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41 Sonar systems | Focus design limitations, as the precision of measurements correlates directly to the baselines between hydrophones. A bigger baseline means more accurate calculations, so shrinking the size of the USBL’s array – which is always the largest component in the system – without widening its margins of error is a persistent design challenge However, the use of DSPs and similar new technologies in blank-sheet designs have greatly helped to make acoustic navigation solutions less dependent on excessively bulky electronics. Doppler velocity logs In addition to localisation systems such as USBL, the use of acoustic Doppler current profiling systems such as Doppler velocity logs (DVLs) has long been key to UUV navigation by recording the vehicle’s speed over the seafloor. They work by measuring the Doppler shift in emitted pulses – typically a ping on four beams. Changes in wavelength and frequency between acoustic transmissions and their returns are correlated with the speed of movement. Most DVL systems send their beams at a 30 º decline, meaning the DVL has to be relatively near the seabed to detect the return wave. That means it is generally the most likely item to be damaged by impacts or debris close to the underbelly of the UUV, yet the further it is from the seafloor, the lower the resolution of its measurements. An alternative approach has been developed though that sends two pings from the sensor instead, which are received by the transducer at slightly different times. The correlation between the two signals, and particularly the displacement between them, indicates the UUV’s speed and velocity. It also uses a different beam form and width from DVLs, which gives it more flexibility as it can measure speed and velocity from up to 300 m from the seafloor, and can be integrated anywhere on a UUV so long as its acoustic pulses are unobstructed. Sub-bottom profilers Sub-bottom profiling (SBP) sonars have been around since the 1970s, but historically they have been too large and power-hungry to be used in UUVs, particularly since the cost of building and operating a UUV typically means the developer and operator try to package as many acoustic sensors into the hull as possible, to derive a wider range of valuable data. As a result, SBP r&d has focused heavily on miniaturising the technology, more so than perhaps any other acoustic system. At the same time, a growing amount of effort is going into developing 3D SBPs, as 2D sub-bottom profiling often gives insufficient detail and precision for some archaeological and mining projects. To achieve 3D sub-bottom profiling, the measurements need an extremely high angular resolution – ideally less than 2 º of error – and a low-frequency sonar. That typically means too large and costly a payload, so to circumvent that, more and more so-called parametric SBP sonars are being developed for UUVs. The parametric effect is non-linear and refers to how, when transmitting powerful acoustic waves at two different frequencies close to each other – 100 and 110 kHz, say – the physical properties of the seawater causes a 10 kHz acoustic wave to be generated with the resolution and angular aperture of the 100 kHz beam. By harnessing this effect, a low- frequency, high-resolution beam would be generated from a pair of relatively compact transducers. To generate 3D SBP imagery though, the beam must be electronically steerable – achieving that is an ongoing design challenge. Projects aimed at developing 3D parametric SBPs are due to be available in the next few years though. Conclusion As end-users turn their eyes towards improving the return on their investment in UUVs, demands will grow for subsea vehicles that can gather multiple streams of data and double-check their accuracies against one another, all while navigating and localising itself autonomously. Advances in sonar will therefore drive future UUVs to become larger than ever, to carry multiple sensor payloads as well as the energy storage needed to power them for months at a time. Acknowledgements The author would like to thank Frederic Mosca and Hubert Pelletier at iXblue, Tim Jensen at Teledyne Marine and Alex Tennant at Advanced Navigation for their help with researching this article. Unmanned Systems Technology | December/January 2020 USBL designers continuously look for ways to shrink the size of their systems’ arrays without reducing their accuracy (Courtesy of Sonardyne)