Browsing by Author "Yeh, Ta-Kang"

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Clarifying the Relationship between Quality of Global Positioning System Data and Precision of Positioning
(2010-02-01)
Yeh, Ta-Kang
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Chen, Yi-Jao
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Chung, Yi-Da
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Feng, Chung-Wei
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Xu, Guochang
This study selected five indexes to determine the relationship between data quality and positioning precision. These indexes are multipath on L1, multipath on L2, observations by cycle slips, clock offset, and clock stability. To reduce the number of common errors, six types of global positioning system (GPS) receivers are selected and set up in an Ultra-Short Distance Network at the National Measurement Laboratory, Taiwan, for 3 days. The first three indicators are calculated with the Teqc software, and the last two indicators are computed using the Bernese software. Moreover, the observations from three GPS tracking stations, namely, short baseline (about 25 km), middle baseline (about 150 km), and long baseline (around 2000 km), were downloaded from the Internet and processed using Bernese software. Analytical results indicate that clock stability and observations divided by cycle slips are found to be very important positioning factors. Clock offset and multipath effects do not significantly affect the GPS results.
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
;
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.
GPS Height and Gravity Variations Due to Ocean Tidal Loading Around Taiwan (vol 29, pg 37, 2008)
(2009-02-01)
Yeh, Ta-Kang
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Hwang, Cheinway
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Xu, Guochang
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土木工程學系
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Department of Civil Engineering
GPS Height Variations Affected by Ocean Tidal Loading Along the Coast of Taiwan
(2016-05-15)
Chung, Y. D.
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Yeh, Ta-Kang
;
Xu, G.
;
Chen, C. S.
;
Hwang, C.
The Global Positioning System (GPS) uses static relative positioning to mitigate common errors and has the ability to remove the effects of ocean tidal loading (OTL). OTL effects cannot be completely removed by increasing the baseline length during GPS data processing and must be corrected with OTL models, especially in Taiwan, which is surrounded by the ocean. In this paper, we collected GPS data from ten stations that are located along the coast of Taiwan. The periodic effect of OTL is discussed through a wavelet analysis and a harmonic analysis. We also calculated the amplitudes and phases of the semi-diurnal and diurnal tides to understand the effect of OTL on the GPS heights along the coast of Taiwan. The GPS data were divided into short-term (1 month) and long-term (2 years) categories and were calculated using the Bernese 5.0 software. The tidal data and the GPS heights were transferred into a wavelet spectrum for analysis. We found that in the tidal data, the periodic M2 signal was stronger than the K1 signal, which was not the case for the GPS height data. This result indicated that the common errors were not completely removed from the GPS heights and that the corresponding period was approximately 1 day, which might result in a stronger K1 signal. Moreover, five OTL models were chosen and compared in this case. Although the GPS heights with different OTL corrections were very similar, NAO. 99b was the best fitting model for the ocean around Taiwan. Finally, we analyzed the tidal data and the M2 amplitudes and phases of the GPS heights using a harmonic analysis and compared the results with the NAO. 99b model. The comparison indicated that the best agreement occurred in southwestern Taiwan and that the poorest agreement occurred in southeastern Taiwan with a maximum error of 0.3 cm. The phase results showed that the OTL model overlapped with the tidal data in the open ocean around central Taiwan, and both the GPS heights and the heights from the OTL model were 1.5 to 2 h earlier than the tidal data.
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
;
Wu, Chien-Ting
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Wang, Chuan-Sheng
;
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.
Vertical Displacement due to Ocean Tidal Loading Around Taiwan Based on GPS Observations
(2011-08-01)
Yeh, Ta-Kang
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Hwang, Cheinway
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Huang, Jiu-Fu
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Chao, Benjamin Fong
;
Chang, Ming-Han
Ocean tidal loading (OTL) is an important factor in GPS positioning, especially along the vertical direction. OTL influences the precision of GPS positioning and produces height variations of up to 12 cm. In this study, daily UPS data obtained from 27 GPS tracking stations around Taiwan were collected, and four OTL models were applied to relative static GPS positions derived from these data. The UPS data were obtained from 8 - 14 August 2006 (summer) and 1 - 30 January 2007 (winter). The software Bernese 5.0 was utilized for data processing. During the summer, the average GPS error along the vertical axis was 2.72 cm. After correcting the measured heights with the Schwiderski, NAO.99b, CSR4.0 and TPXO.6.2 models, the average errors became 2.30, 2.30, 2.30 and 2.31 cm, respectively. In winter, the average UPS error in the vertical direction was 1.32 cm. After correcting the heights using the above models, the average errors became 0.86, 0.85, 0.89 and 0.84 cm, respectively. The precision of the vertical direction was improved by approximately 15 - 36% on average. In eastern Taiwan, all UPS stations in the vertical direction have amplitudes smaller than 1 cm, because the ocean is more than 2000 m deep and with less shallow and narrow bights and estuaries. On the other hand, the amplitudes of GPS stations in northwestern Taiwan are larger than 1 cm because the ocean is only approximately 200 m deep. The amplitudes in the vertical direction of UPS stations are approximately 1.2 - 2.7 cm on the offshore islands of Taiwan, larger than average in Taiwan. In the future, local corrections from an OTL model are necessary for high accurate GPS positioning.