Matched-field Techniques Research Articles

Broadband Underwater Localization of Multiple Sources Using Basis Pursuit De-Noising

Locating multiple underwater acoustic sources is a problem that can be solved using antenna array beamforming based on the matched field (MF) processing. However, known MF beamforming techniques fail to provide good performance for multiple sources, a high noise power, and/or when the sources are close to each other. This paper proposes an MF technique for solving the localization problem. The proposed technique exploits formulation of the localization problem in terms of sparse representation of a small number of source positions among a much larger number of potential positions. The sparse representation is formulated as the basis pursuit de-noising (BPDN) problem for complex-valued variables. The solution is found as a joint solution to a set of BPDN problems corresponding to the set of source frequencies subject to the joint support. The joint BPDN problem is efficiently solved using the Homotopy approach and coordinate descent search. For further reduction in the complexity, a position grid refinement method is applied. Using simulated and real experimental data, it is shown that the technique can provide accurate source localization for multiple sources. The proposed technique outperforms other MF techniques in resolving sources positioned closely to each other, tolerance to the noise and capability of locating multiple sources.

Passive matched-field inversion using a horizontal planar array

Shallow-water acoustic predictions are severely limited by uncertainty in sediment property characteristics. Inverse methods with controlled active sources and vertical arrays have been used to estimate seabed properties; however, some applications require a covert approach and horizontal bottomed arrays. This study addresses the accuracy of low-frequency (100–200 Hz) matched-field correlations using broadband signals from surface ships with unknown source levels at unknown ranges. Matched-field techniques are applied in a realistic shallow-water environment with a horizontal planar array and high signal-to-noise ratios. The simulations indicate significant potential for accurate estimates of thick-sediment characterizations of grain size out to ranges of tens of water depths in shallow water, despite moderate mismatch conditions in the environmental model. The results show that: (1) the horizontal aperture should contain at least three hydrophones per wavelength to ensure high quality inversions; (2) the horizontal aperture should be several times longer than a vertical aperture; (3) coherent (phase-only) matched-field processing outperforms standard intensity processing by about 2 dB in good input SNR conditions; (4) incorrect assumptions about the assumed sound-speed profile (e.g., incorrect mixed-layer-depth) do not significantly affect the inversion results. [Work sponsored by QinetiQ North America.]

Estimation of the Refractivity Structure of the Lower Troposphere From Measurements on a Terrestrial Multiple-Receiver Radio Link

A matched field technique is used for remote sensing of refractivity profile. The method relies on electromagnetic propagation simulation to estimate the height profile of radio refractivity in the lower atmosphere by comparing theoretical predictions with measurements on a terrestrial radio link. The objective is to discover whether small variations of the air's refractive index can be detected in the lowest part of atmosphere, up to 150 m above the ground. The resultant estimations of the height profile of refractivity are compared with the measured refractivity profile obtained from meteorological sensors at different heights. The results show a close agreement between the estimated and measured refractivity profiles during periods of deep fading events.

SU‐FF‐T‐95: Reduction of Larynx Dose in Head and Neck IMRT: A Restricted Field Approach

Purpose: To develop a Whole‐Field Head and Neck IMRT technique that allows for a considerable reduction in dose to the uninvolved larynx and post cricoid/inferior constrictor structures without sacrificing target coverage or introducing uncertainties associated with matched field techniques. Materials and Methods: Seven coplanar beams are used to treat target volumes located superior to the larynx. The inferior jaws of these beams are fixed at the superior extent of the larynx. Three additional beams, on both the left and right sides, are typically used to treat the entire superior to inferior extent of the disease, but with the medial collimator jaw edges fixed at the edge of the larynx. The three beams chosen on each side, are typically an AP and 2 steep posterior obliques approximating an AP/PA approach in the inferior portion of the target volumes. All beams are placed in a single plan and undergo optimization simultaneously (Eclipse, Varian, Palo Alto, CA). Results: Excellent target coverage is maintained while dramatically reducing dose centrally through the larynx region. Point dose measurements for a sample patient in the larynx, post cricoid and inferior constrictors are 14Gy, 8.8Gy and 8.9Gy respectively. There are also no adverse effects on parotid sparing or dose conformality in the region superior to the larynx. Conclusion: The restricted field head and neck IMRT technique allows for excellent coverage of the target volumes while providing dramatically reduced larynx and post cricoid/inferior constrictor doses when compared to using the optimization algorithm alone in the whole field IMRT technique. In addition, this technique eliminates the inhomogeneity associated with a matched field technique due to collimator position tolerances and avoids matching through disease.

WE‐E‐224C‐06: Differential Smoothing IMRT Planning for Head and Neck Cancer Patients with Mediastinal Involvement

Purpose: Head and neck cancer patients with mediastinal involvement present a planning challenge. The superior fields are ideal for IMRT but possible interplay effects between leaf and mediastinal motion makes IMRT less desirable for the inferior fields. A new, differential smoothing IMRT technique is compared to matched and extended field IMRT plans. Materials and Methods: The differential smoothing IMRT technique treated the superior portion of the target with 7 fixed, low smoothing rate (LSR) beams while the inferior portion was treated by 3–4 highly smoothed, fixed beams with an overlap region. All beams existed in a single IMRT plan and were optimized simultaneously. This technique was used to plan treatments for three head and neck cancer patients with mediastinal involvement. These patients were also planned using two alternative techniques: (1) A matched field technique, with conformal radiation therapy used for the inferior portion, and an LSR IMRT plan used for the superior portion; (2) An extended field LSR IMRT plan treating the full extent of the disease. The plans were compared dosimetrically. Results: The differential smoothing technique provided homogenous, continuous coverage throughout the target volume while the matched field plan demonstrated discontinuous coverage in the match region. The extended field IMRT plan had coverage comparable to the differential smoothing plan. However, patient and leaf motion could compromise coverage in practice. The average MLC leaf opening in the inferior differentially smoothed beams was 1.5–2 times greater than the average opening in any of the LSR beams (1.55cm for LSR beams verses 3.04cm for highly smoothed beams) thereby potentially reducing the impact of patient motion. Lung DVH's were similar for all three techniques. Conclusion: The differential smoothing technique offers continuous, more homogenous coverage than the matched field method and is probably less susceptible to patient motion than the extended field technique.

Diffraction model of short-range longitudinal wakefields in circular and rectangular structures

Diffraction models based on the Green function technique and ‘image’ field concept were developed for analytical calculation of high-frequency radiation in rectangular iris-loaded structures. The models for single-cell and periodic structures were considered as an extension of the Lawson and Sessler-Vainshtein approaches and were applied to circular, infinitely wide planar and ‘muffin-tin’ geometry. The calculated spectral radiation losses are in good agreement with existing formulae. For a 2.5D muffin-tin structure a special semi-analytical frequency domain model was built to compare the diffraction model with the matched field technique based on eigenmodes calculations.

Reduction in computational requirements with matched field processing

The sensitivity of matched field techniques to source location is a significant benefit from a localization accuracy perspective. Unfortunately, the implementation of matched field techniques in real-time systems is hampered by the computationally demanding hypothesis testing of source location in range and depth. Furthermore, operational requirements of such systems generally do not demand a high degree of accuracy. The Minimum Variance with Environmental Perturbation Constraints (MV-EPC) processor developed by Krolik [J. Acoust. Soc. Am. 92, 1408–1419 (1992)] has been used to reduce the sensitivity to environmental uncertainty. This same approach can be used to reduce the sensitivity to source location, thereby reducing the number of source location hypotheses. This produces a computational tractable approach to real-time, robust matched field processing. Illustrative simulations and real data results using SWellEX-96 data are presented.

Electron/photon matched field technique for treatment of orbital disease

Purpose : A number of approaches have been described in the literature for irradiation of malignant and benign diseases of the orbit. Techniques described to date do not deliver a homogeneous dose to the orbital contents while sparing the cornea and lens of excessive dose. This is a result of the geometry encountered in this region and the fact that the target volume, which includes the periorbital and retroorbital tissues but excludes the cornea, anterior chamber, and lens, cannot be readily accommodated by photon beams alone. To improve the dose distribution for these treatments, we have developed a technique that combines a low-energy electron field carefully matched with modified photon fields to achieve acceptable dose coverage and uniformity. Methods and Materials : An anterior electron field and a lateral photon field setup is used to encompass the target volume. Modification of these fields permits accurate matching as well as conformation of the dose distribution to the orbit. A flat-surfaced wax compensator assures uniform electron penetration across the field, and a sunken lead alloy eye block prevents excessive dose to the central structures of the anterior segment. The anterior edge of the photon field is modified by broadening the penumbra using a form of pseudodynamic collimation. Direct measurements using film and ion chamber dosimetry were used to study the characteristics of the fall-off region of the electron field and the penumbra of the photon fields. From the data collected, the technique for accurate field matching and dose uniformity was generated. Results : The isodose curves produced with this treatment technique demonstrate homogeneous dose coverage of the orbit, including the paralenticular region, and sufficient dose sparing of the anterior segment. The posterior lens accumulates less than 40% of the prescribed dose, and the lateral aspect of the lens receives less than 30%. A dose variation in the match region of ±12% is confronted when an unmodified photon field edge is matched with the fall-off of the electron field at the 50% isodose lines. By modifying the penumbra, the dose variation is reduced to ±2%. Treatment setup accuracy is essential. Conclusions : The electron/photon matched field technique offers a uniform isodose distribution for treatment of the orbit that has not been previously achieved. With this technique a homogeneous dose can be delivered to the entire orbit while avoiding a significant dose to the anterior segment and minimizing the risk of morbidity.

Active, wideband detection in an uncertain multipath environment

Active, wideband detection of targets in a dense multipath environment is approached via optimal detection and estimation theory in conjunction with a received signal model described by weighted and delayed replicas of the transmitted signal. The set of weights and delays are known to vary with acoustic medium properties including sound-speed profile, bottom composition and topography and surface area, which are largely unknown or uncertain. The optimum receiver is based on prior distributions of multipath amplitude and delay. An acoustic propagation model drives the optimum receiver by generating these distributions through Monte Carlo simulation. Unlike traditional matched field techniques, the acoustic medium is treated as a random process rather than a deterministic one for a given set of propagation conditions. The sonar system toolset (SST)/generic sonar model (GSM) software package provides the required simulation environment. Illustrative detection examples are presented along with receiver operator characteristic (ROC) performance. [Work supported by Digital System Resources.]