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Objectives: In October 2019, the International GNSS Service(IGS) began the third data reprocessing campaign(repro3). To enable more robust ambiguity resolution(AR) in precise point positioning (PPP), one of IGS important tasks is to combine satellite clock and code/phase bias products. Methods: We elaborated the precise satellite clock product model, code/phase bias product model, as well as prior corrections and specific processes for the combination of global navigation satellite system(GNSS) satellite clock and code/phase bias products. What􀆳s more, entrusted by the IGS Analysis Center Coordinator, we combined the clock/bias products of repro3 from 2000 to 2020 successfully. Results: Through the statistics of the combination, it is concluded that the integrity rate of combined clock can reach more than 95% after stabilization, which is superior to most of the products of IGS Analysis Centers. The average root mean square error of GPS and Galileo satellite navigation system satellite clock reach 6.13 ps and 5.63 ps respectively, which prove that the correction models in the combination can significantly improve the consistency. The average weight of Graz University of Technology participating in the combination reaches 21.83%, which has better precision than that of other analysis centers. The weight of phase/integer clock is higher than that of legacy clock, with better stability and precision. Furthermore, 103 stations with uniform global distribution were selected for the static PPP-AR solution to validate the positioning precision of the combined products. The results showed that the average fixed rates of GPS wide and narrow-lane ambiguity based on combined products are 90.16% and 86.72% respectively, and those of Galileo are 97.11% and 93.77% respectively, which are about 1-5 percentage points higher than those based on Center for Orbit Determination in Europe (COD) repro3 products. The average positioning precision based on combined products in the east, north and high directions is 0.154, 0.178 and 0.542 centimeters respectively, which is 20% to 30% higher than COD. Conclusions: The multi-GNSS satellite clock and code/phase combination method can effectively solve the problems of the consistency among products of analysis centers,and enable more robust ambiguity resolution in PPP-AR. Moreover, the combined products have been officially downloaded (IGSMAIL-8248:https://cddis.nasa.gov/archive/gnss/products/repro3/). [ABSTRACT FROM AUTHOR]

Objectives: High-accuracy and high-reliability satellite orbit is the fundamental requirement for precise applications of low earth orbit (LEO) satellites. Due to the uncalibrated phase delay (UPD) originated from receivers and satellites, the phase ambiguity cannot be estimated as integer values. Fixing phase ambiguity to proper integer number is one of the key issues to improve the precise orbit determination (POD) accuracy. The algorithms and methods of single-difference (SD) ambiguity resolution (AR) based on integer clocks are demonstrated. The reduced dynamic POD and kinematic POD with SD AR are implemented and the performance is analyzed. Methods: Integer clock is the precise clock product that absorbs satellite UPD. By applying integer clock products, the receiver UPD can be eliminated by forming SD am-biguities between two global navigation satellite system (GNSS) satellites. And the SD ambiguity recover its integer nature and can be fixed to integer value by a rounding strategy. The onboard gravity recovery and climate experiment follow on (GRACE-FO )data (GRACE-C/D) and integer clocks from Centre National d’ Etudes Spatiales/Collecte Localisation Satellites (CNES/CLS) of April and May 2019 was employed to evaluate the improvement of reduced dynamic POD and kinematic POD accuracy with SD AR. Results: The wide-lane ambiguity fixing rates of both reduced dynamic and kinematic POD reach 99% and the narrow-lane fixing rates are about 95%. For reduced dynamic POD, the 3D root mean square error (RMSE) values of overlapping orbit difference reduced from 7.1 mm and 7.4 mm to 4.2 mm and 3.6 mm for GRACE-C/D, respectively. The standard deviation values of satellite laser ranging (SLR) residuals decrease from 15.9 mm and 14.4 mm to 10.8 mm and 11.0 mm, with improvements of 32% and 24%. Besides, the RMSE values of K-band ranging residuals reduced from 8.0 mm to 2.9 mm, demonstrating that SD AR can also improve the relative position accuracy of LEO satellites. As for kinematic POD, the 3D RMSE values for orbit difference with precise science orbit (PSO) of ambiguity-float solution are 37.5 mm and 36.4 mm, while those of ambiguity-fixed solution are 27.7 mm and 25.5 mm, with improvements of about 28%. Moreover, the SLR residuals results also improved by 20%. Conclusions: With the integer clock products, SD AR can be achieved using onboard observations of a single LEO satellite. The SD AR can significantly improve both the reduced dynamic and kinematic POD accuracy of LEO satellites. [ABSTRACT FROM AUTHOR]

Objectives: The frequency division multiple access technology of GLONASS causes IFB (inter‑ frequency bias) when the receiver receives signals from different satellites, which makes it difficult to fix the ambiguity of GLONASS. It is an important factor that makes the orbit accuracy of GLONASS inferior to that of GPS （global positioning system） . At present, only CODE (centre for orbit determination in Eu‑ rope) among the IGS (international GNSS service) analysis centers provides GLONASS ambiguity ‑fixed orbits. To improve the orbit accuracy of GLONASS, it is necessary to develop a method to fix ambiguity in GLONASS all baseline and all constellation orbit determination. Methods: Based on a GLONASS ambiguity fixing method for long baseline, GLONASS ultra‑rapid orbit determination is propsed. Firstly, the float ionosphere‑free ambiguity with a wavelength of about 5.3 cm is solved. Then the Inter‑frequency phase bias (IFPB) rate will be determined by interval search method to correct the IFPB. After the IFPB is eliminated, the double difference ionosphere‑free ambiguity can be fixed and then the ambiguity‑fixed ultra‑rapid orbit is obtained. Results and Conclusions: The effect of this method in the GLONASS ultra‑rapid orbit determination is verified and analyzed by using globally distributed stations. The results show that the IFPB rates are relatively stable within one month. The fixing ‑rate of GLONASS satellite ambiguity after correcting IFPB decreases with the increase of baseline length. The highest fixing‑rate is 95%, the lowest is 88%, and the average is 94%. After the ambiguity is fixed, compared with the CODE product, the accuracy of the ultra‑rapid orbit is increased by 26.8% and 18% for the calculation part and the 3‒6 hour forecast part respectively, the accuracy of the inner precision is increased by 20% and 13% for the calculation part and the 3 ‒ 6 hour forecast part respectively, which indicates that this method can effectively fix the GLONASS ambiguity and improve the accuracy of ultra‑rapid orbit. [ABSTRACT FROM AUTHOR]

The experimental satellites of BeiDou global navigation satellite system (BDS‐3) broadcast new B1C / B2a / B2b signals that overlap with GPS L1/L5 signals and European Galileo E1/E5a/E5b signals, which provides the opportunity of tightly combined relative positioning for short baselines with observations from GPS, Galileo, and BDS ‐ 3 experimental satellites. In this contribution， we first evaluate the value and time‐domain stability of differential inter‐system bias (DISB) between overlapping frequencies of GPS, Galileo, and BDS ‐ 3. The results show that the DISB of the same type of receiver is close to 0, and its influence can be ignored in the tightly combined relative positioning. Then we preliminarily evaluated performance of single‐epoch tightly combined relative positioning for short baselines with observations from GPS， Galileo， and BDS‐3 experimental satellites. The results show that, compared with the traditional loosely combined model, the tightly combined model can significantly improve the success rate and reliability of ambiguity resolution. The improvement is especially significant (approximately 25% ‐45%) under environments where the observed satellites for each system are limited and only single‐frequency observations are available. [ABSTRACT FROM AUTHOR]

The satellite Fengyun 3C(FY3C) launched in 2013 and carried a global navigation satellite system (GNSS) occultation sounder, which is the first BDS/GPS compatible radio occultation sounder in the world and records both GPS and BDS measurements. It provides an opportunity for investigating the precision realtime orbit determination based on space‐borne GPS/BDS measurements. In order to realize the real‐time orbit determination based on space‐borne GPS/BDS measurements with decimeter‐level accuracy, this paper carries out in‐depth research in the following aspects. Firstly, the real‐time orbit determination mathematical model based on the space‐borne GPS/BDS carrier phase observations is introduced. Then the properties of the errors in the signal’s propagation path that originates from GPS/BDS broadcast ephemeris are analyzed, and the signal‐in ‐propagation path error(SIPPE) statistics of different kinds of GPS/BDS satellites are also presented. Based on the analysis, a pseudo‐ambiguity is modeled as a stochastic process and estimated in the real‐time orbit determmination(RTOD) filter to absorb the SIPPE error, and then the decimeter‐ level accuracy can be realized. The space‐borne GPS/BDS data from China’ s FY3C satellite are em ‐ ployed for the simulative RTOD test using the SATODS software. Results have demonstrated that the position and velocity accuracy of 30 cm and 0.3 mm/s, respectively, are achieved when using GPS/BDS carrierphase data and broadcast ephemeris. Although the accuracy of BDS broadcast ephemeris is obviously worse than that of GPS, orbit results of the GPS/BDS fusion solution with reasonable stochastic models for pseudo‐ambiguities will be slightly superior to that using single GPS data. For example, when the FY3C satellite flew over Asia/Pacific region where more BDS satellites can be tracked, the orbit accuracy can be improved by 10% with the GPS/BDS fusion solution. [ABSTRACT FROM AUTHOR]

Frequency division multiplexing of GLONASS signals causes inter - frequency bias(IFB) in the receiving equipment. IFB significantly prevents GLONASS ambiguity resolution and limits the accuracy and reliability of GLONASS positioning and orbit determination. Therefore, we present a new method for GLONASS ambiguity resolution. Firsly, it weakens the influence of inter-frequency code bias, and wide-lane ambiguities are calculated directly based on the wide-lane combined observations. Then, according to the range of inter-frequency phase bias rate, a step-by-step search schedule is designed to remove the impacts of inter-frequency phase bias on wide-lane and narrow-lane ambiguities. Finally, the ambiguity resolution can be achieved successfully. An IGS experiment network is carried out to verify the validity of this method. Experimental results show that the maximum, minimum and mean success rate of fixed ambiguities within the month were 95.4%, 88.8% and 93.45%, respectively. After ambiguity resolution, the repeatability and root mean square error (RMSE) of N, E, U components were improved; especially for E component, the repeatability and RMSE were improved 20% and 14%, respectively. This proves the validity of this method. [ABSTRACT FROM AUTHOR]