Patent ID: 7301467

Claim:
A position detection system for detecting a relay device, comprising: a plurality of first communication devices, each of plurality of the first communication devices including a first transmitter for transmitting a distance-calculation signal for calculating distance to a second communication device, a detector for detecting, by receiving from the second communication device a signal corresponding to the distance-calculation signal, phase difference between the distance-calculation signal when transmitted and the signal corresponding to the distance-calculation signal when received, and a calculator for calculating the distance to the second communication device, based on the phase difference detected by the detector; the second communication device including a second transmitter for transmitting the signal corresponding to the distance-calculation signal transmitted by plurality of the first communication devices; a determining unit for determining whether or not the multiple distances to the second communication device, calculated by plurality of the calculators of plurality of the first communication devices, are below a predetermined distance; and a position detector for detecting the position of the relay device corresponding to the multiple distances on the basis of the determining result when the determining unit has determined that the multiple distances to the second communication device, calculated by plurality of the calculators of plurality of the first communication devices, are at or above the predetermined distance, wherein in a state where communication between plurality of the first communication devices and the second communication device is not possible due to multiple distances between plurality of the first communication devices and the second communication device being at or above the predetermined distance, enables communication between plurality of the first communication devices and the second communication device.