What is LiDAR?

A LiDAR system may use a scan mirror, multiple laser beams, or other means to "scan" the object space. With the ability to provide accurate measurement of distances, LiDAR can be used to solve many different problems.

In remote sensing, LiDAR systems are used to measure scatter, absorption, or re-emission from particles or molecules in the atmosphere. For these purposes, the systems may have specific requirements on the wavelength of the laser beams.  The concentration of a specific molecular species in the atmosphere, e.g. methane and the aerosol loading, can be measured. Rain droplets in the atmosphere can be measured to estimate the distance of a storm and the rain fall rate.

Other LiDAR systems provide profiles of three-dimensional surfaces in the object space. In these systems, the probing laser beams are not tied to specific spectral features. Instead, the wavelength of the laser beams may be chosen to ensure eye safety or to avoid atmospheric spectral features. The probing beam encounters and is reflected by a "hard target" back to the LiDAR receiver.

LiDAR can also be used to determine the velocity of a target. This can be done either through the Doppler technique or measuring the distance to a target in rapid succession. For example, atmospheric wind velocity and the velocity of an automobile can be measured by a LiDAR system.

In addition, LiDAR systems can be used to create a three-dimensional model of a dynamic scene, such as what may be encountered by an autonomous driving vehicle. This can be done in various ways, usually using a scanning technique.

What are the challenges with LiDAR?

Essentially, LiDAR is a ranging device, which measures the distance to a target. The distance is measured by sending a short laser pulse and recording the time lapse between outgoing light pulse and the detection of the reflected (back-scattered) light pulse. 

There are some well-known challenges with operational LiDAR systems. These challenges depend on the type of LiDAR system. Here are some examples:

• The isolation and rejection of signal from the emitted beam - The radiance of the probing beam is generally much greater than that of the return beam. Care must be taken to make sure the probing beam is not reflected or scattered by the system back into the receiver such that the detector is saturated and unable to detect external targets.
  
• Spurious returns from debris in the atmosphere between the transmitter and the intended targets - The debris can cause such a strong spurious return that the return from the intended targets is not reliably detected.
  
• Limitations on available optical power -A system with more power in the beam provides higher accuracy but is more expensive to operate.
  
• Scanning speed-Safety can be an issue when the laser source is operating at a frequency dangerous to human eyes.  This issue is being mitigated by other approaches such as flash LiDAR which illuminate a large area all at once and by operating at eye-safe wavelengths.
  
• Device crosstalk-signals from nearby LiDAR devices might interfere with the signal of interest.  The challenge faced now is how to differentiate signals emitted by other LiDAR devices nearby.   Various approaches with signal chirping and isolation are under development.
  
• Cost and maintenance of LiDAR systems – These systems are more expensive than some alternative types of sensors however there is active development to overcome the high cost and produce systems at lower prices for wider use.  Rejection of returns from unintended objects- This is similar to the rejection of atmospheric spurious signal as mentioned previously. However, it can also happen in clear air scenarios. Addressing this challenge generally involves minimizing the size of the beam at various target distances as well as over the field-of-view received back at the LiDAR receiver.