The principle and technique for large or small displacement measurement by using the method of random coincidence in the turbulent aymosphere

when a collimated laser beam passes through the turbulent atmosphere, several effects induced by the turbulence will be met. They are intensity fluctuation effect, distribution of intensity, beam expansion and the spot dance. But the most important is the spot dance for the displacement measurement. By using optical transform and the displacement-time conversion technique (GUAN-LONG TAN J. Optics, Paris, 1981, vol. 12,4) the intensity of laser beam is converted into the photo-electric pulses at the receiving point, and the motion of laser beam passing through the atmosphere will be converted into the motion of photoelectric pulses on the time axis. In view of the time axis the influence of the turbulence upon the displacement measurement could be replaced by those effects which are of pulse amplitude, pulse shape, and the fluctuation of pulse appearing moment on the time axis. If the pulse shape effect and pulse amplitude effect are eliminated or reduced to the least (G.L. Tan ICL Digest, 1983 GUANGZHOU, CHINA) the rest is the effect of the fluctuation of pulse appearing moment. It is just the influence of spot dance of the laser beam passing through the turbulent atmosphere on the displacement measurement. We investigated the distribution of the spot dance by using a2.1 times 10^{8}numbers of photoelectric pulse. The conclusion is that the spot dance obeys a normal distribution either in the strong turbulence or in the weak turbulence. In general, the spetro-width of spot dance is three times wider in the strong turbulence (at noon 12:---14:) than in the weak turbulence (at night 20:---22:), and the maximum times is about 8.It is evident that the spetro-width of the spot dance is bound to the turbulent intensity. And the procision of the displacement measurement depend on the spetro-width of the spot dance as well.