## 3 Results and Discussions
### Discussion  
It has been mentioned that the largest deformation rate occurred on the long span cable-stayed bridge (Figure 7a). The cable-stayed bridge is known as a temperature sensitive structure [41]. The difference in the temperature of cable and beam will influence the cable force and elevation of the main beam [42]. Although there are no public results on the temperature effects on the Donghai Bridge especially the cable-stayed section, the studies on the similar bridges show that the changes of elevation caused by the variation of temperature are up to several centimeters [43]. The used PALSAR data were acquired at 10:23 pm local time in January, February, April, and July, respectively, and the mean night-time temperature in those months were 1 \({}^{\circ}\)C, 1 \({}^{\circ}\)C, 10 \({}^{\circ}\)C and 23 \({}^{\circ}\)C. The bridge load during the specific acquisition time (i.e., 10:23 p.m.) is much smaller compared to that in daytime. It can be concluded that the large deformation rate on the cable-stayed bridge are mainly caused by the thermal expansion. The width of the cable-stayed bridge is nearly 260 m, resulting in a maximum vertical deflection of 0.52 m. Therefore, the detected motion of 1.2 cm would not cause any damage to the bridge stability.  
In this study we focus on the testing of the performance of TCPInSAR with a small dataset of SAR images, therefore, only four images are employed which cover nearly half a year. However, at least a 1.5-year cycle is required to establish the relationship between the deformation and temperature [44]. This will be conducted in future work by involving a large dataset of SAR images.  
It should be highlighted that cross-heading data can be more beneficial to the monitoring of the Donghai Bridge than the single-track data. The InSAR LOS deformations of a bridge can be constituted by three components, including the vertical component, the longitudinal components (i.e., the deformation along the longitudinal axis of the bridge), and the transversal component (i.e., the deformation perpendicular to the longitudinal axis of the bridge). The longitudinal deformation is mainly induced by thermal expansion [44; 45]. We can observe in Figure 7 that the longitudinal direction of the cable-stayed bridge is nearly north, but the longitudinal direction of the other parts of the bridge (i.e., the pier-stayed bridge) is north by east, nearly parallel to the azimuth direction of the ascending PALSAR data. Therefore, the longitudinal deformation of the pier-stayed bridge might be missed by the ascending data but can be measured by the descending data [46].  
We can also observe in Figure 7b that, in the pier-stayed part of the bridge, there are also some relatively large deformation rates (~1 cm/year). Since the bridge is located in a very rough sea environment it suffers many serious effects. The seasonal winds and the tide could cause the transversal deformation of the bridge, and the foundation settlement could cause the vertical deformation of the bridge. Both components are quite suitable to be monitored by the ascending PALSAR data. Therefore, it can be inferred that the seasonal winds, the tide, and the foundation settlement all contribute to the deformation of the Donghai Bridge in addition to the thermal expansion. More environmental materials and collaboration with the bridge engineers are needed to draw a more reliable conclusion on the safety of the bridge.