Distributed Surveillance Sensor Network

Concept Overview

The purpose of the Distributed Surveillance Sensor Network (DSSN) program is to investigate the applicability of small, inexpensive undersea vehicles to surveillance applications and submarine connectivity. It is based on the concept of a fleet of autonomous undersea vehicles which gather surveillance data and communicate acoustically. Each occasionally docks at an underwater station to dump its data, recharge its batteries, receive any new mission instructions and perhaps remain dormant until its next deployment. The docking station is self powered and is not connected to shore or ship by communications cable. The massive quantity of accumulated data is retrieved at a later time by means of a Flying Plug, a remotely controlled vehicle guided to the docking station by means of a fiber optic microcable (FOMC). The FOMC is the high-bandwidth channel by which the data is recovered and instructions are downloaded to be disseminated to the surveillance fleet. The DSSN program combines the Flying Plug and FOMC, which were developed at SSC San Diego, with other concepts and technology funded by the Office of Naval Research (ONR).

Autonomous Ocean Sampling Network

The conceptual basis for a distributed array of autonomous sensors is provided by the Massachusetts Institute of Technology's Autonomous Ocean Sampling Network (AOSN). AOSN is a distributed, highly mobile, adaptive sensor network composed of a mix of autonomous underwater vehicles (AUV's) which exhibit complementary capabilities. It is being developed for oceanographic characterization. The architecture is very general, hence the mix of AUV's and their payloads can be optimized for specific mission scenarios, making the concept both highly flexible and very powerful.

The AOSN concept is predicated upon the assumption that the geometric growth in signal processing power we are experiencing at the present continues into the future. Besides increasing capabilities and driving costs down, this trend ultimately permits a single hardware device to support multiple applications. For example, digital signal processing (DSP) chips are used to compensate for multipath propagation in the current generation of acoustic modems developed for AOSN. As DSP's become more capable they will be able to support higher reliable data transfer rates. More importantly, as increased processor speed becomes commercially available enough signal processing capability will eventually exist to permit the extraction of information from the multipath signals themselves (which are currently only discriminated against). This capability configures the AUV communications network into a huge multi-static active sonar capable of detecting and localizing anomalies within the volume of seawater supporting the acoustic propagation paths. In time the same basic hardware which was originally employed for data communications can simultaneously detect mines and submarines in the water volume --- with only an upgrade in the silicon! This is a striking, but realistic, example of the efficacy of selecting a system's architecture to take maximum advantage of expected technological evolution.

The Odyssey Vehicle

Odyssey is a low-cost AUV specifically developed by the Massachusetts Institute of Technology, SeaGrant Office for the AOSN Program. Constructed to operate at full ocean depth, Odyssey was designed from the beginning to be both highly capable and inexpensive to mass-produce. At less than two meters in length and not requiring any special handling equipment for launch and recovery, Odyssey can transit at several knots for up to 20 hours due to its ultra-low hydrodynamic drag profile and efficient propulsion system, yielding a very respectable range and oceanographic mission profile. An integral part of the Odyssey is its powerful onboard computer, which is based upon a commercial 68030 processor board. This computer executes a control program based upon a flexible high-level behavioral language developed at MIT, and supports vehicle control in a wide range of conditions and mission profiles. New mission profiles are quickly configured, tested (via a simulator developed by the Charles Stark Draper Laboratory) and entered into the computer's library. A sophisticated acoustic modem (developed by the Woods Hole Oceanographic Institute) is an integral part of the system and is used to support reliable two-way digital communications. A large fraction of Odyssey's internal volume is available for mission sensors. Odyssey is a mature technology which has been successfully deployed and operated in many types of ocean environments, including the arctic.

Concept Demonstration

In FY-95, under ONR sponsorship, SSC San Diego developed an optical docking sensor with processor for installation on SSC San Diego's dedicated Odyssey vehicle (provided by SeaGrant). In March-April 1996 the system was employed in month-long AOSN vehicle docking experiments at Buzzard's Bay, MA, to demonstrate autonomous underwater docking. The ability to reliably mate with an underwater docking station provides the capability for Odyssey to recharge its internal silver-zinc batteries, giving the vehicle greatly extended endurance without requiring its return to the surface. Docking also provides an opportunity for mass data transfer to occur, permitting the vehicle to carry sensors which generate megabytes or gigabytes of data which would be impractical to transmit acoustically.

SSC San Diego was one of three participating groups bringing Odyssey vehicles outfitted with docking sensors to the test. The DSSN (SSC San Diego) approach was based upon optical guidance, the EDC (North Carolina State) system upon magnetic guidance and the Wood's Hole system employed acoustic guidance. The SSC San Diego docking system performed well in the Buzzard's Bay experiment.

Conclusion

Autonomous surveillance systems have the potential to go well beyond the capabilities of existing and even planned surveillance systems if the current paradigm can be superseded. By employing a swarm of small AUV's communicating with each other it is possible to form a distributed sensor (and possibly an effector) network. In this manner it is possible to introduce mobility, dynamic adaptability, redundancy and mutual assistance into the paradigm. Additionally, the mix of AUV's and their payloads becomes a system parameter which can be optimized for prosecution of specific missions. Finally, with cost/performance-ratio anticipated to be the overriding figure of merit for future Navy systems, numerous small, mass-produced AUV's have an inherent advantage over a few large, expensive (and, per unit, more capable) AUV's in many, if not most, mission scenarios. This is particularly true when the large AUV has become so capable (and therefore so expensive) that it is too valuable an asset to be put at-risk, and therefore may not be used in many missions.

The Navy will benefit from surveillance system architectures which intelligently exploit what are expected to remain the US primary commercial technology thrusts for the next decade or more: microelectronics, networking (both signal processing and data communications), robotics and automated mass-production. This is so because such systems are likely to be cost-effective to build and operate. The DSSN architecture is optimally positioned to take full advantage of commercial trends because of its distributed, modular nature and its adaptability to new technologies and economy of scale.