Siwecki, T.L. and Neal, E.H., 1993. S-3: Large active long range variable depth sonar, Sea Technology, November 1993, 39-42
By Thomas Lee Siwecki
President
International Investments Organization
A new type of sonar recently proposed to the Navy will propel surface ASW force's sonar range to about 100 miles. Current ranges are a fraction of this amount.
Called "S-3" (for Siwecki sonar system), its rectangular planar arrays and operating frequency provide the directivity needed for effective long sonar range that is not achievable by the physics of currently used sonar line array technology. The proposed S-3, a surface-ship-towed ASW variable depth active sonar, promises the following performance:
Active interrogation of 34,636 square miles, 360 degrees surrounding the S-3, out to 105 miles, above and below the ocean's seasonal thermolayer every 60 minutes with the tow-ship maneuvering at 15 knots.
Detection of targets as small as the bow aspect of a 20-foot hull diameter submarine in sea state 6 (wave height about 20 feet) at performance range. (Germany sells this sized submarine on the international market)
Tracking and localization of the target continuously in real time within and area of less than 2 square miles.
Probability of detection of 99.8 percent with four pings (false alarm probability: 0 - 1percent).
The S-3 can be towed by existing ASW combat ships (DD-963 Spruance, FFG-7 Oliver Hazard Perry, and FF-1052 Knox classes). The sonar mitigates tow-ship's radiated noise.
Efforts to employ the Navy's many existing line arrays by adding sound transmitters to regain the previous 100 mile ranges (obtainable before non-U.S. submarines took a giant step in quieting) has produced ineffective results. The reason is that the physics of sonar line array technology has low array.
Low array directivity translates into inability to focus, transmit, receive, and steer very narrow sound beams horizontally and vertically in overlapping patterns. The solution to this problem is achieved with directivity.
The S-3 increases sonar performance by employing two rectangular planar arrays collocated at right angles, providing high sound directivity. The projector array transmits and the hydrophone array receives, each focusing very narrow low-frequency sound beams selectably steerable in all directions. These narrow beams mitigate ocean boundary and volumetric reverberation, ambient sea noise, and identify ocean bottom reflections. They further provide realtime target tracking and localization to within small areas.
Departure from Past, Present
Active sonar was developed for surface ships to detect submarines in the l930s. The projector that transmitted the sound and the receiver that collected target backscatter via hydrophones were collocated and attached to the keel of the ship. Sonar performance was limited by platform motion and noise and the higher frequencies dictated by the small sonar dimensions.
In the 1960s, performance was improved by separating the receiver from the hull and towing it. The configuration reduced unwanted noise, permitted lower sonar frequencies (and therefore less sound absorbed by the sea), enabled variable depth permitting adjustment to ocean thermolayer, and abetted search of ship's stern area not possible with the hull-mounted configuration without the ship maneuvering.
In the l980s, performance was improved again by mounting the projector in a towed body, which together with the towed receiver line arrays created a variable depth sonar (VDS) system with both enhanced transmission and reception.
S-3 Sonar
Building on VDS, the proposed S-3 increases sonar performance by collocating at right angles projector and receiver arrays into a single towed body to produce very narrow directional low- frequency sound beams that achieve high multiplicative directivity (HMPD) for the indicated ASW performance. It generates a sound level of 248.2 dB at 750 Hertz (free field) with a 10 percent bandwidth.
The projectors and receivers are collocated into large rectangular arrays. The sound projecting array is positioned vertically and the receiving array horizontally. The arrangement produces high multiplicative directivity and improved sonar performance.
HMPD focuses high levels of directional low frequency sound in very narrow high energy transmit beams to yield long detection ranges. By steering overlapping horizontal and vertical transmit and receive beams above and below the thermolayer, maximum sonar coverage (within the detection range) is obtained regardless of convergence zone conditions.
Real-time concurrent target detection and tracking is achieved by HMPD's high signal-to-noise-ratio which requires coordinate transforms only on received target backscatter, form beams, and display targets. High bearing resolution localizes targets within small regions.
By HMPD's ability to re-interrogate targets with very narrow beams, false targets are reduced. Low off-axis response (mitigating sonar-generated ocean boundary and volumetric reverberation) achieves high probability of target detection and low probability of false alarms.
Hardware, Performance Controls
The system consists of a towed body, a tow cable, cable handling system, detected target displays, signal processor, and electrical power generators.
The towed body, a steel lattice structure enclosed in acoustically transparent, glass-reinforced plastic, is 240 feet in length, 120 feet high, 8 feet wide (extending to 41 feet with the horizontal control surface in the operational mode).
Configured for towing into port (rotated on its side and
horizontal control surfaces folded) it has a draft of 12 feet. It
weighs 750,000 pounds in air and is neutrally buoyant and
level-trimmed in water.
In the event of catastrophic failure, provision is provided for the towed body to surface (there is a drop weight mechanism) and for actuating towed body locating devices (buoys and beacons). An air-operated buoyancy system controls orientation for entering port.
The towed body can operate from 200 to 1,000 feet in depth. It requires 5,000 ship-propulsion horsepower to tow at 15 knots (less at slower speeds, 20 knots is maximum speed). Reportedly the system is installable on most Navy destroyers.
The projector array (120 feet high, 53 feet long) mounted in the
vertical element at the end of the towed body has 340 projectors,
mounted l0x34 on 3-foot centers. The array configuration is
similar to that used on the research submarine Dolphin in
1969. A Kalman controller will monitor pitch, roll, yaw, and heading
for the signal processor. Ship-towed-body communications system
includes FDDI (fiber-optic distributed data interconnects) links
and Ethernet for networking towed-body computers. The 3-inch-diameter tow cable has six electrical power conductors
(3,000 volts), 12 single mode FDDI strands (six for operation and
six for backup), an air conduit, and steel armor. Data load is 15
megabytes. In terms of cable payout, the operational envelope is
600 to 6,000 feet. The proposed winch system requires a vertical drum to handle the
size and amount of tow cable but poses no performance or
technology risks. It requires a level-wind cable guide and
tension monitor. Space at the stern, currently allocated for
variable depth sonar (VDS) and tactical towed array system
(TACTAS), would be used for S-3 handling equipment. The sonar signal processing system (data recording, transmitting,
receiving, adaptive beam forming, and data displays) will use
commercially available hardware and software. Most of the signal processing is done on the tow ship. The
architecture is similar to a local area network. There are four
subsystems—three for data communication and front-end processing
and one for beamforming, raytracing, coordinate transformations,
and localization of acoustic energy. A 15-gigabyte disk storage
array with 8-millimeter tape backup will be provided. The two data displays consoles will have 19-inch high-resolution
color monitors with touchscreen, trackball, and keyboard
interface. There are nine data display formats: active search
primary detection, active contact analysis and track, passive
search, sound velocity trace/ray path, propagation loss figure of
merit, geosituational, system performance monitoring, system
fault localization, and tow control. In addition to towed body depth selection, operational variables
include desired search range (1.2 to 18, 16 to 40, and 36 to 105
nautical miles), bearing search sectors, search patterns, and
depression angles. Search rates for the respective search ranges
are 6, 11, and 28 minutes. Subcontractors to International Investment Organization (110)
(Concord, California) for the development and construction are:
David Taylor Research Center (Bethesda, Maryland) for drawings
for the towed body, cable handling system, and ship alteration
study; South West Marine Inc. (San Francisco) for construction of
the towed body; Syntech Materials Inc. (Springfield, Virginia)
for acoustically matched buoyancy foam; HITCO Inc. (Los Angeles)
for towed body enclosure panels and mountings; Magnavox
Electronic Systems Co. (Fort Wayne, indiana) for sonar
projectors, power amplifiers, hydrophones, and isolation mounts;
South Bay Cable (Idylwild, California) for tow cable; Dynacon
Inc. (College Station, Texas) for the cable handling system;
AIDCO Inc. (San Leandro, California) for electrical and
mechanical components of the sonar system; Trident Systems Inc.
(Fairfax, Virginia) for hardware/software for sonar data
displays; and Universal Computing (San Diego) for sonar data
acquisition, coordinate transformations, beam forming, sonar
signal processing, and system control work.