## 6 Experiments and Results  
The following experiments are based on a high fidelity sonar simulator [41], which allows the definition of a transmitter/receiver array (number and arrangement of sensors), the pulse form and the frequency. Note that a realistic simulation of the received signal needs to be based on a multitude of echoes arising from the water volume, from boundary layers and from the sediment volume as well as from objects in the water, on the bottom and even in the sediment. Unlike in simple ray-tracing based sonar simulations with simple noise added, the generation of appropriate transmitter and receiver signals is hence required in addition to modeling the physical properties of sediment volume, targets and the water/sediment boundary layer to get high fidelity results. Targets are specified in the high fidelity sonar simulator [41] from CAD objects modeled by a collection of facets with parameters depending on the backscattering properties on the surface of the object [42]. The backscattering of each facet is modeled as that of a plane circular element. Orientation and accordingly backscattering strength is defined by a vector normal to the object surface at the facets grid position. The total backscattered signal is then obtained by coherently superimposing the reflections from all facets. In [22], it is shown that realistic and reliable emulation of array signals based on dense grids of small discrete scatterers leads to high-fidelity results. For example, a 3D BF is modeled there, which generates realistic images containing all expected elements. The object used for the experiments here (Figure 10) is a test structure in the form of a mock-up panel for trials in the context of deep-sea oil- and gas-production (OGP) [18], which was used in the EU project \"Effective Dexterous ROV Operations in Presence of Communications Latencies (DexROV)\". In DexROV,the amount of robot operators required offshore (Mediterranean Sea, offshore of Marseille, France) was reduced--hence, reducing cost and inconveniences--by facilitating offshore OPG operations from an onshore control center (in Brussels, Belgium) via a satellite communication link and by reducing the gap between low-level tele-operation and full autonomy, among others by enabling machine perception on-board of the Remotely Operated Vehicle (ROV) itself [43; 44; 45; 46]. The model of the test structure is in the following experiments in a top-down view, which corresponds to the scenario when the ROV is in the initial approach phase, i.e., when sonar is used to localize the target structure from above. _2D Imaging with a 1D Linear Antenna_  
The following SAS processing is based on four array positions using the same signal processing parameters. The array is a 1D linear system with 128 elements with a pulse center-frequency of \(f_{0}=400\) kHz. The pulse length is \(T_{p}=\frac{1}{100\,\mathrm{kHz}}\), the sampling frequency \(f_{s}=200\) kHz and the sensor spacing is always \(\frac{\lambda}{2}\). The resulting SAS reconstruction is shown in Figure 10(e). The significant SAS effect is especially observable when directly comparing it to the quality of the image of a single scan shown in Figure 10(b). A comparison using the standard SAS wavefront reconstruction (Section 2.2) along the same aperture \(u\) is shown in Figure 10(f). In this wavefront reconstruction and the corresponding generated data, ideal phase relations are used. Both our SAS and the ideal phase results show very similar results. The original CAD model is shown in Figure 10(a). The incoherent summation in Figure 10(c) represents no information gain. Another comparison is shown in Figure 10(d) where a theoretical 1D antenna of the full SAS length is used. This result is less accurate, which can be explained by the omnidirectional beampattern of the transmitter. It demonstrates that a resolution gain can be achieved by the combination of a transmitter/receiver beampattern when the grating lobes are well suppressed by null-steering (see Section 4.1 again).