The state-of-art global navigation satellite system (GNSS) occultation sounder (GNOS) onboard the FengYun 3 series C satellite (FY-3C) has been in operation for more than five years. The accumulation of FY-3C GNOS atmospheric data makes it ready to be used in atmosphere and climate research fields. This work first introduces FY-3C GNOS into tropopause research and gives the error evaluation results of long-term FY-3C atmosphere profiles. We compare FY-3C results with Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) and radiosonde results and also present the FY-3C global seasonal tropopause patterns. The mean temperature deviation between FY-3C GNOS temperature profiles and COSMIC temperature profiles from January 2014 to December 2017 is globally less than 0.2 K, and the bias of tropopause height (TPH) and tropopause temperature (TPT) annual cycle derived from both collocated pairs are about 80–100 m and 1–2 K, respectively. Also, the correlation coefficients between FY-3C GNOS tropopause parameters and each radiosonde counterpart are generally larger than 0.9 and the corresponding regression coefficients are close to 1. Multiple climate phenomena shown in seasonal patterns coincide with results of other relevant studies. Our results demonstrate the long-term stability of FY-3C GNOS atmosphere profiles and utility of FY-3C GNOS data in the climate research field.
GNSS single-frequency occultation processing technology has the advantage of simple instrumentation, but it is not clear about the accuracy of the Beidou-based single-frequency occultation processing. This paper verifies the single-frequency occultation processing algorithm of the BeiDou navigation system (BDS) and analyzes its accuracy based on occultation observation data from the FY3E satellite. The research aimed to verify the single-frequency ionospheric relative total electron content (relTEC), analyze the accuracy of the reconstructed second frequency B3∗’s excess phase Doppler, and analyze the accuracy of the refractive index products. Results: (1) As for relTEC and excess phase Doppler, the correlation coefficient between single-frequency occultation processing and dual-frequency occultation processing is greater than 0.95. (2) The relative average deviations of the excess phase Doppler of B3∗ are mostly less than 0.2%, and the relative standard deviations are mostly around 0.5%. (3) The bias index and root mean square index of single/dual-frequency inversion have good consistency compared with ERA5 data. All the results show that the single- and dual-frequency inversion refractive index products have comparable accuracies, and the accuracy of the standard deviation of single-frequency inversion refractive index products over 25 km being slightly lower than that of dual-frequency inversion refractive index products.
Studies have proved that the Global Navigation Satellite System (GNSS) Polarimetric Radio Occultation (PRO) technology has the possibility of detecting rainfall. This study uses GPM DPR products as rainfall rate data to collocate with the latest PAZ satellite observation data, and selects representative rainfall events with a wide range of rainfall and matching with RO events. By selecting 7 raindrop shapes such as TB, and 5 raindrop size distribution models such as MP, the T-matrix method is used to simulate these rainfall events. The Pearson correlation coefficient, root mean square error, and other parameters between the simulated polarimetric phase shift and the observation data calibrated using linear trend, or the observation data calibrated using antenna pattern are calculated respectively. The Pearson correlation coefficients between the simulated value and the calibrated value using linear trend, or the calibrated value using antenna pattern are 0.9994 and 0.9933, respectively. The root mean square error between the simulated value and the calibrated value using linear trend, or the calibrated value using antenna pattern are 0.3429 and 1.2765, respectively. The comparative analysis results show that there is a high correlation between the simulated value and the polarimetric phase shift measured by PAZ and the simulated results are closer to the polarimetric phase shift calibrated using linear trend. The results show that adopting MP or JD as the raindrop size distribution model and SC or PB as the raindrop shape can get higher accuracy when simulating events with a small rainfall rate (below 1 mm·h–1). For events with high rainfall rates (above 1 mm·h–1), selecting the MP or SS raindrop size distribution model and the TB raindrop shape can simulate the best results.
Global Navigation Satellite System (GNSS) Radio Occultation (RO) is a novel detection technique that can provide global ionospheric products with high vertical resolution, high precision, and low cost. In recent years, China has launched the FY3 series of meteorological satellites carrying the first RO payload to simultaneously receive GPS (Global Positioning System) and BDS (BeiDou Navigation System) signals. In the accuracy assessment of RO products observed by GNOS (GNSS Occultation Sounder), the maximum F2-layer electron density (NmF2) of GPS occultation and BDS occultation have a standard deviation (std) of less than 20% in comparison with that of ionosondes. The std of F-layer worst-case ionospheric scintillation index ( $S4_{max}^F$ ) between COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) and FY3/GNOS is less than 0.1. The above results prove the high precision of FY3 ionospheric RO products. The RO products have been applied to preliminary scientific research and applications, e.g., the process of main phase and recovery phase of magnetic storms revealed by the NmF2 observed by FY3C/GNOS, the pre-midnight dynamics of F-layer strong scintillation during magnetic storms revealed by GNOS scintillation data, the ionospheric perturbation driven by Tonga volcano eruption revealed by FY3/GNOS, applications of the RO data for the research of sporadic E layers, evaluation of IRI model in statistics and ionospheric climatological characteristics, etc. With the successive network observation and continuous deployment of FY3 meteorological satellites, the continuous improvement of GNOS payload and the BDS system, massive high-precision ionospheric RO products will be developed and show more significant value.
The development of small-satellite technologies allows the low Earth orbit intersatellite link (LEO-LEO) occultation method to observe the Earth’s atmosphere with global coverage and acceptable costs using electromagnetic signals, in which the L/X/K/M band and short-wave infrared band signals have been well demonstrated to be suitable. We hence need to investigate the impacts of orbital and constellation parameters on the number and spatiotemporal distribution of LEO-LEO occultation events for best-possible LEO-LEO occultation mission design and optimization at the targeted mission size. In this study, firstly, an occultation events location simulation model accounting for the right ascension of the ascending node (RAAN) precession was set up and the concept of a time-dependent global coverage fraction of occultation events was defined. Secondly, numerical experiments were designed to investigate the orbital parameters’ impacts and to assess the performance of LEO-LEO occultation constellations, in which the Earth is divided into 5° × 5° latitude and longitude cells. Finally, the number, timeliness, and global coverage fraction of occultation events for two-orbit and multi-orbit LEO-LEO constellations were calculated and analyzed. The results show that: ① the orbit inclination and RAAN are the main impacting parameters followed by orbital height, while the RAAN precession is a relevant modulation factor; ② co-planar counter-rotating receiving and transmitting satellite orbits are confirmed to be ideal for a two-satellite LEO-LEO constellation; ③ polar and near-polar orbit constellations most readily achieve global coverage of occultation events; near-equator orbit constellations with supplementary receiving and transmitting satellite orbit planes also readily form the occultation event geometry, though the occultation events are mainly distributed over low and low-to-middle latitude zones; and ④ a well-designed larger LEO-LEO occultation constellation, composed of 36–72 satellites, can meet the basic requirements of global numerical weather prediction for occultation numbers and timeliness, yielding 23,000–38,000 occultation events per day and achieving 100% global coverage in 12–18 h.
China's FengYun-3 E (FY-3E) meteorological satellite with the payload Global Navigation Satellite System Occultation Sounder II (GNOS II), which is the upgraded GNSS remote sensor of FY-3C and D satellites' GNOS I, was launched on July 5 th , 2021. Sea surface wind speed(SWS) is the primary product of the Fengyun-3 E GNOS II' new GNSS reflection (GNSS-R) functions. This paper reviews the new GNSS reflection functions of Fengyun-3E meteorological satellite GNOS II and introduces the preliminary GNSS-R level 1 calibration product and level 2 wind speed product.
Earthquake forecasting is the process of forecasting the time, location, and magnitude of an earthquake, hoping to gain some time to prepare to reduce the disasters caused by earthquakes. In this paper, the possible relationship between the maximum electron density, the corresponding critical frequency, and the occurrence of earthquakes is explored by means of radio occultation data based on mechanism analysis and actual earthquake-nearby data. A new disturbance frequency index is proposed in this paper as a novel method to help forecast earthquakes. Forecasting of the location and timing of earthquakes is based on the connection between proven new frequency distributions and earthquakes. The effectiveness of this index is verified by backtracking observation around the 2022 Ya’an earthquake. Using this index, occultation data can forecast the occurrence of earthquakes five days ahead of detection, which can help break the bottleneck in earthquake forecasting.
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