Title: Global Navigation Satellite System‐based positioning technology for structural health monitoring: a review
Abstract: Structural Control and Health MonitoringVolume 27, Issue 1 e2467 REVIEWOpen Access Global Navigation Satellite System-based positioning technology for structural health monitoring: a review Jiayong Yu, Jiayong Yu orcid.org/0000-0001-5246-2846 Key Laboratory for Wind and Bridge Engineering of Hunan Province, Hunan University, Changsha, China Nottingham Geospatial Institute/Sino-UK Geospatial Engineering Centre, The University of Nottingham, Nottingham, UKSearch for more papers by this authorXiaolin Meng, Corresponding Author Xiaolin Meng [email protected] orcid.org/0000-0003-2440-8054 Key Laboratory for Wind and Bridge Engineering of Hunan Province, Hunan University, Changsha, China Nottingham Geospatial Institute/Sino-UK Geospatial Engineering Centre, The University of Nottingham, Nottingham, UK Correspondence Xiaolin Meng, Key Laboratory for Wind and Bridge Engineering of Hunan Province, Hunan University, Changsha 410082, Hunan, China. Email: [email protected] for more papers by this authorBanfu Yan, Banfu Yan Key Laboratory for Wind and Bridge Engineering of Hunan Province, Hunan University, Changsha, ChinaSearch for more papers by this authorBin Xu, Bin Xu orcid.org/0000-0001-8336-3306 College of Civil Engineering, Huaqiao University, Xiamen, ChinaSearch for more papers by this authorQian Fan, Qian Fan College of Civil Engineering, Fuzhou University, Fuzhou, ChinaSearch for more papers by this authorYilin Xie, Yilin Xie Nottingham Geospatial Institute/Sino-UK Geospatial Engineering Centre, The University of Nottingham, Nottingham, UKSearch for more papers by this author Jiayong Yu, Jiayong Yu orcid.org/0000-0001-5246-2846 Key Laboratory for Wind and Bridge Engineering of Hunan Province, Hunan University, Changsha, China Nottingham Geospatial Institute/Sino-UK Geospatial Engineering Centre, The University of Nottingham, Nottingham, UKSearch for more papers by this authorXiaolin Meng, Corresponding Author Xiaolin Meng [email protected] orcid.org/0000-0003-2440-8054 Key Laboratory for Wind and Bridge Engineering of Hunan Province, Hunan University, Changsha, China Nottingham Geospatial Institute/Sino-UK Geospatial Engineering Centre, The University of Nottingham, Nottingham, UK Correspondence Xiaolin Meng, Key Laboratory for Wind and Bridge Engineering of Hunan Province, Hunan University, Changsha 410082, Hunan, China. Email: [email protected] for more papers by this authorBanfu Yan, Banfu Yan Key Laboratory for Wind and Bridge Engineering of Hunan Province, Hunan University, Changsha, ChinaSearch for more papers by this authorBin Xu, Bin Xu orcid.org/0000-0001-8336-3306 College of Civil Engineering, Huaqiao University, Xiamen, ChinaSearch for more papers by this authorQian Fan, Qian Fan College of Civil Engineering, Fuzhou University, Fuzhou, ChinaSearch for more papers by this authorYilin Xie, Yilin Xie Nottingham Geospatial Institute/Sino-UK Geospatial Engineering Centre, The University of Nottingham, Nottingham, UKSearch for more papers by this author First published: 30 October 2019 https://doi.org/10.1002/stc.2467Citations: 56AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Summary Global Navigation Satellite System (GNSS) positioning technology has had widespread applications in the structural health monitoring as its overall performance has improved significantly in the last two decades. It is capable of providing timely and accurate structural vibration information such as dynamic displacements and modal frequencies at higher performance than traditional accelerometers. The studies summarized in this paper focus on the improvement of the multi-sensors and multi-constellation data acquisition techniques, the improvement of multiple approaches for erroneous noise mitigation, and innovative modal parameter identification methods. We also detailed the applications of GNSS on the deformation monitoring for towers, chimneys, tall buildings, and bridges. With continuous enhancements in the algorithm and hardware of GNSS, it is expected that the application of GNSS technology can be expanded to other fields such as bridge cable-force measurements and bridge weight-in-motion as well as structural deformation monitoring. 1 INTRODUCTION Structural health monitoring (SHM) technologies have had widespread applications for the purpose of evaluating the safety of engineering structures during their service lifetime, which ensures the serviceability and sustainability of the structures.1-3 The sensing technology is a critical part of SHM. Two types of sensors have been utilized for SHM of engineering structures, respectively, to identify the local and the overall dynamic response characteristics of structures. The first type of sensors, such as optical fiber, piezoelectric sensors, and intelligent materials, can obtain local dynamic response characteristics of structures with point or line distributions, which are usually attached on the important components of structures. The second type of sensors can monitor the overall dynamic response parameters of structures, such as global navigation satellite system (GNSS),4, 5 accelerometer,6 camera,7, 8 and 3-D laser scanner.9 The accelerometer has difficulties to monitor the structural quasi-static displacements caused by wind and temperature loads and to identify the vibration displacements from accelerations.10 When employed in SHM, the camera and 3-D laser scanner encounter the same drawback that their measurement accuracies rapidly decrease as the measurement sigh-distances increase.9 The GNSS positioning technology has many attractive advantages in SHM with its significantly improved overall performance. For example, its data sampling rate can reach 20 Hz, or even 100 Hz, which meets the monitoring requirement for most large-scale structures; it can continuously acquire data under all weather conditions such as wind, rain, and haze; it can provide not only real-time 3-D displacements but also the time with an accuracy of 30 ns.11-13 The GNSS technology can measure the static displacements at millimeter level accuracy. Meanwhile, its dynamic measurement accuracy is also significantly improved with measurement accuracies of 20 and 10 mm in the vertical and horizontal directions, respectively, in last two decades.14 Some GNSS-based monitoring systems have been successfully established and applied in SHM of the long-span bridges, towers, chimneys, and high-rise buildings.15-21 Nevertheless, many researches have been conducted to improve the GNSS measurement accuracy due to the limitation of ephemeris error, ionospheric error, tropospheric errors, multipath error, receiver measurement noise, and so forth.22 Many data acquiring and processing techniques have emphatically been developed to reduce the measurement cost and improve measurement accuracy in structural dynamic monitoring. The paper summarized not only the data acquisition methods such as multiple sensors, pseudo-satellite augmentation, and multiple antennas technique but also the kinematic solution methods, for example, Post-Processing Kinematic (PPK), Real-Time Kinematic (RTK), Network-based RTK (NRTK), and kinematic Precise Point Positioning (PPP). Finally, the development opportunities of GNSS technology in SHM are illustrated. 2 GNSS KINEMATIC POSITIONING MODES FOR STRUCTURAL MONITORING The expanded satellite navigation systems cover GNSS, regional navigation satellite systems (RNSS), satellite-based augmentation systems (SBAS), and ground-based augmentation system (GBAS).23 The global GNSS consist of America's Global Positioning System (GPS), Russia's GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS), EU's Galileo Navigation Satellite System (Galileo), and China's BeiDou navigation satellite system (BDS) as shown in Table 1. RNSS include Japan's Quasi-Zenith Satellite System (QZSS) and India's Indian Regional Navigation Satellite System (IRNSS). GBAS and SBAS are utilized to improve the performance of GNSS, enhancing their measurement accuracy, integrity, continuity, and effectiveness.24 SBAS contains Wide Area Augmentation System in North America, Satellite Navigation Augmentation System in China, European Geostationary Navigation Overlay Service in Europe, and so forth. GBAS includes local area augmentation system in America, Ground-based Regional Augmentation System in Australia, and so forth.25 Table 1. GNSS systems GNSS system Establishment country Satellite number (plan/launch) Opening time GPS America 24/24 From 1993 GLONASS Russia 24/24 From 1996 BDS-3 China 35/23 From 2020a Galileo EU 30/26 From 2020 QZSS Japan 4/4 From 2018 IRNSS India 7/7 From 2018 Abbreviations: BDS, BeiDou navigation satellite system; EU, European Union; GLONASS, GLObalnaya NAvigatsionnaya Sputnikovaya Sistema; GNSS, Global Navigation Satellite System; GPS, Global Positioning System; IRNSS, Indian Regional Navigation Satellite System; QZSS, Quasi-Zenith Satellite System. a BDS-2 has been operational in the whole Asia-Pacific region since 2012. As shown in Table 2, four modes are usually utilized for the GNSS kinematic solutions in SHM, consisting of PPK, RTK, NRTK, and kinematic PPP.26 The double-differential carrier phase method is used to resolve the GNSS baseline for the former three modes whereas the PPP method is adopted to resolve the observation data for the last mode. Table 2. GNSS solution modes Solution modes Acronym Detail Real-Time Kinematic RTK At least one base station needed to be established at a known point, and rover stations are established at measurement points. The real-time coordinates of measurement points can be computed using at least four satellite signals and correction data broadcasted by base station (Figure 1) Post-Processing Kinematic PPK PPK solution is similar to RTK mode, but compute in post-processing manner instead of real-time. Network-based Real-Time Kinematic NRTK NRTK solution is similar to RTK, but receive correction data from CORS instead of a self-established base station (Figure 2) kinematic Precise Point Positioning PPP Rover stations are established at measurement points. The coordinates of measurement points are resolved using at least four satellite signals and precise ephemeris and clock bias afforded by ground tracking stations. Abbreviations: CORS, Continuously Operating Reference Station; GNSS, Global Navigation Satellite System. In conventional PPK and RTK modes, one or more local base stations need to be established at a stable site close to the monitored target for baseline solutions (Figure 1). The rover stations are established at the measurement points of the monitored target. All GNSS receivers on both rover and base stations will synchronously receive more than four GNSS satellite signals.27 In RTK mode, the base stations will continuously broadcast the RTK correction data; the real-time coordinates of the rover stations (i.e., the measurement points) can be computed on the basis of the several GNSS satellite signals and correction data. The dynamic deformations and natural frequencies of the monitored target can be detected from the coordinate time series. Comparing the PPK and RTK modes, the only difference is that the PPK results are computed in a post-processing manner instead of real time.28, 29 The RTK technique was successfully applied for the first time to the dynamic displacement monitoring of the Humber Bridge near Kingston upon Hull, UK. The results demonstrated remarkable accuracies of a few millimeters in all three directions.30 Figure 1Open in figure viewerPowerPoint GNSS measurement principle in RTK mode. GNSS, Global Navigation Satellite System; RTK, Real-Time Kinematic In a pioneering study, the NRTK technique was applied in dynamic response monitoring of large-scale structures.25 In NRTK mode, the correction data are received from the Continuously Operating Reference Station rather than a self-established base station (Figure 2). The time series of dynamic displacements can be computed from the GNSS solution results in this mode, similar to the RTK mode. Subsequently, the NRTK method was used to monitor the dynamic displacements of the Nottingham Wilford suspension bridge with an amplitude of less than 8 mm.31 Figure 2Open in figure viewerPowerPoint GNSS measurement principle in NRTK mode. CORS, Continuously Operating Reference Station; GNSS, Global Navigation Satellite System; NRTK, Network-based Real-Time Kinematic In kinematic PPP mode, the coordinates of measurement points are resolved using carrier phase observations as well as precise ephemeris and clock bias afforded by ground tracking stations.32 It was demonstrated that high-rate PPP can detect the oscillations with a sub-centimeter level amplitude in horizontal direction at a 2 to 4-mm level accuracy, with an amplitude of 15–20 mm in vertical directions at a sub-centimeter level accuracy by Xu et al.33 The post-processing kinematic PPP method was demonstrated by Yigit et al34 that it can identify vibration displacements and frequencies of engineering structures. Meanwhile, the real-time kinematic GNSS-PPP was successfully used to measure the displacements of the Severn suspension bridge in UK. PPP can replace double difference for structure monitoring when double difference becomes unavailable on account of natural disasters, and so on.35 Numerous studies on the applications of post-processing or real-time PPP techniques for SHM have been performed with similar results by some other researchers.36-39 3 APPLICATIONS OF GNSS FOR STRUCTURAL MONITORING GNSS sensors are an ideal instrument for the deformation monitoring of engineering structures such as the buildings, towers, chimneys, and bridges. Some large-scale structures have equipped GNSS-based SHM sensors to monitor their dynamic response characteristics induced by wind, temperature, and earthquake loads. 3.1 Dynamic monitoring for towers and chimneys Soon after the advent of GNSS technology in 1993, many researches were carried out to monitor the vibrations of towers and chimneys with the GNSS technology (Table 3). Lovse et al40 pioneered GNSS technology-based vibration monitoring method; this method was used to understand wind-induced vibration responses of the Calgary Tower in Canada with a height of 160 m. A fundamental frequency of 0.36 Hz and vibration displacements of 1.6 cm were successfully identified. Breuer et al41 evaluated the ability of GNSS to identify wind-induced vibration displacements of a TV tower and an industrial chimney, with an amplitude of several centimeters. The advantages of GNSS monitoring technology over conventional accelerometers have been proved that displacements can be reliably and accurately identified. Gorski et al42 detected wind-induced dynamics of a 300-m industrial chimney of the Bełchatów Power Station in Poland with GNSS measurements. Three sets of GNSS receivers were fixed at the different location of the chimney, that is, H = 179, 293, and 297 m above the ground (Figure 3). The displacement measurement accuracies were approximately 6–18 mm in the vertical direction and 4–5 mm in the horizontal direction. The first nature frequency of 0.212 Hz was also detected from the GNSS measurements, agreeing well with the finite element model (FEM) predicting results. Other applications of GNSS technology on towers and chimneys were conducted, including a 108-m tower in Tokyo43 and Guangzhou New TV tower.44 Table 3. GNSS-based dynamic monitoring for towers, chimneys and high-rise buildings Resource Structure type Structure name Structural characteristics Identifying structural dynamics from GNSS data Lovse et al40 Tower The Calgary Tower in Canada Height of 160 m Vibration frequency 0.36 Hz; vibration displacement 1.6 cm Breuer et al41 Tower The Stuttgart TV tower in Germany Monitoring point at the height of 155 m above the ground Elliptical movements of along-wind displacement 4 cm and cross-wind displacement 2.5 cm Breuer et al41 Chimney The chimney of Opole power station in Poland Monitoring point at the height of 245 m Elliptical movements Gorski et al42 Chimney The chimney of Bełchatow power station in Poland Three monitoring points at three levels H = 297, 293, and 179 m The first natural frequency 0.212 Hz Li et al43 Tower A tower in Tokyo Height of 108 m Vibration frequency 0.57 Hz; Ni et al44 Tower Guangzhou New TV tower Height of 610 m Maximum displacement of about ±15 cm during three typhoons Celebi et al45 High-rise building The building in Los Angeles 44 stories No information Ogaja et al46 High-rise building The Republic Plaza in Singapore The tallest building in Singapore Verifying the feasibility of GPS method Kijewski-Correa et al47 High-rise building Three tall building in Chicago No information Verifying the potential of GPS to track the motions of building Kijewski-Correa et al48 High-rise building Burj Khalifa building in Dubai Height of 828 m Agreeing well with the accelerometer results Guo et al49 High-rise building The Diwang Tower in Shenzhen, China 68 stories No information Park et al50 High-rise building A high-rise building 66 stories x-axis displacement from −11.7 to 20.9 mm; y-axis displacement from 31.8 to 61 mm Yi et al51 High-rise building A super-tall building in Hong Kong 420 m in height and 88 stories The fundamental frequencies of 0.140 and 0.144 Hz in x and y directions Liu et al52 High-rise building Tianjin 117 Tower in China Height of 597 m No information Abbreviations: GNSS, Global Navigation Satellite System; GPS, Global Positioning System. Figure 3Open in figure viewerPowerPoint View of the industrial chimney of Belchatow Power Plant and installation of the rover receivers on the gallery of the chimney42 3.2 Dynamic monitoring for high-rise buildings Many applications of GNSS technology on high-rise buildings have been performed to identify the dynamic characteristics due to wind, vehicle, and earthquake loads (Table 3). Celebi et al45 confirmed the feasibility of GNSS monitoring technology through two simulation tests and an in-site test of a 44-floor building. The measurement accuracy of relative displacements provided by GNSS were sufficient to identify the dynamic characteristics of the tall building. Ogaja et al46 utilized a RTK-GPS system consisting of two Trimble receivers to measure vibration responses of the Republic Plaza Building in Singapore, complementary to accelerometers. It was confirmed that the GNSS technology can monitor wind, earthquake, or thermally induced responses of large engineering structures. Kijewski-Correa et al.47 established a GNSS monitoring system on high-rise buildings in Chicago and demonstrated the potential of GNSS to accurately measure the wind-induced dynamic characteristics of tall buildings, with an excellent correction between the GNSS and accelerometer data. Also, they established and operated an advanced prototype system for SHM, SmartSync, on the world's tallest building, Burj Khalifa in Dubai.48 The displacement module of SmartSync, consisting of Leica AT504 GG choke-ring GPS antennas and GRX1200 GG receivers, provided displacements of the building at 10 Hz in near real time (Figure 4). In addition, GNSS units were also deployed on the 68-story Diwang Tower in Shenzhen City,49 a 66-story building in Korea,50 an 88-story super-tall building in Hong Kong,51 and a 597-m high Tianjin 117 Tower in China, and so forth.52 Figure 4Open in figure viewerPowerPoint Measured acceleration and displacement responses of the world's tallest building, Burj Khalifa, in Dubai on September 10, 2008, earthquake.48 GPS, Global Positioning System 3.3 Dynamic monitoring for bridges Pioneering researches on the dynamic monitoring of bridges using GNSS begun in the late 1990s. The feasibility of applying the GNSS technology to monitor bridge structures has been verified by many experiments (Table 4). Ashkenazi et al30 carried out GNSS monitoring experiments on the Humber Bridge in UK to verify the feasibility of the GNSS technology application for the dynamic monitoring of bridges. They are confident that RTK GNSS technique can measure the 3-D dynamic displacements of Table 4. GNSS-based dynamic monitoring for bridges Resource Structure type Structure name Structural characteristics identifying structural dynamics From GNSS data Ashkenazi et al30 Brown et al53 Suspension bridge the Humber Bridge in UK Main span of 1,410 m Displacements of 3 mm, 14 cm and 40 cm in longitudinal, lateral, and vertical directions; natural frequencies of 0.116 and 0.052 Hz in vertical and lateral directions Watson et al54 Cable-stayed bridge The Batman Bridge in Australia Main span of 206 m Vertical displacements of 54 ± 3.5 mm; longitudinal displacements of 17 ± 3.5 mm Nakamura et al28 Suspension bridge A Japanese suspension bridge Main span of 720 m and two side spans of 330 m each GNSS measurement errors within 1.6 cm and 2.1 cm in horizontal and vertical directions, respectively Yu et al31 Suspension bridge The Nottingham Wilford bridge in UK Constructed in 1904 with a single span of 69 m The fundamental frequency 1.69 Hz of the bridge, and dynamic displacements within 8 mm Moschas et al55 truss girder bridge A steel pedestrian bridge in Greece 40 m long 4.3 Hz modal frequency, ~6 mm oscillation amplitude Ogundipe et al14 box girder bridge A steel box girder viaduct bridge in UK Total length of 1,400 m, and main span of 173.7 m Vertical modal frequencies of 0.526 and 1.139 Hz, the peak vertical deflections within ±50 mm Lekidis et al56 Cable-stayed bridge the Evripos cable- stayed bridge in Greece 395 m in length with a central span of 215 m Four modal frequencies of 0.369, 0.389, 0.449, and 0.543 Hz Larocca et al57 Suspension bridge The Pierre-Laporte suspension bridge in Canada Total length of 1,040 m, and one center span with a length of 667 m Natural frequency 0.21 Hz of central span, and traffic caused deflections ranging from 4 to 8 cm Schaal et al58 Cable-stayed bridge a cable-stayed wood footbridge in Brazil A 35 m span Response frequency 2.07 Hz and displacement amplitude 12 mm Gaxiola-Camacho et al59 Girder bridge The Juarez bridge in Mexico Length of 200 m Vertical dynamic displacements ranging from 0.011 to 0.25 m Wong et al15 Suspension and cable- stayed bridges The Tsing Ma Bridge, Kap Shui Mun Bridge, and Ting Kau Bridge in Hong Kong Total length of 2,159.5, 820, and 1,177 m, respectively Employment of wind and structural health monitoring system Kashima et al16 Suspension bridge The Akashi Kaikyo Bridge in Japan Three-span and two-hinged suspension bridge with a center span of 1,991 m The wind-induced horizontal displacements with a mean-value 5.17 m and a vibration amplitude 0.78 m Yu et al17 Suspension bridge The Aizhai suspension bridge in China Three spans of 242 + 1,176 + 116 m The longitudinal displacements of the girder linearly related with temperatures Meng et al18 Suspension bridge The forth road bridge in Scotland Total length of 2,512 m with a central span of 1,006 m Employment of the GeoSHM system for SHM Guo et al60 Suspension bridge The Humen bridge in China Total length of 15.762 km and a main span of 888 m One RTK reference station and a total of 12 GPS monitoring stations Li et al61 Cable-stayed bridge Binzhou Yellow River highway bridge in China Total length of 768 m and two main span of 300 m each Employment of four Topcon GPS receivers Kaloop et al62 Cable-stayed bridge The Tianjin Yonghe bridge in China Total length 510 m and a main span 260 m The fundamental frequency 0.418 Hz and damping ratio 6.54% of first mode shape Abbreviation: GeoSHM, Global Navigation Satellite System and Earth Observation for Structural Health Monitoring; GNSS, Global Navigation Satellite System; GPS, Global Positioning System; RTK, Real-Time Kinematic; SHM, structural health monitoring. bridges with an accuracy of only few millimeters. Watson et al54 conducted GNSS monitoring experiments on the Batman Bridge in Northern Tasmania, Australia. They successfully determined both the thermally induced displacements and vehicle-excited high-frequency vibration displacements, with typical vertical displacements of 54 ± 3.5 mm on the main-span and corresponding longitudinal displacements of 17 ± 2 mm on the tower structure. Pioneering researches confirm that GNSS technology can be applied to detect dynamic response of bridges with small vibration amplitudes and low vibration frequencies.43 Furthermore, the GNSS monitoring technology was used to the deformation monitoring of various flexible bridges based on these successful pioneering works. Brown et al53 measured the vibration displacements of the Humber Bridge in UK under known loading conditions using five sets of Ashetech ZXII dual-frequency GNSS units. The GNSS-measured displacements agree very closely with those predicted by the FEM. Moreover, the GNSS-identified vibration frequencies are also similar to those both measured by conventional methods and predicted by FEM. Nakamura et al28 conducted field experiments on a suspension bridge in Japan using GNSS. The wind-induced displacements of bridge girders were directly measured using GNSS with estimated errors of no more than 2.1 cm in each direction. Yu et al.31 conducted numerous GNSS monitoring experiments on the Wilford suspension bridge in the UK to validate the measurement accuracy of GNSS, and the millimeter-level displacements were identified by GNSS, which align well with the accelerometer results (Figure 5). Figure 5Open in figure viewerPowerPoint Comparison of the filtered GNSS displacements with accelerometer-derived displacements. The GNSS displacements were identified from the filtering scheme (a), (b), and (c), corresponding to subfigures of (a–c), respectively.31 GNSS, Global Navigation Satellite System Thereafter, the applications of GNSS technology were extended to rigid bridges with relatively small vibration displacements and high modal frequencies. Moschas et al55 measured dynamic responses of a steel pedestrian footbridge in Athens, Greece, using GNSS. A Topcon receiver with a data sampling rate of 10 Hz and a GeoSIG-type accelerometer were attached at the middle of the center span. Vertical displacements of approximately 6-mm oscillation amplitude and modal frequency of 4.3 Hz were identified from the GNSS data, in agreement with accelerometer measurements. Ogundipe et al14 conducted field tests on a viaduct bridge in UK using GNSS for monitoring relatively small deformations. Five GNSS receivers with choke-ring antennas (Leica 500 and 1200) were attached on the bridge while two reference stations were installed away from the monitored bridges. The vertical deflections with amplitudes of ±50 mm were accurately measured whereas lateral and longitudinal deflections with amplitudes in the order of a few millimeters were also measured with sufficient accuracy. Similar verification experiments were conducted by Lekiddis et al56, Larocca et al57, Schaal et al58 and Vazquez et al.59 All verification experiment results confirmed the feasibility of monitoring structure dynamic responses using high-sampling rate GNSS units. Thus, GNSS monitoring technology has the capability of identifying dynamic characteristics not only of flexible bridges with large vibration displacements and low vibration frequencies but also of relatively rigid bridges with small vibration displacements and high vibration frequencies. 3.4 Employments of GNSS monitoring systems Some long-span flexible bridges (i.e., suspension cable-stayed structures) have equipped GNSS-based structural monitoring systems for monitoring their dynamic responses due to various loads (Table 4). The Hong Kong Highways Department established and operated the Wind and Structural Health Monitoring System to monitor structure conditions of three bridges, that is, the Tsing Ma Bridge, the Ting Kau Bridge, and the Kap Shui Mun Bridge.15 The GPS-On-Structure Instrumentation System (GPS-OSIS) was installed to monitor the 3-D displacements of major components (bridge towers, main cables and bridge deck) of these bridges. GPS-On-Structure Instrumentation System consists of 27 GNSS receivers at measurement points (rover stations) and two GNSS receivers at reference stations. The estimated displacement measurement accuracies of GNSS in each direction were10–20 mm. Kashima et al16 introduced that a GNSS monitoring system was installed and operated for measuring the deformation of both the girder and tower of the Akashi Kaikyo Bridge. Three GNSS receivers were located on the upper surface of the 1A anchorage, the west side of the 2P tower top and the east side of the middle-span stiffness girder, respectively. The GNSS-based monitoring system accurately obtained the lateral displacements with a vibration amplitude of 0.78 m during a typhoon in September 22, 1998. It was confirmed that the GNSS-based monitoring system was capable of monitoring structure conditions of the Akashi Kaikyo Bridge. An SHM system was implemented on the Aizhai suspension bridge in China, which employed nine Leica GMX902GG GNSS receivers for monitoring vibration displacements of the bridge.17 Six GNSS unit