IQ-locked loop tracker

A significant improvement in tracking narrowband acoustic signals at low SNR has been achieved by including low pass filter outputs --Inphase (I) and Quadrature (Q)--in the inner loop of a frequency tracker. Comparisons with conventional digital line trackers such as Phase-Locked Loops, Frequency Discriminators, and Spectral Amplitude Trackers show improvements in tracking thresholds of from 2 to 8 dB for a variety of signals (lines with doppler dynamics, frequency modulation, and multipath dispersion effects). This paper describes the IQ-Locked Loop, explains the technique of choosing coordinates to improve nonlinear estimation at low SNR, and presents comparison results which have implications for detection, communication, and localization as well as frequency and bearing tracking.