An interview with Dr. Rob Crook, Research Director, Sonar Systems here at Forcys.
Solstice Multi-Aperture Sonar, or as we call it, Solstice MAS is a product of a rethink of side scan technology from the bottom up.
It aims to blend the high resolution of Synthetic Aperture Sonar (SAS) with the robustness and the reliability of side scan. The high-quality imagery is dependent on more than just high resolution. Solstice is therefore designed to provide not just high resolution but high SNR and contrast too. To achieve this, Solstice is built around five key technologies. In this interview, we’ll concentrate on Multi Ping Multi Look (MPML) and how this distinguishes Solstice MAS from SAS.
Q. Before we look at Solstice MAS and its advantages over traditional sidescan and synthetic aperture sonar systems, could you please briefly explain what these two actually are?
Sidescan sonar dates back to the 1970s and it’s become a workhorse technology used for commercial surveys or for search, classification, and mapping type MCM operations. It creates useful imagery of the seabed and objects which lie on it.
The longer the array of hydrophones or aperture used by the sidescan sonar, the better the picture resolution. Higher operating frequencies create higher resolution imagery, but that’s at the expense of range. Synthetic aperture sonar, or SAS, seeks to improve this resolution by synthesizing an aperture in the signal processing far longer than the actual physical sonar array. Also, SAS tends to operate at a considerably lower frequency, which helps extend its range.
Q. What experience do Forcys have in this technology?
Our engineers at Forcys have considerable experience in designing and developing synthetic aperture sonar systems. Two of our principal designers led the hardware development for NATO’s so-called MUSCLE SAS program and also led the development of a SAS system for a major international defense company.
Forcys, through Wavefront Systems, acted in a design consultancy role for several other SAS projects over the past decade. From this experience, we’ve learned that SAS can be very effective, but it is not without its drawbacks. It’s relatively expensive, heavy, and power-hungry, and in some fairly commonplace scenarios, SAS can be fragile.
Q. What do you mean when you say SAS can be fragile?
SAS can produce impressive results in the right conditions, such as deeper deployments away from the surface effects or when deployed on larger, more stable platforms. However, SAS can struggle in very shallow water—less than 40 feet deep—or when deployed on smaller, say 9 or 12-inch diameter AUVs. In these situations, feedback from users suggests SAS performance can degrade significantly.
Q. And what would that degradation look like and why does it occur?
Some SAS systems can compromise as much as 50% of their claimed swath in shallow water, or default to the poor resolution associated with its real aperture length when the coherent processing fails. The quality of this data is rarely of operational use and missions have been compromised as a result.
As to the why: SAS performance is adversely affected by higher-order multipath interference commonly encountered in shallow water scenarios. Its performance is degraded by unknown or dynamic sound velocity profiles. It demands high-accuracy bathymetry, without which non-linear platform trajectories will not produce focused images, and it struggles to provide reliable performance, particularly in high cross-currents, due to the impact these have on the SAS micro-navigation.
Now, some of these issues aren’t unique to SAS, of course, but because SAS seeks to extend the range of conventional sidescan sonar, they have a far greater significance for SAS.
Q. And what are the associated operational issues with SAS?
Large, heavy, power-hungry systems; complex mission planning due to its achievable range being dependent upon its speed—with higher speeds reducing available ranges—and often unmanageable quantities of real-time data making real-time processing problematic.
Q. Okay, so tell us, what did you do?
We decided to completely rethink the sidescan tech from the bottom up with the aim of developing a sensor which blended the high resolution of SAS with the robustness and reliability of sidescan.
Q. So, tell us more about Solstice.
Well, high-quality imagery is dependent on more than just high resolution. Solstice is therefore designed to provide not just high-res, but high SNR (signal-to-noise ratio) and contrast. To achieve this, we designed Solstice around five key technologies: MSAT (Multipath Suppression Array Technology), RAC (Real-time Auto Calibration), Motion Compensation, Pixel Perfect Imaging, and last but certainly not least, multi-ping multi-look.
Q. Is this last core technology the one that most clearly distinguishes MAS from SAS?
Yes, it’s what makes MAS unique and distinct from SAS for sure. Our multi-ping multi-look tech incoherently combines returns from multiple pings to greatly enhance the image signal-to-noise ratio, which in turn greatly reduces the distracting speckle-type noise so common in SAS imagery.
This ability to integrate incoherently allows our multi-aperture processing to be far less affected by navigational inaccuracies. This makes Solstice’s imaging performance in shallow water environments on smaller, less stable vehicles far more robust.
Of course, design decisions like this come with trade-offs. In this case, incoherent multi-aperture processing doesn’t increase the image resolution as the multiple apertures are processed, but MAS largely offsets this effect by using a much higher operating frequency than a typical SAS. Its natural real aperture resolution is therefore much better—a better starting point, you might say.
Q. Just to be clear, can you explain precisely what you mean by coherent and incoherent processing, and how are they different?
So, SAS coherent processing uses both the signal phase and amplitude information. Multi-ping multi-look uses incoherent processing, meaning only the amplitude is used for processing of the multiple apertures.
Q. Now what are the operational advantages of Solstice MAS?
Solstice has the ability to image large areas of the seabed at significantly higher ground speeds than SAS. Its low 20-watt power draw dramatically extends search, classify, map mission durations for AUVs, and this allows the sonar to be used alongside identification systems such as Voyis’s laser line scan in our latest L3Harris IVER4 Recon module.
Mission planning is simplified due to the constant range swath, and that all the advanced MAS processing can be performed on board the vehicle itself in real time, producing manageable quantities of data that are available for third-party software packages such as automated target recognition algorithms.
Q. Are you continuing to develop Solstice and can you share any future developments?
We pride ourselves on maintaining relationships with our end customers, listening to their real-world experiences of sensor tech, and using this information to help guide our development and improve our products.
The latest addition to the Solstice family is a bigger brother, Solstice MAS 4000. It’s now being sea trialled and has already achieved SAS-like along-track resolution within a power budget of just 24 watts.
