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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.

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.

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.

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.