# A Wide-Area Deep Ocean Floor Mapping System: Design and Sea Tests
## 5 Toward Autonomous Sparse-Aperture Sonar  
As we have shown that sparse-aperture imaging of the seabed is possible, we are working to disaggregate the sonar array onto multiple ASVs. The first step is to derive the required position knowledge for all the hydrophones in the array to generate sharp and focused imagery. This position knowledge will serve as a requirement for how well each of the ASVs needs to be tracked. Using our tank demonstration sonar array, we confirmed that if we know the positions of all elements to one-tenth of the acoustic wavelength, we can generate clear imagery. Using our ocean data at 33 kHz and additional modeling, we can estimate the array's performance degradation with element position error.  
For example, the wavelength at 33 kHz is 4.5 cm, and from the tank experiments, we would expect sharply focused imagery with a 4.5 mm or less element position error. The power loss with simulated position errors at 33 kHz is shown in Figure 14. We see minimal degradation through the one-tenth wavelength uncertainty and a less than 0.5 dB loss. The 3 dB loss point is around 8 mm, or about one-sixth of a wavelength, and at about one-third of a wavelength, the performance drops precipitously. Therefore, the required position accuracy for hydrophones on the first-generation ASV will be one-tenth of a wavelength, 5 mm, the hope is to develop autofocus techniques to relax this requirement to one-third or one-fifth of a wavelength and reduce the cost of the ASVs.  
We are working to demonstrate a low-cost, precision-relative navigation capability to achieve 5 mm, 3D relative position accuracy. We performed a trade study evaluating many potential relative navigation options: (1) GPS, (2) inertial measurement, (3) optical  
Figure 14: Power loss with position error simulated based on 33 kHz sonar data.  
Figure 13: A comparison of seabed imagery of automated wreck and obstruction information system (AWOIS) site 2112, a sunken barge outside Boston Harbor. The site’s geolocation is indicated in (**a**). (**b**,**c**) Show reference imagery from a commercial side-scan sonar and imagery generated with our large sparse array using our initial low-resolution algorithm, respectively.  
tracking, (4) LiDAR, (5) radio-frequency ranging, and (6) high-frequency (automotive) radar. The key performance parameters are sensor field of view, maximum range, measurement accuracy, and cost. We plan to use a fused GPS and inertial measurement with optical and LiDAR tracking. The stereo-optical measurements can be used to generate the most accurate relative measurements; however, this approach requires many cameras and targets, which may exceed cost constraints. LiDAR has excellent coverage and good potential solution accuracy. Figure 15 shows an example of a \"dummy\" ASV in a tank with GPS and inertial measurements onboard and offboard tracking with an optical sensor and LiDAR. Our current optical system post-processing can measure optical-marker rigid-body edge distances and interior angles with accuracies of 0.33 cm and 0.25\({}^{\circ}\), respectively.  
In collaboration with the Woods Hole Oceanographic Institution, we designed a low-cost ASV with a sonar array on the bottom that could have fiducials installed to enable precision tracking. The first of these ASVs has just been deployed and includes a 21-element passive sonar array, fully autonomous control, fused inertial navigation and GPS, and rigid markers for tracking. Pictures from the sea tests are shown in Figure 16. Optical markers are tracked using stereo cameras and LiDARs to generate precise 6-degrees-of-freedom relative poses of the ASVs.