From Single-pulse to Full-waveform Airborne Laser Scanners: Potential and Practical Challenges

Airborne laser scanning, often referred to as lidar or laser altimetry, is a remote sensing technique which measures the round-trip time of emitted laser pulses to determine the topography of the Earth’s surface. While the first commercially available airborne laser scanners recorded only the time of one backscattered pulse, state-of-the-art systems measure first and last pulse; some are able to measure up to five pulses. This is because there may be several objects within the travel path of the laser pulse that generate multiple echoes. Pulse detection is then used to determine the location of these individual scatterers. In this paper we discuss the physical measurement process and explain the way how distributed targets (such as trees or inclined surfaces) transform the emitted pulse. It is further shown through theoretical experiments that different detectors may yield quite different height information, depending on the type of the target. For example, even in the simple case of a tilted roof (with a tilt angle of 45°) the range values obtained by using different detectors may vary by ~ 0.4 m for a laser footprint size of 1 m. Airborne laser scanner systems that digitise the full waveform of the backscattered pulse would give more control to the user in the interpretation process. It would e.g. be possible to pre-classify the acquired data with respect to the shape of the echoes, to use different detection methods depending on surface cover and the intended application, and to employ more physically-based retrieval methods. * Corresponding author