A basic understanding of sonar technology is important before discussing the applications and design of a towed array sonar (TAS).
Sonar, a short name for Sound Navigation and Ranging, is a technology that utilizes sound propagation, usually underwater, to distinguish objects above and below the water and to measure distances underwater. A sonar device sends out subsurface sound waves and “listens” for a returning echo. This sound data is then relayed to a sonar operator and displayed on a monitor.
Types of Sonars
Active sonars generate a pulse sound called “ping” and then takes note of the reflections of that pulse.
A Functional Diagram of an Active Sonar System
A transmitter generates the outgoing ping. It determines the pulse width, carrier frequency and other data needed.
A passive sonar “listens” without transmitting. This type is used for defense and scientific applications. This type is connected to a large sonic database. A computer system analyzes the data and uses this database to identify objects like vessels, the speed of the ship or type of weapon released
Typical Functional Diagram of a Passive Sonar
The hydrophone array are the sensitive components that detect acoustic energy emitted from the target. They are arranged in an array to boost beam-width. Cylindrical or spherical configurations are the most common. Submarines use the spherical configuration due to its wider vertical field-of-view. Since submarines maybe below what is being tracked, the array must be able to “look up”. The large downward angles are used for bottom bounce detection.
What is a Towed Array Sonar?
A towed array sonar is a vital sensor used by submarines and anti-submarine warfare surface defense. Due to the sounds produced by submarines as they move through water or noise from internal systems in operation, there is a need to detect other sounds from external sources. This sonar is composed of a system of hydrophones and is tugged a considerable distance behind a submarine using a cable. These hydrophones are often kilometers away thus greatly improving the signal to noise ratio. The towed array sonar is effective in the detection and tracking of faint sounds such as the low noise-emitting submarine threats or seismic signals.
This passive TAS has superior resolution and has a wider range than with the hull-mounted sensor. This covers the baffles which is the blind spot of hull-mounted sonars. The downside of this type of sonar is that it limits the speed of the towing body and extreme care is needed to protect the cable from being damaged or cut off.
Not like active sonars, the towed array sonars are passive ones thus will not give out the identity or make or location of the user.
A Brief History of Towed Array Sonar
In World War 1, the towed array sonar was called the “Electric Eel”. This was developed by Harvey Hayes, a U.S. Navy physicist. The Electric Eel was the forerunner of the towed array sonar design. There were two cables each with a dozen hydrophones. Unfortunately, Hayes’ project was discontinued after the WW1. In the 1960s, the U.S. Navy continued the development of the towed array technology in order to identify nuclear-powered submarines of the Soviet Union.
Towed Array Sonar Use and Set-Up
Surface ships normally keep the towed array sonar in drums and is located behind the ship when it is deployed. In the US Navy submarines, these are placed inside an outboard tube on the hull with its opening on the starboard tail.
The hydrophones are placed at exact distances along the cable with the end elements at a considerable distance to be able to triangulate the sound source. Other components are place in an upward or downward angle to triangulate on an estimated vertical depth of the sound. Other arrays are used to work on depth detection. The first hundred meters from a ship’s propeller, no hydrophones are placed. The effectiveness would be reduced by noise, vibration and turbulence.
Surveillance Towed Array Sensor Systems that are utilized by surface vessels have a sonar array that are mounted on a cable that pulls a depth-adjustable remote operated vehicle (ROV). For lower depths, a weighted cable attached to the ROV connector may be dropped.
The hydrophones are used to detect sound sources, but the real value of the array is in the signal processing technique of beamforming and Fourier analysis. This provides distance calculation and direction of the sound. It assists in the identification of the vessel type due to the distinctive, acoustic signature of its internal machineries.
The direction of the towed array sonar is an important consideration. It should be determined if the cable is in a stable position or on a straight line if there is a self-sensing system or GPS or other requirements embedded in the cable. The relative position of the hydrophone is used to monitor the shape of the array and do corrections to the curvature.
Design Considerations of a Towed Array Sonar
The development of a towed array system being a low-frequency component of the sonar operation of submarines is a complex one. Besides the acoustic requirements, attention to the properties of materials used is considered. There are electric and electronic conditions that are taken into account to develop a TAS that is efficient for digital data transfer.
A typical towed array sonar system consists of:
- Array section (HF, MF, LF): length approx. 150 m
- One Vibration Insulation module each at the front end and at the rear end of the array section: length 20 m
- Towing cable approx. 200 m
- Tail rope approx.= 30 m
- Winch on board of the submarine for handling the array including launching device and cable cutter in case of emergency
- Signal processing on board; integrated into the signal processing of other sonar sensors (e.g. Flank Array Sonar – FAS).
This compact system provides optimal detection performance. Another vital consideration is that system components must be matched in order to achieve efficient performance.
Schematic Structure of the Hydrophone Section
Skeleton of a Hydrophone Section
The future for the towed array sonar technology is very bright. It is the only technology capable of detecting faint acoustic targets on a long-range basis. Current developments in the optical fiber technology will impact TAS allowing longer arrays with more hydrophones, better accuracy and sensitivity.
AMETEK SCP has the competence to manufacture diverse types of tow systems including cables for towed array sonar systems. This includes electrical and fiber optic cabling. Contact AMETEK SCP for any requirement.