Search

Global navigation satellite system reflectometry (GNSS-R) is designed to get the quasi-specular reflection of the GNSS signal over the Earth's surface. The reflected signal of GNSS-R recorded on a Delay-Doppler Map (DDM) may then be used to retrieve wind speed, ocean surface roughness, and latent heat flux over the open ocean and to retrieve soil moisture over the land. The L-band signal from GNSS is transmitted via forward (quasi-specular) scattering geometry, obeying the geometric optics (GO) limit of the Kirchhoff approximation (KA). One of the factors on DDM is bistatic radar cross section (BRCS) which represents sea surface roughness. The ocean surface slope and roughness spectrum sensed by GNSS-R response to the energy transferred to the ocean makes it possible for the wind speed to retrieve from the GNSS-R DDM. L-band (~1.5 GHz) microwave is less sensitive to the rain than higher frequency band signals such as Ku- band and C- band. There is a potential to use the L-band signal to retrieve wind speed over not only the fully-developed sea but over limited-fetch sea under more extreme weather systems, such as under tropical cyclones.In this study, we will present a wave-considering retrieving wind speed algorithm for the new GNSS-R satellite, TRITON. TRITON (Wind-Hunter Satellite) is designed and manufactured by Taiwan Space Agency (TASA) and will be launched in the first season of 2023.The wind speed retrieving algorithm contains self-built Level 1b (L1b) and Level 2 (L2) algorithms. The L1b algorithm starts from Level 1a output, power signal DDM in watts. We will introduce the procedure to calibrate the DDM to normalized bistatic radar cross section (NBRCS) in meters and compute the DDM observables (DDMA and LES) for the next level. The performance assessment of the self-developed Level 1 algorithm. Good agreements have been found compared to the CYGNSS results. The correlation coefficient among ~0.5 million DDMA_cygnss and DDMA_Triton points regression is 0.95. The root-mean-squared error is 4.99, with data ranging from 0 to 200, and the scattering index is 0.19.In the L2 algorithm, wind speed will be retrieved in two steps. In the first step, NBRCS will be used to compute the mean square slope (mss) with the help of the Fresnel reflection coefficient. In the second step, the relationship between wave age, mss, and wind speed developed based on the state-of-art microwave remote sensing study will be applied to retrieve wind speed. Uncertainty of the two-step algorithm will be assessed and compared with the results from the existing one-step geophysical model function algorithm to check the improvement. Results under the fully-developed sea and young-sea limited-fetch condition will be presented.Keywords: TRITON, wind speed retrieval, calibration, two-step algorithm, wave age, ocean surface roughness

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.

The global positioning system (GPS) receiver has been one of the most important navigation systems for more than two decades. Although the GPS system was originally designed for near-Earth navigation, currently it is widely used in highly dynamic environments (such as low Earth orbit (LEO)). A space-capable GPS receiver (GPSR) is capable of providing timing and navigation information for spacecraft to determine the orbit and synchronize the onboard timing; therefore, it is one of the essential components of modern spacecraft. However, a space-grade GPSR is technology-sensitive and under export control. In order to overcome export control, the National Space Organization (NSPO) in Taiwan completed the development of a self-reliant space-grade GPSR in 2014. The NSPO GPSR, built in-house, has passed its qualification tests and is ready to fly onboard the Triton satellite. In addition to providing navigation, the GPS/global navigation satellite system (GNSS) is facilitated to many remote sensing missions, such as GNSS radio occultation (GNSS-RO) and GNSS reflectometry (GNSS-R). Based on the design of the NSPO GPSR, the NSPO is actively engaged in the development of the Triton program (a GNSS reflectometry mission). In a GNSS-R mission, the reflected signals are processed to form delay Doppler maps (DDMs) so that various properties (including ocean surface roughness, vegetation, soil moisture, and so on) can be retrieved. This paper describes not only the development of the NSPO GPSR but also the design, development, and special features of the Triton’s GNSS-R mission. Moreover, in order to verify the NSPO GNSS-R receiver, ground/flight tests are deemed essential. Then, data analyses of the airborne GNSS-R tests are presented in this paper.

We assess the impact of orbit modeling on the origin offsets between GRACE kinematic and reduced-dynamic orbits. The origin of the kinematic orbit is the center of IGS network (CN), whereas the origin of the reduced-dynamic orbit is assumed to be the center of mass of the Earth (CM). Theoretically, the origin offset between these two orbits is associated with the geocenter motion. However, the dynamic property of the reduced-dynamic orbit is highly related to orbit parameterizations. The assessment of the F10.7 impact on the geocenter motion is implemented by using different orbit parameterization setups in the reduced-dynamic method. We generate two types of reduced-dynamic orbits using 15 and 240 empirical parameters per day from 2005 to 2012. The empirical parameter used in Bernese GNSS Software is called piece-wise constant empirical acceleration (PCA) and is mainly to absorb the non-gravitational forces mostly related to the atmospheric drag and solar radiation pressure. The differences between kinematic and dynamic orbits can serve as a measurement for geocenter. The RMS value of the geocenter measurement in the 15-PCA case is approximately 3.5 cm and approximately 2 cm in the 240-PCA case. The correlation between the orbit difference and F10.7 is about 0.90 in the 15-PCA case and −0.10 to 0 in the 240-PCA case. This implies that the reduced-dynamic orbit modeled with 240 PCAs absorbs the F10.7 variation, which aliases to the 15-PCA orbit solution. The annual amplitudes of the geocenter motion are 3.1, 3.1 and 2.5 mm in the 15-PCA case, compared to 0.9, 2.0 and 1.3 mm in the 240-PCA case in the X, Y and Z components, respectively. The 15-PCA solution is thus closer to the geocenter motions derived from other space-geodetic techniques. The proposed method is limited to the parameterizations in the reduced-dynamic approach.

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 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.

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.

[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.

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.