An uncertainty assessment of the effect of using FM pulses on MBES depth measurements

Abstract

The most recent generation multi-beam echosounders (MBES) allows to transmit Frequency Modulated (FM) signals, in addition to the more conventional Continuous Wave (CW) pulses. In this contribution, the effect of using these FM pulses for bathymetric measurements is investigated. Advantage of using FM signals is that measurements can be taken at long ranges, due to the use of long pulse lengths combined with matched filtering at reception. However, it is found that using FM pulses results sometimes in a loss of the quality of the depth measurements. An important contributor to the errors when using FM signals is the Doppler frequency shift of the received signal, inducing errors in the matched filtering and beamsteering. The latter, however, also holds when using CW signals. A second contributor is the baseline decorrelation, resulting from not having an infinitely small footprint. Due to the larger pulse lengths of the FM signals, also the footprint is larger and consequently the effect of baseline decorrelation is more pronounced for FM signals. Here, we consider a situation of relevance for measurements in the Dutch North Sea (water depth of 50 m and ship dynamics corresponding to a typical winter season sea state). For this case, the uncertainty induced by the Doppler frequency shift, both for CW and FM signals, is found to contribute significantly to the MBES total error budget. The effect of baseline decorrelation depends fully on the signal shape and parameters.