directory contains the measurements of the light intensity field around the sun (the solar aureole). The data are presented as tables of pixel values.
The 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 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). Its amplitude is an independent measurement of the solar absorption at its 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 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 datasets contain the reading (amplitude) of the violet photometer.
The VISIBLE 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 spatial. In some cases the columns (spatial dimension) are summed to reduce noise.
The VISIBLE_EXT 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 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.
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, 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.
This directory contains general information about the data set, such as involved personnel, instrument description, references, etc.
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.
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 EXTRAS 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 atmosphere'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.
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 result is that the DISR sun sensor was not able to maintain sun lock throughout the descent, and consequently not all data were taken at the proper azimuths relative to the sun.
The sun sensor experience another problem in that its detector became too cold during the descent, such that its 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 probe's 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.
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. Elizabeth McFarlane | General Contact | University of Arizona |