Browsing by Author "Wang, Chuan-Sheng"

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Assessing antenna field of view and receiver clocks of COSMIC and GRACE satellites: lessons for COSMIC-2
(2014-04-01)
Tseng, Tzu-Pang
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Zhang, Kefei
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Hwang, Cheinway
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Hugentobler, Urs
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Wang, Chuan-Sheng
We provide suggestions for the approved COSMIC-2 satellite mission regarding the field of view (FOV) and the clock stability of its future GNSS receiver based on numerical analyses using COSMIC GPS data. While the GRACE GPS receiver is mounted on the zenith direction, the precise orbit determination (POD) antennas of COSMIC are not. The COSMIC antenna design results in a narrow FOV and a reduction in the number of GPS observations. To strengthen the GPS geometry, GPS data from two POD antennas of COSMIC are used to estimate its orbits. The phase residuals of COSMIC are at the centimeter level, compared to the millimeter level of GRACE. The receiver clock corrections of COSMIC and GRACE are at the microsecond and nanosecond levels, respectively. The clock spectra of COSMIC at the frequencies of 0-0.005 Hz contain significant powers, indicating potential systematic errors in its clock corrections. The clock stability, expressed by the Allan deviation, of COSMIC ranges from 10(-9) to 10(-11) over 1 to 10(4) s, compared to 10(-12) to 10(-14) for GRACE. Compared to USO-based clock of GRACE, the clock of COSMIC is degraded in its stability and is linked to the reduction of GPS data quality. Lessons for improvement of COSMIC-2 over COSMIC in FOV and receiver clock stability are given.
Determination of global positioning system (GPS) receiver clock errors: impact on positioning accuracy
(2009-07-01)
Yeh, Ta-Kang
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Hwang, Cheinway
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Xu, Guochang
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Wang, Chuan-Sheng
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Lee, Chien-Chih
Enhancing the positioning precision is the primary pursuit of global positioning system (GPS) users. To achieve this goal, most studies have focused on the relationship between GPS receiver clock errors and GPS positioning precision. This study utilizes undifferentiated phase data to calculate GPS clock errors and to compare with the frequency of cesium clock directly, to verify estimated clock errors by the method used in this paper. The frequency stability calculated from this paper (the indirect method) and measured from the National Standard Time and Frequency Laboratory (NSTFL) of Taiwan (the direct method) match to 1.5 x 10(-12) (the value from this study was smaller than that from NSTFL), suggesting that the proposed technique has reached a certain level of quality. The built-in quartz clocks in the GPS receivers yield relative frequency offsets that are 3-4 orders higher than those of rubidium clocks. The frequency stability of the quartz clocks is on average two orders worse than that of the rubidium clock. Using the rubidium clock instead of the quartz clock, the horizontal and vertical positioning accuracies were improved by 26-78% (0.6-3.6 mm) and 20-34% (1.3-3.0 mm), respectively, for a short baseline. These improvements are 7-25% (0.3-1.7 mm) and 11% (1.7 mm) for a long baseline. Our experiments show that the frequency stability of the clock, rather than relative frequency offset, is the governing factor of positioning accuracy.
Identifying the Relationship between GPS Data Quality and Positioning Precision: Case Study on IGS Tracking Stations
(2012-08-01)
Yeh, Ta-Kang
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Chung, Yi-Da
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Wu, Chien-Ting
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Wang, Chuan-Sheng
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Zhang, Kefei
The number of global positioning system (GPS) tracking stations is increasing, primarily because the stations are multifunctional. In civil engineering, they can be used for precision positioning; in the earth sciences, they can be used to monitor faults and earthquakes; and in the atmospheric sciences, they can be applied to predict perceptible water vapor. Currently, there are more than 400 GPS stations in Taiwan; however, the data obtained through such stations are not being assessed carefully. Experienced scientists and engineers examine the data in advance to see if they qualify for research purposes, but inexperienced users can adopt poor quality data that eventually lead to inaccurate research results. Of the observation stations with receivers that were renewed between 2006 and 2008 in the International GNSS Service Network, four stations (ZIMM, BOR1, NRC1, and NICO) were selected to be the subjects of this research. Six indexes of data quality were observed to calculate the quality of data obtained before and after receiver renewal. Then, analyses were conducted to understand the relationship between the quality indexes and positioning precision. The results showed that after receiver renewal, the positioning precision of the four stations was improved by 1-19%. Therefore, positioning precision is positively affected by the six data quality indexes. It was also discovered that receiver clock error was the most critical factor among the six indexes. In conclusion, if data quality control can be applied to GPS tracking stations, the data obtained will be more reliable for research purposes, and the accuracy of subsequent engineering and science measurements will be improved. DOI: 10.1061/(ASCE)SU.1943-5428.0000077. (C) 2012 American Society of Civil Engineers.
Monotone iterative method for numerical solution of nonlinear ODEs in MOSFET RF circuit simulation
(2005)
Li, Yiming
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Yu, Shao-Ming
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Wang, Chuan-Sheng
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電信工程研究所
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Institute of Communications Engineering
(MOSFET) radio frequency (RF) circuit as a system of nonlinear ordinary differential equations. Then we solve them with the waveform relaxation method, the monotone iterative method, and Runge-Kutta method directly in time domain. With the monotone iterative method, we prove each decoupled and transformed circuit equation converges monotonically. In comparison with the HSPICE outputs, results calculated with our method are stable and robust in both the time and frequency domains. Convergence properties for the monotone iterative and outer iterative loops axe also presented and discussed. This method provides an alternative in the time domain numerical solution of MOSFET RF circuit equations.
Structural properties of pressure-induced structural phase transition of Si-doped GaAs by angular-dispersive X-ray diffraction
(2016-02)
Lin, Kung-Liang
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Lin, Chih-Ming
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Lin, Yu-Sheng
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Jian, Sheng-Rui
;
Liao, Yen-Fa
Pressure-induced phase transitions in n-type silicon-doped gallium arsenide (GaAs: Si) at ambient temperature were investigated by using angular-dispersive X-ray diffraction (ADXRD) under high pressure up to around 18.6 (1) GPa, with a 4:1 (in volume ratio) methanol-ethanol mixture as the pressure-transmitting medium. In situ ADXRD measurements revealed that n-type GaAs: Si starts to transform from zinc-blende structure to an orthorhombic structure [GaAs-II phase], space group Pmm2, at 16.4 (1) GPa. In contrast to previous studies of pure GaAs under pressure, our results show no evidence of structural transition to Fmmm or Cmcm phase. The fitting of volume compression data to the third-order Birch-Murnaghan equation of state yielded that the zero-pressure isothermal bulk moduli and the first-pressure derivatives were 75 (3) GPa and 6.4 (9) for the B3 phase, respectively. After decompressing to the ambient pressure, the GaAs: Si appears to revert to the B3 phase completely. By fitting to the empirical relations, the Knoop microhardness numbers are between H-PK = 6.21 and H-A = 5.85, respectively, which are substantially smaller than the values of 7-7.5 for pure GaAs reported previously. A discontinuous drop in the pressure-dependent lattice parameter, N-N distances, and V/V-0 was observed at a pressure of 11.5 (1) GPa, which was tentatively attributed to the pressure-induced dislocation activities in the crystal grown by vertical gradient freeze method.