- Discrete return (DR) airborne lidar systems can record 4 returns
- Full waveform (FW) can record an unlimited number in theory
- Some systems provide both sensors
- DR is catching up to FW
Excerpts from the paper.
Lidar sensors currently used for 3D vegetation mapping and forestry research can be categorized as either discrete return (DR) or full waveform (FW) systems. They differ from one another mainly with respect to the range measurement method, which implies significant differences in sensor hardware design.
On the application side, it results in distinctively different number of range measurements recorded for each emitted laser pulse and subsequent substantial differences in data processing and analysis tools. As a result, the data collected by DR and FW sensors representing the same 3D vegetation target may look dramatically different.
Most commercially available DR sensors allow for a few, typically four, multiple returns to be recorded for each emitted laser pulse. A FW lidar measures the full profile of a return signal by sampling it in fixed time intervals, typically 1 ns (equivalent to 15 cm sampling distance), with theoretically unlimited number of measurements per every emitted pulse.
Practically, the number of recorded measurements is determined by several factors including sensor hardware and data flow design and may be limited by a number from a few tens to a few hundreds of measurements. This provides a quasi-continuous recording of the reflected energy for each emitted laser pulse.
It is often perceived to create a ―true profile representing the vegetation canopy structure as opposed to DR sensor measurements providing only up to four records typically separated by a few meter distances. However, due to the limited capabilities of processing and analysis software for FW data, more data does not always translate into more or better information for data interpretation.
The information content of the returned laser signal also depends on characteristics of the horizontal illuminated area (laser footprint). The most common commercial sensors are small-footprint systems, either DR or FW sensors. Some manufacturers offer sensors capable of both operational modes: the main DR sensor and an optional FW unit, which may or may not be used during data collection missions.
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