1. A method of detecting an intruder in a protected space using a Doppler motion detector comprising the steps of: transmitting three microwave frequency signals of different frequencies, receiving reflected microwave signals, wherein each reflected microwave signal corresponds to one of the three transmitted microwave frequency signals reflected by a moving intruder in the protected space, generating from the received microwave signals intermediate frequency signal components, wherein each intermediate frequency signal component is representative of one of the received reflected microwave signals, sampling the intermediate frequency signal components, determining a first, second, and third phase angle from the sampled intermediate frequency signal components, determining a first phase difference between the second phase angle and the first phase angle, determining a second phase difference between the third phase angle and the first phase angle, for each of the first and second phase differences, determining two corresponding distance measurements, wherein one distance measurement is a true distance measurement and the other distance measurement is an ambiguous distance measurement, selecting an accurate distance measurement by matching the common true distance measurement of each phase difference.