Patent ID: 7518546

Claim:
A method of measuring a distance, comprising: transmitting predetermined radio waves; generating a first clock signal having a clock frequency b and counting N 3 a number of clocks of the first clock signal; generating a second clock signal having a clock frequency a and delaying the second clock signal until the transmitted radio waves are received to generate a delayed second clock signal; counting N 1 a number of clocks of the delayed second clock signal having the clock frequency a; and measuring the distance using the count values N 1 , N 3 of the clock signals having the clock frequencies a and b, stopping the counting of N 1 and N 3 when the first clock signal and the delayed second clock signal overlap, counting N 2 a number of overlaps between the first clock signal and the delayed second clock signal; and measuring the distance using the counts N 1 , N 2 , and N 3 .