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Seeing with Sound

 

A number of different activities rely on accurate seabed imaging and mapping. Offshore exploration, ocean science, seabed mapping, environmental surveillance and military missions are all examples of such applications with increasing demands for high area coverage rates and high resolution seabed imagery.

Until recently, sidescan sonars and multibeam echo-sounders have been the leading technology for detailed mapping and imaging of the seabed. However, a new technology called Synthetic Aperture Sonars (SAS) provides ultra-high image resolution combined with very efficient area coverage rates. In many applications, SAS provides over 25 times greater image resolution with 300-percent increase in area coverage compared to conventional sidescan sonars.

SAS was initially developed for demanding military applications such as naval mine detection and classification. As SAS technology becomes more affordable, it will find increasing use in civilian markets and become a valuable supplement to, and in some cases, a replacement for existing sonar technology.

Kraken has successfully developed an ultra-high resolution Interferometric Synthetic Aperture Sonar (InSAS) with 3D bathymetric capabilities called AquaPix®.

AquaPix® is capable of providing detailed images with an along-track/across-track resolution better than 3cm out to a range of 300m from each side of an underwater vehicle (600m swath). It can also produce bathymetric data with a resolution better than 25cm out to full range while delivering very high depth accuracy, in compliance with IHO S44 special order requirements.

AquaPix® is primarily designed for use onboard Autonomous Underwater Vehicles (AUVs), Remotely Operated Tow Vehicles (ROTVs), Remotely Operated Vehicles (ROVs) and Tow Bodies.

Interferometric SAS is strongly related to its airborne cousin - interferometric Synthetic Aperture Radar (SAR). While interferometric SAR has transitioned into a commercial off-the-shelf product, interferometric SAS has for a long time remained at the research stage. Some of the reasons for this delay have been the challenges in obtaining very high navigation accuracy through the ocean, as well as the high-computational cost of SAS imaging software.

The introduction of stable Unmanned Underwater Vehicles (UUVs), cheaper and more powerful data- collection and processing electronics, combined with advanced micro-navigation and auto-positioning methods has recently brought Interferometric SAS forward as a viable alternative to Side Scan Sonars and Multibeam Echo-Sounders for seabed imaging.

Interferometric SAS hardware (transducer arrays and electronics), image processing and Interferometric SAS (InSAS) processing have been a research topic at the NATO Undersea Research Centre (NURC) in La Spezia, Italy for many years.

Kraken was able to leverage this research during the development of AquaPix® – the world’s most advanced Interferometric Synthetic Aperture Sonar (InSAS) - capable of generating practical image resolutions of 3cm across swath widths of 600m. In parallel with the hardware design, senior sonar scientists at Kraken have developed a complete software package for InSAS imaging called INSIGHT (INterferometric Sas Imaging Georeferenced High-fidelity Toolbox).

Both AquaPix® and INSIGHT were developed by Kraken’s team of scientists and engineers over a record time span of less than 18 months. The first system was successfully integrated and deployed onboard DRDC’s Arctic Explorer AUV in Halifax, Nova Scotia in August, 2012. All of the InSAS software processing was performed by Kraken’s INSIGHT toolbox.

Synthetic Aperture Sonar Processing

Interferometric Synthetic Aperture Sonar technology is divided into two independent techniques:

  1. Synthetic Aperture Sonar Processing
  2. Interferometric Processing

Synthetic Aperture Sonar Processing

SAS offer high resolution imagery at longer ranges than conventional side-scan sonars. This is done by replacing traditional sonar hardware with sophisticated signal processing software.

The principle of SAS is that a long receiver array, (much longer than a physical one) is "synthesized" in the along-track direction. This is achieved by coherent recombination of many pings that overlap the area of interest. Typically, a 300 kHz SAS would operate with 60 pings to form the synthetic aperture at 265m. This represents a major savings in sonar hardware and enables much higher resolution than is otherwise possible with conventional 300 kHz sonars.

In order to make SAS work, the relative position of the sensors must be known to within a fraction of a wavelength (typically 1/16 λ required) over the entire synthetic aperture. At 300 kHz, λ =5mm so this requirement is nearly impossible to meet with current navigation sensors. As the array is formed from a moving platform with uncertainties in positional accuracy, the calibration and correction of element positions is very challenging.

To overcome these challenges, Kraken has developed a number of micro-navigation techniques for this purpose that give extremely accurate positioning of the sonar array. In addition to being a requirement for SAS imagery, micro-navigation can be used to aid high-accuracy positioning of both the sonar and the underwater vehicle itself.

Once the challenge of accurately computing the positions of the sonar array is overcome, the data must also be beam-formed into a SAS image. As the synthetic aperture array may contain many hundreds of synthetic elements, this beamforming often places a heavy demand on computational processing resources. The INSIGHT toolbox implements beamforming techniques to obtain SAS imagery using a unique algorithm called Widebeam Azimuth Compression which has the same accuracy as much slower backprojection or delay and sum beamformers while being much faster, allowing faster-than- real time processing rates using general purpose graphical processing units. Kraken is also undertaking development of an Embedded Real Time SAS Processor. This will significantly reduce post mission analysis requirements while increasing operational effectiveness and efficiency in seabed imaging applications.

Interferometric Processing

An interferometric sensor such as the AquaPix® system can be used to estimate the relative seafloor height (which can then be processed into bathymetry maps). The vertical direction to the seafloor is calculated from the estimated time difference of arrival from the two vertically displaced receiver arrays. Interferometry operates through noting that relative height differences give a phase shift at the carrier wavelength.

Interferometric height-estimation can give height estimates at the same resolution as the original imagery. One of the biggest challenges is in resolving ambiguities caused by the cyclic phase shift measurements, i.e. unwrapping the phase estimates. The number of phase wraps is dependent on the carrier wavelength and the interferometric baseline. However, the large bandwidth employed in the AquaPix® system is used to resolve these ambiguities. This allows rapid collection of large swathes of bathymetric information at low processing cost, while simultaneously allowing extremely detailed bathymetric imagery to be generated for the selected areas of interest.

Examples of SAS imagery and 3D bathymetric images are shown below. These results are from sea-trials carried out with the AquaPix® system installed on DRDC’s Arctic Explorer AUV from August 7 – 16, 2012 in the area of Bedford Basin in Halifax, Nova Scotia. The AUV was typically operated at 10m altitude over a 20 - 50m deep seafloor.

Operational sea trial area near the Bedford Institute of Oceanography

Example of SAS imagery from a single pass of the survey area, showing a small rock.

3D bathymetry of the survey area, showing small rock elevation and seabed slope.

Imagery of Volvos that were dumped in Halifax Harbour in 1969.

Imagery of sea floor at 180m range.

Imagery of sea floor at 220m range. Visible seabed scour mark.

AquaPix® Provides a Compelling Price and Performance Value Proposition

AquaPix® utilizes military-proven SAS algorithms and robust sonar designs that feature advanced signal processing software and the latest in micro-electronics technology. The system provides very cost-effective and ultra-high resolution imagery that’s ideal for mine countermeasures, Q-route surveys, wreck searches, cable route survey, pipeline survey and a wide variety of other seabed imaging missions.

Features & Capabilities

  • Ultra-high resolution - 3cm across entire swath
  • Superior area coverage rate
  • Swath widths of up to 600m – 15x sonar altitude
  • Low to high speed operation
  • Shallow water to full ocean depth (option)
  • Co-registered imagery and 3D bathymetry
  • No need for expensive inertial navigation system
  • Advanced sonar array design mitigates multipath
  • Very tolerant of UUV yaw / crabbing action

Operational Benefits

  • Lowest total cost of ownership
  • Modular design integrates to AUV, ROV, ROTV and towfish
  • Superior image resolution and area coverage rate over SSS
  • High speed collection of geo-referenced data
  • Compliant to IHO SP44 hydrographic survey standard
  • Interfaces to industry-standard visualization software
  • SAS image processing operates 2x real-time
  • Enables in-stride ATR for enhanced operational performance
  • Robust sonar design uses latest technologies