# Review on Active and Passive Remote Sensing Techniques for Road Extraction
## 2 Overview of the Existing Data Acquisition Techniques for Road Extraction
### SAR Imaging Technology  
#### 2.3.1 Data Acquisition Methods and Characteristics  
SAR is an active remote sensing technology that uses microwaves with wavelengths of few centimetres as opposed to LiDAR, which functions using optical wavelengths (ultraviolet, visible, near-infrared or shortwave infrared light). Both these sensors measure the distance between the instrument and the target using the time delay of the echoes. One of the main benefits of SAR is the acquisition of fine and detailed images through the clouds; moreover, this sensor can even work at night. In SAR, initially, short-pulsed microwave radiation is emitted and backscattered by the target; this backscattered signal from the illuminated area is then recorded. A SAR system with a long virtual antenna can generate fine spatial resolution. Among the currently available SAR systems, the spaceborne SAR sensors can provide sub-metre spatial resolutions. Notably, the long \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Name** & **References** & **Platform** & \
\begin{tabular}{c} **Wavelength Range (\(\mu\)m)** \\ \end{tabular} & **Channel** &
\begin{tabular}{c} **Spectral** \\ **Resolution** \\ **(nm)** \\ \end{tabular} & **FOV/Swath** \\ \hline AISA-FENIX 1K & [122], 2018 & Airborne & 0.38–0.97, & 348, & \(\leq\)4.5, & 0.68 & 40\({}^{\circ}\) \\ APEX & [123], 2015 & Airborne & 0.94–2.54 & 198 & 5–10 & 0.489 & 28.1\({}^{\circ}\) \\ AVIRIS-NG & [124; 125], 2016 & Airborne & 0.38–2.52 & 430 & 5 & 1 & 34\({}^{\circ}\) \\ CASI-1500 & & 0.38–1.05, & & 2.3, & 2.3, & 2.3, & 2.3, & 2.19 \\ SASI-1000A & & 0.95–2.45, & 288, & 15, & 0.49, 1.22, 1.19 & 40\({}^{\circ}\) \\ TASI-600A & & & 8–11.5 & 100,32 & 110 & & \\  & & 0.4–0.95, & 256, & 2.34, & 0.25, & \\ AMMIS & [127; 128], 2019, 2020 & Airborne & 0.95–2.5, & 512, & 3, & 0.5, & 40\({}^{\circ}\) \\  & & 8–12.5 & 128 & 32 & 1 & \\  & & 0.4–1, & & 5, & & \\ SYSPHE & [129], 2016 & Airborne & 0.95–2.5, & 6.1, & & \\  & & 3–5.4, & 560 (total) & 11 cm\({}^{-1}\) & 0.25 & 15\({}^{\circ}\) \\  & & 8–11.18 & & 5 cm\({}^{-1}\) & & \\ HSI & [130], 1996 & LEWIS & 0.4–1, & 128, & 5, & \\  & Satellite & 1–2.5 & 256 & 5.8 & 0.057 & 7.68 km \\ Hyperion & [131], 2003 & EO-1 & 0.4–1, & 242 (total) & 10 & 0.043 & 7.7 km \\  & Satellite & 0.9–2.5 & & & & \\ CHRIS & [132], 2004 & PROBA-1 & 0.4–1.05 & 18/62 & 1.25–11 & 0.03 & 18.6 km \\  & & Satellite & 1.00–2.92 & 544 (total) & 6.55 & 0.061 & \textgreater{}7.5 km \\ AHSI & [134], 2019 & Gaofen-5 & 0.39–2.51 & 330 (total) & 10 & 0.043 & 60 km \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Typical airborne and spaceborne hyperspectral sensors. antenna aperture is realised in the cross-range direction and that the range resolution is given by the pulse width (in general, the bandwidth of the signal). Another particularity of the SAR compared with optical sensors is the low correlation between range and spatial resolution. SAR uses a side-looking imaging geometry to generate 2D image data of the target area. In target areas with uneven topography, the side-looking imaging geometry distorts the SAR images because of various factors such as foreshortening, layover and radar shadows [144]. These distortions are challenging for mapping applications, especially in urban areas with high-rise buildings. SAR sensors collect data in a complex domain, and the acquired data can be converted into intensity and phase information. SAR data are usually presented as 2D intensity images that provide information on the amount of backscattered signal. Since the backscattered signal is a combination of signals from multiple scatterers, the images have granular noise called speckle. Sensors transmit and receive horizontally and vertically polarised signals and provide either single, dual or quad polarisation data [145; 146; 147]. Phase information is used in SAR polarimetry and interferometry.