descent imager/spectral radiometer (disr)
TRANSCRIPT
University of Colorado Boulder Colorado Center for Astrodynamics Research
Descent Imager/Spectral Radiometer (DISR)!Instrument on Huygens Probe to Titan
Ben K. Bradley
ASTR 5835 Planetary Seminar on Spacecraft Instrumentation 11/08/2011
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Outline
l Huygens Mission Overview
l Science Goals
l Instrument Design
l Results
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Spacecraft and Trajectory
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Huygens Mission Profile
l Huygens probe separated from Cassini and landed on Titan
l Designed to enter and brake in Titan’s atmosphere and parachute
down to the surface
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DISR Science Objectives
l Measure distribution of photochemical haze in Titan's atmosphere.
n Measure the size, shape, vertical distribution, and absorption as a function of wavelength of the haze particles to better understand their production and life cycle in Titan's atmosphere.
l Understand the heat balance of Titan's atmosphere.
n Measure the absorption of sunlight as a function of altitude to understand how much sunlight is absorbed in the atmosphere and how much is absorbed at the ground to support the small greenhouse effect observed in the atmosphere. DISR will also measure the wind speed directly from the horizontal drift of the probe relative to features seen on the surface in our images.
l Understand the nature of the surface and its interaction with the atmosphere.
n Obtain some 700 images of the surface at resolutions varying from 150 meters to less than one meter. Assemble these images into some 20 panoramic mosaics of the surface as seen from altitudes varying from 150 km to a few hundred meters. Also measure the reflection spectrum of the surface in several thousand locations in an attempt to catalog the surface reflectivity and composition.
l Understand the composition of the atmosphere.
n Measure the mixing ratio of methane as a function of altitude using the DISR measurements of the atmospheric absorptions as a function of wavelength throughout our descent from 150 km to the surface.
http://www.lpl.arizona.edu/DISR/objectives.htm
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Instrument Overview
l DISR is made up of 7 instruments
l Divided into 2 categories
n (1) measure atmospheric aerosol particles makeup
n (2) take pictures of the surface
l The DISR is one of the most complicated optical instruments to ever be sent on a space mission
l Developed at the Lunar and Planetary Laboratory at the University of Arizona under the direction of Martin Tomasko
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Instrument Overview
l (1) Visible Spectrometer
l (2) IR Spectrometer
l (3) Violet Photometer
l (4) Solar Aureole Camera
l (5) Surface Science Lamp
l (6) Sun Sensor
l (7) Imaging System
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The Trick
l It was not possible to include separate instruments devoted to each of the scientific measurements so fiber optics was used.
l The fiber optic conduit brings light from the 3 imagers, solar aureole camera, and visible spectrometer to the CCD detector.
CCD
12mm x 5.9mm
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(1) Visible Spectrometer
l Measures the light spectrum of upward and downward sunlight as a function of altitude during the descent.
l Measurement of the profile of the absorption of solar energy.
l Defines the composition and nature of Titan's surface by measuring the absorption of reflected sunlight.
l Provides a vertical profile of methane distribution.
l Measures the optical properties, size, and vertical distribution of aerosols (particles in the atmosphere) and clouds.
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(2) IR Spectrometer
l The IR spectrometer contains two parts: n an upward-looking infrared spectrometer (ULIS) and n a downward-looking infrared spectrometer (DLIS), which are useful in measuring the
upward and downward fluxes
l Otherwise extremely similar to the Visible Spectrometer
Indium-Gallium-Arsenide Array Detector
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(3) Violet Photometer
l Allows measurements similar to those of the other spectrometers at shorter wavelengths.
l Enables DISR to cover nearly all the solar energy within Titan’s atmosphere.
l Determines the absorption properties of photochemical aerosols in Titan's stratosphere.
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(4) Solar Aureole Camera
l Measures the intensity of scattered sunlight near the sun. The size of this “aureole” of light around the sun is sensitive to particle size.
l In effect, the solar aureole camera will retrieve particle size distribution
data for different levels of Titan's atmosphere.
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(5) Surface Science Lamp
l Illuminates Titan's surface, since very little sunlight is able to reach the surface at many wavelengths.
l The SSL is a 20-watt lamp designed like a flashlight to produce a beam of light. It is activated at a height of 700 meters above Titan's surface, as determined by the radar altimeter.
l Allows for continuous measurements of the spectral reflectivity of the surface infrared spectrometer during the last few minutes of the descent.
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(6) Sun Sensor
l The timing of all the upward-looking DISR measurements are all based on the position of the sun in the sky. The location of the sun provides a standard orientation for the other instruments' data.
l As the sun crosses this instrument's field of view, it passes through three slits. Each slit produces a “pulse” signal, which appears in the detector. The slits are arranged such that the elevation of the sun in Titan’s sky can be measured.
Measures the flux in the direct solar beam at 950 nm
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(7) Imaging System
l Comprised of High resolution, Medium resolution, and side-looking imagers.
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Image Stitching
l A view of the ground from 10 km showing boundaries of individual images
l As the probe rotates, sets of 3 images are obtained at 12 azimuths
l Panoramic images created from these
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Results
l Images of surface showing evidence of rain and fluid
flow
l Measurements of heights of terrain
l Measurement of reflection spectrum of surface
l Measurements of zonal and meridianal wind
l Size, optical properties, vertical distribution of haze
aerosols
l Measurements of methane absorption coefficients for
long paths at low temperature
l Measurement of methane mixing rate
l Solar heating rate and thermal cooling rate compared
at landing site for heat balance studies
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Terrain
l Bright highlands cut by rain-fed streams draining into a lower, dark, dry lakebed
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Terrain
l (right) Rounded cobbles of ice in the dry lake beg provide evidence of fluid flow
l (bottom) Surface reflection spectrum
Methane + Ethane + Propane
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DISR Wind Speed Measurements
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Huygens Descent and Data Collection Video
http://www.lpl.arizona.edu/DISR/Multimedia/
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References
l Tomasko, M.G., et. al., “The Descent Imager/Spectral Radiometer (DISR) Experiment on the Huygens Entry Probe of Titan,” Space Science Reviews, Vol. 104, pp. 469-551, 2002.
l Tomasko, M.G., “Results from the Descent Imager/Spectral Radiometer (DISR) Experiment on Huygens,” presentation.
l http://http://www.lpl.arizona.edu/DISR/
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Sub-Instrument Details
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Data and CCD
l After A/D conversion, the data are reduced from 12 to 8 bits/pixel by a
square-root algorithm
l Image data are compressed in a lossy hardware compressor by factors
between 3 and 8:1
l The CCD is at a temperature of 260 K at Titan entry and then begins to cool
to 200 K