J.P. Donohoe

Comparison of finite difference time domain results for scattered EM fields: Yee algorithm vs. a characteristic based algorithm

Two finite-difference time-domain (FDTD) algorithms for computing scattered electromagnetic fields are compared. The Yee algorithm is compared to a characteristic-based algorithm using a simple two-dimensional geometry. The scattered fields of a conducting strip give an incident transverse magnetic (TM) short-duration Gaussian pulse are computed using both algorithms. The characteristics of the scattered fields are compared.<<ETX>>

Stationary and cyclostationary random process models

Cyclostationary random process modeling is an area of signal processing that has been the subject of numerous journal papers. W.A. Gardner (1988) devoted half of a book to cyclic spectral analysis. Cyclostationarity, however, has not received very much attention at regional conferences in the recent past, which implies that it is not being utilized by many practising engineers. Therefore, this paper reviews both stationary and cyclostationary random process models. It will be seen that cyclostationary models are more complete than stationary random process models for many manmade signals. Since these signals are best modeled as cyclostationary random processes, signal processors that exploit cyclostationarity can, in principle, have performance superior to traditional processors that utilize only stationary statistical models.<<ETX>>

Simple edge basis functions for open-ended bodies of revolution

A relatively simple and widely-used basis function in the method of moments solution for surface currents on a conducting scatterer is the piecewise linear function (triangle function). However, the triangle function does not exhibit the proper edge behavior for surface current and charge at a conducting knife-edge. The proper edge behavior is incorporated into the basis function set via a minor modification. The effect of the modified basis set on the computation of the axial electric field in the aperture of open-ended bodies of revolution is considered. Both straight and tapered cylinders are investigated. The convergence properties of the edge-modified triangle basis set are found to be superior to those of the normal triangle basis function set.

Matched FSK/PSK radar

In this paper, a novel radar signaling scheme designated as matched FSK/PSK signaling is introduced. An overall radar system which implements matched FSK/PSK signaling along with the appropriate processing scheme is given. Radars of this type are shown to be capable of generating FSK/PSK signals optimally matched to any arbitrary target response. The radar signals consist of bursts of pulses, whose frequencies are chosen by a random selection process with a probability density function based on the radar targets spectral content, and whose phases are a pseudo-random spreading sequence. The capability of the radar to match its illumination to a target could prove to be vital in tracking low observable targets. Additionally, the spectral agility of the matched FSK/PSK system allows for spectrally adaptive signaling. Adaptive signaling could not only aid target tracking, but also provide information about the target that could be used for target identification purposes.<<ETX>>

Simulation of target responses to high frequency ultra wideband radar signals using the physical optics impulse response

High-frequency ultrawideband radar signals are constructed by modulating a microwave or millimeter wave carrier with a broad bandwidth signal with the percentage bandwidth of the modulating signal being typically more than 20% of the carrier frequency. The wideband properties of the signal render the single-number radar cross section (RCS) inadequate for target response simulations. Instead, a characteristic function describing the response of the target to the waveform must be obtained in either the time or frequency domain. A first approximation to the temporal impulse response of a target can be obtained from physical optics (PO). The targets response can then be obtained by convolving the signal with the temporal impulse response of the target. A computational method for obtaining a PO signal response is described, and the results are compared to analytic responses for some primitive volumes (flat plate, cone, sphere).

Correlation properties of Gaussian FSK/PSK radar signals

Hybrid radar signals which are simultaneously frequency-shift-keyed (FSK) and phase-shift-keyed (PSK) are investigated with regard to their range correlation properties. The firing order of the discrete frequencies within a single burst is selected at random from a pseudo-Gaussian distribution. The range correlation properties of FSK and FSK/PSK signals are illustrated using computer simulations of the autocorrelation functions of several representative signals. It is observed that the range sidelobes of the autocorrelation function associated with a Gaussian FSK signal can be improved through the application of a pseudo-noise bipolar PSK sequence.<<ETX>>

Ambiguity function properties of frequency-hopped radar/sonar signals

Properties of the ambiguity function for frequency-hopped radar/sonar signals with firing orders based on Costas arrays are analyzed. The locations of range-Doppler coincidence values are obtained using ambiguity function plots of long-duration signals containing large numbers of cycles per subpulse. The general shape of the ambiguity function for short-duration signals containing small numbers of cycles per subpulse is deduced from that of the corresponding long-duration signal. Costas sequences with closely spaced coincidence locations are shown to produce slightly higher sidelobe levels than Costas sequences with more widely spaced coincidence distributions.<<ETX>>

A comparison of air breakdown criteria for slightly divergent fields

The commonly used air breakdown criteria for slightly divergent fields are compared. The various criteria can be classified into two distinct groups with regard to the basic breakdown mechanism employed. The Townsend (1915) breakdown mechanism is described by a succession of developing electron avalanches between the electrodes. The streamer mechanism of breakdown is based on a single electron avalanche and its inherent exponential growth. The primary assumption in the majority of these breakdown criteria is that the integral of the effective ionization coefficient /spl alpha/ over the critical avalanche length Z/sub 0/, is equal to a constant K. It is shown that the value of K is not a constant even for gaps with uniform fields. Of the six breakdown criteria considered, the Pedersen (1967) criterion is shown to have the most solid basis in fundamental physics.

The effect of radar signal construction on detectability

Three spread spectrum radar signals with similar power spectral densities are analyzed using cyclostationary signal processing techniques. It is shown that squaring these signals yields information about the carrier frequencies, chip rates and hop frequencies. It is also shown that, in an environment that contains additive white noise, increasing the number of frequencies used to construct the signal decreases the detectability of the sub-pulse frequency spectral lines.<<ETX>>

Simplified performance estimation of FSK/PSK hybrid signaling radar systems

FSK/PSK signaling radar systems make use of a pulse compression signaling scheme formed by a combination of two common modulation techniques: frequency shift keying (FSK) and phase shift keying (PSK). This technique of radar signaling can achieve high time-bandwidth products-the primary measure of radar signal processing gain. Since the ability of most radars to detect moving targets against a ground clutter background is often phase noise limited, if is feared that the phase noise of the oscillators required for synthesis of the signal component frequencies could result in a performance degradation that outweighs the performance improvement obtained from the complex signaling scheme. In this paper, the effects of phase noise on the digital Doppler processor of an FSK/PSK signaling radar system are analyzed in a very general fashion. These results can be applied to other types of radar systems that use other forms of pulse compression and also to systems that use no pulse compression. The analysis is based on a complete radar system stochastic process model and simplifying assumptions are made based on practical component characteristics. The paper concludes with a helpful example.<<ETX>>