Synthetic-aperture sonar

The along-track resolution can approach half the length of one sonar element, though is downward limited by 1/4 wavelength.

When moving along a straight line, those pings that have the image position within the beamwidth constitute the synthetic array.

By coherent reorganization of the data from all the pings, a synthetic-aperture image is produced with improved along-track resolution.

In contrast to conventional side-scan sonar (SSS), SAS processing provides range-independent along-track resolution.

For academics, the IEEE Journal of Oceanic Engineering article: Synthetic Aperture Sonar, A Review of Current Status[4] gives an overview of the history and an extensive list of references for the community achievements up to 2009.

Due to currents, heave or sway, a sensor platform may undergo lateral movement known as "crabbing", which have the potential to heavily impact SAS image formation.

SAS arrays may not be the best choice for a sensor platform in rough terrain nor areas where one can expect currents from the sides.

When operating a SAS system in shallow waters, multiple reflections may come back to the sensor from the sea surface, impact the quality of the data.

This also depends on the seafloor conditions, sound velocity profile as well as how rough the sea surface is.

One way to alleviate this issue is to angle the beams up slightly—to reduce reflections from the nearest bottom.

This means that the maximum range and resolution depends primarily on the transmit frequency.

A higher transmit frequency gives increased along-track resolution but reduced range.

Synthetic-aperture sonars (SAS) on the other hand, limited by cost and complexity, allows free selection of these parameters, providing the potential for long range as well as high resolution.

in a traditional side-scan sonar will deteriorate with range in the far field, an object will be imaged with a higher resolution when closer to the sensor, and less when further away.

This means that a traditional side-scan sonar with high along-track resolution will require a very long array length for a distant target.

Attenuation of the acoustic energy as frequency is increased and wavelength thus decreased, reduces the effective range.

A synthetic-aperture sonar creates a synthetic array of a long length, moving preferably in a straight line, providing a theoretical along-track resolution of a few centimeters.

In practice, resolution will be somewhat worse, but still much better than an equivalent sized traditional side-scan sonar.

The range of a synthetic-aperture sonar depends on the transmission loss of an acoustic ping as well as the number of elements in the array and the speed of the sensor platform.

In very shallow waters, multipath is another limiting factor for the range of both SSS and SAS; this effect can be reduced by carefully shaping the transmit beam pattern to avoid bouncing off pings of the surface.

Some systems allow real-time processing at a reduced resolution, which allows for in-situ mission updates based on observations, as well as providing a machine learning platform for object classification.

Synthetic-aperture sonar deployed from autonomous underwater vehicles has proven useful for detecting unexploded ordnance[8][9] as well as naval mines.

[10] Synthetic-aperture sonar deployed from autonomous underwater vehicles has been used to find sunken ships and debris.

Detection of carbon dioxide gas seeps has been using synthetic-aperture sonar coupled with advanced signal processing has been proven possible, and is an ongoing research topic.

[12] Hunting for lost fishing gear, pots and nets has been done using synthetic-aperture sonar on an AUV in Norway.