Discrete point recording remains unchanged - full waveform digitiser is an extra. Nothing has changed for the AGC/mainboard of the ALS so the raw laser .SCN files will be the same as before. Extra files will be created that contain the waveform data.
The digitiser starts recording from the first returned peak for the number of required samples (256/128 @ 1ns or 256/128/64 @ 2ns)
If first return is noise (cloud/seagull) then the digitiser will start recording there and not at the target.
measured heights / distances for different sampling rates:
Sample rate
equivalent height
256 @ 1ns
38.4m
128 @ 1ns
19.2m
256 @ 2ns
76.8m
128 @ 2ns
38.4m
64 @ 2ns
19.2m
Benefits of full waveform
Better idea of the nature of the return e.g. tree canopy height is more accurate than from 1 discrete point
Extraction of points from the waveform missed by the discrete measuring
(a future benefit) pulse stretching at swath edges compared to nadir: indicating sloped terrain, improved classification
Maximum rate of acquiring digitised full waveform data is 120KHz. If a higher rate is used then the waveform data will record for every other emitted pulse. But the SCN files will still be recorded at the high rate.
Pulse length (sampling rate) should be chosen based on the maximum height of targets in the area. E.G. if all low lying targets then there is no point using 256 samples at 1 ns - better to use 128 at 1 ns.
Software
ALSPP outputs in LAS 1.3 format
Includes a simple waveform viewer
Displays waveform and discrete points
There is a "constant" timing offset between the waveform and discrete points - thought to be due to processing time