The ionosphere plays a critical role in the electromagnetic waves in communication systems such as the global positioning system (GPS). However, it is suspected that the strong convection during the tropical cyclone (TC) events can be a trigger to anomalous electron density variation in the ionosphere. This study analyzed the variation of three ionosphere-related parameters based on the GPS data including scintillation index S4, cycle slips, and total electron content (TEC) rate (TECR) during the tropical cyclone event (the 2013 TC Usagi) in the Hong Kong region. The results showed that the ionosphere-related parameters had a consistent significant increase on the second day after the Usagi made landfall near Hong Kong. Consequently, the positioning performance of GPS precise point positioning (PPP) and relative positioning modes was degraded. The degradation was ~ 138%, ~ 181%, and ~ 460% in the east (root mean square (RMS) 0.050 m), north (RMS 0.045 m), and up (RMS 0.185 m), respectively, compared with the RMS of 0.021 m in the east, 0.016 m in the north, and 0.033 m in the up on the normal day. Regarding the relative positioning, the positioning errors in the east (RMS 0.134 m) and north (RMS 0.118 m) directions were ~ 7.1 and ~ 7.9 times, respectively, as large as the RMS of 0.019 m in the east and 0.015 m in the north on the normal day. The positioning errors in the up (RMS 0.513 m) direction were ~ 12.2 times larger than the RMS of 0.042 m on the normal day.
The powerful convection in the lower atmosphere, for example, tropical cyclones (TCs), has a high probability of causing ionospheric disturbances. We observed evident ionospheric disturbances during the passage of Super Typhoon Hato in 2017 by analyzing the highest elevation vertical total electron content (HeVTEC) time series, that is, the VTEC from the Global Positioning System (GPS) satellite with the highest elevation at each epoch, retrieved from a single GPS station in Hong Kong. The results demonstrate that the daily maximum of HeVTEC time series on each day during the TC period experienced a large fluctuation ranging from 33.5 TEC unit (TECU) to the peak value 62.0 TECU. The peak value 62.0 TECU occurred on the TC landfall day 23 August 2017, approximately twice as high as that of non‐TC‐impacted days. We also found that the TEC spatial gradients above the landfall area increased by around 50% and 200% in the north‐to‐south and west‐to‐east directions, respectively. We also examined the daily mean bias (MB) of VTEC above Hong Kong with respect to the mean VTEC during the past 27 days from 20 July to 15 August in 2017. The largest VTEC MB was observed in the west of the landfall area along the TC moving direction, on the TC landfall day. Our findings provided the evidence that the Hato's landfall over the coast near Hong Kong caused the apparent ionospheric disturbances above the landfall area.
The tropospheric delay is an important error source in the Global Positioning System (GPS) positioning and navigation applications. Although most of the tropospheric delays can be removed in the double-differencing (DD) positioning mode, their remaining residuals can still contaminate the positioning accuracy and become unpredictable when tropospheric condition encounters severe variations such as during a tropical cyclone (TC) event. We investigated the positioning performance of five baselines with lengths ranging from 7.8 km to 49.9 km during the 2017 TC Hato. The results showed that the TC Hato brought a significant disturbance to the GPS baseline positioning results, particularly in the vertical (up) component. The TC Hato started to affect Hong Kong and the root mean squares (RMS) of GPS positioning errors increased dramatically from about 30 mm to 140 mm.when it was at a distance of 400-600 km from Hong Kong on August 22, 2017. We found that the vertical positioning errors on that day have the major periods: 2.7 h, 3.0 h, 3.4 h, 4.0 h, and 4.8 h.Examining the wet and hydrostatic parts of the tropospheric delays via the continuous wavelet spectral analysis, we found that the periodical variation of the positioning results on August 22 was caused by the periodical variation of the precipitable water vapor (PWV). The variation of differenced PWV between two baseline stations had consistent periods of 2-5 h. Besides, the periods of differenced PWV time series are in good agreement with the spiral rainband in the TC. This finding suggests that the TC Hato induce periodical PWV variations at two GPS stations of baseline, which resulted in GPS positioning errors of the same periods.
The ionospheric effect plays a crucial role in the radio communications. For ionospheric observing and monitoring, the Global Navigation Satellite System (GNSS) has been widely utilized. The ionospheric condition can be characterized by the Total Electron Contents (TEC) and TEC Rate (TECR) calculated from the GNSS measurements. Currently, GNSS-based ionospheric observing and monitoring largely depend on a global fiducial network of GNSS receivers such as the International GNSS Service (IGS) network. We propose a new approach to observe the ionosphere by deploying a GNSS receiver on a Hong Kong Mass Transit Railway (MTR) train. We assessed the TECR derived from the MTR-based GNSS receiver by comparing it with the TECR derived from a static GNSS receiver. The results show that the Root-Mean-Squares (RMS) errors of the TECR derived from the MTR-based GNSS receiver is consistently approximately 23% higher than that derived from the static GNSS receiver. Despite the increased error, the findings suggest that the GNSS observation on a fast-moving platform is a feasible approach to observe the ionosphere over a large region in a rapid and cost-effective way.
The study of space weather is increasingly important as space weather can affect a vast array of technologies and activities in space and on Earth, including Critical National Infrastructure (CNI) systems such as power grids (Watari, 2015), the oil and gas industry (Viljanen et al., 2006), communications (Kelly et al., 2014, ground transportation (Eroshenko et al., 2010), satellite infrastructure (Loto'aniu et al., 2015), and Global Navigation Satellite Systems (GNSS) (Humphreys et al., 2010). The Sun is the ultimate cause of space weather, which occasionally erupts solar flares (Svestka, 2012), Coronal Mass Ejections (CME) (Webb & Howard, 2012), and Solar Energetic Particles (SEP) (Ryan et al., 2000). The interaction between CME and the Earth's magnetic field can produce major geomagnetic storms (Gonzalez et al., 1999). Radio blackouts, solar radiation storms, and geomagnetic storms are recognized as the three fundamental types of space weather (Eastwood et al., 2017
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