# An Overview of Autonomous Vehicles Sensors and Their Vulnerability to Weather Conditions
## 3 Weather Effects
### Precipitation (Rain, Snow, Hail, Sleet)  
Precipitation is water (liquid or frozen) that falls back to the ground after condensing in the colder atmosphere. A droplet's size and distribution defines the intensity of precipitation (measured in millimeters/hour (mm/hr)), which in turn affects the mechanism's electromagnetic signals that propagate through the precipitated medium. According to [30], the maximum diameter a rain droplet can have is 6 mm. If the rain droplet diameter exceeds this amount, the air resistance coupled with the terminal velocity of the droplet will exceed its cohesive force and tear it into smaller pieces. According to [PERSON]'s solution to Maxwell's equation, any transmission wavelength (\(\lambda\)) that is similar or smaller to the droplet diameter of 6 mm will be subject to Mie scattering [31]. Mie scattering can affect the propagation of the EM signal in two ways: first, the absorption of EM energy by water drops and vapor causing attenuation; and second, the rain volume back scattering or rain clutter can generate false alarms or mask actual targets in front of the sensor. LiDARs transmitting in the 905 nm and 1550 nm wavebands will be heavily affected by Mie scattering from rain. Figure 9 shows the visibility deterioration on LiDAR during different rain intensities [13]. As can be observed from Figure 9a, LiDAR visibility in clear conditions, which should have been \(\sim\)2 km at these wavebands, has been deteriorated to approximately 1.2 km for the 905 nm band and 0.9 km for the 1550 nm band in 2 mm/h rain. When the rain rate increases to 25 mm/h, the 2 km visibility drops to 0.7 km and 0.45 km for wavelengths 905 nm and 1550 nm, respectively, as illustrated in Figure 9b. The wetness of the target decreases the visibility by an additional addend of 0.1 km. However, within the range of 250 m usually required for rangefinders on AVs, LiDAR susceptibility to rain is not as noticeable until more severe rain rates occur, as shown in Figure 10 [30]. For 77 GHz RADAR systems used in AVs (\(\lambda\approx 3.9\) mm), the effect of attenuation is not significant at short distances [32]. It ranges from 0.0016 dB/m for 1 mm/h to 0.032 dB/m at 100 mm/h [33]. However, rain backscattering or rain clutter can decrease the maximum range of detectability, as discussed in [34; 35]. Moreover, since the received backscatter from the rain depends on \(R^{2}\) instead of \(R^{4}\) for the target echo where \(R\) is the range, rain clutter  
Figure 10: Variation of the signal/noise ratio as a function of the rain rate and distance between LiDAR and target for a rain droplet radius equal to 3 mm. Figure 9: The visibility effects of differing rain intensities and target wetness on 905 nm and 1550 nm waves vs. normal conditions for (a) 2 mm/h and (b) 25 mm/h rain rate. can exceed the detection threshold and results in false alarms. Figure 11 summarizes the results of an experiment conducted by [33] in order to evaluate the effects of rain volume backscattering on 77 GHz RADARS used in AVs. It demonstrates rain RADAR cross section (RCS) and its received power for different rain rates and ranges and for narrow and wide beam RADAR systems. GNSS is generally not affected by local weather conditions. These satellite systems operate at a frequency (approximately 1.575 GHz) that is mostly unaffected by weather [36]. However, the windshield wipers will occasionally block reception, making it impossible for a GNSS device to identify a complete navigation data string from satellites. As a result, the GNSS receiver may not properly decode the incoming string and may provide erroneous data [37]. Camera systems onboard AVs relies on scene brightness to determine the intensity of image pixels. Different weather conditions can introduce sharp intensity fluctuations that can result in quality degradation of images and videos. For instance, snow and heavy rain can obscure edges of an object, rendering it unrecognizable.