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The burgeoning development of low Earth orbit (LEO) constellations for positioning, navigation, and timing (PNT) in recent years has brought about new opportunities for ionospheric sounding below the LEO satellite altitudes. This study established the regional bottomside ionospheric map (RBIM) using navigation augmentation signals from two CENTISPACE LEO satellites during April 1–7, 2023, under moderate solar activity and quiet geomagnetic conditions. Accuracy was subsequently validated through comparison with multiple datasets, including global and regional ionospheric maps (GIMs and RIMs) constructed from ground-based Global Navigation Satellite System (GNSS) observations, as well as the differential slant bottomside electron content (dSBEC) derived from LEO observations. The vertical bottomside electron content (VBEC) fit by two distinct methods, i.e., grid map and polynomial methods, achieved fitting residuals with root-mean-square (rms) values of 1.2 and 0.7 TECU, respectively. The RBIM precision evaluated by LEO dSBEC reaches 1.0 TECU. Comparing the VBEC from our RBIM to the GIM/RIM indicates an rms level mostly within 3–8 TECU. More importantly, the established RBIM facilitates the analysis of the proportional variation of the VBEC over the total electron content (TEC). For the one-week LEO observations, an average VBEC proportion is derived at 84% during daytime and 56% during nighttime, indicating a reduction of 33.3%. This result is more realistic than that calculated from the empirical International Reference Ionosphere (IRI-2020) model (5.3%) and is in good agreement with the previous studies. Thus, the RBIM can not only benefit LEO navigation augmentation but also provide significant observations on the vertical distribution of electron content.

The accurate assessment of railway irregularities plays a pivotal role in ensuring both operational safety and passenger comfort, especially in the context of high-speed railways. Traditional static methods for monitoring these irregularities, such as absolute measurement trolley systems relying on total stations, have proven to be inefficient. On the other hand, relative measurement systems like the global navigation satellite system/inertial navigation system (GNSS/INS) integrated concept offer high operational efficiency but come with substantial costs associated with navigation-grade inertial measurement unit (IMU) systems. In this research endeavor, we introduce a novel multisensor fusion system centered around a relative inspection trolley. This innovative approach serves to significantly reduce equipment expenditures while simultaneously elevating the accuracy of irregularity detection. The core components of this system comprise real-time kinematic (RTK) GNSS, a cost-effective micro-electric mechanical system (MEMS) IMU, an odometer, and two laser modules. The integration of various advanced techniques, including zero velocity update (ZUPT), Ranch-Tung-Striebel smoother (RTS), nonholonomic constraint (NHC), odometer calibration (OC), and prior knowledge (PK), collectively enhances the efficiency and precision of railway irregularity measurements. The results obtained from extensive testing on railway irregularities demonstrate the remarkable capabilities of the proposed system. It exhibits a measurement precision of approximately 0.2, 1.0, 0.7, 0.5, and 0.5 mm for rail gauge, cross level, twist, vertical irregularities, and lateral irregularities, respectively. Consequently, our suggested technique stands as a compelling choice for the precise quantification of railway irregularities, offering a compelling solution for the railway industry.

Inter-satellite link (ISL) plays an essential role in current and future Global Navigation Satellite System (GNSS). In this study, we investigate the impact of ISL observations on precise orbit determination for BeiDou-3 Navigation Satellite System (BDS-3) Medium Earth Orbit (MEO) satellites based on different Extended CODE Orbit Models (ECOM). Thanks to the better observation geometry of the Ka-band ISL data compared to the L-band data for BDS-3 MEO satellites, the ISL solution substantially reduces Orbit Boundary Discontinuity (OBD) errors, except for C30, which suffers from unstable Ka-band hardware delay. From the external quality analysis, ISL significantly enhances the reliability of the orbit of MEO satellites manufactured by the China Academy of Space Technology (CAST). The standard deviation (STD) of the satellite laser ranging (SLR) residuals is approximately 2.5 cm, and the root mean square (RMS) is reduced by 10–23% compared to L-band solutions. Besides, the Sun-elongation angle dependent systematic error in SLR residuals nearly vanishes based on the reduced 5-parameter ECOM (ECOM1) or extended 7-parameter ECOM (ECOM2) with ISL data. This is because the ISL reduces the correlation between state parameters and solar radiation pressure (SRP) parameters as well as those among SRP parameters, leading to a more accurate estimation of both orbit and SRP perturbations, particularly those along Bdirection. This confirms that the deficiency of the SRP models for BDS-3 CAST satellites can be compensated by using better observation geometry from ISL data. On the other hand, for the satellite manufactured by Shanghai Engineering Center for Microsatellites (SECM), the ISL allows for a more accurate estimation of the Bc1parameter in the ECOM1 model. This only reduces linear systematic error, possibly because the impact generated by the satellite bus cannot be entirely absorbed by the B-direction parameters.

Compressive x-ray tomosynthesis (CXT) uses a set of encoded projection measurements from different incident angles to reconstruct the object under inspection. We consider the variable motion of objects on a conveyor mechanism and establish an imaging model based on the sensing geometry of a dynamic CXT system. Then, a numerical algorithm is proposed to optimize the structured illumination series to improve reconstruction accuracy with reduced radiation dose. Compared with the state-of-the-art method, the proposed strategy increases the degrees of optimization freedom by jointly optimizing the coding mask patterns, locations of x-ray sources, and exposure moments in the CXT system, thus obtaining better reconstruction performance. A genetic algorithm is applied to achieve the optimization results. It shows that the proposed method outperforms the traditional CXT approach by further improving reconstruction performance under comparable radiation dose.

Previous studies have not evaluated the systematic errors implied in the third generation of BeiDou-3 Navigation Satellite System (BDS-3) broadcast ephemeris. In this paper we evaluate the systematic pattern described by the Helmert transformation parameters, including translations, rotations, and scale. BDS-3 broadcast and precise ephemerides from December 2019 to 2022 are collected, and the characteristics of the transformation parameters as well as their effects on the signal in space error are analysed. The annual variation in the z-translation is obtained, and the similar amplitudes of 5.5 cm and phases of approximate 300 days are obtained for different years. When the rotation parameters are considered in the orbit comparison, the Root Mean Square (RMS) errors of the along- and cross-track orbital differences decrease from 29.1 to 12.5 cm and from 30.6 to 9.2 cm, respectively, because the three rotation parameters compensate for the majority of the errors in the BDS-3 broadcast ephemeris. Moreover, the high correlations in the obtained rotation parameters among the three orbital planes suggest that the orientation of the BDS-3 broadcast ephemeris is influenced by common model errors, i.e., uncertainty of Earth Rotation Parameters (ERPs). Further research is required because an offset of 1.5 × 10–9for the scale parameter is observed. A degraded User Range Error (URE) for epochs of up to 84% is attained when the systematic pattern is considered, though the impact of the systematic pattern indicated by the z-translation and rotation parameters on the URE is less than 5.0 cm. With the refinement of the ERPs implemented in the new generation of broadcast ephemeris, we anticipate that the broadcast ephemeris performance of BDS-3 will be improved.

The Haiyang-2D altimetry mission of China is one of the first Low Earth Orbit (LEO) satellites that can receive new B1C/B2a signals from the BeiDou-3 Navigation Satellite System (BDS-3) for Precise Orbit Determination (POD). In this work, the achievable accuracy of the single-receiver ambiguity resolution for onboard LEO satellites is studied based on the real measurements of new BDS-3 frequencies. Under normal conditions, six BDS-3 satellites on average are visible. However, the multipath of the B1C/B2a code observations presents some patchy patterns that cause near-field variations with an amplitude of approximately 40 cm and deteriorate the ambiguity-fixed rate. By modeling those errors, for the B2a code, a remarkable reduction of 53% in the Root Mean Square (RMS) is achieved at high elevations, along with an increase of 8% in the ambiguity-fixed rates. Additionally, an analysis of the onboard antenna's phase center offsets reveals that when compared to the solutions with float ambiguities, the estimated values in the antenna’s Zdirection in the solutions with fixed ambiguities are notably smaller. The independent validation of the resulting POD using satellite laser ranging at 16 selected high-performance stations shows that the residuals are reduced by a minimum of 15.4% for ambiguity-fixed solutions with an RMS consistency of approximately 2.2 cm. Furthermore, when compared to the DORIS-derived orbits, a 4.3 cm 3D RMS consistency is achieved for the BDS-3-derived orbits, and the along-track bias is reduced from 2.9 to 0.4 cm using ambiguity fixing.

By end‐2018, the Chinese BeiDou satellite system (BDS) has entered its third phase of global scale coverage (BDS‐3). This study presents and validates a modified modeling aspect concerning an intersatellite link (ISL) hardware delay estimation. The precise orbit determination (POD) results of BDS‐3 demonstrate this aspect along with calibrated phase center offsets (PCOs) of the ISL terminal to help improve the orbit accuracy. After onboarding the ISL data, two additional sets of POD solutions using regional and global ground stations are conducted. About a 40% improvement can be achieved for regional cases and 20% for global cases after ISL data were used for POD. The best POD performance solution is obtained with observations from both the global ground stations and ISL when the averaged 3D root mean squares of the orbit overlapping differences can reach around 20 cm and the signal‐in‐space range errors (SISREs) obtained by 6‐hour predicted clock and orbit solutions show an average value of 25 cm.

We present a new method of integrating Global Positioning System (GPS) and accelerometer data for high‐rate seismogeodesy. This method is based on the GPS variometric approach which can obtain seismic waves in real time using only the readily available broadcast products and a single receiver. Collocated 1‐Hz GPS and 200‐Hz accelerometer data from the 2016 MW7.8 Kaikōura earthquake were analyzed to verify the effectiveness of this method. Results reveal that this method can provide broadband and more accurate velocities and displacements qualified to detect Pwave arrivals. Moreover, this method can effectively avoid the aliasing problem present in the 1‐Hz GPS waveforms. We therefore conclude that this new method can be a powerful tool to capture seismic waves in real time, which is crucial for the purpose of tsunami early warning and earthquake rapid response. A new method of integrating GPS and strong‐motion data only based on readily available broadcast products is proposedThis new method can provide more accurate and broadband velocities and displacements qualified to detect Pwave arrivalsThis new method can effectively avoid the aliasing problem present in the 1‐Hz GPS‐only seismic waveforms The last decade has witnessed great contribution of high‐rate Global Positioning System (GPS) to earthquake studies. Dense GPS monitoring networks have proliferated in recent years, with outstanding examples such as UNAVCO's PBO (Plate Boundary Observatory) and Japan's GEONET (GPS Earth Observation Network). However, further promotion of GPS applications on earthquakes is still confronted with challenges because GPS remains a sensor with relatively low bandwidth and high noise level. In the current study, we present a new method of integrating GPS and seismic data which takes full advantage of the complementary characteristics of GPS and seismic instruments. Previous seismogeodetic combination methods use the technique of relative positioning or precise point positioning, while this study uses the GPS variometric approach which is more advantageous for real‐time positioning. Using this method, seismic signals can be accurately determined at the order of 0.3–0.4 mm/s for the velocities and a few centimeters for the displacements. Moreover, the bandwidth has been expanded to cover both static offsets and extremely high frequency signals. This study is of great values for the purpose of tsunami early warning, earthquake rapid response, and geoscience studies.

The Gravity Recovery And Climate Experiment (GRACE) mission has achieved a quantum leap in knowledge of the Earth's gravity field. However, current gravity field solutions still cannot reach the prelaunch baseline accuracy. One of the reasons for that is the presence of colored noise in GRACE data, which is typically ignored in the classical dynamic approach to gravity field modeling. In this research, we propose to account for colored noise in the classical dynamic approach by applying the frequency‐dependent data weighting (FDDW) scheme, so that enhanced estimates of gravity field solutions are produced. The monthly solutions are compared with those produced using the standard least squares adjustment without a data weighting scheme. The comparison is performed in both spectral and spatial domains, showing the positive effect of the FDDW scheme in all considered cases. For instance, the cumulative geoid height errors up to degree 96 are reduced by 18%. In the spatial domain, the FDDW scheme lowers noise level in mass changes over the oceans, Mississippi river basin, and Greenland by 20, 38, and 23%, respectively, when compared to the without a data weighting scheme. In addition, the consistency of mass changes over the Mississippi and Congo river basins with those inferred from the state‐of‐the‐art hydrology model WaterGAP is substantially improved when the FDDW scheme is applied. These results indicate that modeling colored noise in the GRACE data allows to significantly improve the recovered monthly solutions. This finding is likely applicable also to the GRACE Follow‐On mission. Colored noise in GRACE data is accounted for in the dynamic approach by applying the frequency‐dependent data weighting (FDDW) schemeApplication of the FDDW scheme substantially reduces the noise level in the obtained GRACE monthly gravity field solutionsThe FDDW scheme significantly improves consistency between GRACE and the state‐of‐the‐art hydrology model WaterGAP over the Mississippi and Congo river basins

The conventional dynamic approach for gravity filed modelling has been implemented in the PANDA (Position and Navigation Data Analyst) software. A variant of the so-called ‘two-step’ method for gravity field modelling is adopted for this purpose, where the GRACE (Gravity Recovery and Climate Experiment) orbits are derived from the GPS (Global Positioning System) data in a first step followed by a simultaneous determination of dynamic orbit and gravity filed from the GPS-derived orbits and K-band range-rate measurements in a second step. In this way, the monthly gravity field solutions complete to degree and order 96 are produced for the period Jan. 2005 to Dec. 2010. Their performance is assessed by comparing them with the official solutions, i.e., CSR RL05, GFZ RL05a and JPL RL05. A comparison in the spectral domain in terms of geoid heights reveals that the obtained solutions present the smallest degree amplitudes at degree 30–75. A further analysis of mass changes in the spatial domain demonstrates that the main signals observed from the obtained solutions are in great agreement with those from the official solutions. Remarkably, the correlation coefficients of mass changes in large river basins from the official solutions with respect to those from the obtained solutions are all above 0.97. These results demonstrate that the obtained solutions are comparable to the official solutions.

Ionospheric delay modeling is not only important for Global Navigation Satellite System (GNSS) based space weather study and monitoring, but also an efficient tool to speed up the convergence time of Precise Point Positioning (PPP). In this study, a novel model, denoted as Quasi-4-Dimension Ionospheric Modeling (Q4DIM) is proposed for wide-area high precision ionospheric delay correction. In Q4DIM, the Line Of Sight (LOS) ionospheric delays from a GNSS station network are divided into different clusters according to not only the location of latitude and longitude, but also satellite elevation and azimuth. Both Global Ionosphere Map (GIM) and Slant Ionospheric Delay (SID) models that are traditionally used for wide-area and regional ionospheric delay modeling, respectively, can be regarded as the special cases of Q4DIM by defining proper grids in latitude, longitude, elevation, and azimuth. Thus, Q4DIM presents a resilient model that is capable for both wide-area coverage and high precision. Four different sets of clusters are defined to illustrate the properties of Q4DIM based on 200 EUREF Permanent Network (EPN) stations. The results indicate that Q4DIM is compatible with the GIM products. Moreover, it is proved that by inducting the elevation and azimuth angle dependent residuals, the precision of the 2-dimensional GIM-like model, i.e., Q4DIM 2-Dimensional (Q4DIM-2D), is improved from around 1.5 Total Electron Content Units (TECU) to better than 0.5 TECU. In addition, treating Q4DIM as a 4-dimensional matrix in latitude, longitude, elevation, and azimuth, whose sparsity is less than 5%, can result in its feasibility in a bandwidth-sensitive applications, e.g., satellite-based Precising Point Positioning Real-Time Kinematic (PPP-RTK) service. Finally, the advantages of Q4DIM in PPP over the 2-dimensional models are demonstrated with the one month's data from 30 EPN stations in both high solar activity year 2014 and low solar activity year 2020.

Since the first pair of BeiDou satellites was deployed in 2000, China has made continuous efforts to establish its own independent BeiDou Navigation Satellite System (BDS) to provide the regional radio determination satellite service as well as regional and global radio navigation satellite services, which rely on the high quality of orbit and clock products. This article summarizes the achievements in the precise orbit determination (POD) of BDS satellites in the past decade with the focus on observation and orbit dynamic models. First, the disclosed metadata of BDS satellites is presented and the contribution to BDS POD is addressed. The complete optical properties of the satellite bus as well as solar panels are derived based on the absorbed parameters as well the material properties. Secondly, the status and tracking capabilities of the L-band data from accessible ground networks are presented, while some low earth orbiter satellites with onboard BDS tracking capability are listed. The topological structure and measurement scheme of BDS Inter-Satellite-Link (ISL) data are described. After highlighting the progress on observation models as well as orbit perturbations for BDS, e.g., phase center corrections, satellite attitude, and solar radiation pressure, different POD strategies used for BDS are summarized. In addition, the urgent requirement for error modeling of the ISL data is emphasized based on the analysis of the observation noises, and the incompatible characteristics of orbit and clock derived with L-band and ISL data are illuminated and discussed. The further researches on the improvement of phase center calibration and orbit dynamic models, the refinement of ISL observation models, and the potential contribution of BDS to the estimation of geodetic parameters based on L-band or ISL data are identified. With this, it is promising that BDS can achieve better performance and provides vital contributions to the geodesy and navigation.

Graphical Abstract:

The variometric approach is investigated to measure real‐time seismic waves induced by the 2015 Mw7.8 Nepal earthquake with high‐rate multi‐GNSS observations, especially with the contribution of newly available BDS. The velocity estimation using GPS + BDS shows an additional improvement of around 20% with respect to GPS‐only solutions. We also reconstruct displacements by integrating GNSS‐derived velocities after a linear trend removal (IGV). The displacement waveforms with accuracy of better than 5 cm are derived when postprocessed GPS precise point positioning results are used as ground truth, even if those stations have strong ground motions and static offsets of up to 1–2 m. GNSS‐derived velocity and displacement waveforms with the variometric approach are in good agreement with results from strong motion data. We therefore conclude that it is feasible to capture real‐time seismic waves with multi‐GNSS observations using the IGV‐enhanced variometric approach, which has critical implications for earthquake early warning, tsunami forecasting, and rapid hazard assessment. First work to use 1 Hz BDS observations to measure earthquake ground motionsVelocity estimation has an additional 20% improvement with GPS + BDS solutionsDisplacements derived with accuracy better than 5 cm for stations experiencing 1–2 m ground motions

The GPS, DORIS, and SLR instruments are installed on Haiyang 2A (HY2A) altimetry satellite for Precise Orbit Determination (POD). Among these instruments, the codeless GPS receiver is the state-of-art Chinese indigenous onboard receiver, and it is the first one successfully used for Low Earth Orbit (LEO) satellite. Firstly, the contribution assesses the performance of the receiver through an analysis of data integrity, numbers of all tracked and valid measurements as well as multipath errors. The receiver generally shows good performance and quality despite a few flaws. For example, L2 observations are often missing in low elevations, particularly during the ascent of GPS satellites, and the multipath errors of P1 show a slightly abnormal pattern. Secondly, the PCO (Phase Center Offset) and PCV (Phase Center Variation) of the antenna of the GPS receiver are determined in this contribution. A significant leap for Z-component of PCO up to −1.2 cm has been found on 10 October 2011. Thirdly, the obtained PCO and PCV maps are used for GPS only POD solutions. The post-fit residuals of ionosphere-free phase combinations reduce almost 50%, and the radial orbit differences with respect to CNES (Centre National d’Etudes Spatiales) Precise Orbit Ephemeris (POEs) improve about 13.9%. The orbits are validated using the SLR data, and the RMS of SLR Observed minus Computed (O-C) residuals reduces from 17.5 to 15.9 mm. These improvements are with respect to the orbits determined without PCO and PCV. Fourthly, six types of solutions are determined for HY2A satellite using different combinations of GPS, DORIS, and SLR data. Statistics of SLR O-C residuals and cross-comparison of orbits obtained in the contribution and the CNES POEs indicate that the radial accuracy of these orbits is at the 1.0 cm level for HY2A orbit solutions, which is much better than the scientific requirements of this mission. It is noticed that the GPS observations dominate the achievable accuracy of POD, and the combination of multiple types of observations can reduce orbit errors caused by data gaps and maintain more stable and continuous orbits.

Abstract: Chinese Beidou satellite navigation system constellation currently consists of eight Beidou satellites and can provide preliminary service of navigation and positioning in the Asia-Pacific Region. Based on the self-developed software Position And Navigation Data Analysis(PANDA) and Beidou Experimental Tracking Stations (BETS), which are built by Wuhan University, the study of Beidou precise orbit determination, static precise point positioning (PPP), and high precision relative positioning, and differential positioning are carried out comprehensively. Results show that the radial precision of the Beidou satellite orbit determination is better than 10 centimeters. The RMS of static PPP can reach several centimeters to even millimeters for baseline relative positioning. The precision of kinematic pseudo-range differential positioning and RTK mode positioning are 2–4 m and 5–10 cm respectively, which are close to the level of GPS precise positioning. Research in this paper verifies that, with support of ground reference station network, Beidou satellite navigation system can provide precise positioning from several decimeters to meters in the wide area and several centimeters in the regional area. These promising results would be helpful for the implementation and applications of Beidou satellite navigation system.

Abstract: In general, China is short of water resources and some regions even experience a shortage of daily water supply. This could threaten the stability and economic development of the nation. A study on the water storage variations is especially important for the water management and storage prediction in three largest river basins of China, namely, Yangtze, Yellow, and Zhujiang, where the most dense population and leading economic regions are located. The satellite gravity mission GRACE (Gravity Recovery and Climate Experiment) provides an opportunity to macroscopically identify water (or mass) variations in the Earth’s system with a spatial resolution of 300–400 km and a temporal resolution of about one month. We use the first release of the DEOS (Delft Institute of Earth Observation and Space Systems) Mass Transport (DMT-1) model based on GRACE data to analyze water storage changes in the three river basins. The DMT-1 model consists of monthly solutions, which are computed using an innovative methodology. The methodology includes, in particular, the application of a statistically optimal Wiener-type filter based on full variance-covariance matrices of noise and signal. This results in particularly sharp mass variation maps. Taking one monthly solution as an example, we compare the results derived from the DMT-1 model with ones produced with the standard post-processing scheme based on a combination of the de-striping and Gaussian filtering. The comparison shows that the DMT-1 model outperforms the other models and is suitable for the analysis of the mass changes in river basins. A subset of the DMT-1 solutions in the interval between February 2003 and May 2008 is used to estimate the secular trends and seasonal variations for the three river basins. The estimated trends show that the water storage of the Yellow River basin does not have significant changes, while the Zhujiang and Yangtze river basins have a large and statistically significant water storage increase. The estimation of seasonal variations demonstrates that the water storage variations in Yangtze and Zhujiang river basins are almost in the same phase. The amplitude of variations in the Zhujiang River basin is larger than that in Yangtze. No clear annual variations are observed in the Yellow River basin. The observed water storage variations generally coincide with the observations and conclusions presented in the hydrological reports of the Chinese Ministry of Water Resources.

The Precise Point Positioning (PPP) technique uses a single Global Navigation Satellite System (GNSS) receiver to collect carrier-phase and code observations and perform centimeter-accuracy positioning together with the precise satellite orbit and clock corrections provided. According to the observations used, there are basically two approaches, namely, the ionosphere-free combination approach and the raw observation approach. The former eliminates the ionosphere effects in the observation domain, while the latter estimates the ionosphere effects using uncombined and undifferenced observations, i.e., so-called raw observations. These traditional techniques do not fix carrier-phase ambiguities to integers, if the additional corrections of satellite hardware biases are not provided to the users. To derive the corrections of hardware biases in network side, the ionosphere-free combination operation is often used to obtain the ionosphere-free ambiguities from the L1 and L2 ones produced even with the raw observation approach in earlier studies. This contribution introduces a variant of the raw observation approach that does not use any ionosphere-free (or narrow-lane) combination operator to derive satellite hardware bias and compute PPP ambiguity float and fixed solution. The reparameterization and the manipulation of design matrix coefficients are described. A computational procedure is developed to derive the satellite hardware biases on WL and L1 directly. The PPP ambiguity-fixed solutions are obtained also directly with WL/L1 integer ambiguity resolutions. The proposed method is applied to process the data of a GNSS network covering a large part of China. We produce the satellite biases of BeiDou, GPS and Galileo. The results demonstrate that both accuracy and convergence are significantly improved with integer ambiguity resolution. The BeiDou contributions on accuracy and convergence are also assessed. It is disclosed for the first time that BeiDou only ambiguity-fixed solutions achieve the similar accuracy with that of GPS/Galileo combined, at least in mainland China. The numerical analysis demonstrates that the best solutions are achieved by GPS/Galileo/BeiDou solutions. The accuracy in horizontal components is better than 6 mm, and in the height component better than 20 mm (one sigma). The mean convergence time for reliable ambiguity-fixing is about 1.37 min with 0.12 min standard deviation among stations without using ionosphere corrections and the third frequency measurements. The contribution of BDS is numerically highlighted.

To meet the need for precise orbit determination (POD) of the LEO satellites equipped with a single-frequency (SF) GPS receiver and deepen the research on POD with SF GPS data, the problem of cycle-slip detection for SF GPS data was solved, and the reduced-dynamic orbits for HY-2A satellite and ZY-3 satellite were determinated with two different approaches using the SF GPS observations. The precision of the derived orbits was assessed by the analysis of the observation residuals, comparison with the precise orbits derived from dual-frequency (DF) GPS data and validation with satellite laser ranging (SLR) measurements. The results indicate that the 3D precision of the SF orbit without ionospheric delay correction is between 2dm and 3dm for HY-2A and about lm for ZY3, and the notable improvement with the group and phase ionospheric correction (GRAPHIC) observation is achieved to eliminate the first order ionospheric delay and the 3D precision can reach ldm for HY-2A and 1~2dm for ZY-3. The results demonstrate that the sub decimeter level accuracy can be achieved for POD of LEO equipped with a SF GPS receiver. [ABSTRACT FROM AUTHOR]