Method and apparatus for improved ranging

Abstract

A high resolution ranging method is described utilizing a novel modulated waveform, hereafter referred to as coherent burst modulation. In the coherent burst method, high frequency modulation of an acoustic or electromagnetic transmitter, such as a laser, is performed at a modulation frequency. This modulation frequency is transmitted quasi-continuously in the form of interrupted bursts of radiation. Energy from the transmitter is directed onto a target, interacts with the target, and the returning energy is collected. The encoded burst pattern contained in the collected return signal is detected coherently by a receiver that is tuned so as to be principally sensitive to the modulation frequency. The receiver signal is processed to determine target range using both time-of-flight of the burst envelope and phase shift of the high frequency modulation. This approach effectively decouples the maximum unambiguous range and range resolution relationship of earlier methods, thereby allowing high precision ranging to be conducted at arbitrarily long distances using at least one burst of encoded energy. Performance of such method and apparatus is significantly improved through use of phase alternation methods that compensate for non-ideal behavior of the ranging apparatus or of the target and its environment. Such phase alternation methods may be achieved by varying the phase of the transmitter or receiver channels. Moreover, methods for reduction of potential uncertainty in absolute range measurement are taught that make use of coherent signal components at twice the nominal modulation frequency.