Multibeam Echosounder With Orthogonal Waveforms: Feasibility and Potential Benefits

Abstract

Modern multibeam echosounders (MBE) employ frequency-division techniques (FDT) to ensonify multiple sectors within the same ping cycle. This leads to improved performance in coverage rate, and yaw and pitch stabilization. However, it introduces a bias among sectors because MBES systems are frequency dependent. It also reduces the maximum pulse bandwidth compared to a single-sector sonar. In this study, we consider the code-division technique (CDT) as a solution to this problem. A set of orthogonal coded pulses are received within the same frequency band, and each sector is separated with a matched filter. We assess the feasibility of the technique through two stages. 1) First, we formulate an analytical model describing the power and crosstalk budgets of any multisector MBES. The model can then be used to design transmission sequences fitting these budgets. 2) Then, we display the practical usage of the technique for MBES imaging and mapping through simulated case studies. For the same total timebandwidth budget, we compare the performance of FDT, CDT, and multicarrier CDT (MC-CDT), a hybrid method employing CDT and FDT, which is robust to strong dynamic backscatter. This study considers only bottom detection based on signal amplitude. Our results show that it is possible to share a larger frequency bandwidth between multiple sectors while maintaining an acceptable bottom detection performance similar to FDT. Our choice of time-bandwidth product with CDT offers a crosstalk suppression of −25 dB between sectors, but may display low-magnitude residual artefacts in the water-column data. MC-CDT provides a significant gain of pulse bandwidth while it offers interband separation performance comparable to FDT and reduces significantly the water-column artefacts.

MIMO synthetic aperture sonar: Benefits and limitations

“…The up/down‐chirp waveform pair is near optimum both regarding imaging performance and cross‐talk performance. The peak auto‐correlation to peak cross‐correlation power ratio (PCCR) between these two waveforms is [6] $$$$\begin{equation} PCCR \approx \frac{1}{BT}, \end{equation}$$$$where B is the signal bandwidth, and T is the pulse length.…”

“…Multiple Input Multiple Output (MIMO) orthogonally coded waveforms sharing the same bandwidth is an attractive alternative that has gained massive popularity in radar [3] and in Synthetic Aperture Radar (SAR) [4]. MIMO has also been suggested in sonar [5,6], and in SAS [7][8][9]. In the latter case, the disadvantages are not fully described.…”

Continuous Phase Modulation of Phase Coded Transmit Waveforms using Multi-Tone Sinusoidal Frequency Modulation

“…This field of research has been enabled by the development of several parameterized modulation techniques such as Phase-Coding (PC) and Frequency Shift-Keying (FSK) [2] which facilitate the synthesis of waveforms with novel characteristics. Waveform diversity has increasingly become a topic of interest to the active sonar community with diverse sets of waveforms being employed for Multi-Beam Echo Sounding (MBES) [3] and Multiple-Input Multiple Output (MIMO) sonar applications [4]. These efforts highlight the need for continued development of parameterized waveform models to further enable waveform diversity for active sonar systems.…”