A footprint of 2 to 4 m (see Section 2) is intermediate between narrow footprint (0.15 m) used by commercial lidars for very accurate urban building description and other dedicated research airborne canopy lidars Site URL List 1|]# (~10 m, e.g., [8,9]) and spaceborne lidar like ICESat (60 m, e.g., ).The purpose of the present paper is to demonstrate the performance of a UV lidar for canopy characterization. First, we use the full waveform canopy data to reconstruct the 3D structure of delineated and characterized forest areas in the Landes forest (one of these areas is used for long term monitoring and research activities by Institut National pour la Recherche Agronomique).
Second, we validate the lidar retrievals using in situ measurements taken from the ground by field foresters (e.g., ).
Section 2 presents the lidar instrumentation and the test areas in the Landes forest where the field experiments were conducted. Section 3 introduces the methodology used to retrieve: canopy tops, apparent tree tops, apparent crown bases and undergrowth heights. Section 4 shows the lidar-derived canopy structural parameters for each of the test areas and provides with comparisons to in situ measurements. The performance assessment is conducted in two steps by: (i) a statistical comparison between canopy lidar and in situ observations (of a forest stand and several sample plots), and then (ii) a one-to-one comparison between the canopy lidar and in situ observations.
A summary and perspectives for future work are given in Section 5.2.?Experimental Setup2.1.
Canopy Lidar Payload Onboard an ULAThe UV lidar onboard the ULA looks downward close to nadir (see Figure 1). It is built around a compact 355 nm tripled Nd-YAG laser that provides 16 mJ with 5 ns pulse duration at 20 Hz pulse repetition frequency (see Table 1). Eye safety conditions are met at the exit of the optical head (Figure 1(a)). The full waveform Cilengitide lidar signal is digitized at a 100 MHz sampling frequency or 1.5 m sampling resolution along the lidar line-of-sight. The lidar signal acquisitions are recorded during 1 s (20 consecutive shots) and then the data are stored on an on-board portable computer for 1 s.
The shot to shot separation at ground is about 1 m for an ULA horizontal velocity of 20 m s?1. The laser footprint at ground level has a nearly circular Anacetrapib shape of 2.4 m diameter for a 300 m flying altitude above ground level (agl) and nadir pointing. Depending on the ULA attitude while in the air, the successive laser footprints move randomly within 10 to 40 m around the ULA ground track (see Section 3.4). A global positioning system (GPS) tracks the ULA position with an accuracy of 5 m.