• Journal of the Korea Institute of Military Science and Technology
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• v.5 no.1
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• pp.59-64
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• 2002

Passive sonar system forms the various beams in any desired directions to obtain the improvement in Signal-to-Noise(S/N) ratio, bearing detection and localization of targets, and the attenuation of interferences from other directions. Detection of modern underwater targets is becoming increasingly difficult as noise reduction technology leads to considerably low-level acoustic emissions. Therefore, the improvement of beamforming is very important to detect modern underwater targets at the long range in the complex environmental sea. Also, to react to the fast attack mobiles such as torpedoes, port and starboard discrimination is required to be performed very quickly. In this paper, we proposed the implementation of omnidirectional target bearing detector without port and starboard ambiguity to detect effectively the low-level underwater targets. The port and starboard discrimination is performed by cardioid processing and the improvement of beamforming utilizes the cross correlation matrix of individual hydrophone pairs of linear array sensors. The sea test result shows that the system implemented is good for the detection of the low-level underwater targets.

• The Journal of Korea Robotics Society
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• v.16 no.2
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• pp.94-102
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• 2021

This study addresses a method for 3D reconstruction using acoustic data with heterogeneous sonar devices: Forward-Looking Multibeam Sonar (FLMS) and Profiling Sonar (PS). The challenges in sonar image processing are perceptual ambiguity, the loss of elevation information, and low signal to noise ratio, which are caused by the ranging and intensity-based image generation mechanism of sonars. The conventional approaches utilize additional constraints such as Lambertian reflection and redundant data at various positions, but they are vulnerable to environmental conditions. Our approach is to use two sonars that have a complementary data type. Typically, the sonars provide reliable information in the horizontal but, the loss of elevation information degrades the quality of data in the vertical. To overcome the characteristic of sonar devices, we adopt the crossed installation in such a way that the PS is laid down on its side and mounted on the top of FLMS. From the installation, FLMS scans horizontal information and PS obtains a vertical profile of the front area of AUV. For the fusion of the two sonar data, we propose the probabilistic approach. A likelihood map using geometric constraints between two sonar devices is built and a monte-carlo experiment using a derived model is conducted to extract 3D points. To verify the proposed method, we conducted a simulation and field test. As a result, a consistent seabed map was obtained. This method can be utilized for 3D seabed mapping with an AUV.

• The Journal of the Acoustical Society of Korea
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• v.37 no.4
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• pp.188-195
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• 2018

A new directional linear array structure and its signal processing method are presented to resolve the left/right ambiguity inherent in a linear array. The array structure combines an ordinary acoustic sensor array with a DIFAR (Directional Frequency Analysis and Recording) sensor array, keeping a linear array configuration and gaining a instantaneous left/right discrimination. It presents better PSRR (Port-Starboard Rejection Ratio) in low frequency band and low SNR (Signal to Noise Ratio) situation as compared with a conventional twin linear array, and good compromise to easily upgrade an existing linear array system to a new one with a left/right discrimination capability.

• The Journal of the Acoustical Society of Korea
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• v.26 no.8
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• pp.415-425
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• 2007

This paper proposes a method of underwater source localization using the wideband interference patterns matching. By matching two interference patterns in the spectrogram, it is estimated a ratio of the range from source to sensor5, and then this ratio is applied to the Apollonius circle. The Apollonius circle is defined as the locus of all points whose distances from two fixed points are in a constant value so that it is possible to represent the locus of potential source location. The Apollonius circle alone, however still keeps the ambiguity against the correct source location. Therefore another equation is necessary to estimate the unique locus of the source location. By estimating time differences of signal arrivals between source and sensors, the hyperbola equation is used to get the cross point of the two equations, where the point being assumed to be the source position. Simulations are performed to get performances of the proposed algorithm. Also, comparisons with real sea experiment data are made to prove applicability of the algorithm in real environment. The results show that the proposed algorithm successfully estimates the source position within an error bound of 10%.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.28 no.4
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• pp.409-415
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• 2015

For the design of Concrete-Filled Steel Tube(CFST) columns, the outside diameter D to the steel tube thickness t ratio(D/t ratio) is limited to prevent the local buckling of steel tubes. Each design code proposes the respective model to compute the maximum D/t ratio using the yield strength of steel $f_y$ or $f_y$ and the elastic modulus of steel E. Considering the uncertainty in $f_y$ and E, the reliability index ${beta}$ for the local buckling of a CFST section can be calculated by formulating the limit state function including the maximum D/t models. The resulted ${beta}$ depends on the maximum D/t model used for the reliability analysis. This variability in reliability analysis is due to ambiguity in choosing computational models and it is called as "modelling uncertainty." This uncertainty can be considered as "non-specificity" of an epistemic uncertainty and modelled by constructing possibility distribution functions. In this study, three different computation models for the maximum D/t ratio are used to conduct reliability analyses for the local buckling of a CFST section and the reliability index ${beta}$ will be computed respectively. The "non-specific ${beta}s$" will be modelled by possibility distribution function and a metric, degree of confirmation, is measured from the possibility distribution function. It is shown that the degree of confirmation increases when ${beta}$ decreases. Conclusively, a new set of reliability indices associated with a degree of confirmation is determined and it is allowed to decide reliability index for the local buckling of a CFST section with an acceptable confirmation level.

• Journal of the Korean Society of Propulsion Engineers
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• v.4 no.4
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• pp.18-25
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• 2000

A two-color particle image velocimetry (PIV) has been developed for measuring two dimensional velocity flowfields and applied to a Mach 2.0 supersonic nozzle. This technique is similar to a single-color PIV technique except that two different color laser beams are used to solve the directional ambiguity problem. A green-color laser sheet (532 nm: 2nd harmonic beam of YAG laser) and a red-color laser sheet (619 nm: output beam from YAG pumped Dye laser using Rhodamine 640) are employed to illuminate the seeded particles. A high resolution (3060${\times}$2036) digital color CCD camera is used to record the particle positions. This system eliminates the photographic-film processing time and subsequent digitization time as well as the complexities associated with conventional image shifting techniques for solving directional ambiguity problem. The two-color PIV also has the advantage that velocity distributions in high speed flowfields can be measured simply and accurately by varying the time interval between two different laser beams due to its high signal-to-noise ratio and thereby less requirement of panicle pair numbers for a velocity vector in one interrogation spot. The velocity distribution in the Mach 2.0 supersonic nozzle has been measured and the over-expanded shock cell structure can be predicted by the strain rate field. These results are compared and analyzed with the schlieren photograph for the velocity distributions and shock location.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.28 no.3
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• pp.186-194
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• 2017

In this paper, we analyze the main performance of satellite's Synthetic Aperture Radar system for high resolution and wide swath. We have used the radiation pattern of reflector antenna with array feed and comparison between the conventional ScanSAR mode and SweepSAR mode has been carried out. The SweepSAR mode is a high-resolution wide-swath mode that transmits beams over a wide range and receives echo signals through sequential beamforming based on SCORE(SCan On REceive). In this paper, we analyzed the operating principle and characteristics of satellite's SweepSAR mode and simulate system performances. In addition, in order to increase the utilization of image, performances analysis for multiple frequency bands(C-band, X-band) has been considered.

• The Journal of the Acoustical Society of Korea
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• v.14 no.6
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• pp.27-31
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• 1995

In this paper, the determination method of available bandwidth for automation of the split-spectrum processing(SSP) has been studied. The SSP is used for the visibility enhancement of the ultrasonic signal with grain noise. Even though the SSP has proved useful in signal-to-noise ratio enhancement, its application and automation have been limited due to ambiguity in the determination of available bandwidth. Until recently, it is the usual practice to optimize the available bandwidth by trial and error. The spectral histogram is the statistical distribution of the spectral windows that is selected by the minimization algorithm with the whole band of the spectrum of the received ultrasonic signal. Since the available bandwidth can be determined adaptively using spectral histogram, this method can be used for automation of the SSP. In order to evaluate the determination technique of the available bandwidth using spectral histogram, this method is applied to experimental ultrasonic data. The experimental results show that the spectral histogram is an efficient method for determination of the available bandwidth and automation of the SSP.

• Water for future
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• v.10 no.1
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• pp.95-100
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• 1977

The empirical area-reduction method by W.M. Borland and C.R. Miller and its revised procedure by W.T. Moody were made of fitting the area and storage curves to the Van't Hul distributions. It should be noted that the reservoir is classified into one of the four standard types on the basis of the topographical feature of the reservoir in application of the method. In other words, this method did not take into account several considerafble factors affecting the mode of sediment deposition, but only the shape of the reservoir as a governign factor. This is why the method occasionally creates ambiguity in classification and accordingly leads to unexpected mode of deposition. This paper describes a generating an formula to decide the standard classification of four types Van's Hul distributions, taking into consideration quantitatively sediment-loss percent and capacity-inflow ratio as well as the shape of the reservoirs by multiple regression analysis using the least square method to get a better fit to the design curves. The result is expressed as $Y=-1.95+55.8X_1+0.14X_2+0.12X_3$ in which the the values of Y locate the standard type I through type IV in the range from ten to forty with the interval of ten. The regression analysis was correlated well with the standard errors of estimate of around two except for the case of the type IV. This formula does not give big difference from the Borland's work in general sityation, but it demonstrates acceptable results, giving somewhat precise replys for the specific reservoirs. Its application to the Soyang Lake, one of the largest reservoirs in the country, defined clearly the type II, while the original method located it in the boundary of the type II and type III.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.43 no.12 s.354
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• pp.16-23
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• 2006

This paper presents the development of the fast Direction Finder using direct conversion method, which can intercept for short pulse signal of less' than 1 msec. in RF Down Converter, and CVDF(Correlative Vector Direction Finding) algorithm, which estimates DoA (Direction of Arrival). The configuration and characteristics of direction finder using 5-channel equi-spaced circular array antenna are presented and the direct conversion techniques for removing tuning time using I/Q demodulator are described. The CRLB of our model is derived, the principles of 2 kind of CVDF algorithm are explained and their characteristics are compared with CRLB w.r.t the number of samples and spacing ratio. The RF Down Converter prototype using direct conversion method is manufactured, the 2 kind of CVDF algorithm are applied and their performance are analyzed. Finally it is confirmed the LSE based CVDF algorithm is better than correlation-coefficient based except for ambiguity protection capabilities.