# An Overview of Autonomous Vehicles Sensors and Their Vulnerability to Weather Conditions
## 2 Autonomous Vehicles Sensors Ecosystem
### Lidar  
LiDAR is an acronym for Light Detection and Ranging, which is a technology developed in the 1970s to be deployed on space platforms and airborne platforms. In a similar fashion to RADARs, LiDAR systems base their operation on measuring the time it takes a pulse of light, in infrared or near-infrared ranges, emitted from a laser diode until it is received by the system's receiver, which is also known as the time-of-flight (ToF) principle. In ToF technology, the LiDAR generates a pulse of light with a specified duration (\(\tau\)) that, at the time of emission, triggers the internal clock in a timing circuit. The reflected light pulse from the target is detected by a photodetector, which produces an electrical output that disables the clock. The distance to the reflection point may be calculated by using this electronically measured round-trip ToF (\(\Delta t\)) [13], as indicated in Equations (2) and (3):  
\[P(R)=P_{0}\rho\frac{A_{0}}{\pi R^{2}}\mu_{0}\exp{-2\gamma R} \tag{2}\]  
\[R=\frac{1}{2n}c\Delta t \tag{3}\]  
Figure 5: High-level block diagram of an FMCW RADAR system.  
Figure 6: (a) A 77 GHz radar with PCB antennas and (b) 24 GHz radar with horn antennas.  
where \(P_{0}\) is the optical peak power of the emitted laser pulse, \(\rho\) is the reflectivity of the target, \(A_{0}\) is the receiver's aperture area, \(\mu_{0}\) is the detection optics spectral transmission, \(\gamma\) is the atmospheric extinction coefficient, \(c\) is the speed of light in a vacuum, and \(n\) is the index of refraction of the propagation medium (\(\sim\)1 for air). Figure 7 demonstrates a high-level block diagram for ToF LiDAR.  
The main wavelengths used in LiDARs are 905 nm and 1550 nm, which are determined by atmospheric transmission windows and the availability of high-power pulsed sources [13]. At the early development stages of AVs, 905 nm pulsed LiDAR systems were chosen for the task due to their availability. However, these systems have some serious limitations, such as high cost, inefficient mechanical scanning, interference from other light sources, and eye-safety power restrictions that limit their detection range to \(\sim\)100 m. This prompted the shift to the retinal-safe 1550 nm band, because the water in the atmosphere begins to absorb energy from 1400 nm, which allows pulse powers high enough to range from 200 to 300 m [13]. LiDARs used for vehicles belong to Class-1 [14] and are safe under all conditions of normal use.  
Creating a three-dimensional (3D) profile (typically 360\({}^{\circ}\) in azimuth \(\times\) 20\({}^{\circ}\) in elevation) of the environment surrounding the AV requires a raster-scanned laser beams (scanning LiDAR) or flooding the scene with light and collecting the returns (flash LiDAR). For scanning LiDARs, the platform emits pulses from a set of diodes mounted on a rotating pod or by using a rotating multi-faceted mirror. The moving parts in these designs (rotates at 300-900 rpm) represent points of high failure rate in rough driving environments. Other approaches that can reduce the need for mechanical steering include the use of microelectromechanical systems (MEMS) mirror to steer the lens electrically or the use of optical phased array (OPA) technology. On the other hand, flash LiDARs flood the scene within the FOV of the detector with light. The detector is an array of avalanche photodiodes (APDs) where each independently measures ToF to the target feature imaged on that APD.