## 2 Stand-Alone Ground-Based Technology Applications in Transport Infrastructure Monitoring
\begin{table}
\begin{tabular}{c c c} \hline \hline
**NDT Technology** & **Description** & **References** \\ \hline Accelerometers & Accelerometers can record three-dimensional movement and could potentially be used to remotely monitor cattle behaviour. These devices collect data based on pre-defined recording intervals called epochs. Accelerometers are one type of sensor used to measure the vibration on bridge decks & [2; 21; 22] \\ \hline Strain Gauges & Strain gauges are used to measure strains on target objects. Generally, strain gauges consist of an insulating flexible backing supporting a metallic foil pattern. As the object is deformed, the foil is deformed, causing its electrical resistance to change & [3; 23] \\ \hline Wireless Network Systems & Data collected by wireless network monitoring systems allow identification of modal frequencies and mode shapes of bridges & [24] \\ \hline Infrared Thermography & This technique is based on a process where heat at any temperature is converted into a thermal image using specialised scanning cameras & [25; 26] \\ \hline Laser Scanner & The laser scanner, also referred to as LiDAR, is used for 3D data acquisition of both topographic and close-range objects. The equipment allows for an automated dense sampling of the object surface within a short time range & [9; 10; 27] \\ \hline Global Positioning System (GPS) & One of the global navigation satellite systems (GNSS) provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites & [4] \\ \hline Ground-Penetrating Radar (GPR) & The energy reflected by dielectric discontinuities in the subsurface is recorded through a receiving antenna and it is subsequently processed and displayed through a display unit & [6; 7; 8; 28; 29] \\ \hline Core Drilling & Geometric information on the internal configuration of the structure (i.e., bridges) are provided from the material extracted. Cores are visually analysed to collect information about layer thickness and hollow sections, amongst others & [30; 31] \\ \hline Sonic Tomography & This technique is an improvement of the sonic transmission test method, as tests are performed along paths non-perpendicular to the wall surface, as well as in a direct mode & [32; 33] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Non-destructive testing (NDT) methods and sensors used in transport infrastructure monitoring. Compared to high-resolution datasets, medium-resolution datasets (i.e., acquired in C-Band) stand as more easily accessible information by end-users for large-scale analyses and the measurements of settlements at the centimetre scale. Use of the medium-resolution imagery allows for lighter data processing and management flows, and it permits to investigate wider areas and sections of the transportation network. Nevertheless, several limitations still exist on the use of SAR medium-resolution data for transport infrastructure monitoring, including:  
1. the ground pixel resolution, which does not allow the allocation of PS displacements from a randomly given object to its actual position on the ground;
2. the accuracy of the measurements, due to the relatively limited frequency range of the sensors. Under specific limiting conditions, associating the PS data to the actual structural element can be challenging. This occurs especially in urban areas with buildings located in the vicinity of transport infrastructure, where errors can be introduced for the data interpretation stage. Within this framework, the use of satellite remote sensing for the systematic monitoring of transportation assets, as well as its accuracy against other conventional ground-based methods, has been widely discussed for the past 15 years. To illustrate the development of MT-InSAR techniques for transport infrastructure monitoring, relevant literature has been here reviewed using the \"Scopus\" scientific database.