Modern vehicles are often equipped with an array of sensors to allow them to detect objects and gather information about their surroundings. The three main types of sensors are cameras, LIDAR sensors, and RADAR sensors. While cameras are more familiar to us from a host of other applications, LIDAR and RADAR sensors are generally less well understood. In this post we will look at how LIDAR and RADAR sensors work, the benefits and drawbacks of each of these technologies, and how they are put to different uses on vehicles.
What is LIDAR?
LIDAR stands for Light Detection and Ranging, and is used to measure the distance between the sensor and objects in the surrounding environment. Typically, a LIDAR sensor sends pulses of light waves into the environment. The pulses will reflect off objects and surfaces in the surrounding area, and a sensor will detect the returning light. The sensor will calculate the time taken for the pulse to return to the sensor, and from this calculate the distance to the object the light has reflected off. LIDAR sensors will repeat this process millions of times per second, and this allows for a detailed 3D image of the surrounding area to be created in real-time.
The LIDAR sensors used on vehicles will fire anywhere from 8 to 108 laser beams in a series of pulses, with each beam pulse emitting billions of photons per second. This process generates a huge amount of data points, and the resulting calculations are processed almost simultaneously. Because of this LIDAR can accurately determine the shape and size of objects and track their movement.
LIDAR sensors can be solid state or rotating. Spinning sensors will often be located at the top of the vehicle and are designed to capture a 360-degree field of view. Solid-state LIDAR sensors on the other hand are fixed, and point in a single direction with 90 to 120 degrees field of view. Solid-state sensors are cheaper to produce, but several fixed sensor units are required to achieve the coverage that a single spinning unit can provide. LIDAR systems usually have a range of somewhere between 250 and 400 metres. This allows the system to identify objects and their positions well before reaching them.
What is RADAR?
RADAR stands for Radio Detection and Ranging, and is also used to measure the distance between the sensor and objects in the surrounding environment. There are two primary methods of measuring distance using radar. The first is known as the direct propagation method and measures the delay associated with reception of the reflected signal which can be correlated to the distance of the reflecting object as a function of the speed of light and the period or rather, the time delay in the transmission and receiving of the waves.
The second method is known as the indirect propagation method or the Frequency Modulated Continuous Wave (FMCW) method. For indirect propagation, a modulated frequency is sent and received, the difference in the frequency can be used to directly determine the distance as well as the relative speed of the object.
Because they operate at wavelengths on the order of a few millimetres, automotive radar systems perform well at detecting objects that are several centimetres or larger. They are also good at ignoring objects that are small relative to a wavelength, such as the water droplets in fog.
LIDAR vs RADAR
As we’ve described above, LIDAR and RADAR are essentially two different technologies which are used to achieve the same goals. So, what’s the difference between their capabilities, and what are their respective pros and cons?
One of the most established companies working in the field of self-driving cars is Waymo, who develop their own LIDAR systems. Waymo's lidar has become so advanced that it can detect pedestrians and also figure out what direction they're facing. This enables a self-driving car to more accurately predict where the pedestrian will walk. This level of accuracy also allows the Waymo Pacificas to see hand signals from bicyclists and drive accordingly. That's the next-best thing to human eyes.
This level of precision is the key advantage of LIDAR over RADAR, but there are three advantages that RADAR systems enjoy over LIDAR systems.
The first is range, with RADAR systems effectively ‘seeing’ further than LIDAR systems. This is particularly important for larger commercial vehicles which will often require more advance notice in order to be able to slow sufficiently to avoid a collision. Longer range is also needed in order to fully understand the driving environment and make proper decisions along the highway, like when to change lanes.
Secondly, as they often involve more moving parts, LIDAR scanners are less reliable over time than RADAR sensors. In trucks, the manufacturer typically wants to see components last a million miles and you have a better chance of that with a solid-state system than you do with something with rotating parts.
Lastly, RADAR sensors are significantly less costly than their LIDAR alternatives. For many years, LIDAR sensors were not an option for most manufacturers because of their high cost. A high-end automotive LIDAR by Velodyne used to cost $75,000 for one car. However, in recent years, LIDAR has been undergoing a significant price reduction with many companies working to make LIDAR systems more affordable. In early 2020, Velodyne released a solid-state LiDAR with no moving parts for $100 called Velabit on CES 2020.
RADAR systems, on the other hand, have always been a cheaper option compared to LIDAR, with a price as low as $50 for an automotive millimetre-wave RADAR sensor module.
Conclusion
If LIDAR can someday match the affordability and reliability that radar has achieved, and match the range of RADAR sensors, then it's likely LIDAR will become the industry standard due to the increased detail achievable. Until that happens, we're likely to have a mixed bag in the industry between cheap, reliable, long-range RADAR and advanced, high-tech, high-detail LIDAR.