In this article, Dr Rob Crook, Director of Research at Wavefront Systems, discusses why Multi Aperture Sonar (MAS) is preferable to Synthetic Aperture Sonar (SAS), and when is it preferable to use one over the other.
When and why should we use multi aperture sonar over synthetic aperture sonar?
Solstice is Wavefront’s Multi Aperture Sonar (MAS) product. It behaves like a top performing traditional side-scan sonar but is processed very differently. Solstice uses the multibeam input from 32 elements to dynamically extend the focus along the whole length of the swath. When using multi apertures, the data is additionally improved and the signal-to-noise ratio is enhanced. Additionally, the array in Solstice is designed to suppress multi-path effects. It improves the shadows and highlights that are critical in helping detect and classify objects, generating narrow 0.15°along-track beams and boasting a remarkable 200m swath. For Solstice to work best, it has been designed to be mounted on an AUV, drawing very little power.
By using an AUV with Solstice, you will see further and at a higher resolution than the more traditional side-scans. However, when we compare MAS to Synthetic Aperture Sonar (SAS), we see it has a shorter range and offers less resolution.
So why change from SAS to MAS?
When building autonomous underwater vehicles, compromises need to be made. The budget for Space, Weight and Power (SWaP) is limited. For ships whose primary mission is to survey, where power consumption is not an issue, the use of SAS makes sense. It can plough away and by drawing 100s of Watts, it makes only a small dent in their power budget.
When it comes to low-logistic systems, we cannot say the same. For popular two-person portable systems, there are strict SWaP budgets. Even the SAS systems will demand twice, and maybe even three times, more power than the Solstice.
These SAS systems are heavy and large in size. Attempting to fit them on a 9” diameter is virtually impossible. The size of a SAS means that, even on larger AUVs, their footprint is such that operators must be prepared to compromise on overall mission endurance. Mounting a SAS means less ‘SWaP’ for your other instruments!
This is where Solstice’s MAS solution excels. By drawing an incredibly low 21 Watts (which includes data acquisition), it becomes an equivalent to the leading AUV side-scan sonars for power. However, it offers a range and accuracy which is unlike any other side-scan sonar. When it comes to precision, over the initial 50-meter swath, Solstice is as stunning as any of the leading SAS systems. At the 75-meter range, the accuracy surpasses the closest side-scan sonar by more than two times. And, unlike our competitors, Solstice can provide data over a 200m swath with a 100m range on each side. For a sonar that offers motion compensation and interferometry, Solstice is tiny and light, making it ideally suited to a two-person portable application.
In conclusion, this means MAS is ideally suited for rapid, low-logistics, long endurance, high-resolution deployments on AUVs capable of operation from vessels by smaller crews. In other words, 95% of AUVs working in the littoral zone!