## 2 Literature Review  
There are various remote sensing (RS) structural inspection techniques that can be used in monitoring and damage assessment of a bridge based on the level of importance and accuracy. The majority of remote sensing inspections are done through imaging sensors; different kinds of imaging sensors can perceive different kinds of information. Red Green Blue (RGB) or visible spectrum imaging sensor is employed for damage quantification [22]. Similarly, multispectral imaging sensors for damage assessment [23], hyperspectral imaging sensors for concrete properties [24], light detection and ranging (LiDAR) sensors or laser scanning for determining the crack depth [25] and near infrared (NIR) or thermal imaging sensors for identifying the moisture ingress and delamination of bridge deck [26]. These techniques are classified depending upon the type of data acquisition platform at varying altitudes and imaging sensors employed.  
The level of accuracy relies on the operational altitude of sensors and the resolution of imaging data. The data acquisition platforms as shown in Figure 1 are categorized based on high accuracy and low accuracy. Inspectors and engineers chose the data acquisition platform based on the type of monitoring, budget allocated and time span.  
(i) High accuracy:  
1. Terrestrial Cameras: terrestrial cameras are mounted on a tripod and they have the capability to continuously acquire the data for longer periods. Different sensors, such as visible range (RGB), laser, thermal and multispectral can be mounted on the same data acquisition platform. Terrestrial cameras and laser scanners to produce 3D models through point clouds can be operated even in varied climatic conditions.
2. Robot: Robotics can be operated directly on top of the bridge deck to produce high-resolution data sets using sensors, such as RGB, laser, ground penetrating radar (GPR) and multispectral. GPR has a unique capability in exploring the internal core condition of the RCC bridge deck to inspect the further serviceability of the structure.
3. Unmanned Aerial Vehicle (UAV): UAV/drone operational altitude is low and restricted to height of 400 ft (120 m) as per the guidelines issued by the Director General of Civil Aviation (DGCA), so it can be operated very near to the target object. UAV can acquire high resolution digital, multispectral and thermal imaging datasets along with laser point cloud data to inspect the bridge.  
(ii) Low accuracy:  
1. Unmanned/Manned aircraft: Aircraft flying range is above 1 km to 10 km and flies at supersonic speeds. The data acquired at specified speed and altitude generates coarse resolution images of the structure that can actually be used for preparing a rough estimate of a damaged structure during natural calamity. Only high-resolution visible  
Figure 1: Different remote sensing platforms based on the operational altitudes and data sets produced.  
range RGB and Multispectral imaging data acquired through aircraft is considered for damage assessment of a bridge. 2. Satellite: Satellites are operated at higher altitudes of 10-1000 km, and the revisit time on a particular location is more than 5 days, which may not be available at desired dates. Data sets obtained are of coarser resolution and may contain cloud cover that makes the bridge monitoring a hard task [27].  
All remote sensing acquisition systems are well designed to assess the condition above the bridge deck. Although there are certain advantages and disadvantages to each data acquisition system, UAV monitoring systems stand out as they can be operated even below the bridge deck and also at inaccessible locations.