A new Algorithm for High-Frequency Resolution Sonar

A0084 Hironori Susaki Research Laboratory, Furuno Electric Co., Ltd.

We examined a method of measuring low velocities with a high velocity

resolution, or frequency resolution, for the continuous wave Doppler

sonar by use of a simple circuit configuration employing a digital

signal processing technique. However, the continuous Doppler sonar is

capable of measuring a high frequency resolution, restricted to a

short range, because of the limitation of the transmission power. We

propose a novel algorithm for the pulse Doppler sonar with a high-

frequency resolution, the same as the continuous Doppler sonar but

without the circuit modification of the pulse Doppler sonar. The

conventional pulse Doppler sonar measurement procedures are as follows:

1)transmit pulse-burst signal, 2)calculate the ensemble average of the

Doppler shift frequency from each echo signal. The proposed algorithm

intentionally utilizes an aliasing phenomenon, which usually causes

measurement problems, to achieve high-frequency resolution with the

conventional pulse Doppler measurement method. The new algorithm

consists of two steps as follows: Step 1: pick up the frequency bin

(coarse Doppler frequency) of a desired signal by the fast Fourier

transform (FFT) used in the conventional pulse Doppler sonar. Step 2:

pick up one piece of data from each echo signal, which corresponds to

a pulse-burst transmission, then gather and calculate FFT on these

data. The Doppler shift frequency is obtained by searching for a peak

frequency of the power spectrum. However, the lower Doppler shift

frequency is usually affected by the aliasing in this FFT method so

that the fine Doppler shift frequency is determined by the combination

with the aliased Doppler shift frequency and the frequency bin

information gained in step 1. This new algorithm can also improve the

temporal response.