# An Overview of Autonomous Vehicles Sensors and Their Vulnerability to Weather Conditions
## 2 Autonomous Vehicles Sensors Ecosystem
### Radar  
RADAR is an acronym for Radio Detection and Ranging technology, which is a device that uses radio waves for object-detection within a certain range. When the transmitted waves intercept with an object along its propagation path, they are reflected by its surface where the RADAR antenna collects the backscattered signal (echo) within its field of view (FOV). The round-trip delay time, together with the known velocity of radio waves, provides a precise determination of the object's distance and velocity from the RADAR system. The RADAR range equation that relates the received echo power (\(P_{r}\)) to the distance of an object R meters away is shown in Equation (1) [9]:  
\[P_{r}(R)=\frac{P_{t}G^{2}\lambda^{2}\sigma L}{(4\pi)^{3}R^{4}} \tag{1}\]  
where \(P_{t}\) is the transmitted power, \(G\) is the gain, \(\lambda\) is the wavelength, \(\sigma\) is the cross section of the target, and L represents all the losses lumped together, including multipath, atmospheric, and environmental losses.  
The RADAR systems for AVs operate at 24, 74, 77, and 79 GHz (millimeter wave (MMW)), which are separated out to work for short-range, medium-range, and long-range RADAR (SRR, MRR, and LRR, respectively) [9]. An LRR can be implemented to detect far targets or objects in front of the ego car, and MRR and SRR, on the other hand, are used for parking assistance or side view detection [10]. Among a plethora of RADAR technologies available nowadays, linear Frequency-modulated continuous-wave (L-FMCW) RADARs are commonly used in AVs due to their simplicity. Figure 5 shows a top-level block diagram of an FMCW RADAR, where the voltage-controlled oscillator (VCO) module generates an L-FMCW chirp signal that is amplified by the power amplifier (PA) and transmitted by the antenna. The receiving antenna captures the echo signal and the low noise amplifier (LNA) amplifies it before mixing it with the VCO signal in order to generate the intermediate frequency (IF) or beat signal. The analog to digital converter (ADC) then digitizes the signal and passes it to the digital signal processing (DSP) module.  
Figure 4: Autonomous vehicles sensor ecosystem (Image source: The Economist).  
Currently, most AV RADAR systems use an array of micro antennas capable of generating a set of antenna lobes. For instance, a 77 GHz radar with printed circuit board (PCB) antennas and a 24 GHz radar with horn antennas are shown in Figure 6[11]. This has become more common with system-on-chip (SOC) architectures, which allows digital beam forming, among other techniques, to restrict the receiver's spatial FOV and reduces the electromagnetic interference from other active sensors operating at similar center frequencies. The advancements of smart vehicles have triggered the use of this type of device in the automotive sector in order to improve safety. Adaptive cruise control (ACC), collision avoidance systems (CAS), blind spot detection (BSD), and lane change assist (LCA) are some examples. Matlab was used to create the initial step in establishing a complete testbed for AV sensor testing and verification, with an emphasis on radar systems under all environmental conditions of relevance [12].