computes the relative distance by multiplying the velocity of light by one half the time difference between the time of the emission module 222 emitting the pulse signal and the time of the receiving module 224 receiving the reflected signal the digital signal 401 for example when the signal level u201c 1 u201d first shows up might be the reflected signal from the target if 3 clock signal cycles have passed between emitting the signal and receiving the reflected signal and the clock signal cycle is 0 11 microseconds then the signal cycle is 0 11 u00d7 3 0 33 microseconds the time for the laser light to travel to the target and back one time therefore the distance is time 0 33 2 0 167 microsecond multiplied by the velocity of light 3 u00d7 108 meters per second which is 50 meters all the possible distance of the targets of the digital signals 401 402 and 403 can be derived in the same way if the x axis denotes the distance and the y axis denotes the accumulated showing up times then the distribution will be as shown in fig5 according to the ratio of the strength of the target signal i e the strength of the signal 100 meter far to the strength of the noise in every distance and in the form of logarithm db a s n chart in which the x.