2.3 - Performance limits and design procedure for all-weather terrestrial rangefinders

This paper provides an assessment of the maximum all-weather operational range for "single-ping" terrestrial laser rangefinders. It is shown that this range is a function of a single parameter involving the transmitted pulse energy, receiver aperture area, photoelectric quantum efficiency, and operating wavelength. By virtue of this convenient parameter, trade-offs between different laser materials and system parameters can be made by inspection, as well as comparative evaluations of different range-finder designs. The major uncontrollable factor which affects the range performance of these devices is weather conditions; the effects arise from signal loss due to atmospheric extinction and spurious returns due to backscatter from atmospheric particles. A first-order theory for the analysis of the effect of weather on the performance of pulsed laser rangefinders shows that the maximum backscatter varies as1/R³compared with a1/R²or1/R⁴dependence of minimum signal. The effects of backscatter can be minimized by incorporation of a time-programmed gain circuit in the receiver. Calculations for the design of optimum time-programmed gain characteristics are given, as well as experimental results with actual systems designed by this procedure.