Improved Range Resolution Research Articles

Improvement of range resolution by spectral extrapolation.

Under various simplifying assumptions, the reflected signal y(t) in the interrogation of a substance by an ultrasonic wave is a convolution of the transmitted signal x(t) with a function h(t) that is related to the reflection coefficient of the medium in the direction of propagation. The function h(t) can, in principle, be determined by deconvolution. However, since the band B of the spectrum X(ω) of x(t) is finite, the frequency components of h(t) outside B cannot be found reliably. In this paper, a method is presented for extrapolating X(ω) beyond B. The resulting increase in resolution is limited only by the level of noise. The method is particularly effective if h(t) is a sum of impulses.

Signal processing in acoustic imaging

A review of signal processing methods which can be used to improve the effectiveness of systems designed for acoustic imaging and bearing estimation is presented. Topics covered include a) signal processing for increased resolution, b) the processing of stochastic acoustic signals, c) image processing, enhancement, and pattern recognition. The discussion of resolution processing includes lateral resolution improvement by both superresolution techniques and aperture synthesis, and improvement of both range and Doppler resolution. The stochastic signal-processing section addresses adaptive processing, as well as methods of imaging in the case of incoherent, noisy signals.

Range-Doppler Resolution of Electromagnetic Walsh Waves in Radar

A number of papers have appeared during recent years about electromagnetic waves produced by antenna currents with the time variation of Walsh functions rather than that of sinusodial functions. Experimental transmitters and receivers for such waves have been developed. There are a variety of effects of Walsh waves that do not exist for sinusoidal waves or are so obscure that they are generally not recognized. This makes a discussion of these effects difficult for the nonspecialist. The range-Doppler resolution in radar lends itself more readily to a discussion, since Walsh waves bring in this case not a new effect, but only a substantial improvement of a well-known one. It is shown that our present technology should permit an improvement of about one order of magnitude in the Doppler resolution by using carriers derived from Walsh or other two-valued functions rather than sinusoidal functions. The theoretical improvement of the relative range resolution is substantially greater since it no longer depends upon the duration of the modulating signal but upon the period of the carrier and the switching time of the carrier. The absolute range measurement is not changed.

Lateral Resolution of a Broad-Band Dual-Element Ultrasonic Transducer

In the pulsed ultrasonic Doppler blood flowmeter, two types of resolution are of importance; these are the range resolution and the lateral resolution. A random signal correlation type of a pulsed ultrasonic system has been developed recently. In this system broad-band noise is used as the transmitted signal. Since the range resolution and the bandwidth of the transmitted signal are inversely related to each other, an improvement in range resolution is thus achieved. The lateral resolution of the system depends upon the dimensions of the ultrasonic transducer face. This may be improved by focusing the ultrasonic beam with lenses. However, this technique increases the complexity of the system and is associated with the risk of generating acoustic intensities harmful to living tissues. To obviate these difficulties, a broad-band dual-element ultrasonic transducer was developed. This paper describes the dual-element transducer and presents experimental results comparing the lateral resolution of the dual-element transducer with the existing single-element transducer. A tenfold improvement in the lateral resolution in the nearfield was achieved, while a fivefold improvement was achieved in the farfield.

A theorem for wide bandwidth echo-location systems

Echo-location systems are unavoidably subject to range-velocity ambiguities. Although the total amount (volume) of ambiguity increases with increasing signal bandwidth, it is shown here that ambiguity density decreases with increasing bandwidth. This result, which is stated in the paper as a theorem, means that for systems observing targets with restricted relative velocities, the effective volume of ambiguity decreases as the bandwidth is increased. It is suggested that this may help one to understand the performance of mammals with advanced echo-location senses, since increasing the bandwidth not only reduces effective ambiguity volume but also improves range resolution and reverberation suppression. For a given bandwidth the effective ambiguity volume is minimized if the energy spectrum of the echo-location signal is uniform throughout the band.

Beacon target processing for air traffic control

The beacon subsystem is one of many sources providing information to the air traffic control system. In automatic systems, various types of target information are derived from the beacon replies in a special purpose computer known generally as a processor. An important aspect of processor performance is its ability to detect targets in a noisy environment and to accurately determine the location of these targets. Among the important measures of target detector performance are the false alarm rate, the probability of detection, and azimuth accuracy. The intimate relationship between these three characteristics is described in this paper and specific examples are given using a sequential observer target detector. It is shown that increasing the range resolution of the system also benefits target false alarm rate and azimuth accuracy. Thus, as advances in technology permit improved range resolution, greater overall system performance can be expected. Nevertheless, with current technology, the azimuth accuracy which can be achieved with a statistical detector is better than the azimuth equivalent of one pulse repetition period even in a noisy environment.