My post a couple of weeks ago discussed the changes in UAV platform technology and a bit about aerial imaging, including a mention of LiDAR. In this post I’m going to talk more about aerial LiDAR technologies and what you might see as future trends,
Most of what we see in aerial LiDAR collection is linear LiDAR, a technology where range is measured from the returns of single laser pulses. The sensitivity and inherent sampling rates are low, requiring a large number (possibly thousands) of photons to register a return, forcing a high power consumption, which can be costly at high resolutions, Linear LiDAR suffers from data occlusions and shadows, limited foliage penetration and range resolution (the separation of targets).
The alternative methods are described as single photon or Geiger mode systems. These LiDAR systems work on the principle that a single pulse is split into multiple sub-pulses, which upon return, are received by multiple segmented detectors. This means that the ground reflects the individual photons from a larger area, and at a higher effective pulse rate than are found in linear LiDAR systems. The sample rates are high, being up to several hundred million per second, with the sensors sampling the same spot on the ground multiple times. This greater density improves coverage, foliage penetration, and reduces shadows and voids. There is improved separation of vertical targets.
Systems of this type use less power, and can collect more dense data at higher altitudes over larger areas. Some of the names you may hear in connection with Geiger mode systems are Flash, Avalanche Photo Diode (APD), and Photon Counting LiDAR.
There have been some misconceptions over this Geiger mode technology. It is often thought that this is new and unproven. Actually, it’s been around for over fifteen years, but has not gained commercial success until more recently due to the lack of availability of some of the hardware components for commercial use. It does work in daylight conditions with the decrease in performance being minimal, contrary to popular belief. Geiger mode data is more noisy, but this can be overcome with different methods of processing the data. Granted, the data sets can be massive, so this is not yet a workstation environment.
So, the question is, will this type of technology replace the current linear mode LiDAR technologies now in use? The answer is unclear, for now. With lower costs of data acquisition, Geiger mode LiDAR may be a better choice for larger area surveys. Linear LiDAR, while offering lower point density, is more accurate at this time, so you will still want to use it for the highest accuracy measurements. It also provides rich attribute data for things like classification and feature extraction. Geiger mode systems don’t provide intensity data for the returns in the way that current linear LiDAR technologies do. While Geiger mode systems may be cost effective for large areas, traditional methods are more cost effective for smaller jobs. As single photon systems evolve, these differences may start to disappear as hardware costs change and the accuracy and fidelity of these systems increases.
Right now, the competitive future of these technologies is up in the air (no pun intended). It will be interesting to see how this plays out.
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