Browsing by Author "Zhang, Kefei"

Now showing 1 - 2 of 2

Assessing antenna field of view and receiver clocks of COSMIC and GRACE satellites: lessons for COSMIC-2
(2014-04-01)
Tseng, Tzu-Pang
;
Zhang, Kefei
;
Hwang, Cheinway
;
Hugentobler, Urs
;
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.
Identifying the Relationship between GPS Data Quality and Positioning Precision: Case Study on IGS Tracking Stations
(2012-08-01)
Yeh, Ta-Kang
;
Chung, Yi-Da
;
Wu, Chien-Ting
;
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.