Anti-aircraft artillery simulator

Air defense systems protect land and maritime resources from air attack. Depending on the regional characteristics and type of conflicting forces, air defense threats vary considerably. In regional conflicts, where forces with similar capabilities are involved and no air-superiority can be achieved, the role of air defense systems becomes critical. In combat terrains containing mountains (in mainland or in small islands), the man-operated or computer-controlled (using passive sensors) anti-aircraft artillery can be highly effective. The simulator presented in this work aims to exploit the capabilities provided by current commercial-off-the-shelf (COTS) communication and multimedia technologies for providing a training environment that improves the personnel capability for effective use of man-controlled anti-aircraft weapons.     

Cassini Radio Science

Cassini radio science investigations will be conducted both during the cruise (gravitational wave and conjunction experiments) and the Saturnian tour of the mission (atmospheric and ionospheric occultations, ring occultations, determinations of masses and gravity fields). New technologies in the construction of the instrument, which consists of a portion on-board the spacecraft and another portion on the ground, including the use of the Ka-band signal in addition to that of the S- and X-bands, open opportunities for important discoveries in each of the above scientific areas, due to increased accuracy, resolution, sensitivity, and dynamic range.This revised version was published online in July 2005 with a corrected cover date.     

X-rays from the first massive black holes

X-ray studies of high-redshift (z > 4) active galaxies have advanced substantially over the past few years, largely due to results from the new generation of X-ray observatories. As of this writing X-ray emission has been detected from nearly 60 high-redshift active galaxies. This paper reviews the observational results and their implications for models of the first massive black holes, and it discusses future prospects for the field.     

Theoretical Bounds on Random Errors in Satellite Doppler Navigation

Analytic expressions of the random errors in a satellite Doppler navigation system are developed. The errors in the along-track and across-track coordinates are expressed as functions of the geometry (range and angle), pass-length, and signal-to-noise ratio (SNR). Such an analytic result becomes possible due to an explicit solution to a simplified rectilinear model of the satellite pass over a flat Earth. It is shown that the explicit solution performance in the presence of noise is identical to that of the iterative solution, used in the real navigation problem.     

Optimum Linear Estimation of Stochastic Signals in the Presence of Multiplicative Noise

This paper considers optimum (MMSE) linear recursive estimation of stochastic signals in the presence of multiplicative noise in addition to measurement noise. Often problems associated with phenomena such as fading or reflection of the transmitted signal at an ionospheric layer, and also situations involving sampling, gating, or amplitude modulation, can be cast into such formulation. The different kinds of estimation problems treated include one-stage prediction, filtering, and smoothing. Algorithms are presented for discrete time as well as for continuous time estimation.     

Dynamic Modeling of Brushless dc Motors for Aerospace Actuation

A discrete time model for simulation of the dynamics of samarium cobalt-type permanent magnet brushless dc machines is presented. The similation model includes modeling of the interaction between these machines and their attached power conditioners. These are transistorized conditioner units. This model is part of an overall discrete-time analysis of the dynamic performance of electromechanical actuators, which was conducted as part of prototype development of such actuators studied and built for NASA-Johnson Space Center as a prospective alternative to hydraulic actuators presently used in shuttle orbiter applications. The resulting numerical simulations of the various machine and power conditioner current and voltage waveforms gave excellent correlation to the actual waveforms collected from actual hardware experimental testing. These results, numerical and experimental, are presented here for machine motoring, regeneration and dynamic braking modes. Application of the resulting model to the determination of machine current and torque profiles during closed-loop actuator operation were also analyzed and the results are given here. These results are given in light of an overall view of the actuator system components. The applicability of this method of analysis to design optimization and trouble-shooting in such prototype development is also discussed in light of the results at hand.     

Signal Reconstruction from the Output Phases and Magnitudes of Two-Dimensional Filters

Algorithms are presented for reconstruction of a two-dimensional complex signal by knowing the phases and magnitudes of different filtered versions of the signal for a special class of filters. The algorithms have simple geometric interpretation and are easily extended to N-dimensional systems.