Low Frequency SAS 2D transmitter array calibration

This paper describes the calibration experiments that took place at Loch Goil, Scotland, as part of the HR-LFSAS project. The aim of the trials was to calibrate a 2D low frequency array and to measure its beampattern. One of the main challenges of the calibration was the sampling frequency at the transmitting side introducing strong spectrum periodisation in the recorded signal. The artefacts impact heavily the received signal at frequencies close to the Nyquist frequency. Utilising the high sampling frequency of the receive hydrophone, we propose here an algorithm to reconstruct the pulse based on a down-sampling and up-sampling procedure. We also derive a metric based on spectrum relative energy to assess the quantitative efficiency of a DAC (Digital-to-Analog Convertor) system as a function of the ratio between its sampling frequency and the pulse maximum frequency. Finally, we show that under certain conditions, a system can be calibrated beyond the Nyquist frequency imposed by the DAC.