Linear Frequency Modulation Pulse Research Articles

Using Triplet Arrays for Broadband Reverberation Analysis and Inversions

Some directional characteristics of observed clutter and reverberation are presented using line arrays with cardioid elements. It is shown that the cardioid arrays break the left/right ambiguity for reverberation sources above ~600-700 Hz. Data analysis uses a cardioid beamforming algorithm developed by researchers at the NATO Undersea Research Centre (NURC, La Spezia, Italy) but the normalization for this algorithm is derived by the author. It is shown that this normalized algorithm has an upper frequency limitation not previously noted. Reverberation data are taken from two recent experiments. One was the 2004 Boundary Characterization Experiment near the Malta Plateau. The area is rich in clutter objects like wrecks and mud-volcanoes and has some subbottom features that may be important. Sources were monostatic coherent pulses and impulsive sources. The receiver was the NURC cardioid array. The other experiment was the U.S. Office of Naval Research's 2003 Geoclutter effort to study shallow-water bottom reverberation and clutter in the STRATAFORM area off New Jersey. Sources were bistatic coherent pulses. The receiver was the five-octave research array (FORA). The STRATAFORM has benign surface morphology but strong clutter is observed. Some highlights of the reverberant returns from that area are discussed that include returns from probable fish schools. From these two data sets, it is shown that up to 15-20-dB left/right discrimination is attainable on broadside cardioid beams in the 1200-1750-Hz region for both linear frequency modulation (LFM) pulses and sound-underwater-source (SUS) broadband data. Also, a (previously developed) constrained simulated annealing (SA) inversion technique is applied to the directional reverberation data from the NURC array. A two-layer fluid model of the bottom is assumed. The algorithm is best at estimating compressional speeds, layer thickness, and attenuations in that order. Extracted bottom parameters at the site are compared with independent inversion results previously reported by Holland. Inverting reverberation from directional arrays is shown to give somewhat better quality results since, unlike standard line arrays, the reverberation data apply to only one bearing along the seafloor.

Linear Frequency-Modulated Pulse Compressor Based on a Three-Mirror Ring Cavity

A pulse compressor using a three-mirror cavity is tested at a frequency of 34.27 GHz and a low power level. The cavity is fed by a wave beam reflected from the corrugated mirror. The fourfold compression of a rectangular chirp is reached with an efficiency of ≈60%.

Detection of polyphase pulse compression waveforms using the radon-ambiguity transform

The recently developed Radon-ambiguity transform (RAT) detects unknown linear frequency modulated (LFM) signals by computing line integrals through the origin of the signal's ambiguity function (AF) magnitude. It is shown that this method also detects the step LFM and frequency-derived polyphase pulse compression waveforms with varying performance degradation. Simulations are provided to estimate the detection loss relative to the LFM.

Range, radial velocity, and acceleration MLE using radar LFM pulse train

An efficient implementation of the maximum likelihood estimator (MLE) is presented for the estimation of target range, radial velocity, and acceleration when the radar waveform consists of a wideband linear frequency modulated (LFM) pulse train. Analytic properties of the associated wideband ambiguity function are derived; in particular the ambiguity function, with acceleration set to zero, is derived in closed form. Convexity and symmetry properties of the ambiguity function over range, velocity, and acceleration are presented; these are useful for determining region and speed of convergence for recursive algorithms used to compute the MLE. In addition, the Cramer-Rao bound (CRB) is computed in closed form which shows that the velocity bound is decoupled from the corresponding bounds in range and acceleration. A fast MLE is then proposed which uses the Hough transform (HT) to initialize the MLE algorithm. Monte Carlo simulations show that the MLE attains the CRB for low to moderate signal-to-noise depending on the a priori estimates of range, velocity, and acceleration.

Pulse modulation and frequency effects on long-range sonar displays of fish

Early work on long-range sonar displays of fish is reviewed, for observations in the Perranporth Sea area. There is a concentration on the complications in interpretation arising from the use of a variety of pulse types and carrier frequencies. A useful technique was the interleaved transmission of different pulses or frequencies, permitting a comparison of virtually synoptic records. For linear frequency-modulation pulses there are dramatic changes in display appearance when pulse duration or pulse bandwidth are changed, which must be allowed for in the inversion giving fish distribution and behavior. There are further differences and problems with noise-modulation pulses; although these can perform well, they do often give very poor pictures. In contrast to the pulse-type effects there is a relatively small dependence of detailed appearance on carrier frequency for frequencies 1, 2, and 3 kHz; reflecting the slow dependence of fish school target strength on frequency. Of course system performance does depend on frequency as it does on the whole system engineering. In spite of the complications it is possible to find useful information on fish numbers, school numbers, movement, target depth, and behavior generally.

Time-frequency analysis of model based pulse scattering from a rough fluid-elastic interface

A null field T-matrix formalism for plane-wave scattering from a doubly infinite fluid–solid interface with doubly periodic surface roughness [Bishop and Smith, J. Acoust. Soc. Am. 94, 1560–1583 (1993)] is extended to include scattering of broadband acoustic pulses. The scattered pressure field is expressed in terms of a Fourier integral over the spectra of the incident pulse and the T matrix. Time series are calculated for the scatter of a Gaussian amplitude-modulated linear frequency-modulated pulse from a fluid–elastic interface with sinusoidal and triangular roughness. The time series are analyzed using short time Fourier, Wigner, wavelet packet, and local cosine T-F transforms. The results are displayed graphically in a T-F plot in which the T-F plane is covered by rectangular boxes of dimensions Δt×Δω. These results are used to evaluate the ability of each of the transforms to address inverse scattering and identification problems. It is shown that the T-F analysis provided by the wavelet packet may be used to distinguish between the scatter from surfaces of different roughness and is superior to the T-F analysis provided by both the short time Fourier and Wigner transforms.

Monostatic and bistatic reverberation results using linear frequency-modulated pulses

Estimates of reverberant scattering strength are presented as a function of range and bearing for selected monostatic and bistatic geometries of a reverberation experiment. Diffuse scattering strengths were found to be comparable (within ∼3 dB) for both monostatic and bistatic geometries. This experiment was performed off the west coast of Sardinia in approximately 2800 m of water during February of 1990. The receiver was a horizontal array of approximately 20 wavelengths. Source and receiver depths ranged from 50 to 120 m. The signals used consisted of linear frequency-modulated (FM) pulses centered near 1 kHz with durations ranging from 8 to 16 s and bandwidths ranging from 32 to 64 Hz. Monostatic and bistatic scattering strength values are estimated using uncorrelated (i.e., not match-filtered) pulses. The scattering strengths presented vary from −29 to −48 dB for ranges up to 40 km (but not data near fathometer returns). A method is described that shows different correlator gains against noise and reverberation. Comparisons of measured and modeled reverberation are presented using the generic sonar model (GSM) and curve fits with the decay in monostatic diffuse backscatter show good agreement with Lambert’s rule using a −32 dB constant. Finally, GSM is used to estimate grazing angles from which plots of measured scattering strength versus grazing angle in the 10°–26° interval are presented. Using this method, a Lambert’s rule constant of ∼−30 dB was a better fit to the data.

Variable domain transform and the detection of frequency modulation pulses

In this article a method is described for the detection of linear frequency modulation (FM) pulses with unknown frequency rate parameters in a noisy environment. The method, which is equivalent to the wavelet transform, enhances the signal towards noise. The technique is based on the variation of the domain after which a Fourier transform is applied, or for short called ‘‘variable domain transform.’’ The enhancement is in the order of pulse compression. The computational speed is of the order of that of the fast Fourier transform. The method is also applicable for other signal forms. A criterion is given for the type of signals that can be processed in that manner. An actual signal is analyzed and discussed.

Scattering of Linear FM Acoustic Pulses from Aluminum Spheres in Water

Experimentally recorded farfield echoes from linear frequency-modulated pulses incident on precision aluminum spheres in water are compared with computed echo forms. Good agreement was found using different pulse lengths and swept bandwidths. Spheres having ratios of internal to external radius of 0.95, 0.8, and 0.0 were studied, with either air or water in the hollow interior. Multiple periodic secondary echo components observed under some pulse conditions are interpreted in terms of circumferential waves and internal reflections.

The CLFM: A Method of Generating Linear Frequency-Coded Radar Pulses

A simple method of generating a coherent linear frequency-modulated pulse, suitable for use in a radar pulse-compression system, is described. The method, termed CLFM, employs a swept-frequency oscillator with closed-loop control of its phase. The phase-error voltages for the loop are obtained by sampling the waveform at regular intervals. The sampling technique itself is unconventional and avoids the need for large bandwidth circuitry. Results obtained with a demonstration model of the CLFM, generating a signal with a time-bandwidth product of 1000, are described.