• Journal of the Institute of Electronics Engineers of Korea TC
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• v.47 no.8
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• pp.16-23
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• 2010

In this paper, we propose an efficient estimation algorithm to find unknown location of an earth station for a geostationary satellite system. A cross ambiguity function (CAF), using time difference of arrival (TDOA) and frequency difference arrival (FDOA), is usually used to estimate the unknown location of an unauthorized earth station which may invoke interference to the existing satellite systems. However, a practical estimation of the location data requires tremendous computational time of CAF, and this prohibits direct utilization of CAF. For this reason, we propose a computationally efficient algorithm which utilizes characteristics of TDOA and FDOA within CAF. The proposed algorithm greatly enhances the computational efficiency without any performance degradation. In addition, we demonstrate the simulation results on the estimation performance by the resolution of the CAF estimation. The results provided in this paper will be utilized at the real system implementation.

• Proceedings of the Korean Society of Dyers and Finishers Conference
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• 2011.03a
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• pp.53-53
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• 2011

Poly(lactic acid)(이하 PLA라 칭함)는 1970-80년대의 초기 연구에서는 높은 제조비용과 희귀성에 의해 봉합사와 약물전달시스템으로 용도가 제한적이었으나 1980년대를 거쳐 1990년 이후 농업의 혁신적인 변화를 거쳐 옥수수의 다양한 측면의 이용의 하나로서 6-7년 전부터 양산화에 성공하여, 의류, 필름 및 플라스틱의 여러 가지 분야에서 적용가능성을 모색하고 있다. PLA의 장점은 석유가 아닌 천연 원료에서 얻을 수 있으며, 기존의 합성섬유와는 달리 일정한 조건하에서 미생물 등에 의해 물과 이산화탄소로 90% 이상 분해되는 친환경적인 소재이다. 합성섬유에서 의류용으로 대부분 사용되는 폴리에스테르(이하 PET라 칭함)와 유사한 물성을 가지고 있는 PLA섬유는 PET섬유와 유사한 분산염료로 염색할 수 있다. 따라서 PLA섬유는 분산염료에 의한 염색법을 중심으로 연구되어지고 있으나, PET 섬유의 융점이 $254^{\circ}C$부근인 반면, PLA섬유는 $160-170^{\circ}C$부근이다. 이로 인해 PLA를 섬유로 용도전개에 있어서 약점으로 작용하고 있다. 그러나 PLA섬유는 특유의 경량감과 새로운 촉감 등의 많은 장점을 지니고 있어 여러 가지 용도전개가 되어지고 있다. 초음파는 사람이 들을 수 없는, 사람마다 한계값은 다르지만, 주파수 약 25kHz이상의 음파를 말하며, 주파수가 높고 파장이 짧기 때문에 강한 진동을 수반한다. 초음파의 이용 원리는 Cavitation이라는 현상으로 생성된 기포가 터지면서 강력한 에너지를 방출한다. 이것은 강력하여 사방으로 퍼져 물질표면이나 내부에 영향을 미친다. 섬유에서는 표면에 뭍은 불순물을 제거하는 세정 등에 응용되고 있으며, 염색분야에서도 다양하게 연구되어지고 있다. 본 연구에서는 초음파에 의한 PLA섬유의 염색성을 조사하였다. PLA섬유를 정련하여 초음파작용의 유무에 따라 염색하여 그 결과를 고찰하였다. 초음파 작용에 따른 분산염료의 표면 색변화는 없었으며, 섬유 표면 색 농도의 차이는 초음파 작용에 의하여 저온에서는 차이가 나타나지만, 고온인 $80^{\circ}C$에서는 오히려 낮아졌다.

• Journal of Advanced Navigation Technology
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• v.20 no.4
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• pp.298-304
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• 2016

In this paper, a wavelet packet transform method is proposed for improving the altitude control performance of quadrotor UAV using an ultrasonic rangefinder. A ground tests are conducted using an ultrasonic rangefinder that is much used for vertical takeoff and landing. An ultrasonic rangefinder suffers from signal's spike due to specular reflectance and acoustic noise. The occurred spikes in short time span need to be analyzed at both sides time and frequency domain. The analyzed spikes of the ultrasonic rangefinder using a wavelet packet transform. Compared with the discrete wavelet transform, the wavelet packet decomposition can obtain more abundant time-frequency localization information, so it is more suitable for analyzing and processing ultrasonic signals spike. Experimental results show that it can effectively remove the spikes of the ultrasonic rangefinder.

• Journal of the Korean Society for Nondestructive Testing
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• v.21 no.2
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• pp.152-162
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• 2001

Laser-generated ultrasonic technique which is one of the reliable nondestructive evaluation techniques has been applied to evaluate the integrity of structures by analyzing the characteristics of signal obtained from surface crack. Therefore, the signal analysis of the laser-generated ultrasonics is absolutely necessary for the accurate and quantitative estimation of the surface defects. In this study, one-sided measurement by laser-generated ultrasonic has been applied to evaluate the depth of the surface-breaking crack in the materials. However, since the ultrasonic waveform excited by pulse laser is very difficult to distinguish the defect signals, it is necessary to consider the signal analyses of the transient waveform. Wavelet Transform(WT) is a powerful tool for processing transient signals with temporally varying spectra that helps to resolve high and low frequency transient components effectively. In this paper, the analyses of the surface-breaking crack of the ultrasonic signal excited by pulse laser are presented by employing the WT analyses.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.3A
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• pp.237-246
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• 2007

Clock Synchronization is one of the most basic factors to be considered when we implement an indoor synchronization network for indoor positioning. In this paper, we present a new synchronization algorithm which does not employ time stamps in order to reduce the hardware complexity and data overhead. In addition to that, we describe an algorithm that is designed to compensate the frequency drift giving an serious impact on the synchronization performance. The performance evaluation of the proposed algorithm is achieved by investigating MTIE (Maximum Time Interval Error) values through simulations. In the simulations, the frequency drift values of the practical oscillators are used. From the simulation results, it is investigated that we can achieve the synchronization performance under 10 ns when we use 1 second synchronization interval with 1 ns resolution and TCXOs (Tmperature Compensated Cristal Oscillators) both in the master clock and the slave clock.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.32.4-33
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• 2016

활동성은하핵(AGN)의 거대 블랙홀 주변에 존재하는 플라스마 디스크의 구조나 물리적인 상태를 관측으로 직접 찾는 것은 AGN 중심부분에서의 제트 형성, 방출과정이나 에너지 수송과정을 조사하기 위해 중요한다. 지금껏 주로 센치미터파장 영역의 다주파 VLBI 관측으로 우리은하에서 가까운 AGN 속에 존재하는 10 pc 정도의 플라스마 디스크가 발견되어 있다만, AGN의 활동성을 정하고 있는 1 pc 이하의 스케일에서의 디스크 구조를 직접 관측한 결과는 아직 없다. 우리는 2015년8월부터 KVN 및 KVN과 일본 VERA로 구성되는 한일공동 VLBI 관측망(KaVA)을 이용해서 전파 은하 3C 84(z = 0.0176, 1 mas = 0.36 pc)의 밀리미터파장 모니터링을 진행하고 있다. KVN과 KaVA를 이용하면 1 pc 이하의 스케일로 3C 84의 중심구조를 고감도에서 분해할 수 있다. 이번 발표에서는 KVN 및 KaVA로 거의 동시에 실시한 관측결과를 중심으로 보고한다. 관측은 2016년2월22일(KaVA 43 GHz) 및 23일(KVN 86 GHz)에 실시되었다. 양 주파수의 이미지에서 종래의 센치미터 ~ 밀리미터파장 VLBI관측으로도 검출되어 있는 중심핵(C1) 및 남쪽에 약 3 mas 떨어져서 위치하는 로브(C3) 성분 뿐만 아니라 C1으로부터 북쪽에 약 2.5 mas 떨어져서 위치하는 새로운 성분(N1)을 검출하였다. N1의 검출 수준은 43, 86 GHz 모두 $6{\sigma}$이며, 양 주파수 사이에서 광학적으로 두꺼운 스펙트럼을 가지고 있다. 과거의 관측으로 측정된 C3의 겉보기 속도는 빛의 속도의 약 23%이며, 남북 로브의 구조와 운동의 대칭을 가정하면 N1이 도플러 분사출 효과 때문에 어두워지고 있는 가능성은 낮다. 따라서 C3에 대응하는 북쪽 N1로브로부터의 복사가 블랙홀 주변의 플라스마 디스크로 인해 저주파수에서 강한 흡수를 받고 있는 결과고 생각된다.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.26 no.7
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• pp.765-773
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• 2016

The vehicle interior noise caused by exterior fluid flow field is one of critical issues for product developers in a design stage. Especially, turbulence and vortex flow around A-pillar and side mirror affect vehicle interior noise through a side window. The reliable numerical prediction of the noise in a vehicle cabin due to exterior flow requires distinguishing between the aerodynamic (incompressible) and the acoustic (compressible) surface pressures as well as accurate computation of surface pressure due to this flow, since the transmission characteristics of incompressible and compressible pressure waves are quite different from each other. In this paper, effective signal processing technique is proposed to separate them. First, the exterior flow field is computed by applying computational aeroacoustics techniques based on the Lattice Boltzmann method. Then, the wavenumber-frequency analysis is performed for the time-space pressure signals in order to characterize pressure fluctuations on the surface of a vehicle side window. The wavenumber-frequency diagrams of the power spectral density shows clearly two distinct regions corresponding to the hydrodynamic and the acoustic components of the surface pressure fluctuations. Lastly, decomposition of surface pressure fluctuation into incompressible and compressible ones is successfully accomplished by taking the inverse Fourier transform on the wavenumber-frequency diagrams.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.19 no.7
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• pp.1203-1212
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• 1994

This paper proposes a variable block size vector quantization based on a biorthogonal wavelet transform for image compression. An image is first decomposed with the biorthogonal wavelet transform into multiresolution image and the wavelet coefficients of the middle frequency bands are segmented using the quadtree sturcture to extract the perceptually important regions in the middle frequency bands. A sedges of middle frequency bands exist the corresponding position of high frequency bands, the complicated quadtree structure of middle frequency bands is equally applied to the high frequency bands. Therefore the overhaed information of the quadtree codes needed to segment the high frequency bands can be reduced. The segmented subblocks are encoded with the codebook designed at the each scales and directions. The simulation results showed that the proposed methods could reproduce higher quality image with bit rate reduced about 20(%) than of the preceding VQ method and sufficiently reduce the bolck effect and the edge degradation.

• The Journal of the Korea Contents Association
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• v.6 no.7
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• pp.90-98
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• 2006

Digital watermarking is a technique embedding hidden information into multimedia data imperceptibly such as images and sounds. Generally an original image is transformed and coded watermark data is embedded in frequency domain watermarking models. In this paper, We propose a new color image watermarking technique using Fresnel transform. A watermark image is Fresnel - transformed and the intensity of transformed pattern is embedded into color image. In our watermarking model, an original image is converted from RGB components into YCrCb components and then the values of real number and imaginary number of a Fresnel-transformed pattern of a watermark image are embedded into Y component. The watermarking experiments were conducted to show the validity of the proposed method using PSNR value, and the results show that our method has the robustness against lossy compression like JPEG.

• The Journal of the Acoustical Society of Korea
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• v.36 no.1
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• pp.49-56
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• 2017

This paper proposes a method to improve the left right discrimination performance by eliminating the imaginary target based on the frequency features of the beam pattern for bow array. The beamwidth of the imaginary target is wider than that of the real target. If an azimuth axis is considered as a time axis, the real and the imaginary targets can be assumed as high and low frequencies, respectively. To eliminate the imaginary target which has a low frequency component, we design a cut-off frequency of the High Pass Filter (HPF) using the back-lobe imaginary beamwidth. The real target is estimated by eliminating the imaginary target by applying HPF to the entire power of the beamformer output. Computer simulations show that the proposed method can increase the left right discrimination performance above 8 dB on average.