Abstract
In high-precision GPS precise point positioning (PPP) time transfer, errors caused by the effect of ionosphere delay have to be corrected. Usually the ionosphere-free combinations of the pseudo code and the carrier phase is used in GPS PPP data processing, and it effectively eliminates the effect of the first-order ionospheric delay. This study quantitatively analyzes the errors caused by higher-order ionospheric (Ion2+) delays in precise PPP time transfer. Data of two 7-day test periods, including low and moderate ionospheric conditions, from 20 stations located in middle- and low-latitude, were analyzed. The difference in clock solution with and without the Ion2+ correction, including the receiver clock solution and time-link clock solution, was deeply analyzed and discussed. The difference sequence shows a constant bias plus some variations with a diurnal variation. For the difference of the receiver clock solutions, the mean standard deviation of the variations is 3.92 ps in low-latitude, which is much larger than that of 2.59 ps in mid-latitude due to the influence of the larger ionospheric electron density on the low-latitude. The maximum constant bias reached more than 15 ps and was negative at most stations in the northern hemisphere, while it was positive at most stations located in the southern hemisphere. The difference in the time-link solutions correlates not only with time and region, but also with the length of the time-links. The largest difference in the long time-link SYDN-PTBB, BJNM-SYDN, AMC2-SYDN, etc., reaches more than 25 ps, while that of the short time-link IENG-PTBB, BRUX-PTBB, etc., is less than 3.5 ps. Therefore, the Ion2+ correction is necessary for high-precision PPP time transfer over long time-links, especially time-links made by one station located in the northern hemisphere and another located in the south hemisphere; however, it could be ignored for short time-links.
Highlights
Time transfer is based on the processing of transmitted time signals from one station to another
Dual-frequency Global Positioning System (GPS) signals have been successfully used for precise positioning and timing
The goal of this paper is to evaluate the effect of the Ion2 and Ion3 (Ion2+) delay terms on GPS precise point positioning (PPP) time transfer
Summary
Time transfer is based on the processing of transmitted time signals from one station to another. Dual-frequency Global Positioning System (GPS) signals have been successfully used for precise positioning and timing. The main methods of precise time transfer using GPS are common view (CV), all-in-view (AV), and precise point positioning (PPP). The current GPS PPP is used as a routine method for precise time and frequency transfer in many time laboratories around the world [1,2]. GPS PPP is a joint analysis of dual-frequency pseudo code and carrier phase observation from a single GPS station. It calculates a high-precision receiver coordinate and clock and tropospheric delay, among others. Ionospheric delay is an important error in GPS PPP positioning and timing application [7,8,9]. The goal of this paper is to evaluate the effect of the Ion and Ion (Ion2+) delay terms on GPS PPP time transfer
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