## 1 Introduction  
The self-weight of girder and vehicle load are usually transmitted to the bridge tower by stay cable or main cable. To satisfy the stability of the vehicle and the rational distribution of the cable force, the bridge tower has been designed as higher tower and larger cross-section in engineering [1]. As the deadweight of the structure increases, it can easily deform the foundation of the bridge tower and affect the vehicle riding comfort. In addition, the foundation settlement of the bridge tower can affect the construction formwork positioning, steel anchor beam positioning, and alignment control of the main girder in the construction process. As a consequence, the continuous monitoring of the foundation displacement of the long-span bridge is the necessary guarantee for the bridge safety during the construction and operation periods. Currently, traditional foundation displacement monitoring is usually adopted for the precise leveling. However, due to various disadvantages of manual monitoring, such as heavy workload, error accumulation, long observation period, etc., especially in the service period of bridges, it is difficult to achieve rapid, accurate, and real-time monitoring of foundation displacement.  
The technological improvement in Global Navigation Satellite System (GNSS) techniques and advances in data processing algorithms, compared with the traditional deformation monitoring methods, have provided real-time 3D absolute displacements, continuously operating under all-weather conditions, and no need for line-of-sight between stations. With the increase of sampling frequency and dual-frequency GPS real-time kinematic, GNSS technology has been carried out to examine the ability of displacement detection of static and dynamic response of high-rise buildings [2] and long-span bridges [3]. Due to the satellite ephemeris error, tropospheric delay error, multipath effect error, etc., the obtained data with real-time GNSS techniques has a short-term accuracy of 10 mm. However, at times, foundation displacement monitoring requires greater accuracies during construction and operation. Then, the way of collecting data directly through GNSS cannot meet the accuracy requirements in real-time monitoring of foundation displacement and prediction of the final settlement.  
GNSS system usually adopts short baseline (i.e., less than 5 km) when it is applied to deformation monitoring of structures [4]. In relative positioning of short baselines, the double difference (DD) method causes some errors to be eliminated or decreased [5], such as receiver and satellite clock errors, orbital errors and ionosphere/troposphere errors, whereas the multipath error is the main error in DD residual of LS solutions [6]. Multipath errors differ from receiver to receiver and are not correlated. Then the DD technique cannot reduce multipath disturbances due to the lack of correlation. In dynamic de-formation monitoring, on account that the position of satellite antenna and its surrounding environment are basically unchanged, the signal noise can be decreased by employing wavelet de-noising [7], empirical mode decomposition [8] (EMD), and filtering algorithm [9] through the multipath interference-effect.  
The focus of this study is to demonstrate the performance of the continuous monitoring of the foundation displacement in long-span bridges with GNSS technology. This pa-per is organized as follows. Section 2 offers brief descriptions of the relevant theories on de-nosing approach. The stability and accuracy tests are summarized in Section 3. A new method is proposed to realize real-time monitoring of foundation displacement and prediction of final settlement in Section 4. In Section 5, based on the Hutong Yangtze River Bridge, the quality of the proposed method is proved by analysis and comparison with the precise level method in the construction process. Finally, Section 6 summarizes the key findings.