Showing papers by "Jet Propulsion Laboratory published in 1989"

Voyager 2 at neptune: imaging science results.

Abstract: Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptune's atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptune's zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.

Towards intelligent autonomous control systems: Architecture and fundamental issues

Abstract: Autonomous control systems are designed to perform well under significant uncertainties in the system and environment for extended periods of time, and they must be able to compensate for system failures without external intervention. Intelligent autonomous control systems use techniques from the field of artificial intelligence to achieve this autonomy. Such control systems evolve from conventional control systems by adding intelligent components, and their development requires interdisciplinary research. A hierarchical functional intelligent autonomous control architecture is introduced here and its functions are described in detail. The fundamental issues in autonomous control system modelling and analysis are discussed.

Infrared Observations of the Neptunian System

Abstract: Voyager 2's IR observations of Neptune encompass thermal emissions and broadband radiometer measurements of reflected solar radiation. Temperature maps were obtained for the planet between 80 deg S and 30 deg N for two atmospheric layers, one in the lower stratosphere and the other in the troposphere. The relatively warm pole and equator, with cooler midlatitudes, are qualitatively similar to Uranus, despite the two planets' very different obliquities and internal heat fluxes. Powerful wavelike longitudinal thermal structure is noted, of which some appears to be associated with the Great Dark Spot; a localized cold region uncorrelated with any visible feature is found in the lower stratosphere.

First plasma wave observations at neptune.

Abstract: The Voyager 2 plasma wave instrument detected many familiar plasma waves during the encounter with Neptune, including electron plasma oscillations in the solar wind upstream of the bow shock, electrostatic turbulence at the bow shock, and chorus, hiss, electron cyclotron waves, and upper hybrid resonance waves in the inner magnetosphere. Low-frequency radio emissions, believed to be generated by mode conversion from the upper hybrid resonance emissions, were also observed propagating outward in a disklike beam along the magnetic equatorial plane. At the two ring plane crossings many small micrometer-sized dust particles were detected striking the spacecraft. The maximum impact rates were about 280 impacts per second at the inbound ring plane crossing, and about 110 impacts per second at the outbound ring plane crossing. Most of the particles are concentrated in a dense disk, about 1000 kilometers thick, centered on the equatorial plane. However, a broader, more tenuous distribution also extends many tens of thousands of kilometers from the equatorial plane, including over the northern polar region.

Orbital migration of protoplanets - The inertial limit

Abstract: The dynamical evolution of a disk and the orbital migration of an embedded protoplanet are examined. It is shown how the migration of a protoplanet due to density wave torques can suppress the tendency for tidal truncation of the disk. A critical mass is determined as a function of the disk properties that represents the limiting mass that can sustain drift without stalling. This inertial limit is derived analytically, using a quasi-steady state theory, and confirmed by numerical experiment. This result contradicts the claim of Lin and Papaloizou (1986) that such a limit does not exist. Orbital mobility of objects due to density wave torques may have played an important role in the early evolution of the solar system.

Limitation Of Optimism In The Time Warp Operating System

Abstract: Optimistic systems execute events out of order and must undo their errors to produce correct results. Undoing incorrect work can be expensive. By restraining their optimism, such systems might execute fewer events out of order and thereby run faster. This paper examines two methods tested in the Time Warp Operating System. The first method explicitly prevents events from executing in the far simulation future. The second method tries to identify objects that are doing work that has to be undone; such objects are allowed to execute less often. Experimental results show that only modest gains were realized, and that even these gains were unpredictable. While other methods remain untested, the value of limiting optimism seems small.

Ocean Radar Backscatter Relationship with Near-Surface Winds: A Case Study during FASINEX

Abstract: A case study of the ocean radar backscatter dependence on near-surface wind and wind stress is presented using the data obtained on February 18, 1986 during the Frontal Air-Sea Interaction Experiment. The particular wind-wave conditions and their variations across a sharp sea surface temperature front are described. The small change in wind speed across the front cannot account for the large change in wind stress implying significant changes in the drag coefficient and surface roughness length. The results strengthen the hypothesis that radar backscatter is closely correlated to wind stress, and therefore, could be used for remote sensing of the wind stress itself over the global oceans.

Unfurlable satellite antennas: A review

Abstract: Une revue des antennes deployables pour satellites est presentee. Apres un survol des exigences typiques pour les applications a des missions spatiales futures, les concepts developpes aux Etats-Unis et en Europe, utilisant des surfaces a treillis ou gonflables et rendues rigides dans l’espace, sont passees en revue. Les antennes de haute precision utilisant des panneaux ou petales rigides ne font pas partie de l’etude. Les methodes de modelisation et d’analyse radioelectrique des antennes a reflecteur deployable a treillis sont ensuite discutees. Finalement, les techniques de mesures au sol et en orbite des antennes deployables sont passees en revue.

A hypercube Ray-tracer

Abstract: We describe a hypercube ray-tracing program for rendering computer graphics. For small models, which fit in the memory of a single processor, the ray-tracer uses a scattered decomposition of pixels to balance the load, and achieves a very high efficiency. The more interesting case of large models, which cannot be stored in a single processor, requires a decomposition of the model data as well as the pixels. We present algorithms for constructing a decomposition based upon information about the frequency with which different elements of the model are accessed. The resulting decomposition is approximately optimized to minimize communication and achieve load balance.

Adaptive narrow-band interference rejection in a DS spread-spectrum intercept receiver using transform domain signal processing techniques

Abstract: An intercept receiver which uses a transform-domain-processing filter is described. This receiver detects direct-sequence binary-phase-shift-keyed (DS-BPSK) spread-spectrum signals in the presence of narrowband interference by employing adaptive narrowband interference rejection techniques. The improvement in the system performance over that of conventional detection techniques is shown by presenting the results of experimental measurements of probability of detection versus false alarm for an enhanced total power detector. Also presented are certain results corresponding to detection of the spectral lines generated at twice the carrier frequency, wherein the goal is often not just signal detection, but also carrier frequency estimation. The receiver uses one of two transform-domain-processing techniques for adaptive narrowband interference rejection. In the first technique, the narrowband interference is detected and excised in the transform domain by using an adaptive notch filter. In the second technique, the interference is suppressed using soft-limiting in the transform domain. >

Morphology of calcite crystals in clast coatings from four soils in the Mojave desert region

Abstract: Pedogenic calcite-crystal coatings on clasts were examined in four soils along an altitudinal gradient on Kyle Canyon alluvium in southern Nevada. Clast coatings were studied rather than matrix carbonate to avoid the effects of soil matrix on crystallization. Six crystal sizes and shapes were recognized and distinguished. Equant micrite was the dominant crystal form with similar abundance at all elevations. The distributions of five categories of spar and microspar appear to be influenced by altitudinally induced changes in effective moisture. In the drier, lower elevation soils, crystals were equant or parallel prismatic with irregular, interlocking boundaries while in the more moist, higher elevation soils they were randomly oriented, euhedral, prismatic, and fibrous. There was little support for the supposition that Mg(+2) substitution or increased (Mg + Ca)/HCO3 ratios in the precipitating solution produced crystal elongation.

Pioneer and Voyager observations of the solar wind at large heliocentric distances and latitudes

Abstract: Data obtained from the electrostatic analyzers aboard the Pioneer 10 and 11 spacecraft and from the Faraday cup aboard Voyager 2 were used to study spatial gradients in the distant solar wind. Prior to mid-1985, both spacecraft observed nearly identical solar wind structures. After day 150 of 1985, the velocity structure at Voyager 2 became flatter, and the Voyager 2 velocities were smaller than those observed by Pioneer 11. It is suggested that these changes in the solar wind at low latitudes may be related to a change which occurred in the coronal hole structure in early 1985.

More on the decoder error probability for Reed-Solomon codes

Abstract: A combinatorial technique similar to the principle of inclusion and exclusion is used to obtain an exact formula for P/sub E/(u), the decoder error probability for Reed-Solomon codes. The P/sub E/(u) for the (255, 223) Reed-Solomon code used by NASA and for the (31, 15) Reed-Solomon code (JTIDS code) are calculated using the exact formula and are observed to approach the Qs of the codes rapidly as u gets large. An upper bound for the expression mod P/sub E/(u)/Q-1 mod is derived and shown to decrease nearly exponentially as u increases. >

Cross-profiling as an efficient technique in simulating parallel computer systems

Abstract: The simulation of parallel computers is discussed. In particular, a technique, called cross-profiling, that can be used to improve the efficiency of these simulations substantially is presented and evaluated. Cross-profiling, along with another simulation technique called execution-driven simulation, can be used to simulate one processor's instruction set (the target processor) with another processor that has a different instruction set (the host processor). Moreover, this technique is significantly faster than the usual method where several host instructions are executed to emulate the effect of executing a single instruction of the target processor. The cross-profiling technique is introduced, and it is shown how it can be combined with execution-driven simulation. The results of several experiments that demonstrate that the error introduced by cross-profiling is typically less than 1% are presented. >

Experimental test of the variability of G using Viking lander ranging data

Abstract: Results are presented from the analysis of solar system astrometric data, notably the range data to the Viking landers on Mars. A least-squares fit of the parameters of the solar system model to these data limits a simple time variation in the effective Newtonian gravitational constant to (0.2 ± 0.4) × 10−11 year−1 and a rate of drift of atomic clocks relative to the implicit clock of relativistic dynamics to (0.1 ± 0.8) × 10−11 year−1. The error limits quoted are the result of uncertainties in the masses of the asteroids.

Reflectance characteristics of reference materials used in lidar hard target calibration

Abstract: Lidar hard target calibration is discussed, emphasizing the transfer target methodology. Characteristics of example calibration target surfaces are described in light of the four reflectance mechanisms: specular, diffuse, retroreflection, and off-specular reflectance. This ideal transfer target is one which can be described as entirely diffusely reflecting, i.e., Lambertian. Correction for retroreflection is required when using the flowers of sulfur transfer target at CO(2) laser wavelengths. Corrections for specular and retroreflection for the integrating sphere are negligible; however, the off-specular reflectance of rough metal surfaces used in the integrating spheres is only qualitatively compared with the diffuse reflectance of flowers of sulfur.

Dynamical History of the Oort Cloud

Abstract: Dynamical studies during the past decade have resulted in an almost explosive increase in our understanding of the Oort cloud of comets, which surrounds the solar system Cometary orbits in the cloud evolve under the complex interaction of stellar, galactic, and giant molecular cloud perturbations, as well as planetary and nongravitational perturbations when the orbits re-enter the planetary region Evidence has continued to build for a dense, inner Oort cloud of comets which acts as a reservoir to replenish the outer cloud as comets there are stripped away A ring of comets beyond the orbit of Neptune, which may be the source of the short-period comets, is also likely Both the estimated number and mass of comets in the Oort cloud have grown such that the total mass may be comparable to the mass of the planets Temporal variations in the flux of comets from the Oort cloud into the planetary region by a factor of 50% are typical, and by factors of 20 to 200 are possible The most intense cometary “showers” may have serious implications for biological extinction events on Earth as well as for the impact history of planets and satellite systems Comets in the Oort cloud are processed by galactic cosmic rays, heated by nearby Supernovae, eroded by interstellar dust impacts, and disrupted by mutual collisions (in the inner cloud) A detailed estimate of the Oort cloud’s dynamical history is not possible because of the inability to reconstruct the Sun’s varying galactic motion over the history of the solar system, and because of uncertainty over where comets actually formed However, it is likely that a substantial fraction of the original Oort cloud population has been lost to interstellar space We are approaching the time when Oort clouds around other stars may be detectable, though searches to date have so far been negative

Traveling-wave induction launchers

Abstract: An analysis of traveling-wave induction launchers shows that induction is a feasible method of producing armature current and that efficient accelerators can be built without sliding contacts or arcs. In a traveling-wave induction launcher the armature current is induced by a slip speed between the armature and a traveling magnetic field. At 9-m/s slip speed a 9-kg projectile with an aluminum armature weighing 25% of the total mass can be accelerated to 3000 m/s in a 5-m-long barrel with a total ohmic loss in the barrel coils and armature of 4% of the launch kinetic energy and with an average armature temperature rise of 220 degrees C, but a peak excitation frequency of 8600 Hz is required. With a 2-kg launch mass the ohmic loss is 7%. A launcher system optimized for rotating generators would have a peak frequency of 4850 Hz; with an aluminum armature weighing 33% of the launch mass and a slip speed of 30 m/s the total ohmic loss in the generators, cables, and accelerator would be 43% of the launch kinetic energy, and the average armature temperature rise would be 510 degrees C. >

Mesh-matrix analysis method for electromagnetic launchers

Abstract: The mesh-matrix method is a procedure for calculating the current distribution in the conductors of electromagnetic launchers with coil or flat-plate geometry. Once the current distribution is known the launcher performance can be calculated. The method divides the conductors into parallel current paths, or meshes, and finds the current in each mesh by matrix inversion. The author presents procedures for writing equations for the current and voltage relations for a few meshes to serve as a pattern for writing the computer code. An available subroutine package provides routines for field and flux coefficients and equation solution. >

Slow shock characteristics as a function of distance from the X‐line in the magnetotail

Abstract: Both particle and MHD simulations are performed to study the characteristics of slow shocks in the magnetotail. The particle simulations indicate that switch-off shocks exhibit large amplitude rotational wave trains, while magnetotail slow shocks with an intermediate Mach number M(An) less than M(c) of about 0.98 do not display such rotational wave trains. The MHD simulations show that the spontaneous reconnection process in the near-earth plasma sheet leads to the formation of a pair of slow shocks tailward of the reconnection line (X-line). The properties of slow shocks are found to vary as a function of the distance from X-line due to the formation of plasmoid. Slow shocks in most regions of magnetotail are found to be nonswitch-off shocks with M(An) of less than 0.98. The present results are used to discuss the lack of large amplitude rotational wave trains at slow shocks in the deep magnetotail.

The induction of probabilistic rule sets—the Itrule algorithm

Abstract: In this paper we address the problem of learning sets of probabilistic rules. While attention has been focused for some time now on the learning of fixed classification rule structures such as decision trees, the problem of identifying useful sets of probabilistic production rules is relatively new. We discuss our recent work in this area and conclude that learning and inference algorithms need to be more closely coupled.