The evolution of LiDAR
LiDAR systems are typically constructed from a number of components:
- a range-finding system
- scanning optics to direct the laser pulses
- a position and orientation system to record the origination point of the laser pulse.
These systems use relatively high energy in each emitted laser pulse. Each pulse travels from the aircraft to the ground, from where it is reflected back to the scanner.
By using more energy per pulse, a stronger reflection can be recorded because more photons are reflected by the terrain below the aircraft. The output from linear-mode systems is impressive and these systems provide data with high spatial and radiometric precision. The technology does, however, impose some limitations on the maximum effective pulse rates that can be achieved.
Higher pulse rates
The pulse repetition rate is an important parameter to define the acceptable flying height and flying speed during data acquisition. A higher pulse repetition rate allows for faster flying while maintaining a similar point density.
As the pulse rate of linear-mode LiDAR systems increases, so, too, does the electrical power consumption. In addition, there will be greater heat generation by the lasers used.
The ability to generate increasingly higher average optical output, required for ever-higher pulse rates, is an engineering challenge. Besides accuracy and pulse repetition rate, the sensor design needs to consider not only total electrical power consumption and system cooling, but also size, weight and eye safety.
In order to take a next step in airborne LiDAR system development, the required energy per pulse must be reduced. This can be achieved by changing the nature and technology of the range-finding system. Next-generation LiDAR technologies, including SPL systems, rely on new range-finding techniques to achieve lower energy consumption and higher pulse rates.
From space to earth
SPL technology was originally developed for Earth satellite ranging and has proven to generate accurate range measurements using a minimal amount of laser energy in each pulse. Compared to currently available linear-mode LiDAR systems, SPL systems contain a laser splitter, which splits each laser pulse into an array of 10x10 small laser beams (beamlets). For these 100 beamlets, the travel time of the photons to the ground and back is measured individually. The addition of highly-sensitive photon detectors deployed within the SPL system enable detection of a single returning photon with much less required energy.
The SPL system can generate 60,000 pulses per second. Since each pulse is split into 100 beamlets, this results in an effective pulse rate of 6.0 MHz - significantly higher than can be achieved with linear-mode LiDAR.
Multiple returns with individual points
Linear-mode LiDAR systems allow for the registration of peaks from various target reflections within the full return waveform, which can be processed to retrieve multiple returns. As SPL systems do not capture a continuous wave but count the individual photons instead, such a full waveform is not available. It is still possible, however, to retrieve multiple returns thanks to the very short channel recovery times of 1.6 nanoseconds.
This means that the photon counter is reset every 1.6 nanoseconds to count if any new photons return from the beamlet. These are then regarded as a new return. The result is a true multi-return LiDAR system with short inter-return separations of 24 centimetres. As a result, SPL systems can acquire high density point clouds of 12 to 30 points per square metre with many returns underneath canopies.
The point density varies inversely with the flying height. If the flying height is doubled, the covered swath will double, but the point density will be cut in half. An SPL instrument flying at 200 knots at 4,000 metres above ground will produce a point density of roughly 20 points per square metre.
Introducing the Leica SPL100
Linear-mode LiDAR remains the industry standard for airborne mapping, yet SPL technology is gaining acceptance for large projects. For instance, the U.S. Geological Service (USGS) 3D Elevation Program (3DEP), which aims to systematically collect enhanced elevation data in the form of high-quality LiDAR data, has explored SPL technology. The system has proven to meet the accuracy standard for USGS QL1 data, which corresponds to a height precision better than 10 cm for non-vegetated areas.
With this in mind, Leica Geosystems introduced its first commercially available SPL airborne systems in the Leica SPL100 earlier this year. The newest sensor in the company’s airborne portfolio is the first to be released using Sigma Space technology since its acquisition by Hexagon in 2016.
The new SPL100 is one half of the new reality capture solution, Real-Terrain. Combined with HxMap, the scalable post-processing workflow software, the new solution enables the efficient collection and rapid processing of large area LiDAR data sets. The efficiency gained by SPL100 acquisition and HxMap data processing enable larger and more frequent LiDAR data acquisition for applications such as dense vegetation mapping and change detection.
"SPL technology brings up to 10 times the efficiency of prior offerings to our flying partners and customers. It is now possible to deliver extremely high point densities over large areas, enabling the digitisation of the world around us in detail previously not possible,” said John Welter, Leica Geosystems Content and Engineering Services and Geospatial Solutions Division president. “Leica RealTerrain is the next evolution in providing high quality airborne information; both advancing the field and shaping the future of digital realities."
The future of LiDAR
SPL technology continues to advance over time in terms of accuracy and radiometric capabilities. This will result in an expansion in application areas for which SPL technology is suitable.
It is also expected that the effective pulse rate of SPL systems will continue to improve, just as the effective pulse rate for linear-mode systems has steadily improved over the past two decades. With current performance levels at 6 million points per second, SPL systems could potentially be capturing 1 billion points per second in less than a decade.
Bringing down the cost per point through higher effective pulse rates is the best way to address large-area, high-point-density projects in the future. As the use of SPL technology becomes appropriate in more and more applications, we will see positive changes throughout industries, such as increased efficiency in resource management, more effective infrastructure planning and better preparation for natural disasters.
A version of this story first appeared in GIM International.