High accuracy distance measurement by two-wavelength pulsed laser sources.
The two-wavelength method, proposed by P. L. Bender and J. C. Owens, makes it possible to achieve fractional accuracies of better than 10(-6) (1 ppm) in ranging. The technique has only been applied to continuous laser sources. This paper attempts to ascertain whether high power pulsed lasers can be successfully used with this method and to check the system feasibility from an experimental viewpoint. Instead of phase comparison techniques (typical of cw systems), the pulse sources involve direct transit time measurements. The consequent average of N measurements can reduce the error on the distance mean value by an N(-(1/2)) factor. The main cause of error concerns threshold crossing in pulse discrimination, especially in the presence of amplitude fluctuations. Causes of error affecting such measurements are analyzed. Results are reported of measurements at distances of 5 km and 13 km by pulsed ruby laser PTM.emission (lambda = 6943 A) and its second harmonic (lambda = 3472 A) using a timer with 20-psec accuracy. Such measurements show that two-wavelength pulsed laser telemeters can reach 1-ppm accuracies for distances exceeding a few kilometers: over large distances (greater than 50 km, under favorable visibility conditions), such high power pulsed temeters appear to be the only ones to achieve this accuracy.
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