Unambiguous Doppler frequency estimation in an MTI radar

Abstract

In pulse radars, the Doppler frequency estimation is used to determine the object velocity. Many estimation methods have been developed for signals sampled with a rate higher than required by the Shannon sampling theorem. However, in a long range radar the minimum sampling interval is determined by the time needed for the microwave pulse to travel to and from the range perimeter. The frequencies of the Doppler return signal may significantly exceed the resulting unambiguous frequency range. Thus, an MTI (moving target indicator) radar lacks the ability to measure velocity unambiguously. In MTI radar systems periodically nonuniform (staggered) sampling is used to avoid the blind speed effect. M different sampling intervals are used in one stagger cycle. We show that in this case, signal frequency can be recovered from the samples. The method proposed exploits the properties of integer remainders derived from the Chinese remainder theorem. With a proper choice of sampling intervals, it allows for ambiguity resolving in a wide frequency range and with low error sensitivity.