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S4 index is the rapid modification of signals when propagating through small-scale structures in the ionosphere, called ionospheric irregularities. The rapid modification of the signal causes the scintillation of the power. The purpose of this letter is to confirm the superposition property of the S4 index by using the ray-tracing method to simulate the signal propagation path through the ionospheric irregularities and then calculate the amplitude and S4 index of the received signal. The relation between the ionospheric scintillation S4 index and the signal propagation path can be understood. Then, the assimilation method for the S4 index can be developed to monitor the ionospheric irregularity in the future.

Inversion of radio occultation (RO) measurements to atmospheric parameters in the neutral atmosphere utilizes the assumption of spherical symmetry by implementation of the Abel transform. The main contribution to the retrieved refractional angle and other geophysical parameters comes from gaseous properties of the atmosphere. The atmospheric refraction is expressed by a function of air pressure, air temperature, and water vapor pressure. Such commonly adopted methodology results in highly comparable RO retrievals with background models. However, in the lowermost troposphere referred to as planetary boundary layer, inversion in spherically symmetric atmosphere is an ill‐conditioned problem. The presence of superrefractions introduces negative errors in the RO‐retrieved refractivity (N‐bias). We show that significant refractivity gradients are frequently collocated with clouds over oceans in tropical and subtropical regions. Based on gridded monthly means we show that superrefractions usually occur at altitudes up to 2 km and the largest cloud fractions tend to suspend at underlying layers. The magnitude of clouds expressed in terms of refractivity units can exceed 1.5, which corresponds to 0.5% in terms of fractional differences. We use both geometrical optics and wave optics techniques to illustrate propagation mechanisms in RO retrievals. Simulation experiments suggest that RO inversions in cloudy planetary boundary layer lead to larger negative N‐biases. Low‐level clouds retrieved from numerical weather prediction model could therefore be used as an indicator of erroneous RO observations. A better agreement with RO refractivity could be achieved by incorporating cloud variables into background fields especially over the Pacific and Atlantic Oceans.

This study investigates the Global Navigation Satellite System (GNSS) radio occultation (RO) data from FORMOSAT-7/COSMIC-2 (FS7/C2), which provides considerably more and deeper profiles at lower latitudes than those from the former FORMOSAT-3/COSMIC (FS3/C). The statistical analysis of six-month RO data shows that the rate of penetration depth below 1 km height within ±45° latitudes can reach 80% for FS7/C2, significantly higher than 40% for FS3/C. For verification, FS7/C2 RO data are compared with the observations from chartered missions that provided aircraft dropsondes and on-board radiosondes, with closer observation times and distances from the oceanic RO occultation over the South China Sea and near a typhoon circulation region. The collocated comparisons indicate that FS7/C2 RO data are reliable, with small deviations from the ground-truth observations. The RO profiles are compared with collocated radiosondes, RO data from other missions, global analyses of ERA5 and National Centers for Environmental Prediction (NCEP) final (FNL), and satellite retrievals of NOAA Unique Combined Atmospheric Processing System (NCAPS). The comparisons exhibit consistent vertical variations, showing absolute mean differences and standard deviations of temperature profiles less than 0.5 °C and 1.5 °C, respectively, and deviations of water vapor pressure within 2 hPa in the lower troposphere. From the latitudinal distributions of mean difference and standard deviation (STD), the intertropical convergence zone (ITCZ) is evidentially shown in the comparisons, especially for the NUCAPS, which shows a larger deviation in moisture when compared to FS7/C2 RO data. The sensitivity of data collocation in time departure and spatial distance among different datasets are presented in this study as well.

In this study, we determine the near real-time (NRT) clocks of the Global Positioning System (GPS) and Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS) satellites in the Taiwan RO Process System (TROPS), which is mainly designed for the data processing in both the FORMOSAT-3/COSMIC (F3C) and FORMOSAT-7/COSMIC-2 (F7C2) satellite missions. The accuracy of GNSS clocks defines the quality of the atmospheric excess phase, which is used for the retrieval of bending angle profiles in GNSS radio occultation (RO) observations. The accuracy of the NRT GNSS clocks is assessed by comparing the clock rate, clock stability and clock-induced positioning error on receivers with the final solutions given by the European Space Agency (ESA). Overall, the standard deviations of the clock rates from TROPS agree with those from ESA within 0.05 mm/s over 2304 clock solutions. Additionally, we find that the clock stability of the GPS Block IIF type (3 × 10−13) is an order of magnitude better than that of IIR Block types (3 × 10−12) over a time interval of 30 s. In comparison, the stabilities of GLONASS clocks are approximately 3 × 10−12. We quantify the NRT clock error on the receiver positioning by using the precise point positioning technique obtained from the Bernese GNSS software. The 3-dimensional clock-induced positioning error is approximately 3.3, 3.2 and 0.9 cm for station AUCK and 6.9, 6.3 and 3.1 cm for station NRC1 for the GPS-only, GLONASS-only and GPS + GLONASS cases, respectively. For GNSS-RO applications, the bending angle profiles derived using TROPS GPS clocks agree with the COSMIC Data Analysis and Archive Center products to within 0.01–1.00 μrad. However, this is not the case for the GLONASS clock, because the GLONASS clock-induced errors on the RO profile are 10–100 times greater than those induced by the GPS clock. This suggests that different weightings should be used for RO applications, such as data assimilation, when different satellite clocks are involved in GNSS-RO retrievals. This study serves as a reference for assessing the impact of GNSS clocks on both GNSS-POD (precise orbit determination) and GNSS-RO in preparation for the F7C2 satellite mission.

The COSMIC-2 mission is a follow-on mission of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) with an upgraded payload for improved radio occultation (RO) applications. The objective of this paper is to develop a near-real-time (NRT) orbit determination system, called NRT National Chiao Tung University (NCTU) system, to support COSMIC-2 in atmospheric applications and verify the orbit product of COSMIC. The system is capable of automatic determinations of the NRT GPS clocks and LEO orbit and clock. To assess the NRT (NCTU) system, we use eight days of COSMIC data (March 24-31, 2011), which contain a total of 331 GPS observation sessions and 12 393 RO observable files. The parallel scheduling for independent GPS and LEO estimations and automatic time matching improves the computational efficiency by 64% compared to the sequential scheduling. Orbit difference analyses suggest a 10-cm accuracy for the COSMIC orbits from the NRT (NCTU) system, and it is consistent as the NRT University Corporation for Atmospheric Research (URCA) system. The mean velocity accuracy from the NRT orbits of COSMIC is 0.168 mm/s, corresponding to an error of about 0.051 μrad in the bending angle. The rms differences in the NRT COSMIC clock and in GPS clocks between the NRT (NCTU) and the postprocessing products are 3.742 and 1.427 ns. The GPS clocks determined from a partial ground GPS network [from NRT (NCTU)] and a full one [from NRT (UCAR)] result in mean rms frequency stabilities of 6.1E-12 and 2.7E-12, respectively, corresponding to range fluctuations of 5.5 and 2.4 cm and bending angle errors of 3.75 and 1.66 μrad .

The formation of the sporadic E (Es) layer can be interpreted in several different ways, with wind shear theory and the meteor ionization mechanism being the most commonly used explanations. Nevertheless, neither the wind shear theory nor the meteor ionization mechanism alone can completely explain the formation of the Es layer. The meteor ionization mechanism cannot interpret the different activity in this layer between the Northern and Southern Hemispheres, while the wind shear theory cannot explain the source of the large amount of ionized particles in the Es layer. In this study, the activity in the Es layer is compared with information about meteors and the global vertical speed of ionized particles. The information about meteors is obtained from International Meteor Organization and Radio Meteor Observing Bulletin. The global vertical speed information for ionized particles is calculated using the International Geomagnetic Reference Field model, Horizontal Wind Model (HWM07), and Mass Spectrometer-Incoherent Scatter model. The activity in the Es layer is based on the value of the irregular degree index, which is derived from the signal-to-noise ratio obtained from Formosa Satellite Mission-3/Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC) Global Positioning System radio occultation mission. Taking both wind shear theory and the meteor ionization mechanism together, the source of the ionized particles in the Es layer and the difference in the activity in the Es layer between Northern and Southern Hemispheres can thus be explained more completely.

Long-term stability, global coverage and high resolution are characteristics that make the Global Navigation Satellite System (GNSS) radio occultation (RO) technique well-suitable to serve as anchor measurements for observing the Earth’s atmosphere. The concept of occultation soundings utilizes a receiver placed on a low Earth orbit to measure the accumulated atmospheric contribution along the limb in terms of a phase delay. A high sampling rate allows to reconstruct profiles of geophysical parameters through an inversion process of occultation signals. However, such measurements require a careful processing in order to provide accurate retrievals in the neutral atmosphere. The following development describes specific aspects in radio occultation methodology implemented in the retrieval chain from phase data to profiles of dry pressure and dry temperature. Independent retrievals from nearreal time measurements are compared with occultation products provided by official processing centers to demonstrate reliability of the solution. The region within the upper troposphere and lower stratosphere (UTLS) is particularly represented by a low uncertainty being within 0.5% (K). A comparison with radiosondes shows a significant contribution of a water vapor term in the lower troposphere that comes from the dry air assumption in occultation profiles of pressure and temperature. Radiosonde measurements reproduced to refractivity profiles show very high agreement with occultation soundings, which is generally below 5%. A superior accuracy of RO refractivity is observed in the upper troposphere, where retrievals are consistent with radiosondes to 1%.

Radio-occultation (RO) technique, which has been used in planetary science, is a method to obtain the profiles of atmosphere and the global atmospheric data. In 2006, Taiwan launched six low-Earth-orbit satellites as a RO constellation mission, known as FORMOSAT-3. In order to thoroughly understand the process of the RO data retrieval and obtain as much as possible the information for the weather prediction, a National Central University Radio Occultation (NCURO) retrieval algorithm has been developed. The focus of the algorithm development is on the correction of the excess phase of the signal received with open-loop (OL) technique and the criteria for assessment of the data quality. When the OL is activated, the excess phase of the Global Positioning System (GPS) signal is modulated with navigation messages of satellites. In our algorithms, two methods are incorporated to recover the excess phase. Moreover, as the altitude of the received signal decreases, the quality of the GPS signal generally deteriorates, and eventually, the signal is too noisy to be processed. In order to assess the quality of the signal, instead of the signal-to-noise ratio, the degree of unclearness is defined and used in the algorithm. In this paper, the algorithm including the phase-correction methods and the criteria for the quality assessment will be described. The data retrieval using the algorithm will be compared with those obtained from the COSMIC Data Analysis and Archive Center at the University Corporation for Atmosphere Research and Pingtung radiosonde measurement. Some intermediate results of the NCURO algorithm will also be demonstrated.

[Abstract] The FORMOSAT-3/COSMIC spacecraft constellation consisting of six LEO satellites is the world’s first operational GPS radio occultation mission. The mission is jointly developed by Taiwan’s National Space Organization (NSPO) and United States’ UCAR in collaboration with NASA’s Jet Propulsion Laboratory and Naval Research Laboratory for three onboard payloads including GPS Occultation Receiver (GOX), TriBand Beacon (TBB), and Tiny Ionospheric Photometer (TIP). The FORMOSAT

The FORMOSAT-3 mission, also known as constellation observing system for meteorology, ionosphere, and climate (COSMIC), is the third major project of the Formosa satellite (FORMOSAT) series implemented by the National Space Organization of Taiwan. FORMOSAT-3/COSMIC is a joint Taiwan/U.S. mission consisting of six identical low Earth orbit satellites. All six cluster satellites were successfully launched by a single Minotaur launch vehicle on April 15, 2006. The retrieved Global Positioning System (GPS) radio occultation (RO) data have been freely available online to the science community since shortly after the completion of satellite bus in-orbit checkout. Having completed the verification and validation, the worldwide science communities are highly satisfied with the RO data. Scientists have hailed the RO sensors as offering the most accurate, precise, and stable thermometers in space. After one year in orbit, all six FORMOSAT-3/COSMIC satellites were in good condition (except FM2, which had power shortage issues) and were on their way toward the final constellation of six separate orbit planes with 30 deg separation. Four out of six satellites had already reached their final mission orbit of 800 km by mid-May 2007. Together, the six satellites have generated a total of more than 2500 RO data per day. However, only 50%-70% of the RO data as received one year after launch could be retrieved into useful atmosphere profiles. The retrieved RO data, about 1800 per day on average, have been assimilated into numerical weather prediction models by many major weather forecast centers and research institutes. This paper provides an overview of the constellation mission, the spacecraft system performance after one year in orbit, the technical challenges we have encountered, and the performance enhancements we have accomplished.

The Global Positioning System (GPS) is used to provide hourly measurements of precipitable water (PW) in Taiwan during the passage of tropical cyclones. Typhoon Zeb, which caused serious damage in the Philippines, Taiwan, and Japan in mid-October 1998, is used as an example in this paper. GPS data are analyzed from the Central Weather Bureau’s (CWB’s) three weather stations in Taiwan, and from a site in Tsukuba, Japan. Bernese version 4.0 software is utilized to solve GPS signals for total delay due to the neutral atmosphere at the three CWB sites. Wet delay is obtained by subtracting surface pressure derived dry delay from total delay. Wet delay is then converted to PW through a simple calculation. GPS-observed PW time series demonstrate that PW is, in general, high before and during the occurrence of the typhoon, and low after the typhoon. PW increased from about 5 cm on DoY 285 (October 13) to near 8 cm or so on DoY 288 (October 16) when the typhoon was striking Taiwan, and, then, decreased to 2–3 cm after passage of the typhoon. In addition, GPS-observed PW depletion from 8 cm on DoY 288 to about 3 cm on DoY 290 is found to be consistent with radiosonde observations acquired at the Taipei weather station.

The sensing of absolute precipitable water vapor (PW) by the Global Positioning System (GPS) and a Water Vapor Radiometer (WVR) is presented. The GPS approach requires a priori knowledge of the relationship between the weighted mean temperature of the atmosphere and surface temperature whose regression relationship is derived based on ten-year climatological data observed by radiosonde and surface meteorological instruments. Similarly, the WVR scheme needs a priori information of the relationship between sky brightness temperature and PW whose regression relationship is characterized based on the same set of climatological data. GPS-derived PW are compared with those observed by WVR and radiosondes. The GPS and WVR data were collected at the Taipei weather station of Taiwan Central Weather Bureau (CWB) from March 18 to 24, 1998. To obtain the estimates of absolute PW at the Taipei site, GPS data acquired from Tsukuba, Japan, at a distance of 2155 km from Taipei were utilized. It is found that GPS-derived PW agrees reasonably well with observations by the WVR and radiosondes. The average of GPS-derived PW is 3.38 cm with a standard deviation of 0.39 cm. The difference between the average GPS-derived and WVR-observed PW is 0.27 cm with a bias of −4 cm, while the difference between the average GPS-derived and radiosonde-observed PW is somewhat larger, 0.36 cm with a bias of −0.42 cm. These differences are larger than differences reported at higher latitudes in regions with lower average humidity.

The Low-latitude Ionospheric Tomography Network (LITN), a chain of six stations located along the 121 °E meridian receiving signals from the Transit NNSS satellites, carries out tomographic investigations of the ionosphere in this equatorial anomaly region. The technical aspects of the network are introduced. Because of the steep latitudinal gradients of the ionosphere in this region, a special procedure has been developed to facilitate the reconstruction process. Model simulations are carried out to help formulate the best reconstruction algorithm. Initial results from the first set of data from the full network are presented, and the diurnal behavior of the anomaly discussed. Comparisons of the reconstructed electron density profiles with those derived from ionograms and the reconstructed vertical TECs with those observed appear to indicate that the LITN can be used to provide a two-dimensional image of the ionosphere in the equatorial region.

[1] Using the Global Positioning System radio occultation (GPSRO) technique, the observation of the global ionosphere becomes possible. The irregularity in the ionospheric sporadic-E (Es) layer, which is probably caused by wind shear, can be investigated by analyzing the signal-to-noise ratio (SNR) of RO signal. In this study, the relation between the amplitude of RO signals and the electron density profiles of the ionosphere is simulated, and RO data recorded in the time period from mid-2008 to mid-2011 are used for the analysis. Based on the simulation results, the multiple-layer-type (MLT) and the single-layer-type (SLT) Es layers which are defined by the shape of SNR, are used to analyze the global distribution of Es layer. The seasonal MLT Es layer is compared with the seasonal wind shear, which is obtained from the Horizontal Wind Model (HWM07). Furthermore, the seasonal MLT Es layer is compared with the SLT Es layer, and the global altitude distributions of MLT and SLT Es layers are similar while the magnitude distributions are different. Unlike the MLT Es layer, the global distribution of the SLT Es layer is similar to the distribution of E region peak electron density (NmE), which is related to the solar zenith angle.

In this research, a ray tracing model is built up to simulate the propagation of signal in global positioning system (GPS) radio occultation (RO) mission. When GPS signal propagate through the Earth's atmosphere, it will be bent and delayed due to the gradient atmosphere refractive index and received by low Earth orbit (LEO) satellite. Then the parameter profiles of atmosphere can be retrieved by using the received signal. In the previous research, in order to simplify the simulation, the Earth's atmosphere is assumed as spherical symmetry and the positions of GPS and LEO satellites are not considered in simulations. In the model, the shape of the Earth is assumed as an ellipse. The information of European Centre for Medium-Range Weather Forecasts (ECMWF) is used to construct the refractive index of Earth's atmosphere. And two aiming algorithm are developed to control the initial propagating direction of GPS signal to begin from the prescribed GPS satellite position and end at the LEO satellite position. The model is tested and verified by comparing with analytical and observational data.

The FORMOSAT-3/COSMIC spacecraft constellation consisting of six LEO satellites is the world's first operational GPS Radio Occultation (RO) mission. The mission is jointly developed by Taiwan¡¦s National Space Organization (NSPO) and the United States¡¦UCAR in collaboration with NSF, USAF, NOAA, NASA, NASA's Jet Propulsion Laboratory, and the US Naval Research Laboratory. The FORMOSAT-3/COSMIC satellites were successfully launched from Vandenberg US AFB in California at 0140 UTC 15 April 2006 into the same orbit plane of the designated 516 km altitude. The mission goal is to deploy the six satellites into six orbit planes at 800 km altitude with a 30-degree separation for evenly distributed global coverage. All six FORMOSAT-3/COSMIC satellites are currently maintaining a satisfactory good state-of-health. Five out of six satellites have reached their final mission orbit of 800 km as of November 2007. The data as received by FORMOSAT-3/COSMIC satellites constellation have been processed in near real time into 2500 good ionospheric profiles and 1800 good atmospheric profiles per day. These have outnumbered the worldwide radiosondes (~900 mostly over land) launched from the ground per day. The processed atmospheric RO data have been assimilated into the Numerical Weather Prediction (NWP) models for real-time weather prediction and typhoon/hurricane forecasting by many major weather centers in the world. This paper describes the FORMOSAT-3/COSMIC satellite constellation system performance and the mission results that span the period from April 2006 to October 2007; and reviews the prospect of a future follow-on mission.

An extensive database of CHAMP neutral density measurements during 2001-2005 is used to re-examine latitudinal and diurnal variations of the exosphere temperature. Exosphere temperatures were derived from the measured neutral densities based on the hydrostatic assumption. A linear regression analysis was conducted to model exosphere temperatures during periods of magnetic quiet times. To improve the previous neutral temperature models, we generalize the latitudinal and diurnal dependences of temperature on solar flux. When compared with the Jacchia 1971 model (J71), the new model shows more detailed diurnal variation. Our results indicate that dayside temperatures extend into the evening sector with small gradients until midnight. In the post midnight sector temperature drops sharply to reach a minimum near 02:00 local time and recovers gradually to daytime values near local noon. The results provide an improved understanding of thermosphere responses to solar flux and more accurate specification of thermospheric densities.

National Space Organization (NSPO) of Taiwan in the past ten years has successfully launched three different types of satellites. The latest one is FORMOSAT- 3/COSMIC, a joint Taiwan-US project, which was successfully launched on April 15, 2006. The FORMOSAT-3 is a constellation of 6 satellites and each satellite is equipped with GPS Occultation Receiver (GOX), Tri-Band Beacon (TBB), and Tiny Ionospheric Photometer (TIP). The constellation is on the way to its final orbit configuration. All satellites remain healthy except spacecraft flight model no. 2 (FM2) with a problem of power shortage and FM3 currently staying at an orbit of 711 km due to a mechanism issue to be solved. The current sounding profiles retrieved from the GPS occultation measurements are over an average of 1800 daily. The sounding profiles have been used to study atmospheric and ionospheric structures and total electron content, and assimilated into numerical atmospheric and space weather predictions models to improve the accuracy of prediction. Taiwan has taken an active role and grown remarkably well in remote sensing researches during the last decade. Taiwan will continue to share her resources with the international community for scientific research and applications. In this paper, we will present an overview of significant contributions of the FORMOSAT-3/COSMIC satellite mission to the global community. We will also describe the constellation challenges and novel operations solutions and the future follow-on mission planning.

We propose a new configuration for using a triangle-wave signal to drive the electro-optic modulator in an electro-optic heterodyne interferometer system. The new configuration is adapted to measure the phase retardation of a wave plate and the optical rotation angle of a chiral medium. By adding optic elements, the second-harmonic component amplitude of the interferometer photodetector output signal became proportional to the phase retardation or optical rotation angle of the samples being tested.

Fine structures in the vertical gradient of the electron density have been retrieved by means of analysis of the amplitude of radio occultation (RO) data in sporadic E-layers (heights interval 85–110 km). Maximum values of the positive gradients 45.109, 48.109, 25.109, 29.109 [m -3km-1] are located at levels 92, 105 km (GPS/MET event 0393) and 93.5, 100 km (GPS/MET event 0583). Variations in the vertical gradient of refractivity in the atmosphere have been found for CHAMP RO event 09. Vertical gradient of refractivity changes in interval ±5 N-units/km (height interval 3–10 km) and ±0.5 Nunits/km between levels 11 and 18 km. Vertical distribution of temperature gradient between level 3–37 km reveals features at height 4–6, 9–10 km with positive values of about 6–9 °K/km. Amplitude RO data analysis may be used for detailed retrieving vertical gradients of refractivity and temperature in the atmosphere and electron density in the lower ionosphere during CHAMP and future COSMIC RO missions.

In this paper, we present a newly developed algorithm for inferring atmospheric profiles of refractivity and dry temperature profiles from GPS occultation data. Our approach differs from the previous algorithms presented in the literature in that the solutions are solved through a 3-dimensional vector analysis rather than a 2-dimensional approach. The retrieved atmospheric profiles are compared with those predicted by ECMWF (European Centre for Medium-Range Weather Forecasts) and NCEP (National Centers for Environmental Prediction). The comparison shows that our algorithm infers reasonably well with the profiles from altitudes of ≈5 to 40 km. The profiles deviate from the expected one because of ignorance of relatively high humidity near the Earth's surface. The task of reducing the deviation could be achieved through an iterating approach of solution finding between the retrieval algorithm and atmospheric models and shall be further explored in future.

A three-dimensional ray tracing model with aiming algorithms for global positioning system (GPS) signal is proposed to make simulations conform to the realistic radio occultation (RO) signal propagation. The two aiming algorithms used in this study ensure the initial and end point ray trajectories are located in the prescribed region. In past studies, the ray tracing techniques applied to the RO signal simulation usually assumed a spherically symmetrical atmosphere for simplicity. The exact GPS and low earth orbit (LEO) satellite locations are not considered in the simulation. These two assumptions make the simulation unrealistic for GPS signal propagation in the RO technique. In the proposed model, the shape of the earth is assumed as an ellipse. The information from European Centre for Medium-Range Weather Forecasts (ECMWF) analysis is used to setup the atmosphere in the simulation. Two aiming algorithms are developed to determine the initial signal propagating direction to make the simulated signal start from the prescribed GPS satellite position and end in the close vicinity of the LEO satellite position. An ideal spherical symmetric atmospheric structure is used to verify the ray tracing model. The fractional difference between real and simulated refractivity results is less than 0.1%. Otherwise, the GPS and LEO satellite position in the Formosat-3/COSMIC observation and the ECMWF analysis, considering the earth¡¦s flattening, is also used to verify the aiming algorithms. All of the simulated signals end in close vicinity to the LEO satellite position in the simulation results.

Six identical micro-satellites of the FORMOSAT-3/COSMIC (Formosa Satellite #3 and Constellation Observing System for Meteorology, Ionosphere and Climate: FS-3/C) mission were successfully launched on 14 April 2006. The FS-3/C mission provides the first satellite constellation for monitoring global weather using the Global Positioning System (GPS) radio occultation (RO) technique. The mission¡¦s primary scientificific goal is to obtain near-real time profiles of the bending angle and refractivity in the neutral atmosphere and in the ionosphere. In April, 2007 the FS-3/C mission provide about RO soundings of 2000 atmospheric vertical profiles per day in a nearly uniform distribution around the globe. The lowest altitude penetration for more than 80% of RO soundings reached below 1 kmin altitude. Most soundings have penetration below 800m altitude in the equatorial region and below 200 m altitude in polar regions. The quality and accuracy of the RO sounding profiles are in good agreement with the CHAMP(CHAllenging Minisatellite Payload) RO soundings and direct measurements using dropsondes. The FS-3/C RO sounding observations are used to support operational global weather prediction, climate monitoring and research, space weather forecasting, and ionosphere and gravity research.

The ionospheric total electron contents(TEC)during the total eclipseof October 24,1995 were observed by means of the Global Positioning System(GPS)receivers located at Wuchang and Guangzhou.The observationsshowed that there were obvious effects of the eclipse on the ionosphere.The eclipse gave rise to decreases in the TEC in comparison with an establishedTEC reference level,refered to as the negative deviations of the TEC.The deeper the obscuration degree was,the larger was the negative deviation.Furthermore,the restoration of the negative deviations was delayedby about 2 hours after the last contact of the eclipse.

Spatial and temporal distributions of precipitable water (PW) during the passage of a typhoon are monitored by ground-based GPS (Global Positioning System) receivers deployed by Central Weather Bureau (CWB) of Taiwan. GPS-observed PW time series demonstrate an expected trend that the PW amount is in general high before and during the occurrence of the typhoon, and low after the typhoon event. That is, PW increased from about 5 cm on DoY (Day of Year) 285 (October 13) to near 8 cm or so on DoY 288 (October 16) when Typhoon Zeb was striking Taiwan, and, then, decreased to 2-3 cm after passage of Zeb. GPS-observed PW is compared with that predicted by the Penn State-NCAR nonhydrostatic mesoscale model version 5 (MM5). With an acceptable assumption that GPS-observed PW is regarded as reference, MM5-predicted PW mimics that observed by GPS except that there are some discrepancies between them. This indicates that assimilation of GPS-observed PW into atmospheric circulation models is potentially helpful to numerical weather prediction (NWP) models.