Synthetic aperture sonar
Synthetic aperture sonar (SAS) is a form of sonar in which sophisticated post-processing of sonar data are used in ways closely analogous to synthetic aperture radar. 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. 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 constitutes 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, SAS processing provides range-independent along-track resolution. At maximum range the resolution can be magnitudes better than that of side-scan sonars.
For further reading, the Open Access Article: Introduction to Synthetic Aperture Sonar can be recommended as introduction for people familiar with Sonar Systems. A 2013 technology review  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 gives an overview of the history and an extensive list of references for the community achievements up to 2009.
- Roy Edgar Hansen (2011), Introduction to Synthetic Aperture Sonar, Sonar Systems, N. Z. Kolev (Ed.), ISBN 978-953-307-345-3, InTech. Open access article, available from: http://www.intechopen.com/articles/show/title/introduction-to-synthetic-aperture-sonar.
- R. E. Hansen, Synthetic Aperture Sonar Technology Review, Marine Technology Society Journal, Volume 47, Number 5, September/October 2013, pp. 117-127 
- 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.
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