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The Cassini RADAR is a multimode instrument built both by NASA/JPL and ASI/Alenia Spazio. The instrument operates in radiometer-only, scatterometer, altimeter, and imaging modes. This paper will describe this interesting unique radar and how it will be used to collect data at Titan.
The Alaska SAR Facility (ASF) has been receiving, processing, archiving, and distributing data for Earth scientists and operations since it began receiving data in 1991. Four radar satellites are now being handled. Recent developments have served to increase the level of services of ASF to the Earth science community considerably. These developments are discussed.
The Galileo spacecraft was launched on October 18, 1989. After a 3.7 billion kilometer journey lasting just over six years, the Galileo Orbiter and Probe arrived at Jupiter on December 7, 1995. The atmospheric Probe survived its atmospheric entry and successfully transmitted data to the Orbiter flying overhead. To date, the Orbiter has successfully completed the first three of the ten planned satellite encounters. Navigation for the Galileo Mission has proved to be a unique and challenging task. The challenges and results of navigating Galileo through the interplanetary transfer to Jupiter, delivery of the atmospheric entry Probe, Jupiter orbit insertion, and the orbital tour are presented in this paper.
The only physical observations which predate the identifications of 253 Mathilde as a potential flyby target of NEAR were measurements of its absolute magnitude in reflected light and thermal radiometry by IRAS. Mathilde has been revealed to be an unusual object, of a class not previously visited by a spacecraft.
Since 1991 a suite of international satellites have collected large amounts of high-resolution microwave radar images over Arctic and Antarctic sea ice. Together with complementary synthetic aperture radar (SAR) 100 m resolution microwave imaging, these data provide a powerful tool for addressing the characteristics of sea ice which directly influence the polar oceans and climate.
In recent years, a great deal of research has been exerted to developing NDE methods for the characterization of the material properties of composites as well as other space structural materials. The need for information about such parameters as the elastic properties, density, and thickness are critical to the safe design and operation of such structural materials. Ultrasonics using immersion methods has played an important role in these efforts due to its capability, cost effectiveness, and ease of use. The authors designed a series of ultrasonic oblique insonification experiments in order to develop a practical field applicable NDE method for space structures.
The first Danish satellite, rsted, will be launched in August of 1997. The scientific objective of sted is to perform a precision mapping of the Earth's magnetic field. Attitude data for the payload and the satellite are provided by the Advanced Stellar Compass (ASC) star tracker. The ASC consists of a CCD star camera and a capable microprocessor which operates by comparing the star image frames taken by the camera to its internal star catalogs.
NASA's SPICE information system for archiving, distributing, and accessing spacecraft navigation, orientation, and other ancillary data is described. A proposal is made for the further evolution of this concept to an internationally useful standard, to be.
The Galileo Mission is designed to study all aspects of the huge, complex Jupiter system. Its primary objectives are: 1.) Jupiter, 2.) the Galilean satellites, and 3.) the Magnetosphere. Since its arrival in the Jupiter system on 7 December 1995, Galileo has provided a nearly continuous stream of data concerning all its objectives and has already made several major discoveries. Initial results from the Probe and from the Orbiter are described in this paper.
Before the arrival of the Galileo spacecraft in the jovian system, there was little information on the interior structures of Jupiter's four Galilean satellites. Results obtained during the Galileo Mission are described.
The potential of wide-band-gap III-V nitrides as ultraviolet sensors and light emitters has prompted an increasing amount of work recently, including the fabrication of the first UV sensors from as-deposited single crystal GaN. We have used high resolution transmission electron microscopy (TEM) to study the microstructure of two novel developments of wide-band-gap III-V nitrides: the growth of ultra-short period GaN/AlN superlattices; and the incorporation of SiC layers into Al(sub x)Ga(sub 1-x)N structures. By varying the relative periods in a GaN/AlN superlattice, the band gap of the composite can be tailored to lie between the elemental values of 365 nm for GaN and 200 nm for AlN. The group IV semiconductor, SiC, has a wide band-gap and has a close lattice match (less than 3 %) to Al(sub x)Ga(sub 1-x)N for growth on the basal plane. Demonstration of epitaxial growth for Al(sub x)Ga(sub 1-x)N/SiC multilayers would introduce a wide band-gap analog to the already existing family of III-V and Si(sub 1-x)Ge(sub x) heteroepitaxial growth systems. Although good quality growth of GaN on SiC substrates has been demonstrated, Al(sub x)Ga(sub 1-x)N/SiC multilayer structures have never been grown and the interfacial structure is unknown.
This paper describes a technique to locate and characterize loose joints in a large truss structure to generate an accurate structural model. The joint looseness is modeled as a gap in the member that opens and closes depending on the loading. Arbitrarily placed actuators are used to prestress the structure to first linearize the response. Next the actuator displacements are systematically reduced while monitoring the displacement response. The gap locations are determined by comparing the measured displacements with sets of calculated displacements and the sizes are estimated by monitoring the gap member length changes using the appropriate linear force-displacement relationship for the load level. The effect of measurement error in the truss displacements and the actuator length changes are investigated.
We are developing a system for synthesizing image sequences the simulate the facial motion of a speaker. To perform this synthesis, we are pursuing two major areas of effort. We are developing the necessary computer graphics technology to synthesize a realistic image sequence of a person speaking selected speech sequences. Next, we are developing a model that expresses the relation between spoken phonemes and face/mouth shape. A subject is video taped speaking an arbitrary text that contains expression of the full list of desired database phonemes. The subject is video taped from the front speaking normally, recording both audio and video detail simultaneously. Using the audio track, we identify the specific video frames on the tape relating to each spoken phoneme. From this range we digitize the video frame which represents the extreme of mouth motion/shape. Thus, we construct a database of images of face/mouth shape related to spoken phonemes. A selected audio speech sequence is recorded which is the basis for synthesizing a matching video sequence; the speaker need not be the same as used for constructing the database. The audio sequence is analyzed to determine the spoken phoneme sequence and the relative timing of the enunciation of those phonemes. Synthesizing an image sequence corresponding to the spoken phoneme sequence is accomplished using a graphics technique known as morphing. Image sequence keyframes necessary for this processing are based on the spoken phoneme sequence and timing. We have been successful in synthesizing the facial motion of a native English speaker for a small set of arbitrary speech segments. Our future work will focus on advancement of the face shape/phoneme model and independent control of facial features.
Major improvements currently underway to the Arecibo 305 m antenna and 13 cm wavelength radar system will currently increase the radar system's sensitivity by a factor of about 20. Recent upgrades to the Goldstone 3.5 cm wavelength radar system have also improved its sensitivity. While the Arecibo system will have significantly greater sensitivity than Goldstone, the Arecibo antenna's limited declination coverage of -2(deg) to +38(deg) means that the Goldstone system will be important for the investigation of near-Earth objects with declinations outside this range. Over one hundred mainbelt and near-Earth asteroids per year will be observable with the new Arecibo system, and there will be a number of good imaging opportunities each year. While only a few imaging opportunities are predicted for comets over the next 10 years, the system will be able to respond to unexpected cometary opportunities. We are currently investigating the applicability of VLBI techniques to the direct synthesis of images of asteroids and comets illuminated by a radar signal.
The distribution of unimolecular lifetimes of ion-molecule complexes formed in association reactions has been measured by ion cyclotron double-resonance. The mean unimolecular lifetimes of (H(sub 2)C(sub 6)N(sub 2)(sup +))(sup *) and (CH(sub 3)CN.CH(sub 3)(sup +))(sup *) were determined to be 180 (micro)s and 140 (micro)s respectively. A theoretical examination of the distribution of lifetimes of (CH(sub 3)CN.CH(sub 3)(sup +))(sup *) was conducted using a RRKM model. The RRKM distribution, when modified by experimental constraints, was found to be a good approximation of the experimentally determined lifetime distribution. The lifetimes for unimolecular dissociation and radiative relaxation, and the absolute efficiency of collisional relaxation are also reported.
The potential of wide-band-gap III-V nitrides as solar-blind ultraviolet sensors and light emitters has prompted an increasing amount of work recently, including the fabrication of the first UV sensors from as-deposited single crystal GaN. We have used high resolution transmission electron microscopy (TEM) to study the microstructure of two novel developments of wide-band-gap III-V nitrides: the growth of ultra-short period GaN/AlN super lattices (digital alloys); and the incorporation of SiC layers into Al(sub x)Ga(sub 1-x)N structures. By varying the relative periods in a GaN/AlN superlattice, it should be possible to tailor the band gap of the composite to lie between the elemental values of 365 nm for GaN and 200 nm for AlN. The group IV semiconductor, SiC, has a close lattice match (< 3%) to Al(sub x)Ga(sub 1-x)N for growth on the basal plane. Demonstration of epitaxial growth for Al(sub x)Ga(sub 1-x)N/SiC multilayers could enable an extension of direct band-gap material towards the visible.
Thin films of gold and platinum have been deposited onto superpolished fused silica substrates using thermal evaporation, ion assisted deposition (IAD), and ion assisted sputtering. The influence of ion beam flux, thin film material, and deposition rate on the films microroughness have been investigated. Short range surface microroughness of the films has been examined using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Long range surface microroughness has been characterized using an angle resolved optical scatterometer. Results indicate that ion beam deposited coatings have improved microstructure over thermally evaporated films.
The use of metal hydrides as negative electrodes in alkaline rechargeable cells is becoming increasingly popular, due to several advantages offered by the metal hydrides over conventional anode materials (such as Zn, Cd) in terms of specific energy environmental cycle life and compatibility. Besides, the similarities in the cell voltage pressure characteristics, and charge control methods of the Ni-MH cells to the commonly used Ni-Cd point to a projected take over of 25% of the Ni-Cd market for consumer electronics by the Ni-MH cells in the next couple of years. Two classes of metal hydrides alloys based on rare earth metals (AB(sub 5)) and titanium (AB(sub 2)) are being currently developed at various laboratories. AB(sub 2) alloys exhibit higher specific energy than the AB(sub 5) alloys but the state of the art commercial Ni-MH cells are predominately manufactured using AB(sub 5) alloys.