A major advantage of lidar technology is its ability to penetrate thick vegetation such as forest canopies to gather surface elevation data and detect objects hidden to the human eye or other electro-optical methods—objects such as concealed buildings, roads, or weapons.
Photogrammetric techniques, by contrast, cannot discern objects obscured by vegetation. Photogrammetry is only able to create elevation models based on two-dimensional images and is unable to see through dense forest or brush.
Lidar, of course, does not actually see through vegetation. Rather, it sees through holes in the foliage. Some of the multiple laser pulses it emits simply find openings between leaves and branches, in much the same way that sunlight filters through the forest canopy, continuing down to the ground.
The problem with traditional linear-mode lidar, however, is that some pulses do hit leaves and branches. These pulses can either break into multiple return signals or overpower the fainter signals from light that does find openings and make back. For this reason, measurements of objects hidden beneath foliage have been difficult to acquire using traditional linear-mode lidar.
Next-Generation Geiger-mode Lidar
Today’s next-generation Geiger-mode lidar, however, is changing that. In contrast to traditional Linear-mode lidar, Geiger-mode lidar uses a photodiode array to flood an area with infrared light. Each diode in the array is sensitive enough to detect a single photon reflected from the illuminated area. The array is mounted on a scanner that rotates at an angle to create a cone-shaped field of view.
This results in a circular ground field of view rather than a line as with linear-mode lidar. As the scanner rotates, the photodiode array flashes up to 50,000 times every second and takes 4,096 measurements per flash from multiple angles, equating to 205 million samples per second. As a result, each square meter of terrain can be sampled thousands of times in a single over-flight.
Geiger-mode’s high density and multi-angle looks offer a better chance of seeing through foliage. Even with significant occlusion of objects beneath the canopy, detailed extraction of their features is possible with Geiger-mode lidar.
Geiger-mode lidar, developed by L3Harris Corporation in conjunction with Lincoln Laboratories at MIT, relies only on a single photon returning to the detector to measure distance to the ground. This provides a significant advantage over linear-mode lidar, which requires the return of thousands of photons. Linear-mode lidar requires a much higher-powered laser to be flown at a lower altitude and slower speed. Geiger-mode lidar, on the other hand, can be flown faster and at a higher altitude, allowing it to see more of the earth below and collect more at a time.
The ability to see through forest canopy and to conduct wide-area collection faster and more efficiently can be a boon in locating illegal drug farms or in combatting the multi-billion dollar illicit global trade in wildlife and the international crime syndicates and terrorist groups that utilize this trade as a low-risk source of income to finance their operations.
The illicit global wildlife trade has not only led to unprecedented biodiversity loss but has also become a serious a threat to human health, as witnessed by the COVID-19 pandemic, since three-quarters of all emerging infectious diseases are zoonotic, according to the United Nations Environment Programme.
About the Author
Paul Nash, MA, PhD is a strategic communications professional. He obtained his PhD from the University of Edinburgh and is a member of the Canadian Coast Guard Auxiliary, serving as director of a search and rescue unit in the Central and Arctic region.