Fuel cell status: 1996

The technology of fuel cells is heating up. A world that, ten years ago, was unaware of the concept can now witness approximately 200 fuel cell units operating in 15 countries. Energy planners and decision makers are becoming aware that, in addition to a continual increase in installations, the reliability of early commercial units is outstanding and the cost is dropping. They have begun to ask whether fuel cells might fit into their future. While the fuel cell concept is simple, determining which type of fuel cell to consider may prove taxing. The multiplicity of fuel cells and their development programs, coupled with the amount of subject material and claims-versus-reality, may seem overwhelming. Fuel cell commercialization activities in North America are the focus of five manufacturers that are developing four types [fuel cells are typed by electrolyte: the 200°C phosphoric acid (PAFC); the 80°C proton exchange membrane (PEM); the 650°C molten carbonate (MCFC); and the 1,000°C solid oxide (SOFC) fuel cells]. Each fuel cell promises the attractive combination of fairly high efficiency and superior environmental performance compared to the presently available fossil-fueled electric generation technologies. As a result, fuel cells are particularly easy to site. There are additional advantages such as: excellent availability; electrical VAR control; quick ramp rate; remote/unattended operation; and redundancy when multiple units are installed. After earlier success in space, fuel cells are being applied to the commercial sector as on-site cogeneration units mostly fueled by natural gas. They are being considered for larger distributed generators (natural gas) and for vehicular power plants (methanol)     

Analysis of a Multiplicative Feed System for Monopulse Tracking Applications

In angle tracking antenna applications, the angle sensing boresight accuracy capabilities are important. The ability of an antenna to precisely determine the bearing angle to a point source is determined by the slope of the control function pattern at boresight. In the presence of extraneous interference, the magnitudes of the sidelobe and backlobe responses are important. Control pattern slope (angular sensitivity) is primarily a function of aperture illumination. It can be described by a current distribution in intensity and phase at every point. Once distribution is defined, lobe structure is defined by the associated transform. When more than one feed is used, the distribution will be a vector sum of the individual feed distributions. The resulting secondary pattern can be defined in terms of the amplitude distribution and the phase center locations of the contributing feeds. With a four-horn monopulse configuration, the feed phase centers are displaced from the boresight axis. Placing the phase centers on the boresight axis by rotating the feeds through 45 degrees results in a different set of intensity and phase values. A quite different secondary pattern results. The control function obtained by the subtraction of the powers from a paired set of on-axis feeds results in a lower sidelobe level than that obtained with a conventional monopulse combiner for a given feed taper.     

Waveform Detection and Classification with SAW Cepstrum Analysis

A prototype real time cepstrum analyzer incorporating surface acoustic wave (SAW), Fourier transform processors is reported. This system offers sophisticated wideband signal processing for radar, sonar, and communications applications. Practical results demonstrate its capabilities when analyzing bandwidths in excess of 10 MHz in a few microseconds with simulated pulsed RF waveforms in the presence of multipath echoes. Pulse duration, repetition interval, and binary code length are resolved and the potential to characterize unknown chirp waveforms is briefly reported.     

A Modification to Improve the Inverse Filter of Senmoto and Childers

An inverse filter, contained in a recently published paper by Senmoto and Childers, is improved by the application of a shaping function derived by Turin in an earlier paper.     

Characterization of High-Rate QPSK Channel with Adaptive Equalization

A computer simulation model capable of predicting the performance nce of a high-dta-rate end-to-end communication system with adaptive equalization is described in this paper. The model is used to characterize the behavior of NASA's proposed tracking and data relay satellite system (TDRSS), while accounting for signal distortion effects due to operational conditions and the required hardware are components necessary to fulfill mission objectives. Specifically, the performance of the TDRSS high-dta-rate lik (300 Mbps) is defined, both with and without adaptive equalization.     

Automatic Checkout and Monitoring in the AN TPQ-27 Radar System

This paper describes automatic monitoring equipment built in to a precision tracking radar system to detect and isolate faults. The purpose of the monitoring equipment is to minimize the mean time to repair faults and to exercise the overall system for pre-mission alignment and calibration. In addition, it is used to periodically check for performance degradation in key areas of the system. The paper describes the design approach used to meet the above requirements. Three types of signals are monitored: analog, digital, and switch closure. A list of each type is given and the technique used to monitor each is described. The design approach for pre-mission alignment and calibration is outlined. Tests performed are: RF alignment of the boresight axis, range tracking accuracy, angle servo calibration and accuracy, and receiver figure of merit measurements.     

Optimal Precession Control of Randomly Perturbed Spin-Stabilized Satellite

Dynamics of the satellite angular momenta is modeled by ordinary differential equations with additive white noise. A feedback controller er is desired to maintain tolerable satellite momenta and to economize ze the fuel consumption. The stochastic optimal control problem is transformed into an equivalent deterministic optimization problem involving a parabolic partial differential equation. Necessary optimality y conditions are used to develop a computation algorithm. Results regarding optimal controls, expected costs, and confinement probabilities in different noisy environments are presented.     

The Effect of Weighting upon Signal-to-Noise Ratio in Pulse Bursts

The Doppler sidelobes of a received pulse burst may be partially controlled by varying the amplitudes of the pulses in the burst or sequence upon transmission and/or reception. When there is a peak power limitation, weighting the amplitudes produces a loss in signal-to-noise ratio. A general expression is derived for the loss factor under the peak power limitation, and loss factor formulas are given for the following cases: Case A: Full nominal weights upon reception. Uniform weights on transmission. Case B: Square roots of the nominal weights on both transmission and reception. Case C: Full nominal weights on both transmission and reception. These cases are listed in order of increasing loss. Numerical results are tabulated for regular spacings and for the following nominal weights: 1) Dolph-Tchebycheff, 2) Taylor, 3) Hamming, and 4) Hann.