Preventing the saturation phenomenon of detectors used in environmental remote sensing

The nowadays environmental issues motivated us to develop a mobile LiDAR system for two applications: 1°/ the prevention against forest fires in the hilly and inaccessible continuous mountainous forests in north Algeria, where the risk factor of a forest fire is high in summer and 2°/ the study of the atmosphere by monitoring pollutant species and sand aerosols emanating either from cement plants in the Algiers agglomeration and the Sahara. The system mobility enables measurements in different locations. For the first application, we use an eye safe laser wavelength at λ=1.57 μm and an avalanche photodiode (APD) as a detector. For the second application, we use an Nd:YAG laser followed by a second harmonic generator that emits at λ=532 nm and a photomultiplier (PMT) as a detector. The LiDAR is an active technique that often deals with weak return signals. The latter come with an intensity proportional to 1⁄R 2 , where R is the range from the target to the detector. The signal-to-noise ratio (SNR), as an effective detection criterion, needs to be optimized towards better performance. That is, when the SNR is low, the detected signal is increased and vice versa. In the case of forest fires detection, it may happen that strong signals from nearest objects lead the detector to a saturation status. We suggest a method to handle this issue that is base on the control of the detector gain by driving its applied voltage.