Synthetic-aperture sonar

Last updated
Synthetic aperture sonar imagery of the German submarine U-853 U-853 SAS Image.jpg
Synthetic aperture sonar imagery of the German submarine U-853

Synthetic-aperture sonar (SAS) is a form of sonar in which sophisticated post-processing of sonar data is used in ways closely analogous to synthetic-aperture radar.

Contents

Synthetic-aperture sonars combine a number of acoustic pings to form an image with much higher along-track resolution than conventional sonars. The along-track resolution can approach half the length of one sonar element, though is downward limited by 1/4 wavelength. [1]

The principle of synthetic-aperture sonar is to move the sonar while illuminating the same spot on the sea floor with several pings. 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. At maximum range the resolution can be magnitudes better than that of side-scan sonars. [2]

A 2013 technology review [3] with examples and future trends is also available. 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.

The length of the synthetic aperture is

Where R is the range, is the wavelength at center frequency and d is the along-track element size in the array. is a programmable parameter which controls the process beamwidth—the beamwidth actually processed. [1]

Challenges

The SAS system relies on a stable sensor platform, being able to determine to a high accuracy where the sensors are over several meters of travel distance—all the pings captured will be used in the formation of a synthetic aperture. 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. Mission planning and selection of sensor platform can alleviate some of these challenges.

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.

Comparison between SSS and SAS

Traditional side scan sonars (SSS) have along-track resolution, along-track sampling and range closely coupled. 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. [5]

Along-track resolution

Along-track 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.

Along-track resolution is constant at all ranges for a synthetic-aperture sonar system, this means an object should be equally visible at most ranges from the sensor. [6]

Where is the range to target, is the array length, and is the acoustic wavelength, a function of frequency. 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.

Across-track resolution

The across-track (range) resolution of a SAS, with a broadband FM signal, is given by:

Where is the speed of sound in water and is the bandwidth of the transmitted pulse.

Range

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. Transmit frequency is one of the primary factors, and maximum imaging ranges are commonly from 100 meters (220-280 kHz) for HiSAS 2040 up to and beyond 300 meters (60-120 kHz) for HiSAS 1030 in commercially available sonars, depending on configuration. The synthetic-aperture sonars as installed on an Autonomous underwater vehicle or Towed array sonar do commonly have a Nadir gap, as is also the case in traditional side-scan sonars, where no data is available. The size of this gap depends on the lower beam angle. 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.

With frequency sufficiently low to allow reception from maximum range, the ground range is determined by receiver array length and platform speed v:

Where is an overlap factor chosen to allow for ping to ping cross-correlation, is depression angle at maximum range. [7]

Post-processing

Traditional sidescan-sonar is normally available immediately after capturing without any further processing needed, while synthetic-aperture sonars depend on complex post-processing done on powerful computers, increasing the time from data capture to analysis. 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. This also means that data storage rates needed for SAS are profound, from 60 to 90 GB per hour of raw data is common.

Area coverage

Area coverage is one of the most important factors in commercial applications of hydro-acoustics. For both SSS and SAS systems, the instantaneous area coverage for a two-sided system (i.e both port and starboard sensor) is:

Where is the max ground range and is the shortest ground range before the Nadir gap, and is the speed of the sonar. The actual area coverage is somewhat less than this. [7]

Area coverage with a traditional side-scan sonar depends on range and at what range the resolution gets too low for the target goal of the scan. Area coverage with a synthetic-aperture sonar, with an across-track resolution that is constant all the way until the end of the range, is practically closer to the instantaneous area coverage.

Military applications

Synthetic-aperture sonar deployed from autonomous underwater vehicles has proven useful for detecting unexploded ordnance [8] [9] as well as naval mines. [10]

Civilian applications

Synthetic-aperture sonar deployed from autonomous underwater vehicles has been used to find sunken ships and debris. It was among several sensor types used in the search for Malaysia Airlines Flight 370.

This type of sonar is also starting to see use in ocean research. NOAA, Kraken Robotics and ThayerMahan conducted a joint technology demonstration in 2019, [11] where synthetic-aperture sonar was one of the technologies demonstrated.

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. [13]

See also

Related Research Articles

<span class="mw-page-title-main">Radar</span> Object detection system using radio waves

Radar is a system that uses radio waves to determine the distance (ranging), direction, and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track aircraft, ships, spacecraft, guided missiles, motor vehicles, map weather formations, and terrain.

<span class="mw-page-title-main">Sonar</span> Acoustic sensing method

Sonar is a technique that uses sound propagation to navigate, measure distances (ranging), communicate with or detect objects on or under the surface of the water, such as other vessels.

<span class="mw-page-title-main">Phased array</span> Array of antennas creating a steerable beam

In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving the antennas. The general theory of an electromagnetic phased array also finds applications in ultrasonic and medical imaging application and in optics optical phased array.

<span class="mw-page-title-main">Side-scan sonar</span> Tool for seafloor mapping

Side-scan sonar is a category of sonar system that is used to efficiently create an image of large areas of the sea floor.

<span class="mw-page-title-main">Echo sounding</span> Measuring the depth of water by transmitting sound waves into water and timing the return

Echo sounding or depth sounding is the use of sonar for ranging, normally to determine the depth of water (bathymetry). It involves transmitting acoustic waves into water and recording the time interval between emission and return of a pulse; the resulting time of flight, along with knowledge of the speed of sound in water, allows determining the distance between sonar and target. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes.

<span class="mw-page-title-main">Hydrographic survey</span> Science of measurement and description of features which affect maritime activities

Hydrographic survey is the science of measurement and description of features which affect maritime navigation, marine construction, dredging, offshore wind farms, offshore oil exploration and drilling and related activities. Surveys may also be conducted to determine the route of subsea cables such as telecommunications cables, cables associated with wind farms, and HVDC power cables. Strong emphasis is placed on soundings, shorelines, tides, currents, seabed and submerged obstructions that relate to the previously mentioned activities. The term hydrography is used synonymously to describe maritime cartography, which in the final stages of the hydrographic process uses the raw data collected through hydrographic survey into information usable by the end user.

<span class="mw-page-title-main">Synthetic-aperture radar</span> Form of radar used to create images of landscapes

Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide finer spatial resolution than conventional stationary beam-scanning radars. SAR is typically mounted on a moving platform, such as an aircraft or spacecraft, and has its origins in an advanced form of side looking airborne radar (SLAR). The distance the SAR device travels over a target during the period when the target scene is illuminated creates the large synthetic antenna aperture. Typically, the larger the aperture, the higher the image resolution will be, regardless of whether the aperture is physical or synthetic – this allows SAR to create high-resolution images with comparatively small physical antennas. For a fixed antenna size and orientation, objects which are further away remain illuminated longer – therefore SAR has the property of creating larger synthetic apertures for more distant objects, which results in a consistent spatial resolution over a range of viewing distances.

<span class="mw-page-title-main">Sensor array</span> Group of sensors used to increase gain or dimensionality over a single sensor

A sensor array is a group of sensors, usually deployed in a certain geometry pattern, used for collecting and processing electromagnetic or acoustic signals. The advantage of using a sensor array over using a single sensor lies in the fact that an array adds new dimensions to the observation, helping to estimate more parameters and improve the estimation performance. For example an array of radio antenna elements used for beamforming can increase antenna gain in the direction of the signal while decreasing the gain in other directions, i.e., increasing signal-to-noise ratio (SNR) by amplifying the signal coherently. Another example of sensor array application is to estimate the direction of arrival of impinging electromagnetic waves. The related processing method is called array signal processing. A third examples includes chemical sensor arrays, which utilize multiple chemical sensors for fingerprint detection in complex mixtures or sensing environments. Application examples of array signal processing include radar/sonar, wireless communications, seismology, machine condition monitoring, astronomical observations fault diagnosis, etc.

<span class="mw-page-title-main">Beamforming</span> Signal processing technique used in sensor arrays for directional signal transmission or reception

Beamforming or spatial filtering is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in an antenna array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. The improvement compared with omnidirectional reception/transmission is known as the directivity of the array.

<span class="mw-page-title-main">Towed array sonar</span> System of hydrophones

A towed array sonar is a system of hydrophones towed behind a submarine or a surface ship on a cable. Trailing the hydrophones behind the vessel, on a cable that can be kilometers long, keeps the array's sensors away from the ship's own noise sources, greatly improving its signal-to-noise ratio, and hence the effectiveness of detecting and tracking faint contacts, such as quiet, low noise-emitting submarine threats, or seismic signals.

<span class="mw-page-title-main">AN/UQQ-2 Surveillance Towed Array Sensor System</span> Towed array sonar system

The AN/UQQ-2 Surveillance Towed Array Sensor System (SURTASS), colloquially referred to as the ship's "Tail", is a towed array sonar system of the United States Navy.

Radar engineering is the design of technical aspects pertaining to the components of a radar and their ability to detect the return energy from moving scatterers — determining an object's position or obstruction in the environment. This includes field of view in terms of solid angle and maximum unambiguous range and velocity, as well as angular, range and velocity resolution. Radar sensors are classified by application, architecture, radar mode, platform, and propagation window.

Geophysical MASINT is a branch of Measurement and Signature Intelligence (MASINT) that involves phenomena transmitted through the earth and manmade structures including emitted or reflected sounds, pressure waves, vibrations, and magnetic field or ionosphere disturbances.

Bistatic sonar is a sonar configuration in which transmitter and receiver are separated by a distance large enough to be comparable to the distance to the target. Most sonar systems are monostatic, in that the transmitter and receiver are located in the same place. A configuration with multiple receivers is called multistatic.

Leaky-wave antenna (LWA) belong to the more general class of traveling wave antenna, that use a traveling wave on a guiding structure as the main radiating mechanism. Traveling-wave antenna fall into two general categories, slow-wave antennas and fast-wave antennas, which are usually referred to as leaky-wave antennas.

<span class="mw-page-title-main">Side looking airborne radar</span>

Side-looking airborne radar (SLAR) is an aircraft- or satellite-mounted imaging radar pointing perpendicular to the direction of flight. A squinted (nonperpendicular) mode is also possible. SLAR can be fitted with a standard antenna or an antenna using synthetic aperture.

Underwater searches are procedures to find a known or suspected target object or objects in a specified search area under water. They may be carried out underwater by divers, manned submersibles, remotely operated underwater vehicles, or autonomous underwater vehicles, or from the surface by other agents, including surface vessels, aircraft and cadaver dogs.

A track algorithm is a radar and sonar performance enhancement strategy. Tracking algorithms provide the ability to predict future position of multiple moving objects based on the history of the individual positions being reported by sensor systems.

Synthetic aperture ultrasound (SAU) imaging is an advanced form of imaging technology used to form high-resolution images in biomedical ultrasound systems. Ultrasound imaging has become an important and popular medical imaging method, as it is safer and more economical than computer tomography (CT) and magnetic resonance imaging (MRI).

<span class="mw-page-title-main">High Resolution Wide Swath SAR imaging</span>

High Resolution Wide Swath (HRWS) imaging is an important branch in synthetic aperture radar (SAR) imaging, a remote sensing technique capable of providing high resolution images independent of weather conditions and sunlight illumination. This makes SAR very attractive for the systematic observation of dynamic processes on the Earth's surface, which is useful for environmental monitoring, earth resource mapping and military systems.

References

  1. 1 2 Edgar, Roy (2011-09-12), Kolev, Nikolai (ed.), "Introduction to Synthetic Aperture Sonar", Sonar Systems, InTech, doi: 10.5772/23122 , ISBN   978-953-307-345-3 , retrieved 2024-01-23
  2. "Exploration Tools: Synthetic Aperture Sonar: NOAA Office of Ocean Exploration and Research". oceanexplorer.noaa.gov. Retrieved 2024-01-23.
  3. R. E. Hansen, Synthetic Aperture Sonar Technology Review, Marine Technology Society Journal, Volume 47, Number 5, September/October 2013, pp. 117-127
  4. M. P. Hayes and P. T. Gough, Synthetic Aperture Sonar: A Review of Current Status, IEEE J. Ocean. Eng., vol. 34, no. 3, pp. 207-224, July 2009. Access abstract.
  5. Hagen, Per Espen; Hansen, Roy Edgar (October 2009). "Robust synthetic aperture sonar operation for AUVs". Oceans 2009. IEEE: 1–6. doi:10.23919/oceans.2009.5422342. ISBN   978-1-4244-4960-6. S2CID   19698810.
  6. Dillon, Jeremy; Charron, Richard (October 2019). Resolution Measurement for Synthetic Aperture Sonar. IEEE. pp. 1–6. doi:10.23919/OCEANS40490.2019.8962823. ISBN   978-0-578-57618-3. S2CID   209454899.
  7. 1 2 Hagen, Per Espen; Hansen, Roy Edgar (June 2007). "Area Coverage Rate of Synthetic Aperture Sonars". OCEANS 2007 - Europe. IEEE. pp. 1–5. doi:10.1109/oceanse.2007.4302382. ISBN   978-1-4244-0634-0. S2CID   10724314.
  8. Saebo, Torstein Olsmo; Hansen, Roy Edgar; Lorentzen, Ole Jacob (October 2015). "Using an interferometric synthetic aperture sonar to inspect the Skagerrak World War II chemical munitions dump site". OCEANS 2015 - MTS/IEEE Washington. IEEE: 1–10. doi:10.23919/oceans.2015.7401927. ISBN   978-0-9339-5743-5. S2CID   28576855.
  9. Hansen, R.; Geilhufe, Marc; Bakken, E.; Saebo, T. O. (2019-11-28). "Comparison of synthetic aperture sonar images and optical images of UXOs from the Skagerrak chemical munitions dumpsite". S2CID   222226105.{{cite journal}}: Cite journal requires |journal= (help)
  10. Hagen, Per Espen; Størkersen, Nils; Marthinsen, Bjørn-Erik; Sten, Geir; Vestgård, Karstein (January 2008). "Rapid environmental assessment with autonomous underwater vehicles — Examples from HUGIN operations". Journal of Marine Systems. 69 (1–2): 137–145. Bibcode:2008JMS....69..137H. doi:10.1016/j.jmarsys.2007.02.011. ISSN   0924-7963.
  11. US Department of Commerce, National Oceanic and Atmospheric Administration. "2019 Technology Demonstration: NOAA Office of Ocean Exploration and Research". oceanexplorer.noaa.gov. Retrieved 2024-01-23.
  12. Blomberg, Ann Elisabeth Albright; Saebo, Torstein Olsmo; Hansen, Roy Edgar; Pedersen, Rolf Birger; Austeng, Andreas (July 2017). "Automatic Detection of Marine Gas Seeps Using an Interferometric Sidescan Sonar". IEEE Journal of Oceanic Engineering. 42 (3): 590–602. Bibcode:2017IJOE...42..590B. doi:10.1109/JOE.2016.2592559. ISSN   0364-9059. S2CID   26080347.
  13. Moland, Even; Fernández-Chacón, Albert; Sørdalen, Tonje Knutsen; Villegas-Ríos, David; Thorbjørnsen, Susanna Huneide; Halvorsen, Kim Tallaksen; Huserbråten, Mats; Olsen, Esben Moland; Nillos Kleiven, Portia Joy; Kleiven, Alf Ring; Knutsen, Halvor; Espeland, Sigurd Heiberg; Freitas, Carla; Knutsen, Jan Atle (2021-07-08). "Restoration of Abundance and Dynamics of Coastal Fish and Lobster Within Northern Marine Protected Areas Across Two Decades". Frontiers in Marine Science. 8. doi: 10.3389/fmars.2021.674756 . hdl: 10261/254615 . ISSN   2296-7745.