# Assessing the Performance of the Multi-Beam Echo-Sounder Bathymetric Uncertainty Prediction Model
## 3 Description of the Dataset  
For validating the depth uncertainty model, a dedicated survey was carried out by the Ministry of Infrastructure and Water Management of The Netherlands (Rijkswaterstaat) in the Oosterschelde estuary (Eastern Scheldt), The Netherlands. This estuary is located in the province of Zeeland, between Schouwen-Duiveland and Tholen on the north and Noord-Beveland and Zuid-Beveland on the south. The survey areas were selected so as to differ in water depth, while the water depth was close to constant within each area. This allowed for experimentally investigating the effect of water depth on the uncertainties and comparing the measured trends with those predicted. The second parameter that was changed during the survey was the pulse length, as this parameter is known to have an important influence on the uncertainties. The EM2040C dual head [29] from Kongsberg was used with the dual swath acquisition and the equiangular beam spacing modes enabled [30], see Table 1.  
A brief discussion on the systems used for data acquisition and bathymetry processing is in order. A critical element for accurate estimation of the depth below the transducer is the Sound Speed Profile (SSP) in the water column, which varies both spatially and temporally. Therefore, sufficient and accurate measurements of SSPs are required. To ensure the former, the surveyor was asked to acquire a new SSP in case of a difference of more than 2 m/s between the surface sound speed value and the sound speed from the latest full SSP [31]. The sound velocity profiler was manufactured by AML oceanographic, and the uncertainty of its measurements as indicated by the manufacturer was 0.02 m/s, [32]. However, from measurements in different locations (inland waterways and the North Sea), the uncertainty was found to be 0.2 m/s, and hence, this value was chosen as a more realistic description of the system's uncertainty. The sound speed profiles acquired for both datasets were almost constant in the water column equaling 1515 m/s and used in the uncertainty prediction model for the sound speed in the water column and at the receiving array. The datasets were acquired using the Quality Integrated Navigation System (QINSy) (developed by Quality Positioning Services, QPS, BV), and Global Navigation Satellite System (GNSS) sensors received the correction signal from Real-time Kinematic (RTK) services, Netherlands Positioning \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Center frequency (kHz) & 300 \\ Theoretical array length (m) & 0.407 \\ Number of beams (dual swath) (\#) & 800 \\ Beam spacing mode & equiangular \\ Maximum swath width (\({}^{\circ}\)) & 130 \\ Mounting angle of transducer (\({}^{\circ}\)) & 39.89 for Starboard, 40.57 for Port \\ Beam steering reference angle (\({}^{\circ}\)) & 0 \\ Along-track opening angle (\({}^{\circ}\)) & 0.9 \\ Across-track opening angle at nadir (\({}^{\circ}\)) & 0.9 \\ Range resolution (m) & 0.02525 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of EM2040C [29] in the dual head configuration. Service (NETPOS). Using RTK for the vertical positioning in QINSy meant that the depth relative to the chart/vertical datum was directly measured from accurate GNSS observations (For more information on different depth processing algorithms available in QINSy, the interested reader may refer to [33]). Therefore, the water surface level was of no relevance, and accounting for height offsets, such as tide, draft, and height above draft reference, was not necessary as they did not affect the quality of the derived depths. Heave measurements (short-term variations in the transducer depth) were, however, used within the processing software to calculate the height of the vessel center of gravity between two position updates (as the update rate of the Inertial Navigation Sensor (INS) is higher than that of the GNSS system). Therefore, the accuracy of heave measurements acquired by the INS contributed to the uncertainty in the depth estimate.