Wavefront Systems has released an article discussing the importance of choosing the right sonar payload for mine countermeasure operations, highlighting the advantages that its Solstice side-scan sonar offers.
Multi-Aperture Sonar (MAS) is carefully designed and manufactured to conduct effective surveys in support of mine hunting, hydrography, salvage, search and rescue, and unexploded ordnance removal.
The decision on technology is always important but for some applications it’s more critical than others, retired Cdr Justin Hains MBE from our sister company Forcys explains: “When it comes to mine countermeasures the sonar payload is a critical design choice. A good sonar provides MCM teams with actionable data. A bad choice and you are left with a long list of false alarms each needing to be inspected either by a remotely operate vehicle or at the cost of a dive.”
When it comes to side-scan sonars and Synthetic Aperture Sonars (SAS), one of the biggest challenges for engineers is to maximize performance ability in an acoustic environment that is particularly hostile. This is the case in shallow waters, particularly in tidal and riverine habitats. Where higher order multi-path reverberation, unstable velocity of sound profile, significant bathymetry, baseline decorrelation effects and generally far fewer stable platforms are available, all contribute to the creation of an unreliable sonar performance at shorter ranges.
We developed Solstice to address these performance issues faced by traditional side-scan sonars.
How does Solstice overcome acoustic issues in shallow waters?
We designed a multi-aperture array to improve the signal-to-noise ratio, extending the range over other sonars operating at the same frequency. However, it was clear longer ranges in shallow, littoral waters are still susceptible to multi-path reverberation.
Dr Rob Crook, Research Director, explains: “The dominant noise source 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 nonetheless 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.”
How does this work?
Contrast is an essential feature as it helps to differentiate targets from the surroundings. Additionally, Solstice implements dynamic focusing to ensure the image maintains the highest possible resolution at the position in space relative to the sensor. This means 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.
Benefits of Solstice
These design choices and other key benefits lead to significant advantages for Solstice users:
Solstice is simple to use and makes mission planning easy. The area coverage rate increases with speed while the range remains constant. You can understand and use the constant range to plot a survey route and observe the area under consideration. The survey outcome becomes more predictable and easier to manage.
When mounted on an unstable platform or operating over complex seafloor environments, SAS systems are known to be very sensitive. If dynamic changes (such as mud sediment) impact the data quality, this can result in a complete loss of micro-navigation information. The worst outcome would be the SAS reverting to normal side-scan mode. If this were to happen, their low operating frequencies would render the data useless for mine-hunting purposes. Solstice is not sensitive to those dynamic changes providing high-definition data products in challenging conditions.
Unlike SAS, but very much like other side-scans, when using MAS, the range is only limited as a function of the so-called ‘crabbing angle’, but the image quality is still preserved along the whole swath.
Operations in confined spaces and shallow waters (20m to 30m depth) are complex for SAS systems or lower frequency side scans. When the range becomes limited, the multi-path effects from surface returns impact the SNR performance. A common issue for all side-scan sonars, these effects can compromise as much as 50% of their swath. With Solstice MAS, the impact will typically be less than 10%.
Easy to Integrate
Solstice can be combined with other imaging solutions, such as the Voyis Laser Imaging and Camera products along with the EIVA mission planning software.