Synthetic Aperture Sonar
Synthetic aperture sonar achieves significantly higher resolution than conventional sidescan sonars by coherently combining the returns from a number of pings in order to form a large virtual receiver aperture from a relatively small physical receiver aperture. This results in the following key advantages:
- Approximately 10 times the along-track resolution for a given maximum range compared to sidescan
- Resolution is constant with range
- Images are output fully geo-located, requiring little post-processing
Key Challenges
SAS poses two key challenges that must be solved in order to achieve a useful system: platform motion estimation and fast, flexible image formation.
Platform Motion Estimation
Coherent summation of the pings is only possible if the motion of the platform is known with sufficient accuracy. We use proprietary techniques to estimate sway and surge from the acoustic returns. We are also able to estimate yaw acoustically, but better performance can be achieved with the use of a yaw input from an INS or MRU. Latitude and Longitude data is also required for geo-referencing the output, but is not essential for image formation. No markers on the seabed or external location ‘beacons’ are required. See this page for more detail of the effects of imprecise platform motion information on the SAS image.
Image Formation
Image formation by simple direct back-projection provides ultimate image quality and flexibility, but is prohibitively computationally expensive for real-time processing. A number of algorithms from the SAR community ease the computational burden, but at the expense of severe constraints on the allowable platform motion. We employ proprietary algorithms and state-of-the art implementation technologies to tackle the problem without imposing motion constraints. Constraints on the motion of the platform are then solely down to the capabilities of the motion estimation algorithms.
Sonar Requirements
While there is no such thing as a "typical SAS sonar", we can make some statements about the requirements of a sonar system that is capable of SAS:
- Broadband to achieve high range resolution and enable acoustic motion estimation
- Complex baseband sampling at or above the Nyquist rate
- Broad transmit beam width, to isonify a given area with many pings
- Replica of transmitted pulse available, or data supplied pulse compressed
- Multi-element receiver array with data recorded from each channel
- Properly synchronised sampling of receiver channels and known delay between transmission and sampling
- Along-track resolution is approximately limited to the receiver element spacing
- Platform along-track velocity and physical array length must be such that there is some overlap between array positions on consecutive pings
For example:
- 150 kHz centre frequency
- 60 kHz bandwidth
- 20 degree transmitter beamwidth
