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The global navigation satellite system (GNSS) radio occultation (RO) technique is an atmospheric sounding technique that originated in the 1990s. The data provided by this approach are playing a consistently significant role in atmospheric research and related applications. This paper mainly summarizes the applications of RO to numerical weather prediction (NWP) generally and specifically for tropical cyclone (TC) forecast and outlines the prospects of the RO technique. With advantages such as high precision and accuracy, high vertical resolution, full-time and all-weather, and global coverage, RO data have made a remarkable contribution to NWP and TC forecasts. While accounting for only 7% of the total observations in European Centre for Medium-Range Weather Forecasts’ (ECMWF’s) assimilation system, RO has the fourth-largest impact on NWP. The greater the amount of RO data, the better the forecast of NWP. In cases of TC forecasts, assimilating RO data from heights below 6 km and from the upper troposphere and lower stratosphere (UTLS) region contributes to the forecasting accuracy of the track and intensity of TCs in different stages. A statistical analysis showed that assimilating RO data can help restore the critical characteristics of TCs, such as the location and intensity of the eye, eyewall, and rain bands. Moreover, a non-local excess phase assimilation operator can be employed to optimize the assimilation results. With denser RO profiles expected in the future, the accuracy of TC forecast can be further improved. Finally, future trends in RO are discussed, including advanced features, such as polarimetric RO, and RO strategies to increase the number of soundings, such as the use of a cube satellite constellation.

With the accumulation of the ionospheric radio occultation (IRO) data observed by Global Navigation Satellite System (GNSS) occultation sounder (GNOS) onboard FengYun-3C (FY3C) satellite, it is possible to use GNOS IRO data for ionospheric climatology research. Therefore, this work aims to validate the feasibility of FY3C/GNOS IRO products in climatology research by comparison with that of Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), laying the foundation for its application in climatology study. Since previous verification works of FY3C/GNOS were done by comparison with ionosondes, this work matched NmF2/hmF2 of FY3C/GNOS and COSMIC into data pairs to verify the profile-level accuracy of FY3C/GNOS IRO data. The statistical results show that the overall correlation coefficients of both NmF2 and hmF2 are above 0.9, the overall bias and std of NmF2 differences between FY3C/GNOS and COSMIC are −2.19% and 17.48%, respectively, and the bias and std of hmF2 differences are −3.29 and 18.01 km, respectively, indicating a high profile-level precision consistency between FY3C/GNOS and COSMIC. In ionospheric climatology comparison, we divided NmF2/hmF2 of FY3C/GNOS into four seasons, then presented the season median NmF2/hmF2 in 5° × 10° grids and compared them with that of COSMIC. The results show that the ionospheric climatological characteristics of FY3C/GNOS and COSMIC are highly matched, both showing the typical climatological features such as equatorial ionosphere anomaly (EIA), winter anomaly, semiannual anomaly, Weddell Sea anomaly (WSA) and so on, though minor discrepancies do exist like the differences in magnitude of longitude peak structures and WSA, which verifies the reliability of FY3C/GNOS IRO products in ionospheric climatology research.

Tropopause region is a significant layer among the earth's atmosphere, receiving increasing attention from atmosphere and climate researchers. To monitor global tropopause via radio occultation (RO) data, there are mainly two methods, one is the widely used temperature lapse rate method, and the other is bending angle covariance transform method. In this paper, we use FengYun3-C (FY3C) and Meteorological Operational Satellite Program (MetOp) RO data and European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data to analyse the difference of RO tropopause height calculated by the two methods mentioned above. To give an objective and complete analysis, we first take ECMWF lapse rate tropopause (LRT) height (LRTH) as reference to discuss the absolute bias of RO LRTH and RO bending angle tropopause (BAT) height (BATH), and then give the comparison results between RO LRTH and corresponding RO BATH as supplement to analyse the difference between tropopause height derived from the above two methods. The results indicate that BATH show consistent 0.8–1.2 km positive bias over tropics and high latitude region compared with LRTH, and over mid latitude region, results of BATH show less stability. Besides, the mean bias between BATH and LRTH presents different symmetrical characteristic during 2017.12–2018.2 (DJF) and 2018.6–2018.8 (JJA). However, the mean value of both LRTH and BATH show the similar tropopause variation trend, indicating the availability of both two methods.

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.

GNOS is a scientific instrument for monitoring the Earth's neutral atmosphere and ionosphere. It provides vertical profiles of atmospheric temperature, pressure and humidity, as well as ionospheric electron density. GNOS is one of the important payloads on Fengyun3-D low-earth orbit satellite launched at November 15th 2017, and began to work on November 25th 2017. GNOS on FY3-D improved a lot in both function and performance compared with GNOS on FY3-C satellite that had been launched in September 2013. This paper introduces the FY3-D GNOS's improvements compared with FY3-C GNOS, and describes the expected results of FY3-D GNOS. In-orbiting validation is ongoing, and the assessment of the FY3-D GNOS will be finished in the first half of next year. Preliminary results will be given in the conference.

The dual-system compatible Global Navigation Satellite System(GNSS) occultation sounder (GNOS) for Beidou Navigation Satellite System (BDS) and Global Positioning System(GPS) was launched into orbit with FY3C satellite on September 23, 2013, in which we can get an important ionospheric parameter: F2-layer peak electron density (NmF2), it can limit the maximum feasible frequency for ground signal propagation, as well as defining the minimum required frequency for transionospheric radio wave propagation, so it's a crucial parameter for charactering ionosphere and the space weather. In this presentation, we verify the precision of ionospheric BDS radio occultation (BDSRO) data obtained by FY3C/GNOS through NmF2 comparisons between BDSRO and ionosondes. The results show that the correlation coefficient of NmF2 data between BDSRO and ionosondes is 0.96, the bias is 10.21 % and the standard deviation (std) is 19.61 %. The NmF2 precision of BDSRO at daytime is higher than that of nighttime; When Beidou satellites are at Inclined Geosynchronous Satellite Orbit (IGSO), its precision is higher than that at Geosynchronous orbit (GEO) and Medium Earth Orbit (MEO).

The Global Navigation Satellite System Reflectometry (GNSS-R) has been proven to be a powerful technique for retrieving geophysical parameters of ocean and land. Airborne GNSS-R is an important experimental platform, because it is not only needed as validation for spaceborne application, but also possesses the advantages to be capable of remote sensing of small and medium scale targets, such as rivers and lakes. This paper presents an overview of China’s airborne GNSS-R campaign conducted on May 30, 2014, in Henan. The campaign has two objectives, i.e.: (1) to examine the capability of a GNSS-R payload developed by National Space Science Center, Chinese Academy of Sciences (NSSC, CAS) for airborne observations and (2) to study the algorithms for soil moisture and river remote sensing, including altimetry and flow velocity measurement. A previous paper has presented results of soil moisture retrieval as part I, and in this paper, initial results of the Yellow River remote sensing are presented as part II. This paper presents the river altimetry results and explores a new potential application of GNSS-R technology, which is used to detect the flow velocity of the river. The river surface height results observed by code delay altimetry method were consistent with the height results of GPS dual-frequency differential positioning altimetry. The GNSS-R altimetry results showed that decimeter level heights were achieved in 1-min sliding average. Comparing with in situ measurements, the GNSS-R flow velocity result was reasonable; the error was about 0.027 m/s, which indicated the validity and feasibility of using GNSS-R technique to detect river flow velocity.

In Global Navigation Satellite System Radio Occultation (GRO), the tropospheric temperature and humidity can only be retrieved separately from refractivity by co-using a priori humidity or temperature information. Fortunately, the LEO-LEO (Low Earth Orbit) microwave occultation (LMO) exploits both the refraction and absorption of signals to solve the temperature-humidity ambiguity, and so it can retrieve the pressure, temperature, and humidity profiles independently. Furthermore, using LMO, ozone profiles can be retrieved by the signals around ozone absorption lines, and the liquid water and ice cloud variables can also be retrieved as by-products. In this paper, the measurement principle and capabilities of LMO technique are provided as an overview based on available literature; and then a pre-study of LMO including orbit design, frequency channels selection, performance analysis, and transmitter and receiver design/development is presented. The encouraging performance prospects of LMO give us confidence that it is highly worthwhile to pursue a space-borne demonstration mission.

Global Navigation Satellite System (GNSS) Occultation Sounder (GNOS) instruments were designed for the FengYun-3 (FY-3) series of meteorological satellites for sounding the Earth's atmosphere and ionosphere by radio occultation. Meanwhile, another GNSS remote sensing technique named GNSS reflectometry (GNSS-R) has been developed rapidly as well. To integrate these two GNSS remote sensing techniques in one payload, to help save the satellite's space, power and mass, a new instrument named GNOS II has been designed for the FY-3E satellite. The FY-3E satellite is anticipated to monitor the ionosphere, neutral atmosphere, sea wave and wind field by using GNSS signals in the future. So far, these two functional modules have been validated by ground-based, airborne or space-based campaigns. The GNOS II configuration and main performances characteristics are presented in this paper. According to its demonstrated performance, one can draw the conclusion that GNOS II will provide more and higher quality radio occultation data and new reflected GNSS signal delay Doppler maps (DDMs) for sea wave and wind field detection.

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 Feng-Yun-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 rapid advancement of global navigation satellite system (GNSS) occultation technology in recent years has made it one of the most advanced space detection technologies of the 21st century. GNSS radio occultation has many advantages, including all-weather operation, global coverage, high vertical resolution, high precision, long-term stability, and self-calibration. Data products from GNSS occultation sounding can greatly enhance ionospheric observations and contribute to space weather monitoring, forecasting, modeling, and research. In this study, GNSS occultation sounder (GNOS) results from a radio occultation sounding payload aboard the Fengyun 3-C (FY3-C) satellite were compared with ground-based ionosonde observations. Correlation coefficients for peak electron density (NmF2) derived from GNOS Global Position System (GPS) and Beidou navigation system (BDS) products with ionosonde data were higher than 0.9, and standard deviations were less than 20 %. Global ionospheric effects of the strong magnetic storm event in March 2015 were analyzed using GNOS results supported by ionosonde observations. The magnetic storm caused a significant disturbance in NmF2 and hmF2 levels. Suppressed daytime and nighttime NmF2 levels indicated mainly negative storm conditions. In the zone of geomagnetic inclination between 40–80 °, average NmF2 during the geomagnetic storm showed the same basic trends in GNOS measurements, and in observations from 17 ground-based ionosonde stations, and confirmed the negative effect of the event on the ionosphere. The analysis demonstrates the reliability of the GNSS radio occultation sounding instrument GNOS aboard the FY3-C satellite, and confirms the utility of ionosphere products from GNOS for statistical and event-specific ionospheric physical analyses. Future FY3 series satellites, and increasing numbers of Beidou navigation satellites, will provide increasing GNOS occultation data on the ionosphere, which will contribute to ionosphere research and forecasting applications.

With the development of global navigation satellite system (GNSS) remote sensing, radio occultation (RO) has become an important atmospheric observation technique that mainly be used for numerical weather prediction (NWP) and global climate monitoring (GCM). However, residual ionopheric error (RIE) can still be significant after an ionospheric error elimination by linear combination of dual-frequency RO bending angles. Lots of quantification and analysis of RO bending angle RIEs has been conducted by using quasi-realistic end-to-end simulations. In this study, the RIEs in RO observations have been investigated by comparative analysis of COSMIC RO measurements and ECMWF model data. The results illustrate that the RIE is a major source of the bending angle in lower mesosphere and upper stratosphere, and its bias has a clear negative tendency that in line with our empirical studies based on simulation RO data. This study helps to inform future RIE mitigation schemes for ensuring benchmark quality stratospheric RO data.

The GNOS instruments are GNSS radio occultation payloads for atmospheric and ionospheric sounding onboard FY-3 meteorological satellites. The first GNOS instrument has been in orbit since September 23, 2013 when the FY-3C satellite was launched. The next generation GNOS instrument is being developed for the 3rd batch of FY-3 series satellites. This paper presents the new instrument's characteristics in design and main performance in occultation tracking. We show that the phase center variety is low enough for the choke-ring antenna new designed to increase the accuracy of precise orbit determination. A wide coverage atmosphere occultation antenna and a 65nm technology ASIC are used in the new generation GNOS instrument. The new GNOS instrument will acquire higher quality occultation data due to its better performance.

The GNOS (GNSS Occultation Sounder) instrument on FY3 (Feng-Yun-3) meteorology satellite is a GNSS (Global Navigation Satellite System) radio occultation payload for remote sensing of Earth's neutral atmosphere and the ionosphere. GNOS's first flight mission is onboard the Chinese FY3C meteorological satellite, which was launched at 03:07 UTC 23 September 2013. GNOS was switched on 5 days later. From then, GNOS could receive more than 700 (500 GPS + 200 BDS) radio occultation events per day on a global basis. GNOS provides vertical profiles of atmospheric temperature, pressure, and humidity, as well as ionospheric electron density of the earth. These data are being used for NWP (numerical weather prediction), space weather, climate and ionospheric research. GNOS will also be on-board the follow-on FY3 satellites in the future. Some improvements will be implemented, including extended occultation channel number, open loop tracking for L2C and B1I, optimized antenna pattern and so on. This paper focuses on the GNOS design and characteristics for FY3D satellite, which is planned to be launched in 2016. The instrument performance derived from analysis of measurement data in laboratory and mountain-based occultation experiments at Mt. Wuling in Hebei Province is presented. The improvements of GNOS on-board FY3D were validated.

GNOS is a dual-frequency GPS/Beidou receiver developed for GNSS radio occultation studies. It is developed and manufactured by National Space Science Center(NSSC) under a contract with the China Meteorological Administration(CMA) and is designed to provide vertical profiles of atmospheric temperature, pressure, humidity, as well as ionospheric electron density on a global scale. GNOS first flight mission is on-board the Chinese FY3C meteorological satellite, launched on Sep, 2013. It will also be on-board the follow-up FY3 satellites in the future. This paper focuses on the current GNOS design and characteristics for the FY3C mission. The instrument performance derived from analysis of measurement data in laboratory and mountain-based occultation validation experiments is presented. The comparison between Open Loop and Close Loop are shown. The refractivity profiles of GNOS obtained during the mountain-based experiment are also compared with those from radiosondes.