In the below article, Forcys discusses how Solstice, a multi-aperture sonar designed by Forcys’ technology partner Wavefront Systems, can improve the effective area coverage rate and lethality of low-logistic autonomous underwater vehicles in complex littoral zones.
If you have to operate side-scan sonars and Synthetic Aperture Sonars (SAS) in Very Shallow Waters (VSW) or Shallow Waters (SW), the acoustic environment is particularly hostile. Higher order multi-path reverberation, unstable velocity of sound profiles, often unknown, as well as significant bathymetry, baseline decorrelation effects and generally far fewer stable platforms, all add up. The result is far less reliable end sonar products with greater impact to longer range systems. This is particularly acute in tidal and riverine environments. What to do?
Go back to the drawing board
When Solstice was developed in 2010, our technology partner Wavefront Systems decided it was time for a step up in the performance of traditional side-scan sonars. The aim was to deliver a high-frequency, high-resolution, and long-range sonar that would provide a marked improvement in the probability of detection of mine-like objects while minimizing the probability of false alarms.
Solstice was designed to do just that. Step one was to design a multi-aperture array which would improve the signal-to-noise ratio extending the range over other sonars operating at the same frequency. However, longer ranges in shallow waters are susceptible to multi-path reverberation.
Dr Rob Crook, Research Director at Wavefront Systems explains how Solstice overcomes this problem: “The dominant source of noise for all side-scan sonars operating in shallow waters is ‘multi-path’ reverberation. The nature of this noise means many acoustic pathways scattering from spatially unrelated regions of the underwater scene may none-the-less return to the sensor with identical flight-times. The inability of any ‘2D’ (range, bearing) sensor to discriminate between these contemporaneous pathways leads to an inevitable loss of contrast. Multi-path Suppression Array Technology (MSAT) is a physical array-based technology that offers the swathe coverage one would traditionally have associated with wide elevation beam-widths, with the shadow contrast associated with very narrow beams. MSAT allows high shadow contrast right out to the maximum range of the sensor whilst maintaining high quality imagery close to nadir.”
Why is contrast important? It helps to differentiate targets from the surroundings.
In addition, Solstice implements dynamic focusing ensuring that the image will maintain the highest possible resolution at the position in space relative to the sensor, meaning that the resolution will improve as the range to the target decreases. While at longer ranges the interpolated real-time imagery drastically aids human visual perception.
What does it all mean?
The design choices lead to significant advantages for Solstice users. These are some examples of where Solstice excels.
- Simpler to operate: Solstice is simple, the area coverage rate increases with speed while the range remains constant. This makes mission planning easy. You can understand and use the constant range to plot a survey route and you can observe the area that is under consideration. The survey outcome becomes more predictable and simpler to manage.
- More robust: Systems like SAS are known to be very sensitive when mounted on an unstable platform or operating over complex seafloor environments. Any dynamic changes may impact the quality of the SAS data; mud sediments can result in complete loss of micro-navigation data, and in the worst outcome the SAS needs to revert to normal side-scan mode. SAS typically operates at lower frequencies hence this corrupted SAS side-scan data is not suitable for most operations. Solstice MAS does not share this problem.
- High currents: In MAS the range is limited as a function of the so-called ‘crabbing angle’ but the image quality is preserved along the whole swath.
- Shallow waters: Operations in confined spaces and shallow waters (20m to 30 m depth) are difficult for SAS systems or lower frequency side-scans. These systems can become range limited as the multipath effects from surface returns has an impact on the SNR performance and this is common for all side-scan sonars. For some SAS systems, these effects can compromise as much as 50% of their swath but with Solstice MAS, the impact will typically be less than 10%.