Waveform Analysis Techniques in Airborne Laser Scanning

Small-footprint airborne laser scanners with waveform-digitising capabilities are becoming increasingly available. Waveformdigitising is particularly advantageous when the backscattered echo waveform is complex because it allows selecting processing algorithms adjusted to the task. In addition, waveform-digitising laser scanners depict the physical measurement process in its entire complexity. This opens the possibility to derive the backscatter cross section which is a measure of the electromagnetic energy intercepted and reradiated by objects. In this paper approaches for deriving the cross section along the laser ray path are discussed. For data storage and processing reasons a practical approach is to model the waveform as the sum of a number of echoes backscattered from individual scatterers. This approach involves estimating the number of echoes, finding a match between the modelled echoes and the measured waveform, and estimating the cross section using calibration targets. For estimating the number and position of echoes the Average Square Difference Function (ASDF) method, which is a discrete time delay estimation technique, is tested. The results show that ASDF is a promising approach which appears to be less affected by noise compared to more traditional echo detection methods.