## 1 Introduction  
China's strong support for BeiDou satellite navigation and positioning technology and the continuous demand for positioning services require that conventional real-time kinematic (RTK) technology be more optimized both in terms of the solving accuracy of the fixed ambiguity solution, and baseline application distance between the monitoring station and the reference station. Therefore, the network RTK positioning technology based on the Continuously Operating Reference Station (CORS) system arose, compared to the conventional RTK technology, which possesses the advantages of broader applications, more accurate dynamic navigation and positioning, and more reliability. This technology uses multiple reference stations to form a reference station network and provides precise differential correction data (difference between the actual and measured values) for mobile stations within the coverage of the network, thus maximizing the observation baseline length. The accuracy of the ambiguity fixed solution for the network RTK inter-reference station will directly affect the reliability of the monitoring station [1; 2]. When solving the network RTK inter-reference station ambiguity fixed solution timely, the observation value of the network RTK reference station is prone to gross error as the observation environment is subject to more occlusions resulting in poor satellite signals. This makes the parameter estimation results appear clear deviated or even lead to filter divergence, further affecting the overall results of network RTK precious positioning [3]. [PERSON] et al. [4] employed the MW (Melbourne-Wubeena combination) combined observation equation to solve the whole-period ambiguity of the double-difference broad alley at the reference station and then used the integrated error interpolation method to solve the error correction number of the reference station and the mobile station to correct the ambiguity. However, this method is computationally intensive and insensitive to the baseline length. [PERSON] et al. [5] used the combined ionosphere-free observations for the determination of the whole-period ambiguity of the double-difference carrier phase between the reference stations, yet there are still some issues: (1) the problem of the success rate of ambiguity settlement solved by the initially rising satellite; (2) the problem of the success rate of ambiguity settlement when there is a large amount of circumferential hopping information in the observed signal. [PERSON] et al. [6] proposed a fast solution based on a partial ambiguity fixing strategy method, which first shortens the ambiguity solution fixation time among reference stations by establishing an ionospheric model and then screening the optimal subset of the ambiguity set. Reliable estimation of the ambiguity is a key issue in network RTK localization, whereas the joint solution of multiple systems makes the dimension of the whole circumference ambiguity increase steeply, resulting in a more difficult estimation of all ambiguities, especially prominent when the observation environment is poor [7]. Moreover, in a more complex observation environment, gross errors will appear in the observation values, which is the main factor limiting the fast and accurate solution of the ambiguity among network RTK reference stations [8; 9; 10]. Domestic and foreign researchers have demonstrated that fixing all ambiguities at once is unnecessary [11; 12]. This paper first estimates the floating-point ambiguity using the robust extended Kalman filtering technique, then utilizes the optimized partial ambiguity solving method, and fixes the solution of the ambiguity by using the LAMBDA algorithm and the joint test of ratio (R-ratio) and bootstrapping success rate.