Chirped Amplitude Modulation Continuous Wave (AM-CW) Ladar for 3D Imaging and Doppler Tracking

摘要

Radars have used the frequency modulation -continuous wave (FM-CW) waveform for range and Doppler measurements for about 60 years. With the advent of narrow line width lasers in the 1960's, the FM-CW waveform has been used for coherent laser radar for over 40 years. The first and still the most widely used FM-CW waveform is the linear FM-CW waveform for which the frequency of the carrier is swept from a low (high) frequency to a high (low) frequency linearly with time to form a waveform known as an up-(down-) chirp. Both coherent detection and direct detection ladars utilizing amplitude modulation - continuous wave (AM-CW) waveforms with the modulation being at a single frequency (aka a single tone) or at a number of discrete frequencies (aka multi-tone) had been employed prior to and during the 1990's. When used with direct detection, the single frequency AM-CW waveform is generally used with a receiver that measures the phase difference between the outgoing waveform and the received return waveform to determine the range to a target. This measurement can produce excellent range precision, but produces erroneous results for two or more targets at different ranges within a single pixel. The use of multiple frequencies helps to distinguish between multiple targets, but is limited in the number and spacing of the targets that can be distinguished.