Radar target identification of aircraft using polarization-diversefeatures

The characteristics and target identification potential of a representation of the information from two polarization-diverse measurements of the radar backscatter of an unknown target are considered. The locus of these two polarization-diverse waveforms, termed the transient polarization response (TPR), has been shown to be closely related to the geometry of the scattering centers of the target. The polarization-related components of features derived from the TPR concur well with the shape and orientation of the major scattering centers distributed in the downrange profile of the object. This illustrates the intuitively appealing result that the polarization of the backscatter, as represented by the TPR and mapped onto the modified polarization chart, is determined by target geometry. It is shown that both polarization-related and amplitude-related features derived from the TPR are useful for target identification. By using distance measures that depend on various components, the elliptical parametrization information alone is sufficient to allow satisfactory target identification at SNRs of 0 dB and above. The significance of this result is that the absolute amplitudes of the received horizontally polarized and vertically polarized channels need not be known. However, if amplitude information is known, target identification percentages improve     

The solar-terrestrial event of 23 February 1956

The solar flare of 23 February 1956 and the resulting geophysical disturbance ranks as one of the most remarkable solar-terrestrial events of the twentieth century. It sparked many papers and has seldom been equalled. Fifty years after the International Geophysical Year, it seems timely to review the observations of the event from today’s perspective, and to draw on the recollections of scientists who were active at the time.     

Dynamic commutation voltages in a reciprocating linear inductoralternator

The waveform of the induced voltages in linear inductor alternators is quite different from that of rotary machines. From the study of the waveform, a dynamic commutation problem is identified. A general method for the analysis of this dynamic behavior is presented. Two techniques to handle this problem are discussed and illustrated by means of a simple model     

Gamma Rays From Molecular Clouds

High energy -rays from individual giant molecular clouds contain unique information about the hidden sites of acceleration of galactic cosmic rays, and provide a feasible method for study of propagation of cosmic rays in the galactic disk on scales 100 pc. I discuss the spectral features of ⁰-decay -radiation from clouds/targets located in proximity of relatively young proton accelerators, and speculate that such `accelerator+target systems in our Galaxy can be responsible for a subset of unidentified EGRET sources. Also, I argue that the recent observations of high energy -rays from the Orion complex contain evidence that the level of the `sea of galactic cosmic rays may differ significantly from the flux and the spectrum of local (directly detected) particles.     

Programmed-Amplitude Binary Signaling with Decision and Information Feedback

A sequential detection scheme with combined decision and information feedbacks, where the signal amplitude at each iteration may be properly programmed, is analyzed. This approach allows a remarkable gain in transmission rate and a significant reduction in the peak-to-average power ratio, relative to the nonsequential scheme with constant signal amplitude.     

Demodulator Structures for Pulse-Frequency-Modulated Signals

Minimum mean-square-error demodulators for pulse-frequency-modulated signals are considered. The optimum demodulator structure is found and a suboptimum one is proposed that may be implemented by standard circuitry.     

Effects of Automatic Gain Control on Phase-Locked Loop Behavior in the Presence of Interference

The behavior of a second-order phase-locked loop (PLL) in the presence of an interfering signal has been analyzed by different authors with different results. The discrepancy is attributed to the presence or absence of an automatic gain controlled amplifier preceeding the PLL. Simulation results verify that the presence of the automatic gain controlled amplifier reduces the tendency of the PLL to break lock from the desired signal in favor of the interfering signal     

Model-independent requirements for the source of positrons in the galactic centre

Model-independent requirements for the positron source in the galactic centre are formulated. From the known physical processes of positron production the most probable seems to be the e ⁺ – e ^– pair production as a result of photon-photon collisions. When certain conditions are satisfied, the efficiency of positron creation due to this mechanism can reach values 10%, which is comparable with the observed ratio of the annihilation line photon luminosity to the continuum one at E > 511 keV. Such a situation can be realized: (i) in a thermal pair-dominated mildly relativistic plasma, and (ii) on the development of a nonthermal electromagnetic cascade, initiated by relativistic particles in the field of ambient X-rays. Future gamma-ray observations at ultrahigh energies can be crucial to the choice of the model.