Showing papers by "Jet Propulsion Laboratory published in 1982"

A new look at the saturn system: the voyager 2 images.

Abstract: Voyager 2 photography has complemented that of Voyager I in revealing many additional characteristics of Saturn and its satellites and rings. Saturn's atmosphere contains persistent oval cloud features reminiscent of features on Jupiter. Smaller irregular features track out a pattern of zonal winds that is symmetric about Saturn's equator and appears to extend to great depth. Winds are predominantly eastward and reach 500 meters per second at the equator. Titan has several haze layers with significantly varying optical properties and a northern polar "collar" that is dark at short wavelengths. Several satellites have been photographed at substantially improved resolution. Enceladus' surface ranges from old, densely cratered terrain to relatively young, uncratered plains crossed by grooves and faults. Tethys has a crater 400 kilometers in diameter whose floor has domed to match Tethys' surface curvature and a deep trench that extends at least 270° around Tethys' circumference. Hyperion is cratered and irregular in shape. Iapetus' bright, trailing hemisphere includes several dark-floored craters, and Phoebe has a very low albedo and rotates in the direction opposite to that of its orbital revolution with a period of 9 hours. Within Saturn's rings, the "birth" of a spoke has been observed, and surprising azimuthal and time variability is found in the ringlet structure of the outer B ring. These observations lead to speculations about Saturn's internal structure and about the collisional and thermal history of the rings and satellites.

Extreme ultraviolet observations from the Voyager 2 encounter with Saturn

Abstract: Combined analysis of helium (584 A) airglow and the atmospheric occultations of the star delta Scorpii imply a vertical mixing parameter in Saturn's upper atmosphere of K (eddy diffusion coefficient) of approximately 8 x 10 to the 7th sq cm per second, an order of magnitude more vigorous than mixing in Jupiter's upper atmosphere. Atmospheric H2 band absorption of starlight yields a preliminary temperature of 400 K in the exosphere and a temperature near the homopause of 200 K. Certain auroral emissions can be fully explained in terms of electron impact on H2, and auroral morphology suggests a link between the aurora and the Saturn kilometric radiation. Absolute optical depths have been determined for the entire C ring and parts of the A and B rings. A new eccentric ringlet has been detected in the C ring. The extreme ultraviolet reflectance of the rings is fairly uniform at 3.5 to 5 percent. Collisions may control the distribution of H in Titan's H torus, which has a total vertical extent of about 14 Saturn radii normal to the orbit plane.

Modeling and a Correlation Algorithm for Spaceborne SAR Signals

Abstract: A mathematical model of a spaceborne synthetic aperture radar (SAR) response is presented. The associated SAR system performance, in terms of the resolution capability, is also discussed. The analysis of spaceborne SAR target response indicates that the SAR correlation problem is a two-dimensional one with a linear shift-variant response function. A new digital processing algorithm is proposed here in order to realize an economical digital SAR correlation system. The proposed algorithm treats the two-dimensional correlation by a combination of frequency domain fast correlation in the azimuth dimension and a time-domain convolver type of operation in the range dimension. Finally, digitally correlated SEASAT satellite SAR imagery is used in an exemplary sense to validate the SAR response model and the new digital processing technique developed.

The effects of sunspots on solar irradiance

Abstract: It is pointed out that the darkness of a sunspot on the visible hemisphere of the sun will reduce the solar irradiance on the earth. Approaches are discussed for obtaining a crude estimate of the irradiance deficit produced by sunspots and of the total luminosity reduction for the whole global population of sunspots. Attention is given to a photometric sunspot index, a global measure of spot flux deficit, and models for the compensating flux excess. A model is shown for extrapolating visible-hemisphere spot areas to the invisible hemisphere. As an illustration, this extrapolation is used to calculate a very simple model for the reradiation necessary to balance the flux deficit.

Tracking known three-dimensional objects

Abstract: A method of visually tracking a known three-dimensional object is described. Predicted object position and orientation extrapolated from previous tracking data are used to find known features in one or more pictures. The measured image positions of the features are used to adjust the estimates of object position, orientation, velocity, and angular velocity in three dimensions. Filtering over time is included as an integral part of the adjustment, so that the filtering both smooths as appropriate to the measurements and allows stereo depth information to be obtained from multiple cameras taking pictures of a moving object at different times.

Plasma properties of driver gas following interplanetary shocks observed by ISEE-3

Abstract: Plasma fluid parameters calculated from solar wind and magnetic field data obtained on ISEE 3 were studied. The characteristic properties of driver gas following interplanetary shocks was determined. Of 54 shocks observed from August 1978 to February 1980, nine contained a well defined driver gas that was clearly identifiable by a discontinuous decrease in the average proton temperature across a tangential discontinuity. While helium enhancements were present in all of nine of these events, only about half of them contained simultaneous changes in the two quantities. Often the He/H ratio changed over a period of minutes. Simultaneous with the drop in proton temperature the helium and electron temperature decreased abruptly. In some cases the proton temperature depression was accompanied by a moderate increase in magnetic field magnitude with an unusually low variance and by an increase in the ratio of parallel to perpendicular temperature. The drive gas usually displayed a bidirectional flow of suprathermal solar wind electrons at higher energies.

Advanced automation for space missions

Abstract: A NASA/ASEE summer study conducted at the University of Santa Clara in 1980 examined the feasibility of using advanced artificial intelligence and automation technologies in future NASA space missions. Four candidate applications missions were considered: an intelligent earth-sensing information system; an autonomous space exploration system; an automated space manufacturing facility; and a self-replicating, growing lunar factory. The study assessed the various artificial intelligence and machine technologies which must be developed if such sophisticated missions are to become feasible by the century's end. 18 references.

An empirical microwave emissivity model for a foam-covered sea

Abstract: A model for computing microwave emissivity of a wind-driven foam-covered sea is presented. The effect of roughness and foam is modeled by combining early measurement results and theoretical analysis. Recent Seasat-SMMR measurements are used to fine tune the model and derive an "effective" fractional foam coverage expression in terms of frequency and wind speed. The model incorporates polarization characterization and view angle dependence of the foam-cover emissivity. For 48.8\deg incidence angle and wind speed less than 15 m/s, the emissivity values calculated from this model differs by \sim15 percent.

A relationship between chemical structure and the critical temperature

Abstract: The present investigation is concerned with the effect of both molecular weight and chemical structure on the critical temperature. Using data from the comprehensive compilation of critical constants of Kudchadker et al. (1968), a simple relationship could be developed between the critical temperature and chemical structure. This relationship does not require experimental data such as the normal boiling point. It was found that the critical temperature (Tc) is given by an expression containing m and the sum of delta-i, where m is the total number of atoms in the molecule and delta-i is a number whose value is obtained from a table of additive atomic, group, and structural constants.

Terrestrial impact rates for long and short-period comets

Abstract: The present calculations of terrestrial impact rates for longand short-period comets crossing the earth's orbit respectively yield probabilities of 2.2 x 10 to the -9th/perihelion passage (with a probable velocity of 56.6 km/sec), and 6.6 x 10 to the -9th/perihelion passage (probable velocity, 28.9 km/sec). The total cratering rate from both long- and short-period comets is about 15 percent of the observed rate derived from known terrestrial astroblemes, although there is substantial uncertainty in both estimates. The estimated cratering rate from earth-crossing asteroids given by Shoemaker et al. (1979) is about twice the observed rate, and Monte Carlo simulations of the dynamical evolution of the Oort (1950) cloud yield cometary flux rate estimates for the early solar system of the order of 200 times the current flux.

Mineral Identification from Orbit: Initial Results from the Shuttle Multispectral Infrared Radiometer

Abstract: A shuttle-borne radiometer containing ten channels in the reflective infrared has demonstrated that direct identification of carbonates and hydroxyl-bearing minerals is possible by remote measurement from Earth orbit.

The Gravitational Field of a Disk

Abstract: The gravitational potential of a disk is given, as is the gravitational field at a point in space. It is pointed out that formulas for a ring can be obtained as the difference between the results presented here for two different values of the disk radius. Results are obtained in terms of elliptic integrals and it is shown how these functions can be computed efficiently. Formulas necessary for the computation of partial derivatives are also presented.

Jovian winds from Voyager 2. I. Zonal mean circulation.

Abstract: Independent measurements of Jovian cloud motions confirm previously published results on the general structure of Jupiter's zonal mean circulation The new results are based on Voyager 2 images and measurement techniques which are different from those used in previous studies The latitudes of the zonal jets agree with previous results, but there are some differences in the measured speed of the jets which exceed uncertainty estimates These differences may be due to differences in sampling strategies The structure of the zonal mean meridional velocity profile has still not been clearly resolved: mean meridional velocities generally differ from zero by no more than their estimated uncertainty An analysis of successive measurements of the same cloud targets shows that most of the variance of individual velocity measurements is due to true variability of the winds

Bandwidth Limitations on Noiseless Optical Channel Capacity

Abstract: Even in noiseless optical channels one must take into account the fact that the time resolution available is finite. An optimization scheme under the constraint of a given information rate (in nats/second) and minimum time-slot resolution is presented. It is shown that system efficiencies in excess of tens of nats/photon will be extremely difficult to achieve due to fundamental time resolution limitations.

Photovoltaic Module Reliability Improvement through Application Testing and Failure Analysis

Abstract: Field experience obtained through photovoltaic test and application experiments sponsored by the US Department of Energy has offered an opportunity for solar cell module reliability assessment and improvement. Through a formal problem/failure reporting and analysis system, field problems have been identified and characterized. The causes of field failures have been analyzed with respect to environmental and operational stresses and to induced degradation mechanisms. On the basis of these results, manufacturers have improved the reliability of later generations of modules through changes in design, material selection, and manufacturing controls. This paper summarizes field experience, describes key failure modes, and evaluates industry progress in addressing such problems.

Spaceborne imaging radars probe `in depth'

Abstract: Discusses spaceborne radar sensors which allow all-weather, day or night high-resolution imaging of the earth's land and ocean surfaces. The newest of these systems is the synthetic-aperture radar (SAR) which can penetrate dry sands covering desert regions revealing ancient topography buried meters beneath the surface. A comparison of airborne and space radar techniques is given. The Seasat SAR and the shuttle imaging radar-A (SIR-A) sensors are almost identical. They consist of four major electronic subsystems-transmitter, receiver, power converter, and control logic. These sensors are also compared.

The Dependence of Single Event Upset on Proton Energy (15-590 MeV)

Abstract: Low earth orbit satellite and Jupiter orbiter probe semiconductor devices may incur soft errors or single event upsets, manifested as bit flips, during exposure to such nuclear particles or heavy ions as trapped protons with energies ranging up to 1000 MeV. Experimental data is given on the average proton fluence needed to cause a bit flip as a function of proton energy for isoplanar bipolar TTL RAMs. Error dependence data shape and threshold energy can be related to the existing body of theoretical data on energy deposition following proton nuclear reactions. Experimental data also show that the relative cross sectional amplitude for functionally identical devices can be related to the device's power consumption.

Cometary Impacts with the Sun: Physical and Dynamical Considerations.

Abstract: Michels et al. (1982) observed the apparent impacting of the sun by a comet, and Sekanina (1982) showed that the comet, 1979XI, was probably a member of the Kreutz group of sun-grazing comets, although its perihelion of 0.35 solar radii was much smaller than the 1.2-1.9 solar radii common for this comet class. The perihelion change cannot be explained by planetary, stellar, or nongravitational perturbations. The most plausible explanation is collision with another body, probably a comet, at large heliocentric distance. The probability of such an event is, however, extremely small. The sublimation of the comet's nucleus before impact is discussed, and it is suggested that its ultimate destruction probably resulted from the shock of entry into the denser regions of the solar atmosphere, just above the photosphere.

Ancient and modern slopes in the Tharsis region of Mars

Abstract: Ancient slope directions in the Martian Tharsis region are compared with new earth-based radar observations in an effort to detect tectonic deformations. Data were taken from 20-150 pixel/m Viking Orbiter images and from 200 m orthophotomosaics prepared by the U.S. Geological Survey. The positions of 475 lava flows were determined, covering widths between 5-10 km on higher slopes and 15-35 km on lower slopes. Most of the flows originated from four volcanos, although none issued from Olympus Mons, which makes up the central portion of the Plateau. Further radar-derived topography was made of, Arsia Mons and Syria Planum in latitudes 14-21 deg S to find differences in regional gradients and the lava flow directions, to determine if deformations occurred after the lava flows. A lithospheric stability is concluded, indicating no tectonic upheavals since the days of Tharsis Plateau volcanic activity.

The Power Spectral Density of Digital Modulations Transmitted Over Nonlinear Channels

Abstract: This paper examines by analytical methods the power spectral densities of digital modulations (in particular, staggered and unstaggered quadrature modulations) passed through band-limited nonlinear channels. Previously observed (by computer simulation or hardware measurement) behavior of such spectra with regard to the suppression or restoration of its sidelobes after passing through the nonlinearity is verified analytically. Several examples corresponding to specific quadrature modulations and filter-nonlinearity combinations are presented as illustrations of the general results.

Characteristics of RF Resulting from Dielectric Discharges

Abstract: This paper presents measurements of the electromagnetic radiation pulses and the replacement current pulses which result from the breakdown of Mylar and Kapton samples. The observed dependence of the characteristics of the replacement current pulse as a function of electron beam energy is discussed. The characteristics of the electromagnetic radiation pulses are also analysed in the text of this paper. The source of the irradiating electrons is a monoenergetic electron beam whose energy was varied from 12.5 KeV to 25 KeV.

The Performance of M-ary FH-DPSK in the Presence of Partial-Band Multitone Jamming

Abstract: Using a geometric approach, the performance of M -ary FH-DPSK in the presence of partial-band multitone jamming is evaluated. The optimal jamming strategy is determined as a function of the number of signaling levels M and the ensuing results are used to determine worst case bit error probability performance as a function of this same parameter. It is demonstrated that, for M = 2^{m} , the best performance is obtained for M = 4 .

Photoelectron-photoion coincidence study of CF3I. implications for the CW IR laser multiphoton dissociation of CF3I+

Abstract: Photoion-photoelectron coincidence spectra are reported for CF3I in the region of the X2E 3 2 and the X2E 1 2 electronic states of the molecular cation (i.e. 10.2–11.8 eV). Within this energy range CF3+(X1 A1′) + I(X2P 3 2 ) are the only accessible fragments. The kinetics and the translational energy release of this dissociation process are examined. The enthalpy of formation of CF3+ is found to be ΔHf0Θ(CF3+) = 402 ± 5 kJ mol−1, in agreement with earlier literature values. The implications of the present study for the recently reported multiphoton dissociation of CF3I+ are discussed.