Extended range optical distance measuring instrument

Claim 1

An extended range optical distance measuring instrument comprising: first and second terminals disposed at opposite ends of a path whose distance is to be measured, said first terminal including: means for generating a light beam, means for generating a stable frequency signal, means for modulating said light beam with said stable frequency signal, means for directing said modulated light beam over said path to said second terminal, means for frequency multiplying said stable frequency signal by m, and means for radiating said frequency multiplied signal over said path to said second terminal, said second terminal including: means for receiving said modulated light beam, means for generating a first local oscillator signal, means for heterodyning said modulation on said received light beam with said first local oscillator signal and for frequency multiplying the resultant signal by m to provide a first intermediate frequency signal, means for receiving said radiated frequency multiplied signal, means for frequency multiplying said first local oscillator signal by m, means for mixing said received frequency multiplied signal with said frequency multiplied first local oscillator signal to provide a second intermediate frequency signal, means for generating a reference signal of stable frequency equal to said second intermediate frequency, means for comparing said second intermediate frequency signal with said reference signal to provide an error signal, means for applying said error signal to said first local oscillator signal generating means to lock said frequency multiplied first local oscillator signal to said received signal but with a frequency offset equal to said intermediate frequency, and means for radiating said frequency multiplied first local oscillator signal over said path to said first terminal, said first terminal further including: means for receiving said radiated frequency multiplied first local oscillator signal, means for mixing said received signal with said frequency multiplied stable frequency signal and for transmitting the resultant signal over said path to said second terminal via an auxiliary radio transmitter, said second terminal further including: means for receiving said radio transmitted signal and for obtaining from said received signal a third intermediate frequency signal, and means of combining said first, second, and third intermediate frequency signals to provide a signal whose phase is substantially proportional to the optical transit time of said path.