Introduction to Naval Weapons Engineering

ASW Systems

Anti-submarine warfare (ASW) usually, but not always, involves the use of sonar. Although the vagaries of the environment make it difficult to predict and use, there is no other type of energy propagation that travels so far in the ocean without significant losses as acoustics waves. In this section, we describe the principles of operation of the major types of sonar systems and one non-acoustic system (MAD). We begin with the system that most closely resembles the operation of basic radar, namely active sonar.

Active Sonar

A functional diagram of an active sonar system looks like this:

Figure 1. Active
sonar System.

The functional components are described below:

Transmitter. The transmitter generates the outgoing pulse. It determines pulse width, PRF, modulation (optional), and carrier frequency. The output power can be controlled by the operator. The source level may be limited for several reasons. If the transducers are driven with too much power, they can cavitate (drop the pressure so low that the water boils). This is called quenching, and it can destroy the transducer since the normal backpressure is removed when bubbles form on its surface. Since the normal restoring force is gone, the surface of the transducer can travel too far (over-range) and damage itself. The quenching power limit increases with depth due to the increased ambient pressure.

Another common phenomenon that limits the maximum source level is reverberation, which is an echo from the immediate surrounding volume of water. The reverberation level (RL) increases with the source level (SL). At some point the reverberation exceeds the noise level (NL) and will dominate the return signal. Since reverberation always comes back from the same direction you are projecting, the reduction in background noise, quantified by the directivity index (DI) does not apply. When

RL > NL - DI,

the system is said to be reverberation-limited. The figure of merit equation must be modified to reflect this:

FOM_active (reverberation-limited) = SL + TS - RL - DT

When the system becomes reverberation-limited, the display will begin to be dominated by noise near own ship in the direction the active sonar is projecting. The solution is to reduce power to just below the level at which reverberation-limiting occurred.

Transducer array. The individual transducers are simple elements with little or no directionality. They are arranged in an array to improve the directivity index, which improves the figure-of-merit by noise reduction. The array of transducers reduces the beamwidth in the horizontal (or azimuthal) direction, and is usually circular in order to give more or less complete coverage, with the exception of the region directly behind the array (where the ship is). The array is protected from noise by own ship by discontinuing the array in the after regions, and also by putting in sound attenuating material. This region aft of a hull-mounted array, from which the sonar system cannot detect is called the baffles.

The array is also configured to reduce the beamwidth in the vertical direction. Normally a hull-mounted array should only receive sound from the downward direction, not directly ahead, since the noise from the ocean's surface would destroy the sonar's performance.

Figure 2. Vertical beam of typical
cylindrical transducer array.

Beamforming processor. The input/output of each transducer is put through a beamforming processor, which applies time delays or phase shifts to each of the signals in such a way as to create a narrow beam in a particular direction.

Figure 3. Active beamforming.

The width of the beam formed by the beamforming processor will determine the bearing accuracy of the system when searching. In an identical manner to dual-beam tracking systems, sonar tracking systems can improve on this accuracy tremendously, at the expense of the signal-to-noise ratio.

4.) Duplexer. The duplexer performs the same function in an active sonar as in a radar system, namely to protect the receiver from the full transmitter power while the pulse is going out. It can be thought of a switch that toggles between the transmitter and receiver.

5.) Synchronizer. Performs same role as the synchronizer in radar. Provides overall coordination and timing for the system. Reset the display for each new pulse in order to make range measurements.

6.) Receiver. Collects the received energy. The receiver compares the power level to noise with a threshold SNR (DT) in order to determine if the signal will be displayed in a particular beam. If the DT is set too low, there will many false alarms. If it is too high, some detection capability will be lost.
The receiver may also demodulate the return if frequency modulation is used on transmission. Sonar systems often use pulse compression techniques to improve range resolution.

7.) Display. Puts all of the detection information into a visual format. There are several types:

A-scan: the signal along a single beam for a portion of the listening cycle. A target appears as a raised section if it is in the beam.

Figure 4. A-scan display.

PPI: plan position indicator. A top-down (geographic view). The sonar system must sequentially search individual beams which are displayed in their true or relative form. The strength of the return is represented by the intensity on the display.

Figure 5. PPI display.

Passive Sonar Systems

The passive sonar system differs from the active system in many regards. Below is a typical functional diagram:

Figure 6. Passive sonar
system.

Hydrophone array. These are the sensitive elements which detect the acoustic energy emitted from the target. Again, they are arranged into an array to improve the beamwidth. Common configurations are cylindrical or spherical. The cylindrical array operates at a fixed vertical angle, usually downward. The spherical array, which is common on submarines, has a much wider vertical field-of-view. Since the submarine may be below what it is tracking, the array must be able to look upwards to some extent. The large downward angles are only used for bottom bounce detection. Using a beamforming processor (described below) the field-of-view is broken down into individual beams in the vertical and azimuthal directions.

Figure 7. Spherical array showing
multiple vertical beams.

Beamforming processor. Unlike active systems which transmit and receive in a set direction, the passive system must listen to all angles at all times. This requires a very wide beamwidth. At the same time, a narrow beamwidth is required for locating the source and rejecting ambient noise. These two objectives are achieved simultaneously by the passive beamforming processor. The idea is very similar to the active system.

Figure 8. Passive hydrophone
array.

The passive beamforming processor applies a unique set of time delays/phase shifts to the signal to create a particular beam. The difference in a passive system is that this process is repeated several times, each with a different set of time delays/phase shifts, in order to listen to many narrow beams nearly simultaneously. The result is a set of beams that cover the field-of-view of the array.

Figure 9. Passive beamforming.

The beams should not be thought of as coming from the individual hydrophones. In fact, each of the beams so created has a narrow beamwidth that comes from the full aperture of the array, not the individual hydrophones.

Broadband display. The output of the beamforming processor is displayed as a bearing time history (BTH):

Figure 10. Bearing time history
(BTH) display.

The newest information is at the top of the display. The beamwidth of the system determines how accurately the bearing can be measured by such a display. A common beamwidth is about 5^o. The total amount of time displayed from top to bottom can be controlled (to some extent). A quickly updating display that only kept information for a few minutes would be useful for close contacts whose bearings are changing rapidly. On the other hand, a long tie history is more useful for detecting long range contacts, whose bearings are only changing slowly.

4.) Frequency Analyzer. The frequency analyzer breaks the signal into separate frequencies. This is the spectrum of the signal. For processing purposes, the frequencies are divided into small bands known as frequency bins. The width of each bin is called the analysis bandwidth.

Figure 11. Frequency analysis.

Sonar systems can gain considerable signal-to-noise improvements by matching the analysis bandwidth to the bandwidth of narrowband sources. The way to illustrate this is by two counter examples. If the signal processing bandwidth is too wide, then noise from the part of the spectrum beyond the signal is let in and the SNR is degraded. If the bandwidth is too narrow, then part of the signal is excluded, also reducing the SNR. It should be obvious now that the best situation occurs when the bandwidth exactly matches the signal. This is possible when the characteristics of the signal are well known, which they are for most targets.




The frequency analyzer separates (filters) the signal into discrete bins, inside of which the SNR is maximized. The frequency content of the signals from a target information provides vital information about its identity and operation. These frequencies are also subject to the Doppler shift, just like radar, are therefore can provide information about the range rate. This requires that the original frequency be known exactly, which is generally not the case. However, many important facts can be inferred by the changes in the received frequency over time.

Narrowband Display. For a particular beam, the time history of the frequency is called a waterfall display.

Figure 12. Waterfall display.

This can be used to gain additional information from a contact which is already being tracked by another system. In order to search for contacts on the basis of narrowband information alone requires a different type of display. One possibility is to simultaneously display several different beams, each showing a mini-waterfall display, which are called grams.

Figure 13. Narrowband grams.

These are quite useful, but require great concentration on the part of the operator because there is more information displayed at any one time. Many systems require the operator to systematically search the entire field-of-view, looking at only a few beams at a time.

Variable Depth Sonar (VDS)

Variable depth sonars use large transducers that are towed from the ship on a cable with an adjustable scope. The combination of the buoyancy, ship speed and cable scope determine at the depth that the transducer will be at. VDS is used for two main reasons. At increased depth, the source level (SL) can be increased greatly, since the quenching limit is higher. This is due to increased backpressure on the surface of the transducer. Secondly, the VDS can be operated below the layer.

Recall that the combination of positive over negative sound velocity profiles created a layer at the interface. The layer makes it difficult to propagate sound across it. Therefore, ships using hull-mounted sonar systems will be unable to detect submarines operating below the layer, except possibly at short range. However, if the VDS can be place below layer, the ship can take advantage of the deep sound channel while being in the shadow zone of the submarine's sonar.

Towed Array Sonar Systems (TASS)

A towed array is a linear array of hydrophones. The array is towed behind the ship on a cable of variable scope like a VDS. However, it is strictly a passive system.

Figure 14. Towed array.

The signal from the array is led to a beamforming processor which creates several narrow beams. Because the array is linear, there is no vertical directionality. This causes two problems. The first problem occurs when there is bottom bounce propagation. In this case, the direction of the source is not known without further analysis.

The second problem is an ambiguity in relative bearing. The linear array cannot distinguish signals on the left from those on the right.

Figure 15. Bearing ambiguity
with towed array.

The problem of bearing ambiguity can be resolved by maneuvering the ship. When contact is regained, there will again be two ambiguous bearings, but only one of which will match the previous case (assuming that the target hasn't moved much between legs).

Example: a towed array contact is gained at relative bearing 030/330 R, while the ship is on course 045 T. The ship changes course to 135 T and regains the contact at 060/300 R. Find the true bearing to the actual contact.

First leg: the contact is at either 015 T or 075 T (045 030).

Second leg: the contact is at either 075 T or 195 T (135 060)

Therefore the actual contact is at 075 T.

Because the array is not constrained by the size of the ship, towed arrays can be made very long. Therefore, they have very narrow beamwidths, or alternatively, can operate at much lower frequencies. Low frequency capability is particularly advantageous because there are many sources at low frequency, with large source levels, and there is very little loss from absorption.

Sonobuoys

Sonobuoys are small, self-contained sonar systems. They are dropped into the water by aircraft at which point they deploy themselves. The information from the sonobuoy is transmitted to the aircraft by VHF radio link. The information can also be relayed to ships. Signal processing and analysis is performed by equipment in the aircraft, or ship. After some period of operation, the sonobuoy will scuttle itself.

Figure 16. Sonobouy deployment.

There are many types of sonobuoys, depending on their capabilties:

DICASS: directional command activated sonobuoy system. Active system which transmits pulses when commanded by the aircraft.

DIFAR: direction finding acoustic receiver. This is a passive system, with some directionality achieved by a small hydrophone array.

VLAD: vertical thin-line array. A linear, vertical array of DIFAR hydrophones. Has improved directionality in vertical direction. Reduces noise from surface.

Most sonobuoys can operate at several preset depth settings. The shallow setting is used for surface duct propagation, and the deep for sound channel propagation.

Sonobuoys have poor values of directivity index, mostly because of their limited size. Figure-of-merit is not always low, however, because sonobuoys also have very low self-noise. Depending on the environment, and whether or not self-noise is dominant, sonobuoys can actually outperform some larger hull-mounted systems.

Bi-Static Sonar

This is a combination of an active system for transmission at one location with a passive system for reception at another location.

Figure
17. Bi-static sonar.

The passive system does not suffer from the same reverberation limitations as the standard active system. The source level can be very high (up to the quenching limit). Some system use explosive driven projectors with incredible source levels. The source may be well outside the weapons range of the target. Furthermore, the transmission loss is less than the full two-way loss of normal active. Sometimes the projector is a sonobuoy.

Non-Acoustic Detection

Visual

Submarines near the surface are very vulnerable to visual detection. Anything that protrudes above the surface like a periscope, antenna or mast will leave a significant wake if the submarine is moving at any speed over a few knots. Since depth control and steerage is very difficult at low speeds, it is not uncommon for submarines to be at 4 or 5 knots just below the surface. The mast will create a wake, called feather, which is quite visible, and also leave a remnant of its passage, called a scar. The scar is a long streak of foam or bubbles left behind after the object passes. The feather may be a few meters, and the scar tens of meters long. Either may be visible for up to 10 miles, and are easily spotted by low flying aircraft in the vicinity. If the water is especially clear, the hull may be visible for a few hundred feet under water, but is usually not distinguishable unless the water is shallow with a light colored bottom (like white sand).

Radar

Exposed periscopes and masts can be detected by specially designed radars. The radar cross section is very small and is generally not detectable by ordinary radar systems. Furthermore, the sea clutter near the target will generally obscure it. To be effective against a periscope or mast, the radar should have very small range and bearing resolution and must be vertically polarized to match the structure of the target. ISAR has proven to be very effective against submarine periscopes and masts.

Some special radar systems have demonstrated the capability to detect the presence of a submarine by the change in the surface water height as it passes, known as the bernoulli hump. This effect is greatest when the submarine is shallow and moving very fast. These are not real time assets however, since the signal processing takes several hours to complete.

Infrared Detection

Submarines are vulnerable to infrared detection when they are snorkeling, since the diesel exhaust is released close to the surface (as it must be because of the backpressure limitation). The exhaust gases give off a sufficiently strong infrared signature as to be detectable. However, this is only useful if the submarine is snorkeling, which it very infrequent for nuclear powered submarines, and only a few hours a day for diesel-electric submarines.

Magnetic Anomaly Detection (MAD)

Magnetic anomaly detection systems, measure the change in the earth's magnetic field due to the presence of a large amount of ferrous material found in most submarines. The effect can only be detected if the submarine is relatively shallow, and therefore is not a great long range detection system. It can however, provide a precise location of the submarine of sufficient accuracy to permit weapons delivery, which is its main use.