Researchers from the Intelligent Vehicles Group at WMG, University of Warwick, UK, have specifically simulated and evaluated the performance of lidar sensors in rain.
Their work – published in a paper in the IEEE Sensors Journal, entitled, ‘Realistic LiDAR with Noise Model for Real-Time Testing of Automated Vehicles in a Virtual Environment’ – used the WMG 3xD simulator to test an autonomous vehicle’s lidar sensors in different intensities of rain, by driving around a simulation of real roads in and around the city of Coventry.
Lidar sensors work by emitting numerous narrow beams of near-infrared light with circular/elliptical cross sections, which then reflect off objects in their trajectories and return to the detector of the lidar sensor.
One of the issues of lidar is the degradation of its performance in rain. If a lidar beam intersects with a raindrop at a short distance from the transmitter, the raindrop can reflect enough of the beam back to the receiver, therefore detecting the raindrop as an object. The droplets can also absorb some of the emitted light, degrading the range of performance for the sensors.
Using different probabilistic rain models (none, to different intensities) researchers made it ‘rain’ in the WMG 3XD simulator, and measured the lidar sensor’s responses to the rain, making a record of false positive and false negative detections.
They found that as the rain intensity increased it became more difficult for the sensors to detect objects. In a short range from the vehicle (up to 50m), several raindrops were erroneously detected. However in a medium range, (50-100m) this had decreased, but as rainfall increased to up to 50mm per hour, the sensors’ detection of objects decreased in conjunction with a longer range in distance.
“Ultimately we have confirmed that the detection of objects is hindered to lidar sensors the heavier the rain and the further away they are, this means that future research will have to investigate how to ensure lidar sensors can still detect objects sufficiently in noisy environment,” explained Dr Valentina Donzella, from WMG, University of Warwick.
“The developed real-time sensor and noise models will help to further investigate these aspects, and may also inform autonomous vehicles manufacturers’ design choices, as more than one type of sensor will be needed to ensure the vehicle can detect objects in heavy rain,” she concluded.