Range resolution enhancement technique for dual demodulator continuous-time frequency modulation processing

Abstract

Continuous-time frequency modulation (CTFM) technique is extensively used in Sonar signal processing to estimate the range of received echoes. In this, the range resolution is inversely proportional to the bandwidth of the probe signal. A dual-demodulator continuous-time frequency modulation (DD-CTFM) technique that was proposed to improve the range resolution does not offer any significant improvement in range resolution over CTFM in practice. The super-resolution based analysis techniques for CTFM also offer limited improvement over the conventional CTFM technique. This paper describes a new method for CTFM analysis that has the potential to make the range resolution independent of the probe signal bandwidth. The proposed enhanced range resolution dual-demodulator continuous-time frequency modulation (ER-DD-CTFM) algorithm has two phases; one is the training phase and other is the test phase. In the training phase the complex weights are calculated in frequency domain due to all the possible echoes from the range of interest, while in the test phase the information of the echoes that are actually present in the received signal is estimated by solving a set of linear equations. The ER-DD-CTFM algorithm has been analyzed in simulation studies and in experiments conducted in an acoustic anechoic chamber. Both the studies and experiments demonstrate that up-to ten times better range resolution can be obtained using the proposed ER-DD-CTFM algorithm over DFT analysis based CTFM. Further, the improvements in the range resolution compared to Root-MUSIC analysis based CTFM are 2.8 times and 2.35 times for the simulation and experimental study respectively.