James T. Christoff

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.

Synthetic images of proud targets

COASTSYSTA has developed a 3-dimensional target strength model of an arbitrarily shaped object based upon an application of the Kirchoff approximation and the geometric theory of diffraction. A comparison between theory and experiment is made for several scale model targets. In addition it is shown how this target strength model can be incorporated in a sonar performance prediction model to generate synthetic images of proud targets. Synthetic images for both real aperture and synthetic aperture sonars are presented, and the effects of multipaths on shadows are discussed.<<ETX>>

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).

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 ...

Current development of high‐resolution MCM minehunting sonars for small AUVs

The U.S. Navy recently awarded contracts to develop a new generation of autonomous underwater vehicles (AUVs) and high‐performance acoustic sensors that are compatible with these vehicles. These new AUV systems will place minehunting crews and marine mammals well out of dangerous waters in the future. The requirement that these vehicles be small, durable, and easily launched and recovered places severe constraints on the sensor system payloads. The sensor systems must be rugged, low‐powered, and small, yet effective enough to justify the risk to the vehicle. They must also have very low false alarm rates (high clutter rejection) because reacquisition and neutralization are costly, time consuming, and closely related to the number of targets found. These new acoustic sensor systems will use new broadband techniques along with high‐resolution multi‐aspect designs to achieve their goals from such small platforms. The acoustic sensor packages will consist of advanced very high‐resolution Ahead Looking Sonars ...

Synthetic aperture sonar for small unmanned underwater vehicles

The shallow water and very shallow water environment is a very hostile region for sonar operations, however it is still considered one of the best options for mine detection and classification in this region. The water depth and small vehicle size demands that the sonar be small and low power. A conflicting need exists for very high-resolution sonar, which implies a very large aperture, that can discriminate between small mines and natural or benign manmade objects. Resolution cells of approximately 1/16 the size of the targets smallest dimension are generally considered necessary to classify a target with high confidence level and low false classification rate. The synthetic aperture sonar concept can meet the requirement for small size and high resolution. This paper will review the development of the SAS technology at CSS from a simple, single channel first generation SAS to the present real-time dual frequency multi-channel SAS. All these system were designed to be compatible with small underwater vehicles. The SAS signal processing evolved from a very simple delay and sum beamformer to a more complex motion error modulated beamformer. This paper will show images of these systems as they evolved from the earlier 15-cm resolution SAS to the present system producing 2.5 cm and 7.5-cm resolution from a dual high/low frequency SAS, respectively. The present system operates in real-time and has generated the current images using a physical array that is less than 0.6 meters in length and 0.15 meters in height.

Broadband transducer requirements and development for a synthetic aperture sonar

Broadband acoustic signal transmission and reception are key to enhancing the performance of underwater acoustical sensor systems to support functions like underwater acoustic communications and high resolution underwater imaging. In particular, synthetic aperture sonar (SAS) is an attractive technology for high‐resolution imaging because of its ability to produce better cross range resolution than that of a real aperture array sonar of the same length. In addition, SAS systems can produce a cross range resolution that is independent of frequency and range. When the advantages of a SAS system are coupled with those gained from going to a broadband signal space, the performance enhancements can be significant. Specifically improvements in clutter rejection, object classification, and area coverage rate will be achieved. A description of the transducer requirements and expected payoffs for a SAS system designed to operate and image proud and buried objects in shallow waters will be presented. [Work supporte...