[1] The 15-month climatology of medium-scale traveling ionospheric disturbances (MSTIDs) during a solar minimum period has been constructed from observations of a dense GPS receiver array in Central China. In total, 793 MSTID events are identified, with peaks in occurrence at 1500 LT and 0100 LT. The occurrence of MSTIDs decreases following an increase in geomagnetic activity, with 46% of the MSTIDS occurring in the daytime. Daytime MSTIDs are characterized by a major occurrence maximum around the winter solstice and by an equatorward propagation direction. The period, phase velocity, azimuth, and amplitude of daytime MSTIDs are 20-60 min, 100-400 m/s, 130°-270°, and 0.8-1.5%, respectively. The remaining 54% of the MSTIDs occurred at night, and were characterized by a peak in occurrence at the summer solstice and by a southwestward propagation direction. The period, phase velocity, azimuth, and amplitude of nighttime MSTIDs are 20-70 min, 50-230 m/s, 170°-300°, and 2-7%, respectively. The propagation directions and the seasonal behaviors support the view that daytime MSTIDs are an ionospheric manifestation of atmospheric gravity waves from the lower atmosphere, while a possible excitation mechanism of nighttime MSTIDs is the electrodynamics process caused by plasma instability in the F layer.

The GNSS Occultation Sounder instrument onboard the Chinese meteorological satellite Fengyun-3C (FY-3C) tracks both GPS and BDS signals for orbit determination. One month's worth of the onboard dual-frequency GPS and BDS data during March 2015 from the FY-3C satellite is analyzed in this study. The onboard BDS and GPS measurement quality is evaluated in terms of data quantity as well as code multipath error. Severe multipath errors for BDS code ranges are observed especially for high elevations for BDS medium earth orbit satellites (MEOs). The code multipath errors are estimated as piecewise linear model in 2 • ×2 • grid and applied in precise orbit determination (POD) calculations. POD of FY-3C is firstly performed with GPS data, which shows orbit consistency of approximate 2.7 cm in 3D RMS (root mean square) by overlap comparisons; the estimated orbits are then used as reference orbits for evaluating the orbit precision of GPS and BDS combined POD as well as BDS-based POD. It is indicated that inclusion of BDS geosynchronous orbit satellites (GEOs) could degrade POD precision seriously. The precisions of orbit estimates by combined POD and BDS-based POD are 3.4 and 30.1 cm in B Wenwen Li 3D RMS when GEOs are involved, respectively. However, if BDS GEOs are excluded, the combined POD can reach similar precision with respect to GPS POD, showing orbit differences about 0.8 cm, while the orbit precision of BDSbased POD can be improved to 8.4 cm. These results indicate that the POD performance with onboard BDS data alone can reach precision better than 10 cm with only five BDS inclined geosynchronous satellite orbit satellites and three MEOs. As the GNOS receiver can only track six BDS satellites for orbit positioning at its maximum channel, it can be expected that the performance of POD with onboard BDS data can be further improved if more observations are generated without such restrictions.

Abstract. As a new member of the space-based radio occultation sounders, the GNOS (Global Navigation Satellite System Occultation Sounder) mounted on Fengyun-3C (FY-3C) has been carrying out atmospheric sounding since 23 September 2013. GNOS takes approximately 800 daily measurements using GPS (Global Positioning System) and Chinese BDS (BeiDou navigation satellite) signals. In this work, the atmospheric refractivity profiles from GNOS were compared with the ones obtained from the co-located ECMWF (European Centre for Medium-Range Weather Forecasts) reanalysis. The mean bias of the refractivity obtained through GNOS GPS (BDS) was found to be approximately − 0.09 % (−0.04 %) from the near surface to up to 46 km. While the average standard deviation was approximately 1.81 % (1.26 %), it was as low as 0.75 % (0.53 %) in the range of 5-25 km, where best sounding results are usually achieved. Further, COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) and MetOp/ GRAS (GNSS Receiver for Atmospheric Sounding) radio occultation data were compared with the ECMWF reanalysis; the results thus obtained could be used as reference data for GNOS. Our results showed that GNOS/FY-3C meets the design requirements in terms of accuracy and precision of the sounder. It possesses a sounding capability similar to COSMIC and MetOp/GRAS in the vertical range of 0-30 km, though it needs further improvement above 30 km. Overall, it provides a new data source for the global numerical weather prediction (NWP) community.

Abstract. The FY3 (Feng-Yun-3) GNOS (GNSS Occultation Sounder) mission is a GNSS (Global Navigation Satellite System) radio occultation mission of China for remote sensing of Earth's neutral atmosphere and the ionosphere. GNOS will use both the global positioning system (GPS) and the Beidou navigation satellite systems on the China FengYun-3 (FY3) series satellites. The first FY3-C was launched at 03:07 UTC on 23 September 2013. GNOS was developed by the Center for Space Science and Applied Research, Chinese Academy of Sciences (CSSAR). It will provide vertical profiles of atmospheric temperature, pressure, and humidity, as well as ionospheric electron density profiles on a global basis. These data will be used for numerical weather prediction, climate research, and ionospheric research and space weather. This paper describes the FY3 GNOS mission and the GNOS instrument characteristics. It presents simulation results of the number and distribution of GNOS occultation events with the regional Beidou constellation and the full GPS constellation, under the limitation of the GNOS instrument occultation channel number. This paper presents the instrument performance as derived from analysis of measurement data in laboratory and mountain-based occultation validation experiments at Mt. Wuling in Hebei Province. The mountain-based GNSS occultation validation tests show that GNOS can acquire or track low-elevation radio signal for rising or setting occultation events. The refractivity profiles of GNOS obtained during the mountain-based experiment were compared with those from radiosondes. The results show that the refractivity profiles obtained by GNOS are consistent with those from the radiosonde. The rms of the differences between the GNOS and radiosonde refractivities is less than 3 %.

The effects of typhoon Matsa on the ionosphere are studied by using GPS-TEC data observed at about 50 GPS stations. It is shown that the ionosphere has been already influenced and TEC tends to increase before the landing of Matsa, and the difference of TEC from its monthly median over the typhoon area is about 5 TECU. With the landing of Matsa, both the magnitude and the area of increased TEC decrease. One day after the landing of Matsa, TEC reaches its minimum and is lower than the monthly median. In comparison of TEC along the typhoon's path with that along three reference paths far from the typhoon, it is found that typhoon's impact on TEC can be fully distinguished. The evolution of TEC variation has the same tendency as reported typhoon-induced foF2.typhoon Matsa, ionosphere, GPS, coupling, TEC map Citation: