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Wavelength-dependent extinction statistics based on long-term aerosol size distribution measurements
This paper describes a model to calculate the received number of laser photons in lidar experiments, under the influence of detector dark counts, ambient background, afterpulsing and hold-off time. By using characterization data of photon detection efficiency, dark count rate and afterpulsing, for different detector settings, graphs of optimum operation points for different signal and background levels are simulated. Some comparisons to experimental data are performed.
Peak detection approaches for time-correlated single-photon counting three-dimensional lidar systems
In the helicopter trials significant beam wander, scintillations and beam broadening were experienced by narrow divergence laser beams when passing through the down-wash of the hot engine exhaust gases. The measured effects considerably exceed the effects of atmospheric turbulence. Extraction of turbulence parameters for modelling of DIRCM-relevant scenarios show that in most cases the reduction of jamming power and distortion of jamming waveform can be expected to be small. The reduction of effects of turbulence is mainly related to the larger beam divergence and shorter Rayleigh length of DIRCM lasers compared to the experimental probe beams. Measurements using the turboprop platform confirm that tolerable effects on laser beam properties are found when the laser beam passes through the exhaust 15 m behind the outlet where the exhaust gases are starting to cool down.
Modelling efforts have shown that time-resolved computational fluid dynamics (CFD) calculations can be used to study properties of beam propagation in engine exhaust-related turbulence. Because of computational cost and the problem of validating the CFD results the use for system performance simulations is however difficult.
The hot exhaust gases emitted from aircraft engines create extreme optical turbulence in a local region. The effects on countermeasure system performance depend both on the system parameters and on the threat characteristics. With present-day DIRCM systems, the effects of even severe turbulence are often tolerable.
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