Browsing by Author "Xu, Guochang"

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Clarifying the Relationship between Quality of Global Positioning System Data and Precision of Positioning
(2010-02-01)
Yeh, Ta-Kang
;
Chen, Yi-Jao
;
Chung, Yi-Da
;
Feng, Chung-Wei
;
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
;
Hwang, Cheinway
;
Xu, Guochang
;
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
;
Hwang, Cheinway
;
Xu, Guochang
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土木工程學系
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Department of Civil Engineering