TECHNOLOGY, CASE STUDY
Pre-Detection and Pre-Registration Averaging of Full Wave Signals in Airborne LiDAR Bathymetry
A well-known technique to enhance the signal to noise ratio (SNR) of repetitive signals is to average them. The coherent parts of the signal add up constructively while the incoherent parts are averaged out. The prerequisite is that the signals are acquired under conditions of high repeatability, i.e., the signals must be sufficiently similar. In the present technical note, the authors describe an efficient method for maintaining signal similarity by ensuring spatial and temporal proximity of laser waveform signals obtained by a sensor operated from an airborne platform. The method makes use of a few auxiliary parameters such as laser pulse repetition rate, mirror rotation rate, platform altitude and flight speed. The method can be extended to be operated in real time.
All Datasets were acquired by sensors of type VQ-840-G from company RIEGL (Horn, Austria). This sensor type is a compact topo-bathymetric laser scanner for airborne platforms with OWP and full waveform recording for full waveform analysis (FWA) in post-processing. The recording is unconditional, i.e., no trigger condition depending on presence of a target is required to initiate the acquisition of data samples. This is an essential prerequisite for using the method described in this technical note, as the signal before averaging may be too small to trigger the recording.
The laser wavelength of the instrument is in the green at 532 nm with a selectable beam divergence and receiver field of view. The scanner is equipped with a rotating mirror that is arranged in such a way that the nadir angle of the resulting laser beam has little variation over a scan line. The effective scan pattern is nearly elliptic with an off-nadir angle of ±20° perpendicular to flight direction and ±14° in flight direction. The pulse repetition rate is selectable between 50 kHz and 200 kHz, while the angular rotation of the mirror is between 10 and 100 lines per second.
The sensor was also equipped with internal inertial measurement unit and global navigation satellite system (IMU/GNSS) unit.
In this technical note, they describe a variant of waveform stacking, a method for increasing the SNR called pre-detection averaging, which is particularly useful as a pre-processing step of full waveform analysis to derive bathymetry. The higher SNR makes possible higher depth resolution and better point classification due to the smaller amount of spurious points, however at the cost of spatial resolution if the original dataset before averaging is not dense enough. The presented method is fast, requires only a small and constant amount of memory, can be applied to the unregistered dataset and can optionally be performed in real time during flight. The single parameter that needs to be set manually is the degree of averaging. The method can be understood as a means of processing the data with a virtual footprint size that is larger than the native footprint size. When carried out during post-processing, the optimum averaging size can be tuned for best results without the need for new data acquisition.
The full article is published on the MDPI platform, and can be found here.
