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Lidar: Light Detection And Ranging

Measuring the distance to an object with pulsed lasers.
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Description

Light Detection and Ranging (lidar) uses pulsed light to measure the distance to an object. It has a range of applications including automotive sensing (in which case it is often paired with radar and camera), geographic mapping, and biometric identification. Different wavelengths from infrared to ultraviolet are used for different applications, such as near-infrared used for topographic mapping and water-penetrating green light used to measure seafloor and riverbed elevation. Lidar data is a point cloud indicating at what point in space the light was reflected back to the photodetector.

The concept for lidar dates back to the 1930s, and the first system to use the principles was developed in 1961. It has a long history in space exploration, with lidar systems used during the Apollo 15 mission to map the surface of the moon through recent use in the Mars Ingenuity helicopter.

Lidar units are an area of active development, with a lot of startup activity. There is a push to scale down the size of units used in automotive applications from the large, roof-mounted spinning mirrors seen in the early days of autonomous driving trials to smaller, less expensive MEMS-based and solid-state implementations. Improving range and resolution are other key focuses.

In automotive applications, lidar faces certain challenges. It cannot penetrate some weather effects such as heavy fog, rain, or snow. It does, however, provide a more detailed 3D scan compared to other sensors. For this reason, it is often paired with radar or cameras that have their own strengths and weaknesses. (Combining these different forms of information into something actionable is referred to as sensor fusion.)

How lidar works

There are several different approaches to measuring distance. This section is from Automotive Lidar Technologies Battle It Out by Bryon Moyer.

Time of Flight: The most obvious way to detect the distance of an object (formally referred to as “ranging”) is to measure how long light takes to make a round trip from the lidar laser to the photodetector. This is referred to as time-of-flight (ToF). Modulating the lidar pulse helps to extract the reflected light from all the other ambient light that may be received at the same time.

Direct ToF simply starts a really fast counter to detect when the return signal arrives. However, these times are in the picosecond range, making it difficult to do in silicon.

Instead, it’s possible to measure the phase of the return signal as compared with the original signal. That gives a distance — until you get beyond 360 degrees of phase shift. At that point, the distance is ambiguous, since you would see a similar phase shift at each multiple of the wavelength. This approach is referred to as indirect time-of-flight (iToF), and it involves a modulated continuous wave of fixed RF frequency.

Frequency-modulated continuous wave (FMCW): Another approach modulates the frequency on the signal rather than the amplitude. The phase and frequency information in the reflections give distance without an ambiguous range. This is FMCW. In addition, you can get the speed of the object.

A critical aspect of FMCW, however, is that the frequencies are not in the RF range. They’re in the optical range. That makes generation and detection far more complex, requiring more exotic materials like indium phosphide, and more complicated circuitry. While it’s a promising technology, it’s currently larger and more expensive than the alternatives, meaning it would be used only where other technologies couldn’t be used.

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Sensor Fusion Everywhere

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Making Lidar More Useful

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Shifting Auto Architectures

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Simultaneous Localization And Mapping