# A Data-Driven Intermittent Online Coverage Path Planning Method for AUV-Based Bathymetric Mapping
## 4 Experimental Validation in Simulated Environments
### Results  
To test the performance of the proposed approach, a series of experiments were conducted. The target region is bounded by 200 m and 700 m zonally and 200 m and 900 m meridionally. The depth  
Figure 5: Structure of the simulatorof the terrain is modelled between 40 m to 80 m. The AUV is constrained at the depth of 30 m and moving in a constant speed of 1 m/s. We summarize the simulation setup parameters in Table 1.  
We have run 16 simulations with different weights for the three rewards in the terrain shown in Figure 2. As mentioned before, each weight has a range from 0 to 1, and the sum of the weights has to be 1. We use the following rules to select the weights in order to cover all possibilities in our simulation runs. First, we pick \(W_{e}\) from five options, 0, 0.25, 0.5, 0.75, and 1. Then, \(W_{c}\) and \(W_{s}\) are selected from the same options to satisfy the requirement that the sum of the weights has to be 1. The above rule produces 15 pairs of weights. Then, the 16-th pair is selected when all three weights are equal to 0.33. The resultant runtime to reach 99% coverage rate for each run is shown in Figure 6. The sizes of the spheres indicate the total mission runtime for each experiment. In other words, the larger sphere means a longer runtime, therefore, lower coverage efficiency. It is clear that the coverage efficiencies are relatively low when \(W_{c}\) or \(W_{s}\) is 0, which proves the effectiveness of the coverage gain reward and sonar sensing reward. When \(W_{e}\) is set at 0, the vehicle is not encouraged to explore distant area, so the resultant path is similar to lawnmower pattern, except that it is adaptive and greedy. Zero \(W_{e}\) can have a benefit on coverage efficiency according to Figure 6, when we don't care much about interpolating the map. Consequently, the weight setups with zero \(W_{c}\) or zero \(W_{s}\) are excluded, and 7 groups of weights that are encircled in Figure 6 are further inspected, as listed in Table 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Region & 500 m\(\times\)700 m \\ Average depth of terrain & 60 m \\ Depth of the AUV & 30 m \\ Angular coverage of multibeam & 120\({}^{\circ}\) \\ Sampling frequency of multibeam & 2 Hz \\ Velocity of AUV & 1 m/s \\ Initial position of AUV & (200, 200) \\ Forward-looking angle of sonar [32] & 0\({}^{\circ}\) \\ Staring angle of sonar & 90\({}^{\circ}\) (downward-looking) \\ Cell size of workspace & 1 m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Setup of the simulation model. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Run Number** & **1** & **2** & **3** & **4** & **5** & **6** & **7** \\ \hline \(W_{e}\) & 0 & 0 & 0 & 0.25 & 0.25 & 0.5 & 0.33 \\ \(W_{c}\) & 0.75 & 0.5 & 0.25 & 0.25 & 0.5 & 0.25 & 0.33 \\ \(W_{s}\) & 0.25 & 0.5 & 0.75 & 0.5 & 0.25 & 0.25 & 0 Figure 7 shows the overall path and mapping uncertainty in run #2, which has the highest overall coverage efficiency, and Figure 8 compares the results of two typical runs, run #5 and #6,. The sensor confidence of each cell is calculated by Equation (10) and represented by the colors described in the sidebar. The vehicle path is displayed with grayscale lines showing elapsed mission time in the top colorbar while the side colorbar indicating the seafloor depth. The corresponding runtimes for different gray scales are described in the topbar. As the velocity of the AUV is 1 m/s, the runtime also equals the traveling distance. For comparison, we have conducted three lawnmower pattern surveys with inter-distance of 62 m, 50 m, and 42 m. Moreover, two spiral pattern surveys are also conducted with minimum line space of 45m and 33 m. In these runs, the inter-distances are determined by limiting the maximum numbers of transects. For example, the total number of transects is 8 with 62 m inter-distance. Figure 9 shows the result of the lawnmower path with 42 m inter-distance and the result of the spiral path with 46 m zonal inter-distance and 33 m meridional inter-distance.