# A Wide-Area Deep Ocean Floor Mapping System: Design and Sea Tests
## 2 System Overview  
The motivation for developing a large acoustic aperture comes from trade-offs between imaging distance, resolution, operating frequency, and aperture size. To illustrate these trade-offs, consider three scenarios with different aperture sizes and resolution requirements. In Figure 3, the blue curves represent the link budget--the maximum imaging distance as a function of frequency given a maximum sound pressure level (186 dB re 1 uPa for marine life safety) and the minimum required SNR. This link budget limit does not change significantly in the three scenarios. The red and green curves are the range resolution and the diffraction limits for each system, respectively. To achieve 100\(\times\) better range and cross-range resolution, the signal bandwidth needs to be wider and the wavelength needs to be shorter, so both limits shift to the right by two orders of magnitude. The shaded areas indicate possible operating regions for the sonar systems. For instance, when operating near 20 kHz, a 7 m sonar array can achieve a 100 m resolution to at least the target depth of 6 km, a depth greater than 99% of the world's oceans' depths. However, to achieve 1 m resolution, the same sonar array can only reach slightly over 1 km. Therefore, the solution to higher resolution at longer distances is to increase the diffraction limit of the system without increasing frequency, thereby requiring the sonar aperture to be significantly increased. For example, a system using a 100\(\times\) larger array can achieve a 1 m resolution while operating beyond the target depth of 6 km. A larger sonar aperture increases the diffraction limit of the sonar and enables high-resolution imaging from longer ranges. Traditionally, sonar apertures are populated with transducers at half-wavelength spacing. For very large apertures, this approach would be prohibitive in terms of size, weight, power requirements, and cost. Therefore, we  
Figure 3: The performance-bound trade-off between imaging distance, resolution, frequency, and aperture. (**a**) A 7 m sonar array can achieve 100 m resolution at the target depth of 6 km. (**b**) A resolution of 1 m is possible, but the range is limited to 1 km. (**c**) A large 700 m aperture sonar can achieve 1 m resolution at the target depth and beyond. Figure 2: Progression from concept development to sea testing of the sparse aperture. designed a large aperture sonar system with a sparsely populated 2D array of transmit and receive elements. To demonstrate this concept, we implemented a sparse-aperture multiple-input, multiple-output (MIMO) sonar system that comprises pulse generators, a sparse transmit and receive array, multichannel signal conditioners, and signal converters, as shown in Figure 4. The pulse generators have calibration loopback outputs, which are used as a reference to compensate for time jitter and time drift. Receive elements have flat sensitivity over the operating frequency band. Signal conditioner consists of single-ended-to-differential converter for reducing the impact of common-mode noise pickup and harmonics, low-noise amplifiers for improving dynamic range and sensitivity, and anti-aliasing low-pass filter. According to [PERSON] principle, combining radiated pulses from multiple transmitters to perform space-time focusing to improve array performance is possible [21]. However, this method requires transmitters to generate identical waveforms while maintaining precise positioning and time synchronization. Implementing such an approach for the distributed ASV array would significantly increase the cost and hardware complexity, so the concept of transmit focusing was not included in the current system design. To achieve orthogonality with MIMO, we adopted a time-division multiple access (TDMA) transmission scheme in which transmitted waveforms share the same frequency band but at different time slots.