The evolution of LiDAR

Current airborne linear-mode LiDAR systems, such as the Leica Geosystems ALS series, capture 1 million points per second. Increasing the pulse repetition rate is the best way to achieve dense point clouds at lower costs, as the flying speed can be increased. The pulse frequency, however, is constrained by parameters such as energy consumption and eye safety. Single Photon LiDAR (SPL) technology enables a much higher pulse rate to be achieved, since much less energy is needed per pulse.

LIDARsystems 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.

这些系统在每个发射的激光脉冲中使用相对较高的能量。每个脉冲都从飞机到地面传播，从那里反射回扫描仪。

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.

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Higher pulse rates

脉冲重复率是定义数据采集期间可接受的飞行高度和飞行速度的重要参数。较高的脉冲重复率可以使飞行速度更快，同时保持相似的点密度。

随着线性振荡系统的脉搏速率的增加，电力消耗也会增加。此外，使用的激光器将产生更大的热量。

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.

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From space to earth

SPL技术最初是针对地球卫星范围开发的，已证明可以使用每个脉冲中的激光能量最少生成准确的范围测量。与当前可用的线性激光雷达系统相比，SPL系统包含激光拆分器，将每个激光脉冲拆分为10x10小型激光束（Beamlets）的阵列。对于这100个束，光子的行进时间单独测量地面和背部。在SPL系统中部署的高度敏感光子检测器的添加可实现单个返回光子的检测，其所需能量要少得多。

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.

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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.

这意味着光子计数器每1.6纳秒每1.6纳秒重置，以计数任何新的光子从梁中返回。然后将这些视为新的回报。结果是一个真正的多回报激光痛系统，其返回间隔为24厘米。结果，SPL系统可以获取每平方米12至30点的高密度点云，而在檐篷下方有许多回报。

与飞点密度成反比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.

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介绍Leica Spl100

线性模式LIDAR仍然是机载映射的行业标准，但是SPL技术正在接受大型项目。例如，美国地质服务（USGS）3D高程计划（3DEP）旨在以高质量的激光雷达数据的形式系统地收集增强的高程数据，并探索了SPL技术。该系统已证明可以符合USGS QL1数据的准确性标准，该数据对应于非植根区域的高度精度优先大于10 cm。

有鉴于此,徕卡混沌之间引入了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.

新的SPL100是新真人捕获解决方案的一半，即Real-Terrain。bob综合app赌博与HXMAP（可扩展的后处理工作流软件）结合使用，新解决方案可以有效地收集和快速处理大面积LIDAR数据集。SPL100采集和HXMAP数据处理所获得的效率使更大，更频繁的LIDAR数据获取用于诸如密集的植被映射和变化检测等应用。

"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."

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The future of LiDAR

SPL技术在准确性和辐射能力方面继续前进。这将导致SPL技术适合的应用领域的扩展。

还可以预期，SPL系统的有效脉搏速率将继续提高，就像在过去的二十年中，线性模式系统的有效脉搏速率稳步改善一样。由于目前的性能水平为每秒600万点，SPL系统可能会在不到十年的时间内每秒捕获10亿点。

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.