Digital system design in the presence of single event upsets

The authors consider the effects of single event upsets (SEUs) on digital systems, and show techniques for designing reliable systems with current levels of SEU protection. Three main systems are discussed: main memory, logic, and cache memory. A design for the main and cache memory subsystems that are SEU protected is also described. With SEU defined in bit days p, and using single error correction, it is shown that for all subsystems considered, an effective upset rate which is proportional to the product of p² and the time between corrections, or scrub time, can be obtained. Data for memory chip size and performance derived from the gallium-arsenide (GaAs) pilot lines funded by the Defense Advanced Research Projects Agency (DARPA) throughout the 1980s are used     

The heliosphere

The history and current status of heliospheric physics are outlined very briefly. It is emphasized that the existing Voyager Interstellar Mission should go a long way to answering many of the outstanding questions during the next one or two decades, especially those concerning the nature and structure of the solar wind termination shock.     

Evolution of the Martian water cycle.

The current Martian water cycle is extremely asymmetric, with large amounts of vapor subliming off a permanent north polar water ice cap in northern summer, but with no apparent major source of water vapor in the southern hemisphere. Detailed simulations of this process with a three-dimensional circulation model indicate that the summertime interhemispheric exchange (Hadley cell) is very much stronger than transport by eddies in other seasons. As a result, water ice would be distributed globally were it not for the buffering action of regolith soil adsorption which limits the net flux of water vapor off the north polar cap to amounts that are insignificant even on the scale of thousands of years. It has been suggested that the polar layered deposits are the result of exchange on these long time scales, driven by changes in Martian orbital parameters. We therefore are conducting simulations to test the effect of varied orbital parameters on the Martian water cycle. We find that when the perihelion summer pole is charged with a polar water ice cap, large quantities of water are quickly transfered to the aphelion summer pole, setting up an annual cycle that resembles the present one. Thus, the adsorptivity of the Martian regolith may be in the narrow range where it can limit net transport from the aphelion but not the perihelion pole.     

A system for obstacle detection during rotorcraft low altitudeflight

An airborne vehicle such as a rotorcraft must avoid obstacles like antennas, towers, poles, fences, tree branches, and wires strung across the flight path. Automatic detection of the obstacles and generation of appropriate guidance and control actions for the vehicle to avoid these obstacles would facilitate autonomous navigation. The requirements of an obstacle detection system for rotorcraft in low-altitude Nap-of-the-Earth (NOE) flight based on various rotorcraft motion constraints is analyzed here in detail. It is argued that an automated obstacle detection system for the rotorcraft scenario should include both passive and active sensors to be effective. Consequently, it introduces a maximally passive system which involves the use of passive sensors (TV, FLIR) as well as the selective use of an active (laser) sensor. The passive component is concerned with estimating range using optical flow-based motion analysis and binocular stereo. The optical flow-based motion analysis that is combined with on-board inertial navigation system (INS) to compute ranges to visible scene points is described. Experimental results obtained using land vehicle data illustrate the particular approach to motion analysis     

Monte Carlo track structure studies of energy deposition and calculation of initial DSB and RBE.

Estimation of exposure due to environmental and other sources of radiations of high-LET and low-LET is of interest in radiobiology and radiation protection for risk assessment. To account for the differences in effectiveness of different types of radiations various parameters have been used. However, the relative inadequacy of the commonly used parameters, including dose, fluence, linear energy transfer, lineal energy, specific energy and quality factor, has been made manifest by the biological importance of the microscopic track structure and primary modes of interaction. Monte Carlo track structure simulations have been used to calculate the frequency of energy deposition by radiations of high- and low-LET in target sizes similar to DNA and higher order genomic structure. Tracks of monoenergetic heavy ions and electrons were constructed by following the molecular interaction-by-interaction histories of the particles down to 10 eV. Subsequently, geometrical models of these assumed biological targets were randomly exposed to the radiation tracks and the frequency of energy depositions obtained were normalized to unit dose in unit density liquid water (l0(3) kg m-3). From these data and a more sophisticated model of the DNA, absolute yields of both single- and double-strand breaks expressed in number of breaks per dalton per Gray were obtained and compared with the measured yields. The relative biological effectiveness (RBE) for energy depositions in cylindrical targets has been calculated using 100 keV electrons as the reference radiation assuming the electron track-ends contribution is similar to that in 250 kV X-ray or Co60 gamma-ray irradiations.     

A low-cost space-based radar system concept

A space-based radar system concept is described that can provide continuous world-wide, all-weather, day-night observation and tracking of ships, aircraft, vehicles and ground facilities of interest. The system employs a constellation of radar satellites in low-earth orbit to provide continuous world-wide target access. The radars employ reflector antennas, TWT transmitters and high frequency (e.g., X band) to achieve long range with relatively low weight, complexity and cost. The radars operate in moving-target-detection (MTD) and synthetic-aperture-radar (SAR) spotlight imaging modes to observe moving and fixed targets, respectively. The system could support a wide range of military, intelligence, law-enforcement and civilian missions     

Electronically steerable plasma mirror based radar-concept andcharacteristics

An alternative to using a phased array to steer a radar beam is to electronically control the orientation of an inertialess broadband microwave reflector. Recent experiments have demonstrated that a planar plasma mirror immersed in a magnetic field can be formed with electron densities high enough to reflect X-band microwave beams. A plasma mirror performs like a metal mirror, but it is inertialess. Compared to high performance phased array systems, a plasma mirror based radar system is much simpler and is therefore more affordable. Electronic steering of microwave beams using a plasma mirror permits the use of wide instantaneous bandwidth waveforms. Potential areas of application for a plasma mirror based antenna system include ship self-defense, high-resolution radar imaging, target identification, electronic countermeasures, high data rate communications, spread spectrum links and remote sensing. As a reflector, the plasma mirror exhibits extremely low loss and the reflectivity is very nearly 100%. Since a perfectly reflecting object cannot radiate, the noise temperature contribution of the plasma mirror to the antenna temperature is likely to be small. The plasma sheet can be steered in elevation by tilting the magnetic field, and steering in azimuth may be accomplished by designating cathode initiation sites. Switching times between successive mirror orientations may be less than 20 μs     

Ambiguity resolution of multiple targets using pulse-Dopplerwaveforms

To cancel clutter, both medium-PRF waveforms which are ambiguous in both range and Doppler and high-PRF waveforms which are ambiguous in range but unambiguous in ambiguities, a previous paper has shown that superior results for a single target can be achieved by using a clustering algorithm. Here, the problem of multiple targets is considered. A maximum likelihood (ML) technique which incorporates the clustering algorithm is developed for the multiple target problem. Simulation results show that four targets which have the same speed but are at different ranges can be resolved by using a medium-PRF waveform and employing the ML resolution technique     

Fuel cell status, 1994

The high efficiency environmental benefits and other attributes of fuel cells have attracted world-wide attention to the technology. Approximately 250 phosphoric acid fuel cell (PAFC) power units, 35 molten carbonate fuel cell (MCFC) stacks, and 12 solid oxide fuel cell (SOFC) modules have been or are being operated. Total capacity installed or operating is close to 45 MW. Fuel cell development has progressed to where complete power plants have reached nearly 16,000 operating hours and this continues to increase. Developers in the U.S. and Japan have embarked on extensive government and private programs to commercialize the technology in those countries and abroad. By mid-1994, the U.S. sold and shipped to other countries at least 33 PAFC 200 kW plants, 20 675 kW PAFC stacks, two SOFC 25 kW modules, and one MCFC system. Additional units have been produced for the domestic market. There is intense interest in Japan where there are very stringent environmental regulations and fuel prices are high. The fuel cell can respond with its combined attributes of low emissions and relative high efficiency. In Europe, the environmental cleanliness of fuel cell power units holds the promise of preserving the quality of life, motivating support and development of the technology. Canada and Australia have spawned important development programs. Interest continues to increase in other parts of the world. The author reviews the 1994 status and outlines the future development trends in this area