Satellite Laser Ranging System at Geochang Sta

Korea Astronomy and Space Science Institute (KASI) has been developing the space optical and laser tracking (SOLT) system for space geodesy, space situational awareness, and Korean space missions. The SOLT system comprises satellite laser ranging (SLR), adaptive optics (AO), and debris laser tracking (DLT) systems, which share numerous subsystems, such as an optical telescope and tracking mount. It is designed to be capable of laser ranging up to geosynchronous Earth orbit satellites with a laser retro-reflector array, space objects imaging brighter than magnitude 10, and laser tracking low Earth orbit space debris of uncooperative targets. For the realization of multiple functions in a novel configuration, the SOLT system employs a switching mirror that is installed inside the telescope pedestal and feeds the beam path to each system. The SLR and AO systems have already been established at the Geochang station, whereas the DLT system is currently under development and the AO system is being prepared for testing. In this study, the design and development of the SOLT system are addressed and the SLR data quality is evaluated compared to the International Laser Ranging Service (ILRS) tracking stations in terms of single-shot ranging precision. The analysis results indicate that the SLR system has a good ranging performance, to a few millimeters precision. Therefore, it is expected that the SLR system will not only play an important role as a member of the ILRS tracking network, but also contribute to future Korean space missions

Evaluation of a Laser Altimeter using the Pseudo-Random Noise Modulation Technique for Apophis Mission

Apophis is a near-Earth object with a diameter of approximately 340 m, which will come closer to the Earth than a geostationary orbit in 2029, offering a unique opportunity for characterizing the object during the upcoming encounter. Therefore, Korea Astronomy and Space Science Institute has a plan to propose a space mission to explore the Apophis asteroid using scientific instruments such as a laser altimeter. In this study, we evaluate the performance metrics of a laser altimeter using a pseudorandom noise modulation technique for the Apophis mission, in terms of detection probability and ranging accuracy. The closed-form expression of detection probability is provided using the cross correlation between the received pulse trains and pseudo-random binary sequence. And the new ranging accuracy model using Gaussian error propagation is also derived by considering the sampling rate. The operation range is significantly limited by thermal noise rather than background noise, owing to not only the low power laser but also the avalanche photodiode in the analog mode operation. However, it is demonstrated from the numerical simulation that the laser altimeter can achieve the ranging performance required for a proximity operation mode, which employs commercially available components onboard CubeSat-scale satellites for optical communications

Centroid Error Analysis of Beacon Tracking under Atmospheric Turbulence for Optical Communication Links

Optical satellite communication has received considerable attention as a promising alternative to radio frequency communication because of its potential advantages including higher data rates and license free spectrum. Many studies have conducted performance analyses of optical communication channels, but few have investigated beacon tracking channels under atmospheric turbulence. The centroid accuracy of beacon tracking channels is limited by not only noise sources, but also a finite delay time, which also fluctuates due to atmospheric turbulence. Consequently, the centroid error is an important figure of merit when evaluating the performance of a beacon tracking system. In this study, the closed-form expressions were derived for average centroid error and fade probability, based on received photoelectron counts depending on exposure time, taking into account the log-normal tracking channels. We analyzed the angular positioning performance of beacon tracking detectors onboard small satellites in the presence of atmospheric turbulence, in terms of centroid error and fade probability. We found that an optimal exposure time exists, which minimizes the centroid error, and that fade probability is inversely proportional to the exposure time. These are significant properties to consider in the design of beacon tracking systems

Modeling and Analysis of an Echo Laser Pulse Waveform for the Orientation Determination of Space Debris

Orientation information of space debris is required to improve the orbital prediction accuracy for mitigation or elimination of a significant threat to not only human space activities but also operational satellites. Obtaining orientation information is currently achievable by applying photometry, adaptive optics (AO) and satellite laser ranging (SLR) technologies. In this study, a new method is proposed based on an echo laser pulse waveform (ELPW) for the orientation determination of space debris; its feasibility was also investigated by numerical simulations. Unlike the photometry and AO technologies available just under the sun-illumination condition and the SLR technology applicable only for cooperative targets, the ELPW is achievable by using a high power laser regardless of the above measurement constraints. A mathematical model is derived to generate the ELPW, and the beam broadening and spreading due to the atmospheric turbulence is taken into account. The Gaussian decomposition based on a genetic algorithm was employed to the ELPWs in order to analyze the orientation features. It is demonstrated from the numerical simulations that the ELPWs have distinctive shapes characterizing the orientation of space debris and therefore our approach was capable of providing orientation information

Pulse Broadening Effects on Ranging Performance of a Laser Altimeter with Return-to-Zero Pseudorandom Noise Code Modulation

A laser altimeter using code modulation techniques receives a backscattered pulse wider than the transmitted rectangular pulse when scanning a rough or sloped target surface. This leads to degrading the ranging performance in terms of signal-to-noise ratio (SNR) and detection probability. Unlike the pulsed techniques, little work has focused on the pulse broadening effect of the code modulation techniques. In this study, mathematical models were derived to investigate the pulse broadening effect on the ranging performance of a return-to-zero pseudorandom noise (RZPN) laser altimeter. Considering that the impulse response can be approximated by a Gaussian function, the analytical waveform was derived using a new flat-topped multi-Gaussian beam (FMGB) model. The closed-form expressions were also analytically derived for a peak cross-correlation, SNR, and detection probability in terms of the pulse broadening effect. With the use of a three-dimensional model of asteroid Itokawa for practical surface profiles, the analytical expressions were validated by comparing to the results obtained from numerical simulations. It was also demonstrated that the pulse broadening effect dropped down the peak cross-correlation and then deteriorated the ranging performance. These analytical expressions will play an important role in not only designing a laser altimeter using the RZPN code modulation technique but also analyzing its ranging performance

Spinodally Decomposed PbSe-PbTe Nanoparticles for High-Performance Thermoelectrics: Enhanced Phonon Scattering and Unusual Transport Behavior

Dramatic enhancements in the figure of merit have been obtained in bulk thermoelectric materials by doping, band engineering, and nanostructuring. Especially, in p-type thermoelectrics, high figure of merits near 2.0 have been reported in a few papers through the reduction in lattice thermal conductivity and the advancement in power factors. However, there exists no report on the n-type systems showing high figure of merits because of their intrinsically low Seebeck coefficients. Here, we demonstrate that a nanostructured bulk n-type thermoelectric material that was assembled by sintering spinodally decomposed lead chalcogenide nanoparticles having a composition of PbSe_0.5Te_0.5 reaches a high figure of merit of 1.85. The spinodally decomposed nanoparticles permit our thermoelectric material to have extremely low lattice thermal conductivity and a high power factor as a result of nanostructuring, electronic optimization, insertion of an impurity phase and phase change in local areas. We propose that this interesting concept would be one of the promising approaches that overcome limitation arising from the fact that most parameters in the figure of merit are closely correlated

Design and Development of High-Repetition-Rate Satellite Laser Ranging System

The Accurate Ranging System for Geodetic Observation – Mobile (ARGO-M) was successfully developed as the first Korean mobile Satellite Laser Ranging (SLR) system in 2012, and has joined in the International Laser Ranging Service (ILRS) tracking network, DAEdeoK (DAEK) station. The DAEK SLR station was approved as a validated station in April 2014, through the ILRS station “data validation” process. The ARGO-M system is designed to enable 2 kHz laser ranging with millimeter-level precision for geodetic, remote sensing, navigation, and experimental satellites equipped with Laser Retroreflector Arrays (LRAs). In this paper, we present the design and development of a next generation high-repetition-rate SLR system for ARGO-M. The laser ranging rate up to 10 kHz is becoming an important issue in the SLR community to improve ranging precision. To implement high-repetition-rate SLR system, the High-repetition-rate SLR operation system (HSLR-10) was designed and developed using ARGO-M Range Gate Generator (A-RGG), so as to enable laser ranging from 50 Hz to 10 kHz. HSLR-10 includes both hardware controlling software and data post-processing software. This paper shows the design and development of key technologies of high-repetition-rate SLR system. The developed system was tested successfully at DAEK station and then moved to Sejong station, a new Korean SLR station, on July 1, 2015. HSLR-10 will begin normal operations at Sejong station in the near future

Design and Development of High-Repetition-Rate Satellite Laser Ranging System

The Accurate Ranging System for Geodetic Observation – Mobile (ARGO-M) was successfully developed as the first Korean mobile Satellite Laser Ranging (SLR) system in 2012, and has joined in the International Laser Ranging Service (ILRS) tracking network, DAEdeoK (DAEK) station. The DAEK SLR station was approved as a validated station in April 2014, through the ILRS station “data validation” process. The ARGO-M system is designed to enable 2 kHz laser ranging with millimeter-level precision for geodetic, remote sensing, navigation, and experimental satellites equipped with Laser Retroreflector Arrays (LRAs). In this paper, we present the design and development of a next generation high-repetition-rate SLR system for ARGO-M. The laser ranging rate up to 10 kHz is becoming an important issue in the SLR community to improve ranging precision. To implement high-repetition-rate SLR system, the High-repetition-rate SLR operation system (HSLR-10) was designed and developed using ARGO-M Range Gate Generator (A-RGG), so as to enable laser ranging from 50 Hz to 10 kHz. HSLR-10 includes both hardware controlling software and data post-processing software. This paper shows the design and development of key technologies of high-repetition-rate SLR system. The developed system was tested successfully at DAEK station and then moved to Sejong station, a new Korean SLR station, on July 1, 2015. HSLR-10 will begin normal operations at Sejong station in the near future

Flavonoids and a Limonoid from the Fruits of Citrus unshiu and Their Biological Activity

The fruits of Citrus unshiu are one of the most popular and most enjoyed fruits in Korea. As we continue to seek for bioactive metabolites from Korean natural resources, our study on the chemical constituents of the fruits of C. unshiu resulted in the isolation of a new flavonoid glycoside, limocitrunshin <b>1</b>, along with seven other flavonoids <b>2</b>–<b>8</b> and a limonoid <b>9</b>. All structures were identified by spectroscopic methods, namely 1D and 2D NMR, including HSQC, HMBC, and ¹H–¹H COSY experiments, HRMS, and other chemical methods. Compounds <b>3</b>, <b>5</b>, and <b>9</b> are reported to be isolated from this fruit for the first time. The isolated compounds were applied to activity tests to verify their inhibitory effects on inflammation and nephrotoxicity. Compounds <b>6</b> and <b>9</b> showed the most potent inhibitory activity on renal cell damage and nitric oxide production, respectively. Thus, the fruits of C. unshiu could serve as a valuable natural source of bioactive components with health benefits for potential application in functional foods

Correlation of gingival biotypes with clinical parameters

1. 목적 이 연구의 목적은 기존의 평가기준에 따라 관찰자에 의해 주관적으로 판단된 치은형과 실제 치은두께와의 상관관계를 규명하고 임상검사 시에 측정할 수 있는 변수들과 치은두께의 연관성을 평가하는 것이다. 2 방법 211명의 치과대학생(22-43세, 평균 24.7세)을 연구대상으로 하여 상악중절치부위에서 국소마취하에 근관 치료용 파일을 치은에 삽입하여 두께를 측정하고 임상검사를 통해 치주낭깊이, 치태지수, 치은지수, 치은퇴축, 체형 및 비만도, 피부형, 치경부의 형태, 관측자가 임의로 판단한 치은형을 기록하였다. 임상사진으로 치관의 폭경과 길이의 비율, 치은외형의 만곡정도를 조사하고 평행촬영법을 이용한 방사선사진으로 치아의 장평비율 및 치아의 치관 폭경과 치경부 폭경의 비율을 조사하였다. 전체 대상을 치은의 두께를 기준으로 평균두께보다 얇은 군과 두꺼운 군의 두 개의 군으로 분류하여 임상검사 사에 측정한 변수들이 각 군 간에 유의할 만한 차이를 보이는지와 임의로 판단한 치은형이 실제 치은두께와 연관이 있는지를 알아보았다. 통계처리는 Student t-test를 이용하였다. 3. 결과 치주낭 깊이, 치은지수, 체형 및 비만도, 피부형, 치아의 형태, 치경부의 형태, 치아의 장평비율의 경우 실제 측정하여 얻은 치은의 두께와의 상관관계는 통계적으로 유의성이 없었다. 치은의 형태는 치은의 두께와 상관성은 보이고 있으나 통계적으로 유의하지 않았다. 관측자가 임의로 평가한 치은형과 실제 측정치도 유의할만한 일치를 보이지 않고 있다. 4. 결론 치은의 형태는 치은의 두께를 예상하는데 약간의 도움이 될 수 있으나 실제 치은의 두께는 임상적으로 간단히 측정할 수 있는 겸사지수들과 직접적인 상관관계를 보이지 않았다. 따라서 치료결과의 예측에 있어서 치은형을 분류하여 예상하는 것은 큰 도움이 되지 않는다고 할 수 있다