It has recently been shown that lidar (LIght Detection And Ranging) can effectively detect smoke plumes from small bonfires up to distances of 6.5 km, so that the technique can be used for wildfire surveillance. The aim of the present work is to describe a method for calculating the optimal location and minimum number of lidar stations required for the surveillance of a given forest area, taking the hilly terrain of Sintra-Cascais Nature Park (Portugal) as an example. The placement and horizontal scanning of the lidar sensors must be such that the laser beam passes over the ground, while keeping sufficiently low to enable early smoke plume detection, before the smoke is dispersed by the wind. Simultaneously, the laser beam should not hit the ground at distances shorter than the instrument range. To solve the problem, a terrain rendering was created and the best laser-beam zenith angle for each azimuth and the effective range covered by each lidar were calculated. The computations showed that 95.2% of the 146 km<sup>2</sup> of the Nature Park area can be covered by seven detectors with the laser beams scanning at a height of 50 m or less above ground.
Lidar is a promising tool for forest-fire monitoring because this active detection technique allows efficient location of tenuous smoke plumes resulting from forest fires at their early stages. For the technique to be generally usable instrumentation must be eye-safe, i.e. it must operate within the spectral range λ<0.4 or λ>1.4 micrometers . In this paper the lidar efficiency at the wavelengths 0.3472 micrometers (second harmonic of the ruby laser) and 1.54 micrometers (Er:glass laser) are compared using a theoretical model. The results of calculations show that the energy required for smoke-plume detection using 0.3472 micrometers becomes greater than the corresponding value for 1.54 micrometers when the distance exceeds some threshold, which ranges between 2 and 6 km depending on other parameters. Being caused by relatively higher absorption of the UV radiation in the atmosphere, this result is valid for any wavelength in the vicinity of 0.35 micrometers , for example, the third harmonic of Nd:YAG laser and the second harmonic of Ti:sapphire laser.