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Both LiDAR and RADAR and long-range detection systems that can map the terrain of the Earth, provide distance information, and be used to help guide an autonomous vehicle. Whilst both sensing methods use similar principles, there are some key differences when it comes down to which one should be used. In this article, we look at both methods, and the differences between them, including where each technology excels over the other and where each technology is at a disadvantage against the other.
LiDAR
LiDAR stands for ‘Light Detection and Ranging’ and is a remote sensing mechanism that is used in both autonomous vehicles and examining the surface of the Earth. LiDAR sends out light pulses—which originates from a pulsed laser—to measure various distances by collecting the reflected light pulses. The internal systems calculate how long it takes for the light to return to the sensor and then calculates the distance through time of flight measurements—i.e. using the velocity of the light as a guide.
There are four main components to a LiDAR sensor, and these are the laser, the scanners, and optics, photodetectors, and navigation and positioning systems. Depending on the application, lasers with wavelengths between 600-1000 nm or 1550nm can be used, with the latter being used for long-range imaging. 1550 nm has become more popular as the wavelengths do not affect the human eye, nor do they show up on night-vision technology, which makes them useful for military and defense applications.
There are many different types of optics and scanners used within LiDAR sensors, depending on the application. The most common are dual oscillating plane mirrors, dual axis scanners and polygonal mirrors, and which one is chosen often depicts the range that the LiDAR sensor can operate in. Whilst photodetectors are a class of detectors that are designed to detect photons of light, the specific ones used in LiDAR can range from solid-state detectors to photomultiplier detectors.
Depending on the application, LiDAR sensors can be mounted onto a mobile platform, if there is a need to map a wide area. Common applications include satellites and autonomous vehicles. Because the sensor will be frequently moving, navigation and positioning systems are required within the LiDAR system, and the most common types include Global Positioning Systems (GPS) and Inertia Measurement Units (IMU).
RADAR
RADAR stands for ‘Radio Detection and Ranging’ and is a much older technology than LiDAR. RADAR is similar to LiDAR, but RADAR uses a transmitter and antenna that sends out electromagnetic radiation in the form of radio waves. When this radiation is reflected and/or scattered by the target of interest or an object in between (such as precipitation in the air), an antenna (which acts as the receiver) picks it up. This is then processed to determine the location of an object by using the time taken for the signal to return, or it can be used to determine the relative position of meteorological disturbances—such as the location and amount of precipitation—by measuring the scattering of the radiation via the Doppler effect. Whilst RADAR shares some of the same applications as LiDAR—such as autonomous vehicles and geographical mapping—it can also be used for detecting weather patterns and objects that move through the air (such as missiles, aircraft etc).
Like LiDAR, RADAR has many different components. The first is the transmitter, which works in conjunction with waveguides and amplifiers, to produce strong and coherent radio wave signals which can then be transmitted by the antenna. The antenna can be made of a parabolic reflector, planar arrays or electronically steered phased arrays. A duplexer is often used, and this negates the need for there to be both a signal transmitter and a signal receiver, as it enables the antenna to act as both (i.e. send out the signal and receive the reflected/scattered signals). When a duplexer is not used, a separate receiver is required, but the antenna still sends out the signal. Finally, the receiver system processes the information received and converts the identification of objects into a video that can be easily viewed by the operator.
The Key Differences
The cost of the two sensors are very different, and RADAR is a much cheaper option than LiDAR. However, when it comes to accuracy, LiDAR can map positions and distances much more accurately than RADAR can. This is mainly down to the fact that LiDAR uses a large amount of short laser pulses which form point clouds, and these point clouds can then be implemented into various AI, machine learning and big data algorithms to provide more accurate results. This is particularly important for applications involving the public, such as autonomous vehicles, as that extra accuracy can be the difference between a crash and no crash. LiDAR can also detect smaller objects than RADAR can because of the short light pulses.
Whilst LiDAR is more accurate, RADAR can operate in a wider range of environments. LiDAR is limited in night-time and bad weather conditions, whereas RADAR is not sensitive to different environments. This is mainly due to radio waves being used and there being no mechanical parts which can be affected by external debris. RADAR also has a longer operating distance than LiDAR, but it can often interpret small objects to be bigger than they actually are. RADAR sensors also have a quick reaction time and is the reason why they are also used in autonomous vehicles alongside LiDAR. Both methods have their advantages and disadvantages, but the choice often comes down to cost Vs accuracy Vs environmental conditions for each given application.
Sources and Further Reading
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