Jose E. Fernandez

High-frequency/low-frequency synthetic aperture sonar

The HF/LF SAS is a high resolution SAS developed by COASTSYSTA and Northrop Grumman for the shallow water (SW) and very shallow water (VSW) regimes. This sonar suite has recently been delivered to COASTSYSTA and it is currently undergoing field testing. This article describes this sonar and the type of resolution and acoustical images which are expected from this sonar. The application of this sonar to the SW and VSW regimes required the development of a novel method of motion compensation. A description of this method and the type of accuracys expected from this technique are presented. Finally, a look at future broad band system and their predicted performance is presented.

Synthetic aperture sonar on AUV

Synthetic aperture sonars (SAS) and autonomous underwater vehicle (AUV) are two of the most exciting technologies under development by the underwater research and development community. Integration of these technologies promises to produce underwater acoustic imaging systems of tremendous capability and versatility. Currently, several efforts, mostly motivated by military applications, are under way to achieve the realization of these systems. This paper describes one such effort, in which the Coastal Systems Station (CSS) SAS system has been integrated with the Bluefin Reliant AUV. Included in this paper are descriptions of the CSS SAS and Bluefin AUV technology and hardware and of the data processing methods. Also presented are the preliminary results obtained during the first system tests carried out at CSS, Panama City, Florida in March 2003.

Results from a Small Synthetic Aperture Sonar

A Synthetic Aperture Sonar (SAS) is capable of producing range-independent high-resolution imagery from an array which is small in length. The ability of these systems to operate at lower frequencies while maintaining high resolution has made them useful for mapping and searching large areas. The US Navy is now focusing on developing SAS systems into a form robust enough for deployment. Here, we will show results from several sea tests and describe one such system known as the Small Synthetic Aperture Minehunter (SSAM)

Multi-aspect synthetic aperture sonar

Multi-aspect or multi-look processing is known to improve the image quality and target detection capability of high resolution synthetic aperture sonar (SAS) imaging systems. We present in this paper a brief description on the topic and typical results obtained from sea data collected while fielding the High Frequency/Low Frequency Synthetic Aperture Sonar (HF/LFSAS).

Advanced sonar technologies for autonomous mine countermeasures

The U.S. Office of Naval Research is developing synthetic aperture sonars for the detection, localization, and classification of mines, for protection of sea lines of communication and Naval operating areas, and for support of amphibious operations. This paper reviews several evolving mine countermeasures sonars deployed on autonomous undersea vehicles that operate stand-alone and in coordination with complementary sensors, including the Small Synthetic Aperture Minehunter (SSAM), the Autonomous Topographic & Large Area Survey (ATLAS) sonar, and the Buried Object Scanning Sonar (BOSS).

Synthetic aperture sonar development for autonomous underwater vehicles

The combination of synthetic aperture sonar (SAS) and autonomous underwater vehicle (AUV) technologies is producing underwater acoustic imaging systems of tremendous capability and versatility. Under the sponsorship of the Office of Naval Research (ONR), Naval Surface Warfare Center, Panama City (NSWC-PC) and Applied Research Laboratory, Pennsylvania State University (ARL/PSU) are designing and fabricating 21-inch and 12.75-inch diameter AUV compatible Synthetic Aperture Sonar (SAS) systems. These systems are to be integrated and tested with 21-inch and 12.75-inch diameter AUVs built by Bluefin Robotics and Woods Hole Oceanographic Institution (WHOI). This paper provides an overview of the systems and their developmental status. Included is a general description of the motion estimation and compensation technique currently in use and of its potential use for improving the navigational performance of the system. In addition, considerations for cooperative operations between AUV systems are discussed. Examples are given to demonstrate the various points presented

Motion compensation of AUV-based synthetic aperture sonar

To date, ocean-going synthetic aperture sonar (SAS) systems have been deployed exclusively in a configuration where the sonar instrument is housed in a towed body that receives power from and exchanges information with the vessel to which it is attached. Meanwhile, recent years have witnessed the beginnings of maturity with respect to both SAS and autonomous underwater vehicle (AUV) technologies. In order to move away from the towed sonar paradigm, the Coastal Systems Station has recently taken delivery of and begun using the first AUV-based SAS. The AUV was manufactured by Blue n Robotics and the sonar used on this vehicle is the existing CSS LF/HF SAS. This transition is not without its challenges, however, as the operation and dynamic behavior of an AUV is different from that of a towed body. In general, the AUV configuration makes the problem of unwanted platform motion more severe and more difficult to solve. This paper discusses motion compensation in the context of initial evaluations of the performance of the CSS AUV-based SAS system.

Unmanned underwater vehicle broadband synthetic aperture sonar

The experimental synthetic aperture sonar (SAS) in use at the US Navy Coastal Systems Station (CSS) in Panama City, Florida, was first fielded in 1996. Since then, the system has proved itself to be a reliable high-quality sensor. The original configuration provides for simultaneous imaging at two frequency bands; one centered at 20 kHz (10 kHz bandwidth) and the other at 180 kHz (30 kHz bandwidth). The CSS SAS has recently been upgraded with the addition of a new broadband low-frequency transmitter capable of producing a linear FM chirp from 8-55 kHz. This paper discusses the details of the system upgrade and some of the signal processing opportunities afforded by it.

Motion compensation technique for wide beam synthetic aperture sonar

Optimal performance of synthetic aperture sonar (SAS) systems requires accurate motion and medium compensation. Any uncorrected deviations from those assumed during the SAS beam formation process can degrade the beam pattern of the SA in various ways (broadening and distortion of the main lobe, increased side lobes and grating lobes levels, etc.). These would manifest in the imagery in the form of degraded resolution, blurring, target ghosts, etc. An accurate technique capable of estimating motion and medium fluctuations has been developed. The concept is to adaptively track a small patch on the sea bottom, which is in the order of a resolution cell, by steering the SAS beam as the platform moves in its trajectory. Any path length differences to that patch (other than the quadratic function product of the steering process) will be due to relative displacements caused by motion and/or medium fluctuations and can be detected by cross‐correlation methods. This technique has advantages over other data driven ...

Results from a hybrid synthetic aperture sonar motion estimation scheme

Synthetic aperture sonar (SAS) is used to produce high-quality side-scan imagery of the sea floor in which the along-track resolution is constant with range. SAS sensor technology is transitioning from towed vehicles toward more versatile autonomous underwater vehicles (AUVs). Experience to date has shown that the 21-inch AUVs tend to be less stable than the longer towed vehicle of the same diameter used during the U.S. Navys initial SAS technology demo efforts. The corresponding increase in undesirable motion of the AUV platform carrying the sonar has motivated the development of a new generation of motion estimation and compensation schemes. This paper describes the results obtained from one such motion estimation method applied to the U.S. Navys SAS21/Reliant SAS system. The data set used for evaluating the image improvement consists of 31 km of track taken at the 2004 Combined Joint Task Force Exercise (CJTFEX-04) off the North Carolina coast. The motion estimation combines the angular velocity information provided from an inertial navigation unit with the traditional redundant phase center approach in order to accurately estimate the ping-to-ping translation of the SAS array. This approach consistently yields better imagery compared to the previously-used technique.