Ocean Surveillance Research Articles

Model-based processing for a large aperture array

A model-based approach to solve a deep water ocean acoustic signal processing problem based on a state-space representation of the normal-mode propagation model is developed. The design of a model-based processor (MBP) for signal enhancement employing an array consisting of a large number of sensors for a deep ocean surveillance operation is discussed. The processor provides enhanced estimates of the measured pressure-field, modes, and residual (innovations) sequence indicating the performance or adequacy of the propagation model relative to the data. It is shown that due to the structure of the normal-mode model the state-space propagator is not only feasible for this large scale problem, but in fact, can be implemented by a set of decoupled parallel second-order processors, implying a real-time capability. In the paper we discuss the design and application of the processor to a realistic set of simulated pressure-field data developed from a set of experiments and sound speed parameters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Long-term ELF background noise measurements, the existence of window regions, and applications to earthquake precursor emission studies

A Low Frequency Monitoring Network has been under development at Naval Command, Control and Ocean Surveillance Center (NCCOSC), Research, Development, Test and Evaluation Division (RDT & E), San Diego, CA, for the last 2 years. Originally developed to monitor space vehicle induced signatures in the 5–12 Hz region, the network now includes monitoring of anomolous broadband signals in the 0.1–20 Hz region that often precede the occurrence of nearby earthquakes. Observations of such events have led to a hypothesis that geological signals are normally present in the ULF (ultra-low frequency, less than 3 Hz) and ELF (extremely low frequency, 3–3000 Hz) bands in addition to the generally accepted signals from micropulsations and lightning discharges. In Southern California, the geological signals are best observed in the ‘polarimetric window’ regions of (i) 2–5 Hz using horizontally oriented magnetic sensors and, (ii) 2–20 Hz using vertically oriented magnetic sensors. To determine the significance of anomalous events, we conducted a statistical study of the background noise levels. Observations of daily records over a span of several months indicated that the background level in the 1–20 Hz region is stationary. Stationarity is not true below 1 Hz. The APD (amplitude probability density) of 2–12 Hz signals was found to be surprisingly close to a Gaussian distribution. These two results indicated that a determination of long-term averages and variances would be meaningful. Long-term averages and variances were obtained and were used to develop alert-level criteria. Typical deviations over the course of several hours are 1 to 2 dB. The alert-level criteria are used as input parameters in our Automated Monitoring System (AMS), a computerized monitoring and real-time analysis system. The alert levels warn the operator when unusual events are occurring. High alert-levels in conjunction with the fulfilment of several other conditions often presage the occurrence of an earthquake. These other conditions include: (i) persistence of the elevation of the signals; (ii) the existence of resonance lines in the window region of the power spectrum; (iii) the Schumann peaks remaining relatively normal. Several examples of broadband precursor emissions are shown as well as a table listing all nearby moderate/large quakes that occurred during the research period and the correlation with our data base. The table showed that the periods preceding nearby quakes are usually associated with higher than average background levels in the 3–4 Hz region. The general conclusions are that more systematic research is needed, many more monitoring stations are needed, and full tri-axial electric and magnetic monitoring are required.

Sonars and ASW Sensors

Sound being the only form of energy that can be sustained underwater, sonars play a critical role in ocean surveillance and antisubmarine warfare. This review paper presents various aspects of sonar system design. The propagation of sound in sea water and the oceanic parameters that affect such propagation are presented in brief. Transducers, the front end energy converters from sound to electrical and vice versa, and certain issues concerned with them are also presented. Sonar system performance is a triumph of signal processing techniques. The various techniques employed along with the general architecture of the system are also presented here. Antisubmarine warfare uses a variety of platforms for the deployment of sonar systems. A brief appreciation of such systems are also given. For the sake of completion, a few nonacoustic parameter for submarine detection are also indicated. Sonar system research is actively persued all over the world, sonar being the counterpart of radar applicable to underwater e...

Operational prototyping: a new development approach

The two traditional types of software prototyping methods, throwaway prototyping and evolutionary prototyping, are compared, and prototyping's relation to conventional software development is discussed. Operational prototyping, a method that combines throwaway and evolutionary prototyping techniques by layering a rapid prototype over a solid evolutionary base, is described. Operational prototyping's implications for configuration management, quality assurance, and general project management are reviewed. The application of operational prototyping to a prototype ocean surveillance terminal is presented. >

Naval Force Levels: Theory and Practice, with Naval Engineering Challenges for the 1990s

This paper outlines the theory and practice of how naval force levels are created and sustained. A forcelevel equation is shown and the factors of this equation are discussed from the contrasting perspectives of vision and reality. The naval recovery program of the 1980s is discussed in the context of naval forcelevel theory and practice. The second part of the paper discusses naval engineering challenges for the1990s as the United States seeks to maintain strong naval forces. Several different ship types, namely, battle force combatants, aircraft carriers, amphibious ships, and underway replenishment ships are addressed along with the capability for sealift, ocean surveillance, and protection of shipping. Potential innovations, production opportunities, and trends are highlighted. Using the prospective innovations and the theories of force levels developed in the first part of the paper, alternative force structures are shown that are of interest to naval engineers and naval warriors.

Accuracy Control Variation-Merging Equations: A Case Study of Their Application in U.S. Shipyards

As a part of the ongoing move by many U.S. shipyards to adopt zone-oriented ship production, the need for an accuracy control system has been identified. Recent research has indicated that the accuracy control system can form the basis for performing process analyses as a means of improving productivity. This paper reports the results of Maritime Administration-sponsored research to develop a set of variation-merging equations for a specific vessel construction project. The equations are presented for the stern section (block 6) of the U.S. Navy T-AGOS ocean surveillance vessels under construction at Tacoma Boatbuilding Company. Data collected in 1982 provided process means and standard deviations for the following stages of production: part fabrication, panel line, subassembly and subblock assembly. Based on these data and observed assembly sequences, variation-merging equations are written and analyzed for the assembly of subblocks 602 and 603 and for the assembly of block 6 (that is, joining of 602 and 603). The impact on the merged variation of altering the assembly sequence is also analyzed and reported. The analysis is used to predict rework type (gas cutting or back-strip welding), and probabilities. Although these equations are dependent on the shipyard and the design, the techniques and principles for their development are applicable for any shipyard and any design. As such, they form a case study which makes them suitable as a guide for shipyards undertaking analysis of assembly procedures using variation-merging equations.

Radio science and oceanography

The three main radio techniques for remote sensing of the ocean surface, microwave radar, microwave radiometry, and dekametric or HF radar, are surveyed. The generation and characterization of wind waves and swell on the ocean surface, and the first‐order mechanism of Bragg resonant scattering of radio waves by such surfaces are outlined. Doppler HF radar techniques, ground wave and sky wave, developed for utilization of this effect to sense remotely wave, wind direction and surface current velocity are reviewed. Second‐order scattering effects, hydrodynamic and electromagnetic, are also present, and their signatures on the Doppler echo spectrum of an HF radar permit measurement of wave height, wavelength spectrum and directional spectrum. Finally, the scattering mechanisms relevant in satellite‐borne microwave radar are outlined. Bragg resonance occurs with short waves, these being modulated by the longer gravity waves of practical interest. The potential of the radar altimeter, scatterometer and synthetic aperture radar for global ocean surveillance is illustrated by examples from the satellite.

A tomographic approach to multiarray ocean surveillance

One of the most widely applicable areas of signal processing in the last few years has been tomography, or image reconstruction. This area consists of a set of inverse problems characterized by the common trait that the desired quantities are the values within a field which can only be observed by looking through the field. This paper explores the applicability of tomographic ideas to the problems of ocean surveillance. Succinctly stated, beamformer outputs look like integrals of the acoustic field along the paths which include many sources of acoustic energy. By observing many beams from several arrays, the acoustic field in the ocean might be reconstructable. The basic ideas of tomography are presented, with a description of how available measurements in sonar are appropriate for the technique. The ideas are applied to two germane problems: noise mapping and localization.

A compound isolation system for T‐AGOS I class 600‐kW diesel generators

The T‐AGOS 1 class ocean surveillance ships are to tow a sonar array known as the Surveillance Towed Array Sensor (SURTASS). In order for the towed array to operate in a quiet environment, the underwater radiated noise level of the towing ship must be low. This paper presents a design for the installation of four 600‐kW diesel generator units. The radiated noise, due to two operating units, is estimated for a frequency range from 10 to 400 Hz by assuming that the hull acted as 16 simple pistons. Verification of the compound isolation system performance was made by conducting a land‐based test and measuring vibration throughout the system and the force transmitted to the foundation. The estimated radiated noise levels based on test data are compared with those based on mathematical analysis of the isolation system and found to be quite similar.

Ocean surveillance technologies and International payoffs

Abstract This paper discusses the evolution of the existing ocean surveillance technologies and the many new devices in the field. Among the systems dealt with is “Project Seaguard”; of the U.S. Defense Advanced Research Project Agency and its ILLIACIV, the world's largest computer, which coordinates sensor data from 64 worldwide inputs. The author visualizes important future uses of underwater listening devices and ocean monitoring networks for solely peaceful purposes, as, for instance, by warning endangered ships of all nations of hurricanes, tsunamis, and icebergs. More accurate and comprehensive surveys of fishing efforts, pollution prevention, and customs enforcement may also be carried out by the use of present and future technologies. In conclusion, the author proposes the creation of a Global Ocean Management Organization, whose services would greatly beenfit all member states and might even serve as a model for systems verifying multilateral disarmament agreements and the monitoring of crises.

AN/APS-116 Periscope-Detecting Radar

The AN/APS-116 Radar System was developed by the U.S. Navy for ocean surveillance and antisubmarine warfare patrol. The basic system requirement is to reliably detect small periscope-sized targets against strong interfering signals form sea clutter backscatter. The radar system design approach utilized very fine range resolution and a rapid scan antenna. A signal processing scheme was implemented using scan-to-scan decorrelation, with a M-out-of-N threshold for target detection. A brief description of each major unit of the radar is included, along with a summary of the evaluation tests that verified system performance.

Computer-Aided Ship Tracking

A computer-aided manual technique for performing ship tracking and multisensor correlation of ocean surveillance data is described. All types of ship sightings, such as radar and direction finding readings, are associated in a unified procedure. The computer displays all unidentified ship sightings that may be linked to the identified sightings of a particular ship. To resolve ambiguities, an operator assigns identities to one or more of the unidentified sightings, and the computer then recomputes and displays the new sets of linked sightings that satisfy the operator-imposed assignment constraints. The operator can rerun the program several times to take advantage of new sightings or to change the motion models, or he may resolve ambiguities in other ways.