Long range acoustic navigation of cruising AUV based on bearing estimation

In this paper, a basic study on acoustic navigation for long range cruising AUV is described. Typically the position of AUV can be estimated by measuring the range between the AUV and acoustic sources or bearing of the acoustic signal. In long range observation, it is difficult to estimate these parameters in three dimensional. Depth can be directly sensed by depth sensor, then we can estimate the AUV position with measuring the range or bearing only horizontally. In this paper, feasibility of navigation for long range cruising AUV based on bearing estimation is considered. The bearing is estimated by measuring direction of arrival (DOA) of acoustic signal received by a receiver array equipped on the body side of the AUV. The array is horizontal long due to the shape of typical cruising AUV, and it is good to measure especially horizontal DOA. Estimation of the horizontal DOA with two method, delay-and-sum beam-forming (BF) and multiple signal classification (MUSIC) method, were simulated. Carrier frequency of the acoustic signal was assumed as 1kHz, and the signal was received by a planer receiver array, which size is 0.7m for height and 4.9m for width. The acoustic signal was a coded by eighth order maximum length sequence (M-seq.) code. The pulse width was about 1 second. The accuracy of estimation of the horizontal DOA were considered especially about characteristics in Gaussian noise and influence of multipath wave. MUSIC is said to be super resolution method. However, the accuracy of horizontal-DOA estimation is not so good comparing with the beam-forming. The sharpness of the peak in MUSIC spectrum is much sharp rather than beam-forming, but the top of the peak is not stable. On the other hand, when influential multipath exists, MUSIC may be more accurate than BF. However, the result of MUSIC is depending on decision of number of incoming waves. In case of longer range, namely signal to noise ratio (SNR) is low, BF is considered to be simpler, more robust and- - reliable. On the other hand, in case of comparatively short range, such as several dozen km, MUSIC may be more useful, because SNR is higher and multipath wave considered to be more influential.