FM Pulse Research Articles

Measurement and analysis of TS-Frequency relationship on mackerel tuna (Euthynnus affinis) using bandwidth frequency

Mackerel tuna (Euthynnus affinis) is a pelagic fish from Scombridae family and one of the commercially important species. The fish is mostly found in the water column from the surface down to about 50 m. Mackerel tuna spread widely in the tropical and subtropical waters, including the Indian Ocean, Pacific, Mediterranean Sea, and Black Sea. Since the level of exploitation of this fish resource is relatively high, it is critical to have an accurate assessment on the status of the stock. In this paper we report and discuss the measurement results of the acoustic target strength (TS) of Mackerel tuna using acoustic broadband technique. A broadband technique was used to better characterized the basic TS-Frequency relationship of the target, and hence less bias in biomass estimation. Target Strength measurement was performed using SIMRAD EK-80 scientific acoustic instrument with frequency bandwidth 160-240 kHz and sound pulse was transmitted to the target with 1 CW pulse and 8 FM pulse. The fish was tethered and positioned directly below the acoustic transducer. The results of the measurements are presented in terms of TS-Frequency. It is found that the TS-F relationship fluctuates, where the highest TS value -54.64 ± 4.64 dB is in 190-200 kHz frequency interval.

RESOLVING POWER OF MODIFIED IMAGE RECONSTRUCTION METHOD IN SPIN DENSITY PROJECTIONS IN MAGNETIC RESONANCE IMAGING

The well-known method of image reconstruction by projections in magnetic resonance imaging uses spin echo excited by two sinc pulses. The further processing involves forming of the spin echo signal two quadrature components converting them into a digital format and the subsequent Fourier transform. The proposed modified method is based on the second sinc radio pulse substitution for the linear FM pulse. In this case, the mentioned projections are formed by amplitude detection of the spin echo envelope, which significantly simplifies the processing procedure. The aim of the research is to analyze the modified method resolution. The mathematical model is based on Bloch equations. Their solution is carried out on the basis of the device of the spin system state transition matrices. For their calculation, the stepped approximation of the excitation pulse complex envelopes is used. It makes possible to convert the system of linear differential equations with variable coefficients (Bloch equations) to the system of linear differential equations with piecewise constant coefficients. In this case, the equations have analytical solution. Following the obtained solution, the analysis of the method resolution not previously investigated, is performed by means of modeling the spin echo excitation. The conditions are specified when no dynamic distortions influencing received image quality exist. It is shown that resolution is determined by the size of the scan area, the magnitude of the gradient of the applied magnetic field, the pulse duration with linear frequency modulation, as well as gyromagnetic ratio of the core type used. The developed method eliminates the need for Fourier transform over the spin echo signal and pro-vides resolution comparable to the conventional one.

Biosonar interpulse intervals and pulse-echo ambiguity in four species of echolocating bats.

In complex biosonar scenes, the delay of echoes represents the spatial distribution of objects in depth. To avoid overlap of echo streams from successive broadcasts, individual echolocation sounds should only be emitted after all echoes of previous sounds have returned. However, close proximity of obstacles demands rapid pulse updates for steering to avoid collisions, which often means emitting a new sound before all of the previous echoes have returned. When two echo streams overlap, there is ambiguity about assigning echoes to the corresponding broadcasts. In laboratory tests of flight in dense, cluttered scenes, four species of echolocating bats exhibited different patterns of pulse emissions to accommodate potential pulse-echo ambiguity. Miniopterus fuliginosus emitted individual FM pulses only after all echoes of previous pulses had returned, with no alternating between long and short intervals. Pipistrellus abramus and Eptesicus fuscus alternated between emitting long FM pulse intervals to receive all echoes before the next pulse, and short intervals to update the rapidly changing scene while accepting partial overlap of successive echo streams. Rhinolophus ferrumequinum nippon transmitted CF/FM pulses in alternating short and long intervals, usually two to four closely spaced sounds that produced overlapping echo streams, followed by a longer interval that separated echo streams. Rhinolophus f. nippon is a statistical outlier from the three FM species, which are more similar to each other. The repeated overlap of CF/FM echo streams suggests that CF components have a distinct role in rejection of clutter and mitigation of ambiguity.

Investigation into the structure of spectra of signals with linear frequency modulatio

Analytical expressions for the spectrum and the uncertainty function for composite non-uniform signals with FM pulse width (SNLFM-FM signals) are obtained. The technique and results of studies of the SNLFM - FM signals spectral, correlation and structural properties are presented in the manipulation of the phase by various classes of binary discrete sequences.

Regional seabed backscatter mapping using multiple frequencies

The frequency dependence of seabed backscatter has previously been assessed at static sites. While dependencies have been identified, the restricted range of seabed types and issues of absolute calibration have limited inferences. Until recently seabed backscatter measurements from underway mapping sonars (sidescans and multibeams) have predominantly been at a single center frequency, dictated by the range versus resolution compromise best suited for the sonar altitude. With improved range performance using FM pulses, the depth range over which a specific frequency is usable have expanded. Taking advantage of that, two national seafloor mapping programs have switched to routine collection of seabed backscatter with wavelength differences of almost an order of magnitude. The NOAA ship Thomas Jefferson and the Irish government vessel Celtic Explorer now are acquiring data at 45 and 300 kHz and 30 and 200 kHz, respectively. Even though absolute calibration remains a concern (particularly for multi-sector sys...

Interview

Prof. Massimiliano Pieraccini from the University of Florence, Italy, talks to Electronics Letters about the applications of the work behind the paper ‘Interferometric RotoSAR’, page 1451. Prof. Massimiliano Pieraccini I have worked for about fifteen years in the field of radar technology, in particular with ground-based synthetic aperture radar (GB-SAR) and ground penetrating radar (GPR). My research group has designed, built up and tested several prototypes that have become industrial products manufactured by prominent companies. SAR is a powerful imaging technique originally developed for satellite and airborne platforms. Its main advantage in ground-based installation is that it allows the design of radar units with smaller antenna systems: half the size, when compared to conventional equipment. This is an essential feature for designing portable monitoring instrumentation. We have reported the first test of a new interferometric GB-SAR that I have named RotoSAR, as it exploits the movement of a rotating arm to synthesise an aperture size twice its diameter. It is potentially faster than any previous similar radar, and furthermore, it is able to detect the three components of the target's displacements in the field of view. This is an absolute novelty. Previous GBSAR could do that only by installing two or more sets of equipment, with the critical problem that radar image never is exactly the same from different points of view, because of interference and diffraction effects, and this has prevented evaluation of more then one component in the same physical point of the structure under test. An important limit, that until now has prevented a wider use of these radar in engineering practice, is the issue of the civil engineer needing to have not only a single displacement component of the structure under test, but at least two and possibly all three components. This radar responds to this issue. The “killer application” of GB-SAR is as safety equipment in large open quarries. Several hundred are currently operating worldwide with a consolidated market of millions of euros. The aim of these devices is to monitor the small displacements of the quarry slopes, that can be precursors of dangerous collapses. This new radar, faster then previous ones and with a mechanical design that is less stressful on the moving parts, could have a tremendous potential in this field of application. More widely, there are many landslides, that need to be monitored and GB-SAR are frequently used with this aim. Also in this field of application the RotoSAR offers important advantages: greater acquisition speed means more coherent interferometric images, therefore with better capability to detect ground deformations. Currently, we are planning an extensive measurement campaign for in-field testing of this new technology. In particular we would like to evaluate its performances in the monitoring of bridges and large buildings, such as dams. Furthermore, we are engaged in the design of a more advanced prototype able to test the highest possible acquisition speed, that seems to depend more on the mechanical performances than on electronic limits. The idea is to integrate a CWFM radar operating sweeps of 10ms with mechanical equipment designed to provide very fast rotations. It should be able to acquire a complete synthetic image in less then six seconds. A FM pulse radar could further increase the acquisition speed. Another development that we are planning to assess is the possibility of realising DEM (digital elevation model) using the data acquired by two half arcs during a single rotation. The very large baseline could cause difficult problems in data processing, but it could be worth working on it. In ten years I imagine SAR based on this principle operating at very high frequency (e.g. 77 GHz). They could have a diameter of only 20–30 cm and so they could be really portable systems.

Performance of PCM/FM telemetry system based on multi-symbol detection and Turbo product code

Multi-Symbol Detection(MSD) and Turbo Product Code(TPC) can greatly improve the performance of PCM /FM(Pulse Code Modulation / Frequency Modulation) telemetry system. To solve the high computational complexity issues in MSD algorithm,an improved algorithm which reduced the computational complexity of MSD was proposed. Chase decoding algorithm for TPC also reduced the system memories by simplifying the calculation of the soft input information. The simulation results show that despite of 1. 7 dB loss,the improved algorithm still obtains about 8 dB performance gain. Because of lowcomplexity and low system memories,it is more suitable for hardware implementation.

Use of Bessel side lobe modulation of frequency modulated pulse bearing estimation

The beam pattern of a circular transducer is a set of frequency dependent Bessel functions. By using a broad band FM pulse and a 1–3 composite transducer, it appears that the modulation in amplitude and phase due to a reception of the high frequency components can be used to estimate the echo's angle off the transducer axis. The 1–3 composite transducer has a simple amplitude and phase structure.

Broadband synthetic aperture chirp reflection profiling

A newly developed towed chirp subbottom profiler transmits FM pulses with a bandwidth of three octaves to generate high resolution reflection profiles of the seabed. The broad bandwidth of the pulses, generated with two arrays of piston sources, produces the temporal resolution needed for resolving fine sediment layering. A 40 channel horizontal hydrophone array, embedded in vehicle wings, provides the acoustic aperture for enhancing the across track resolution of subsurface features and reducing sediment scattering noise. After application of a matched filter, synthetic aperture processing of hydrophone data generates a large aperture along the track of the sonar vehicle thereby improving along track image resolution and obtaining a further reduction in scattering noise. The reductions in backscattering from sediments yield imagery with improved subsurface penetration. The reflection profiles are stacked envelopes of coherently summed data formed by time domain focusing on planar surfaces oriented over a...

Frequency shifts of echolocation pulses in a cluttered environment: Comparison between Pipistrellus abramus and Miniopterus fuliginosus.

Changes in frequency range (ΔF) of FM echolocation pulses were recorded with an onboard wireless microphone (Telemike) for Pipistrellus abramus and Miniopterus fuliginosus in a cluttered environment created by dense chain‐row obstacles. The duration of echo streams (ESD) from the chains reached ∼40 ms when the bats were flying toward the chain‐row obstacles. P. abramus emitted pulses in pairs (strobe group) with 20–40 ms interpulse intervals (IPIs), which occasionally created pulse‐echo ambiguity (IPI was shorter than ESD). When successive echo streams did not overlap, ΔF within strobe groups was less than 2–3 kHz. When overlap occurred, Δ F increased to 5–6 kHz (F‐test, P < 0.001). By shifting frequency range of successive FM pulses, P. abramus appeared to assign echoes to the corresponding pulses and avoid pulse‐echo ambiguity. Similar finding was previously reported in Eptesicus fuscus. On the other hand, M. fuliginosus never emitted pulses in a strobe group, and IPIs were always adjusted to be longer than ESDs. Thus, the amount of ΔF was less than 2 kHz, which corresponded to that in P. abramus without pulse‐echo ambiguity. These comparative results suggest that echolocation strategy in the cluttered environment is different between two FM bat species. [Work supported by JSPS and ONR.]

Modeling of acoustic reverberation from a diurnally migrating fish aggregation with depth-dependent resonance.

The depth dependence of the resonance frequency of fish with swimbladders is affected by both the change in volume (physostomes only) and pressure of the swimbladder (physostomes and physoclists). Due to these physical processes, the migration of fish upward or downward the water column can cause significant variation in the magnitude of the echoes received by an active sonar, most notably in narrowband applications where the resonance frequency may pass through the sonar band during migration. This variation in received echo level is further complicated by waveguide effects in long-range acoustic scenarios. The sonar simulator toolset, a ray-based sonar simulator, is used to simulate the reverberation of a narrowband, FM pulse by a vertically migrating aggregation of fish over a range of conditions. We examine the results of varying range, depth, and other waveguide parameters on the matched filter output. The predictions distinguish the effect of swimbladder resonance from acoustic waveguide effects on the scattering. A key result illustrated is a 28-dB variation in the received echo due to the resonance frequency passing through the band of a 3-kHz sonar during diurnal migration of the Atlantic herring.

Echolocation signals of a beaked whale at Palmyra Atoll

Acoustic recordings from Palmyra Atoll, northern Line Islands, central Pacific, showed upsweep frequency modulated pulses reminiscent of those produced by beaked whales. These signals had higher frequencies, broader bandwidths, longer pulse durations and shorter inter-pulse intervals than previously described pulses of Blainville's, Cuvier's and Gervais' beaked whales [Zimmer et al. (2005). J. Acoust. Soc. Am. 117, 3919-3927; Johnson et al. (2006). J. Exp. Biol. 209, 5038-5050; Gillespie et al. (2009). J. Acoust. Soc. Am. 125, 3428-3433]. They were distinctly different temporally and spectrally from the unknown beaked whale at Cross Seamount, HI [McDonald et al. (2009). J. Acoust. Soc. Am. 125, 624-627]. Genetics on beaked whale specimens found at Palmyra Atoll suggest the presence of a poorly known beaked whale species. Mesoplodon sp. might be the source of the FM pulses described in this paper. The Palmyra Atoll FM pulse peak frequency was at 44 kHz with a -10 dB bandwidth of 26 kHz. Mean pulse duration was 355 mus and inter-pulse interval was 225 ms, with a bimodal distribution. Buzz sequences were detected with inter-pulse intervals below 20 ms and unmodulated spectra, with about 20 dB lower amplitude than prior FM pulses. These clicks had a 39 kHz bandwidth (-10 dB), peak frequency at 37 kHz, click duration 155 mus, and inter-click interval between 4 and 10 ms.

Detection of spatial cues in linear and logarithmic frequency-modulated sweeps

Frequency- and amplitude-modulated (FM and AM) sounds are the building blocks of complex sounds. In the present study, we investigated the ability of human observers to process spatial information in an important class of FM sounds: broadband directional sweeps common in natural communication signals such as speech. The stimuli consisted of linear or logarithmic unidirectional FM pulses that swept either up or down in frequency at various rates. Spatial localization thresholds monotonically improved as sweep duration decreased and as sweep rate increased, but no difference in performance was observed between logarithmic and linear or between up- and down-frequency sweeps. Counterintuitive reversals in localization were observed which suggested that the localization of high-frequency sweeps may be strongly dominated by amplitude information even in situations in which one might consider timing cues to be critical. Implications of these findings for the localization of complex sounds are discussed.

Estimating bubble density from attenuation measurements through an underwater explosion.

Underwater explosions have been studied intensively in the United States since 1941 [e.g., Cole (1945)]. Research to date primarily focuses on the initial shock and subsequent pressure waves caused by the oscillations of a “gas globe” that is the result of a charge detonation. These phenomena have relatively short timescales (typically less than 2 s). However, as the gas globe rises in the water column and breaks the surface, it leaves behind a residual bubble cloud which has been markedly less studied. A recent experiment measured the spatial and time‐dependent acoustic response of the bubble cloud resulting from a charge detonated at 50‐ft depth. A directional projector was used to propagate a linear FM (5–65‐kHz) pulse through the bubble cloud to an array of hydrophones placed on the opposite side of the charge in order to measure attenuation. This talk will focus on the methods used to estimate bubble density size spectra from the attenuation measurements, those of Commander and McDonald (1991), Carut...

Spin‐echo MRI using π/2 and π hyperbolic secant pulses

AbstractFrequency‐modulated (FM) pulses have practical advantages for spin‐echo experiments, such as the ability to produce a broadband π rotation, with an inhomogeneous radiofrequency (RF) field. However, such use leads to a nonlinear phase of the transverse magnetization, which is why FM pulses like the hyperbolic secant (HS) pulse are not commonly used for multislice spin‐echo magnetic resonance imaging (MRI). Here, a general theory and methods are described for conventional spin‐echo imaging using a π HS pulse for refocusing. Phase profiles produced by the HS pulse are analytically described. The analysis is extended to yield the specific relationships between pulse parameters and gradients, which must be satisfied to compensate the nonlinear phase variation produced with a spin‐echo sequence composed of π/2 and π HS pulses (the π/2 HS − π HS sequence). The latter offers advantages for multislice spin‐echo MRI, including excellent slice‐selection and partial compensation for RF inhomogeneity. Furthermore, it can be implemented with a shorter echo time and lower power deposition than a previously described method using a pair of π HS pulses. Magn Reson Med 61:175–187, 2009. © 2008 Wiley‐Liss, Inc.

Frequency overrepresentation found in the inferior colliculus of Japanese house bats, Pipistrellus abramus.

In this study, we examined the best frequencies of the inferior collicular neurons of Japanese house bats, Pipistrellus abramus. The bats emit long (>10 ms) and shallow FM pulses (quasi‐CF) during their search phase, narrowing the frequency range at around 40 kHz. If the narrowed frequency range is important for detecting a small frequency change or frequency modulation, the bats may need a finer frequency resolution than other frequency ranges. So, the neurons responsive to the frequency range at around 40 kHz must be over‐represented somewhere in the auditory system, typically in the inferior colliculus. In order to examine this hypothesis, the tonotopic organization of the inferior colliculus of P. abramus was measured electrophysiologically by metal microelectrodes. Results show that the frequency range corresponding to their quasi‐CF frequency, at around 40 kHz, appears to be over‐represented in the inferior colliculus. [Research supported by an ONR grant.]

Saclant ASW Research Centre contributions to underwater acoustics during the first sixteen years of its existence: Personal records

In less than one year the NATO Undersea Research Centre, previously called Saclant ASW Research Centre, will celebrate its 50th anniversary. I had the great privilege to be one of the first scientists joining the Centre in 1959 and to work there until 1975. The main purpose of the research conducted in underwater acoustics was to reach a physical understanding of the different processes of sound propagation, including multipath effects, transformations introduced by a multilayered bottom and random scattering by the volume inhomogeneities and by the surface and bottom roughness. Experiments at sea first conducted from the research ships Aragonese and then Maria Paolina used explosives charges and active sonars FM pulses as sound sources. The digital analysis equipment, which had been designed to record, process and facilitate the interpretation of the received signals, had no equivalent at the time. The Centre has been a very active platform of intercommunications for most of the civilian and military organizations, which were involved in this discipline. The result of this continuous exchange of ideas has facilitated the harmonization of the research programs in the different NATO countries and has greatly contributed to the creation of a large international research community in underwater acoustics.

Simplified Homodyne Detection for Linear FM Lidar

ABSTRACTA fiber based lidar system is developed which simplifies the processing of linear FM pulses by using a modulated local oscillator power in the coherent detector. Experiments were conducted on lidar systems with direct, heterodyne and simplified homodyne detection to compare receiver sensitivity. A field experiment using the homodyne system verified the sensitivity estimation on a building target at 370-m range.

Generating simulated reverberation using noiselets

Of the approaches to generating stochastic realizations of reverberation time series, the simple point scatterer model (PSM) is appealing because the statistics evolve naturally as the number of scatterers increases. Unfortunately, the number of eigenrays required is often overwhelming, especially for broadband signals. The more common ‘‘variance factorization model’’ (VFM) uses far fewer eigenrays to compute a time‐dependent estimate of the variance of the reverberation, and applies the spectral factorization theorem to generate a sequence of short‐time spectra, which is inverse transformed and concatenated to form the desired time series. However, the variance and factorization steps scale with the square and the cube, respectively, of the number of receiver channels, so the advantage is lost for element‐level simulations of complex receivers. The ‘‘noiselet model’’ is conceptually similar to the PSM, but the transmit pulse is replaced by an ensemble of precomputed ‘‘noiselets,’’ which are sums of randomly weighted and delayed copies of the original transmit pulse. The number of eigenrays is thus comparable to the VFM, but scaling is linear in the number of channels, rendering this method tractable for real‐time broadband element‐level simulation. Examples are shown for a linear FM pulse at 10 kHz and 400‐Hz bandwidth.

Broadband shallow-water reverberation statistics

Utilizing a wideband transmitting/receiving system, at-sea experimental results for broadband reverberation have been acquired for both harsh as well as moderate ocean acoustic environments. The principal contributor to the backscattered reverberation is bottom features at the shallow-water ocean sites. Replica correlation (so-called match filter) processing of linear FM (LFM) pulse codes is employed to study the degree of reverberation coherence, as measured by the replica correlation coefficient. The database covers several octaves of bandwidth. The corresponding reverberation coherence is computed in terms of the variance of the cross correlation coefficient (cc) as a function of time–bandwidth product. Of particular interest is the possible departure from Rayleigh statistics as signal bandwidth increases. Several bandwidths are compared, using the Kolmogoroff–Smirnof test as a measure of departure from Rayleigh. [This work was sponsored by A. Nucci, ONR 333.]