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With careful consideration of time-varying characteristics of pseudorange and carrier phase hardware biases, more rigorous undifferenced and uncombined observation equations are derived, and the mathematical expressions of the two types of GNSS biases in uncombined mode are given. Based on this, this contribution studied independent parameterization methods of three commonly used triple-frequency precise point positioning (PPP) (i.e., ionosphere-free combination IF1213 and IF123 as well as uncombination UC123) function models in detail, and systematically analyzed the relationship among these models. Subsequently, the positioning performance in terms of positioning accuracy for GPS/BDS/Galileo triple-frequency PPP in static and kinematic modes was assessed. The experimental results showed that the positioning accuracy after convergence for static PPP was better than 1.0 and 1.5 cm; while that for kinematic PPP was better than 2.0 and 5.0 cm in horizontal and vertical components, respectively. Moreover, the positioning performance of triple-frequency and dual-frequency PPP was comparable.

Aiming at the singularity problem in satellite orbit theory,the singularity-free Lagrangian/Gaussian equations of motion is analyzed.Considering the original and physical meaning of the Lagrangian and Gaussian equations of motion,a new Lagrangian/Gaussian singularity-free disturbed equations of motion is proposed and then discussed in three cases:the circular orbit,equatorial orbit,circular and equatorial orbit.Besides,the continuity of these equations is explored.The proposed equations eliminate the zero factor and in this way the singularity problem in the orbital mechanics is solved.

According to the physical characteristics of the BDS on-board atomic clocks, a spectral analysis method is used to detect the periodicity based on the products published by the IGS data Center of Wuhan University from January 1, 2016 to November 1, 2016. The results show that there are periodic fluctuations in the three types of satellite clocks. The main period of GEO and IGSO satellite clock bias is relatively obvious. The testing with nearly a year data showed that the main periods of GEO are 12, 24, 8 and 6 h, but for IGSO are 24, 12, 8 and 6 h, while those for MEO are 12.9, 6.4 and 24 h. Based on the periodic characteristics of different types of clocks and the correction of the starting point deviation of the clock bias, a modified prediction method of satellite clock bias is used to forecast the BeiDou satellite clock bias. The results show that the modified method can significantly improve the prediction accuracy, the 24 h, 12 h, 6 h average forecast accuracy can reach 6.55 ns, 3.17 ns and 1.76 ns respectively.

The precise clock corrections are a prerequisite for applications of the GNSS positioning, navigation and timing (PNT) service. Therefore the combination and assessment for clock products is one of the important tasks of product integration and service center (ISC). First of all, it is introduced that the improved strategy and robust estimation of clock combination. The issues including nonlinear systematic error processing, alignment of clock reference and compensation of linear systematic error are highlighted. Furthermore, two combined products using epoch-wise clock alignment and the "three-step" alignment respectively are compared, and the clock products of recent two years from iGMAS are analyzed and evaluated in detail. Finally, in order to verify the accuracy and consistency of the combined products, PPP tests are implemented using static data from 5 IGS stations. The results show that the clock RMS of combined solutions is the best, which can be used as reference to assess the clock quality of analysis centers (ACs). The results of BDS only static PPP using the final combined products is within 4 cm, which is better than that of most ACs.

Territorial water is a significant part of national sovereignty of China, thus the infrastructures of national space datum and location services should not only cover the land areas, but also the sea areas. China has established relatively complete geodetic datum in land areas over the past decades, including the new developed China Geodetic Coordinate System 2000 (CGCS 2000) and the national gravity datum 2000. However, the currently used geodetic infrastructures have not well covered the sea areas of China. The marine geodetic datum and marine navigation technologies need to be further developed and extended to satisfy the national demands of marine defense and marine economy development in new era of China. This paper mainly reviews the developing states and progress of Chinese marine geodetic datum and marine navigation, analyses key technologies in establishing the national marine geodetic datum. The develop current trends and future directions for the national marine geodetic datum and marine navigation technologies are listed.

A new method for estimating the BeiDou inter-frequency satellite clock bias is proposed, considering the shortage of the current methods. The constant and variable parts of the inter-frequency satellite clock bias are considered in the new method. The data from 10 observation stations are processed to validate the new method. The characterizations of the BeiDou inter-frequency satellite clock bias are also analyzed using the computed results. The results of the BeiDou inter-frequency satellite clock bias indicate that it is stable in the short term. The estimated BeiDou inter-frequency satellite clock bias results are molded. The model results show that the 10 parameters of model for each satellite can express the BeiDou inter-frequency satellite clock bias well and the accuracy reaches cm level. When the model parameters of the first day are used to compute the BeiDou inter-frequency satellite clock bias of the second day, the accuracy also reaches cm level. Based on the stability and modeling, a strategy for the BeiDou satellite clock service is presented to provide the reference of our BeiDou.