LFM Chirp Research Articles

Experimental characterization of the mobile underwater acoustic communication channel in Shallow Water

This paper presents the results for the experimental characterization of the mobile underwater acoustic communication channel in shallow water through the statistics of the fading and the time and frequency dispersion parameters. Starting from measurements performed in Forno Bay, Arraial do Cabo, Rio de Janeiro, Brazil, using three hydrophones in the reception side, two CW signals (3 and 6 kHz) and three sequences of LFM chirps in different bands: 2 to 4 kHz, 5 to 7 kHz and 1 to 8 kHz, the mobile sounding was performed in that channel in order to characterize it as in narrowband as in wideband.

Application of linear frequency modulated laser ultrasonic radar in reflective thickness and defect non-destructive testing

Radar technology plays an important role in modern aviation and navigation. Radar systems use pulse-compression and match-filtering to detect large moving objects in the sky or in water, or small defects hidden inside industrial components, the object of this paper. We introduce a nondestructive testing (NDT) modality based on frequency-domain laser ultrasound (FDLU) by means of implementing radar principles: Linear-frequency-modulated (LFM chirp) excitation CW laser-beam intensity to perform, ultrasonic signal cross-correlation with the reference signal using pulse-compression and match-filtering, leading to reconstruction of time-domain sequences through inverse Fourier transformation at acceptable signal-to-noise ratios. Theoretically, the laser ultrasound radar (LUR) signal was modelled with both one- and three-dimensional thermoelastic equations, a combination of which was used to simultaneously predict the correct location and relative amplitude of experimental targets with relatively simple mathematical expressions that could not be used either in the 1-D (too simplistic) or in the 3-D (too complicated) approach alone. This methodology was further used to detect buried defects inside a metal alloy. The results demonstrated that the LUR system is capable of determining the thickness of the alloy material and quantitatively estimate the subsurface depth of defects in a signal generation process akin to echolocation.

Computer simulation of synthetic aperture sonar or radar for classroom demonstration: Part II

In our presentation, a Mathematica® simulation of Synthetic Aperture Acoustic Radar will demonstrate how point-like targets placed on a smooth surface can be imaged from a collection of acoustic echoes. The transmitter and receiver are collocated and modeled as a single element that stops along a linear track at collection points and hops to the next location (stop and hop approximation). The transmitter on the hypothetical apparatus will transmit a linear frequency modulated LFM chirp pulse that reflect off the targets and are recorded as echoes by the receiver at discrete locations along the track. A matched filter correlation process will perform a pulse compression of the LFM chirp. A series of still frames from the animated simulation will be displayed. These frames show the echoes arriving at different times from the targets in conjunction with a receiver wave recorder screen. The time correlation backprojection algorithm is motivated when pulse compressed arrivals are displayed (at various track locations) including the echo arrival of a hypothetical point target adjusted in the x,y plane on or near one of the actual point targets. [See Yegulap, “Fast backprojection algorithm for SAR,“ in Proc. 1999 IEEE Radar Conf., Waltham, MA, April 20–22, 1999, 60–65].

Investigation of Non-linear Chirp Coding for Improved Second Harmonic Pulse Compression

Non-linear frequency-modulated (NLFM) chirp coding was investigated to improve the pulse compression of the second harmonic chirp signal by reducing the range side lobe level. The problem of spectral overlap between the fundamental component and second harmonic component (SHC) was also investigated. Therefore, two methods were proposed: method I for the non-overlap condition and method II with the pulse inversion technique for the overlap harmonic condition. In both methods, the performance of the NLFM chirp was compared with that of the reference LFM chirp signals. Experiments were performed using a 2.25 MHz transducer mounted coaxially at a distance of 5 cm with a 1 mm hydrophone in a water tank, and the peak negative pressure of 300 kPa was set at the receiver. Both simulations and experimental results revealed that the peak side lobe level (PSL) of the compressed SHC of the NLFM chirp was improved by at least 13 dB in method I and 5 dB in method II when compared with the PSL of LFM chirps. Similarly, the integrated side lobe level (ISL) of the compressed SHC of the NLFM chirp was improved by at least 8 dB when compared with the ISL of LFM chirps. In both methods, the axial main lobe width of the compressed NLFM chirp was comparable to that of the LFM signals. The signal-to-noise ratio of the SHC of NLFM was improved by as much as 0.8 dB, when compared with the SHC of the LFM signal having the same energy level. The results also revealed the robustness of the NLFM chirp under a frequency-dependent attenuation of 0.5 dB/cm·MHz up to a penetration depth of 5 cm and a Doppler shift up to 12 kHz.

On Time Compression Overlap-Add Technique in Linear Frequency Modulation Pulse Compression Radar Systems: Design and Performance Evaluation

This paper introduces a novel approach to incorporate the time compression overlap-add (TC-OLA) technique used in communication systems into linear frequency modulation pulse compression (LFM-PC) radar systems. This technique significantly boosts the signal-to-noise ratio (SNR) and provides a robust processing gain compared with the traditional radar LFM-PC systems. In addition, TC-OLA provides a better immunity against powerful jamming techniques. At the transmitter side, we divide a digitized LFM chirp signal into a controlled number of overlapping segments. We then speed up each segment by increasing the sampling rate to account for the segment overlap. At the receiver side, we apply OLA to reconstruct the signal with a much higher gain. To simulate and evaluate the performance of the new system, we extend the conventional LFM-PC radar model, which includes matched filter (MF) processor, moving target detector (MTD), and two common constant false alarm rate (CFAR) algorithms, by suitably adding TC and OLA blocks at the transmitter and receiver, respectively. Using the TC-OLA-based LFM radar system, we have control over the SNR level and the spectrum spread while preserving the same Doppler shift and target time delay as the conventional LFM radar system. Furthermore, we transform LFM chirp signal into a novel TC signal that inherits LFM properties while possessing better immunity to jamming. Moreover, the proposed radar model relies on high sample rates only when needed and, therefore, does not require changing MF, MTD, and CFAR as is the case for a wideband LFM radar with the same processing gain. Detailed comparisons between the conventional LFM and the wideband LFM radars against the proposed model are also presented.

Estimation of cylinder orientation using autonomous underwater vehicle mapping of bistatic scattered fields

When an aspect-dependent target is insonified by an acoustic source, distinct features are produced in the resulting bistatic scattered field. These features were exploited in a process for estimating target aspect angles that was demonstrated on a real-world data set using models produced using simulation data. Bistatic scattering data was collected during an experiment in November 2014 in Massachusetts Bay using a ship-based acoustic source producing 7-9kHz LFM chirps and a steel pipe target. The true target orientation was unknown, as the target was dropped from the ship with no rotation control. The Autonomous Underwater Vehicle (AUV) Unicorn, fitted with a 16-element nose array and data acquisition payload, was deployed in broadside data collection behaviors around the target, and the ship was moved to create two target aspects. Scattering data was collected for each target aspect angle. A Support Vector Machine (SVM) regression model was trained using simulated scattering bistatic field data from the OASES-SCATT simulation package. This model was used to estimate the target aspect angles using the real data collected by the AUV during the experiment. The aspect angle estimates were consistent with experimental observations of relative source positioning based on ship position.

Time reversal of modal arrivals for broadband signals in horizontally stratified shallow water due to internal waves

Shallow water waveguides in the presence of internal waves can make a time-varying, horizontally stratified medium that can significantly affect broadband pulse propagation. In this paper, we analyze a unique data set obtained during the passage of an internal wave event in a region where a broadband sound source (270 to 330 Hz) was received by an L-shaped hydrophone array about 20 km away in 80 meters on the New Jersey continental shelf. During the time 20:30 and 22:07 GMT, on 17 August 2006, an approaching IW affected the dispersion characteristic of the broadband LFM chirp signal while passing an acoustic track in shallow water waveguide. Modal behavior is examined before and after the internal wave front crossed the source-receiver track. While dispersion characteristics of the signals changed, modal arrival time reversal occurred. The corresponding group and phase velocities that signals experienced during this phenomenon are analyzed using the theory of horizontal rays and vertical modes. These results have motivated theoretical and modeling studies of the waveguide behavior since it was first reported in 2010. [Work supported by ONR322OA.]

Broadband synthetic aperture matched field geoacoustic inversion with a single hydrophone

Traditionally matched-field geoacoustic inversion experiments sampled the acoustic field on long arrays and require powerful transmissions in order to reduce parameter uncertainty. However, single-hydrophone based geoacoustic inversion methods exist. Practically, these methods are attractive compared to the ones using long arrays. In a single hydrophone setup, spatial diversity is traded off for frequency diversity; the source is broadband. This paper uses single-hydrophone frequency coherent matched-field inversion and exploits extended-duration coherent transmissions (multiple LFM chirps) to increase signal to noise ratio. As a result, the overall signal becomes Doppler/motion intolerant. But Doppler can be modeled by including source and hydrophone horizontal motions. To correlate well with the measured field across a receiver trajectory and to incorporate a transmission across source trajectory, Doppler in waveguide and normal mode theory are applied. The method is demonstrated with 100-900 Hz LFM SW06 data with low signal to noise ratio.

Horizontal interference structure of the sound field in the presence of moving internal waves and estimation of angle of horizontal refraction.

The experimental results of Shallow Water 2006 experiment (SW06) and theoretical estimations of the key parameters are presented for observation of non-stationary interference pattern provided by “direct” and reflected acoustic signals (acoustic Lloyd mirror effect) in shallow water. Propagation of low-frequency LFM chirp signals with the center frequency of 300 Hz and the bandwidth of 30 Hz on an acoustic track of about 20 km was studied. Refraction in the horizontal plane is caused by the wave front of the nonlinear internal waves (NIWs) moving across the acoustic track. Analysis of the non-stationary interference pattern on the horizontal and vertical line arrays and theoretical estimations give the angle of horizontal refraction (angle between direct and reflected rays) and the velocity of NIW. Results of estimations are in a good agreement with experimental data. [Work supported by ONR.]

Methods of enhancing the compression of short LFM signals

New digital methods have been presented that provide for enhancing the compression of short LFM chirps under conditions of the matched filtration in the time domain. It was shown that by selecting parameters of a short LFM signal and the pulse response characteristic of the matched filter for the window used and utilizing nonlinear convolution operations, the ratio of the main lobe of output signal to the side lobes can be significantly enhanced with the simultaneous reduction of the main lobe width to one sample.

Influence of small-scale turbulent fluctuations on direct-path coherence from 4 to 20 kHz

A numerical study is presented with results from a sensitivity study of wave front spatial coherence. Specifically, the influence of small-scale (order of tens of meters to tens of centimeters) water-column turbulence on vertical coherence of a direct path, LFM chirp at a source–receiver separation of 500 m is examined. Parameters chosen as variable included turbulence strength, spectral cut-off scale (and combined rms value), source depth, receiver sub-array depths, and background sound speed structure. Preliminary analysis suggests that the background profile may play a more significant role than originally anticipated. The generality of this is considered in the context of measures of profile ‘‘smoothness’’ and ‘‘curvature,’’ as previously quantified by others. Potential implications include the use of background profile knowledge to arrange source–receiver geometry for optimal signal reception when coherent processing is advantageous (e.g., comms, active detection, etc.). The model results also show that the Fresnel scale of the turbulent pertubations may dominate the coherence structure, as predicted by theory. The impact of turbulence strength and frequency appear to behave as expected. [Work supported by ONR.]

Super-resolution algorithm for instantaneous ISARimaging

The super resolution method is investigated for use in instantaneous inverse synthetic aperture radar (ISAR) imaging. The time-varying Doppler spectrum is modelled as that of superimposed LFM chirp signals in a short time interval. A parametric relaxation-based algorithm is proposed for estimating the parameters of the signals. The effectiveness of the algorithm is verified by applying it to ISAR imaging.

Modeling deterministic features in the ARSRP acoustic data

A preliminary study of the match-filtered beam data for the 5-s LFM chirps acquired on the Acoustic Reverberation SRP’93 acoustics cruise has been carried out. The analysis is based on a few selected segments which insonify site A at a 1/2CZ. The deterministic ‘‘bright spot’’ identified 2 years ago in the ’91 data set appears in the ’93 data. Although the characteristics of the event differ because of the different insonification parameters, the event appears in the same location when viewed from a number of azimthus and ranges. Other events within site A are robust when viewed from different ranges along the same azimuth, but disappear with changes in azimuth. The observations, which resolve left–right ambiguity and which account for insonification/bathymetry (TL) issues, indicate that backscatter is strongly dependent on azimuth. It is quite clear at site A that robust deterministic features are present in the 1/2CZ acoustics data. [Work supported by Office of Naval Research.]