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Sonar > All Theory > Flashcards

All Theory Flashcards

1
Q

How are radars used to detect submarines?

A

Periscope detection
Wake detection

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2
Q

What underwater threat are lazers used for?

A

Mines

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3
Q

What is a MAD?

A

Magnetic Anomoly Detector - Detects variations in Earth’s magnetic field caused by subs.

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4
Q

What is the range of SONAR?

A

10 Nm

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5
Q

What are the two main types of sonar?

A

Active and Passive

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6
Q

What do you get from Active sonar that you do not get from passive?

A

Range information

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7
Q

What is a Monostatic SONAR system?

A

Transmitter and receiver are co-located

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8
Q

Bistatic SONAR system

A

Transmitter and receiver are separated by a distance comparable to the distance to the target.

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9
Q

What are the SONAR systems aboard the CPF’s?

A

HMS
CANTASS
NIXIE
Dipping Sonar
Sonobuoys

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10
Q

Draw the Active SONAR block diagram and explain what each component does.

A

Trigger: Ensures the system is synchronized

Beamformer: Steers the transmit beam in the desired direction and listens in the desired direction.

Transmitter: Generates proper waveform at proper frequency and power level.

T/R Switch: Isolates receiver from high power pulses.

Transducer Array: Transmit and receive.

Receiver: Detects echo, amplifies, filters.

Data Manager: Stores data, tracking, TMA

Display: Displays that shit

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11
Q

Draw and label a SONAR pulse train

A
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12
Q

Describe a Longitudinal Wave

A

“L waves”, the displacement of the medium is in the same direction as the direction of travel.

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13
Q

What does the Bulk Modulus (B) represent?

A

Resistance to compression.

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14
Q

Describe a Moving Coil Transducer

A

A solenoid is attached to a flexible piston which moves air.

Typically used for audio speakers.

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15
Q

Describe a magnetorestrictive Transducer

A

Magnetic field induces changes in shape or dimension.

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16
Q

Explain Piezoelectric Transducer

A

Electric field induces changes in shape or dimension.

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17
Q

Describe an Air Gun Array transducer

A

Penumatic chamber is pressurized with compressed air and a piston discharges a high pressure air bubble into the water.

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18
Q

Define Hydrophone vs. Projector vs. Transducer

A

Hydrophone - Receive only
Projector - Transmit only
Transducer - Transmit and receive

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19
Q

What are the negative effects of Cavitation on SONAR?

A

Erosion of projector surface

Loss of acoustic power due to absorption and scattering by the bubbles

Deterioration of the beam pattern

Impedence mismatch between projector and medium (tuned for water, now air)

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20
Q

How do we limit interaction effects between projectors in a SONAR array?

A

Separating elements up to lambda/2

Increase size of elements

Using individual amplifiers

????

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21
Q

At what bearing do you get ambiguous bearings when beamforming?

A

60 degrees

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22
Q

How do we remove ambiguous beams when beamforming?

A

Additional sensors
TMA

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23
Q

What are Grating Lobes in a transducer array?

A

When the hydrophones are too far apart for the wavelength we are receiving, the time between samples across the array is less than 2 samples per period (Nyquist Theorem), causing aliasing in the form of grating lobes. These lobes can be confused for the actual main lobe.

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24
Q

What are the types off losse which cause Transmission Loss?

A

Spreading
Absorption
Scattering

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25
Q

Describe Spherical, Cylindrical, and Mixed spreading

A

Spherical spreading occurs at first, but when the wave reaches the ocean surface and the sea bottom, it becomes Cylindircal, resulting in mixed spreading.

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26
Q

What is the absorption coefficient

A

It tells us the energy loss as a fraction per unit length travelled through water, expressed in dB/km

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27
Q

What are the mechanisms of Absorption in water?

A

Viscosity
Ionic Relaxation
Scattering

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28
Q

Describe how the viscosity of water leads to absorption losses

A

As the pressure wave travels through the water, molecules rub against eachother, converting the kinetic energy into heat via friction, resulting in a raise in temperature.

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29
Q

Describe how Ionic Relaxation leads to absorption losses.

Is it more likely at high or low frequencies and why?

Also what is Relaxation Frequency?

A

Change in state of chemicals absorbed in seawater, triggered by the pressure variations from the pressure wave.

It is more likely at low frequencies because the change in pressure must be slow enough for the change of state to take place. The frequency at which this occurs is called the Relaxation Frequency.

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30
Q

How does scattering result in losses?

A

Inhomogeneities in the index of refraction of water cause re-radation of incident sound waves.

It is effectively constant in water.

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31
Q

What are the three main approximations to modelling wave propagation in water?

A

Ray Tracing
Normal Modes
Parabolic Equation

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32
Q

Describe Ray Tracing

A

A Ray is a line indicating the direction of propagation of the wavefront.

The more two rays spread, the lower the intensity.

Snell’s law tells us how the rays bend in the medium.

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33
Q

Describe Convergence Zones

A

Sound waves will travel down to the isothermal layer and refract back up. They tend to converge around 30, 60, and 90nm.

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34
Q

What are the 3 scenarios where Ray Tracing is not effective?

A

When two rays cross eachother (Caustic)

Shadow Zones

When pressure or raidus of curvature changes over the distance of one wavelength (at low frequencies).

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35
Q

What is a Shadow Zone?

A

Rays do not reach these areas due to the refraction pattern.

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36
Q

What is a Caustic?

A

When rays overlap to create points of increased intensity.

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37
Q

Explain Normal Mode theory

A

Water column is treated like a waveguide. Propagating modes are calculated and summed to obtain a pressure distribution.

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38
Q

What is an advantage of Normal Mode theory over Ray Tracing?

A

Ray Tracing does not account for diffraction.

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39
Q

Explain how Parabolic Equations are used to model wave propagation.

A

Simplifies waves into parabolic equations.

Only takes into account the forward travelling wave.

40
Q

What are the factors which affect the speed of sound in water?

A

Temperature
Salinity
Depth/Pressure

41
Q

Which is the dominant factor affectting the speed of sound in water?

A

Temperature

42
Q

What device do we use to measure the temperature in water?

A

Expendable Bathy Thermograph (XBT)

43
Q

What device do we use to determine the water salinity?

A

Expendable Sound Velocimeter. Used if the salinity is expected to depart significantly.

44
Q

What are the 4 layers of the the ocean thermal structure?

A

Surface layer 0-60m
Seasonal Thermocline 60-300m
Permanent Thermocline 300-900m
Deep Isothermal Layer 900m+

45
Q

Describe the Surface Layer

A

0-60m

Possible to be constant temperature with enough mixing.

Hot during the day, cool during night. Affected by wind and waves.

46
Q

Describe Seasonal Thermocline layer

A

60-300m

Decreasing temperature.

Established in summer as the surface layer heats
Destroyed in winter as the surface cools

47
Q

Describe a Permanent Thermocline

A

300-900m

Colder as it gets deeper.

Slope may depend on location and season.

48
Q

Describe the Deep Isothermal Layer

A

900m+

Constant temperature near 4C

Pressure is the dominant factor affecting speed of sound here.

49
Q
A
50
Q

What is a Shadow Zone?

A

At a layer change where temperature changes form increasing to decreasing (seasonal thermocline), rays can split at the boundary, creating a predictable shadow zone.

51
Q

What is a Surface Duct?

A

Rays are trapped in the surface layer and can travel an extended distance but at shallow depth.

52
Q

What is a Sound Channel

A

When the speed of sound changes from decreasing to increasing, a sound channel can form, deflecting rays towards the centre of the channel. It becomes trapped in the cold layer between warm layers.

53
Q

What is a Convergence Zone

A

When rays are deflected upwards prior to hitting the ocean floor due to the increasing speed of sound uccuring in the deep isothermal layer. When sound rays return to the surface, they tend to converge in a small region called a convergence zone. These appear periodically at 30, 60, and 90 nm.

54
Q

What is Bottom Bounce?

A

Rays reflect off the ocean floor.

Hard flat ocean floors are the best for this.

55
Q

What is a Deep Sound Channel (SOFAR Channel)?

A

A layer of water where the speed of sound is at its minimum.

56
Q

What are 6 factors affecting Target Strength (TS)?

A

Size
Shape
Aspect
Surface Material
Wavelength used
Construction (how thick the reflecting surface wrt the wavelength.

57
Q

What is Target Strength?

A

The ratio of reflected to incident power on the target

58
Q

What are the two main types of Noise?

A

Ambient noise: From natural sources
Self noise: Generated from own ship

59
Q

What are 4 sources of ambient noise?

A

Ocean Turbulence
Shipping Noise
Surface Waves
Ocean Thermal Noise
Explosions
Marine life
Rain
Seismic activity
Shore based activity

60
Q

How do we estimate ambient noise?

A

Wenz Curves

61
Q

Why does noise tend to be at lower frequencies?

A

Lower frequencies are attenuated less

62
Q

What are the two dominant types of noise in ambient noise level?

A

Shipping and Surface noise

63
Q

What are the two types of self noise spectrum?

A

Broadband (large bandwidth)
Tonal (narrowband)

64
Q

What are 3 sources of self-noise from a ship?

A

Machinery
Propeller
Hydrodynamic
Cable Strum of towed devices

65
Q

What is Reverberation?

A

Backscattering of acoustic energy from discontinuuities in the medium.

66
Q

What are the two categoreis of reverb?

A

Boundary Reverb
Volume Reverb

67
Q

What causes volume reverb?

A

Echoes from bubbles, marine life, etc. which occupy a volume of water.

Ship’s wake is also a volume scatterer. Wake homing torpedoes find the wake through volume reverberation.

68
Q

What is Boundary Reverberation?

A

Echoes off flat surfaces such as the top or bottom of the ocean usually.

More significant in littoral water.

69
Q

How do we reduce the effects of reverberation?

A

Use narrower beamwidths
Use Doppler shift to discriminate target and reverb
Use pulse compression

70
Q

How does pulse compression help reduce effects of reverberation?

A

The return pulse will not return in the correct waveform for some reason.

71
Q

What are the two waveforms used by active sonar?

A

CW and FM

72
Q

Describe CW active sonar

A

A pulse of constant frequency and duration is sent out.

Bandwidth = 1/T

73
Q

Describe Frequency modulation in active sonar

A

Frequency of the pulse changes during the duration T

Bandwidth is the amount of frequency variation.

74
Q

What are 4 types of passive sonar analysis?

A

Broadband
Narrowband
DEMON (demodulate propeller noise)
TMA

75
Q

Draw the Active Noise Limited Sonar Equation

A
76
Q

Write out the Active Noise Limited Sonar Equation

A
77
Q

Describe Source Level

A

Amount of sound transmitted by the Sonar

78
Q

Describe Transmission Loss (TL)

A

Sound intensity lost due to spreading and absorption

79
Q

Describe Noise Level (NL)

A

Amount of Isotropic Noise in the water

80
Q

Describe Directivity Index (DI)

A

Focusing ability for listening. Ability to reject noise from directions other than the target.

81
Q

Describe Detection Threshold (DT)

A

SNR required to achieve rated detection performance (measured by Pd and Pfa)

82
Q

Active Sonar Eqns: Echo Level: Write and describe it.

A

LS of equation

Intensity of echo measured at the hydrophone.

83
Q

Active sonar Eqns: Noise Masking Level: Write it and describe it

A

RS of equation

Minimum detectable echo level

84
Q

Echo/Signal Excess: Write it out and describe it

A

LS-RS

Signal in excess of the minimum required for detection

85
Q

Write out the Active Fiture of Merit for NL Sonar Eqn and describe it.

A

SL-NL+DI-DT

Maximum allowable two way transmission

86
Q

Draw the Active Reverb Limited Sonar Equation and write it out.

A
87
Q

Describe Reverb Level (RL)

A

Power from undesired echoes

88
Q

Write out the Active Figure of Merit for RL Sonar Eqn.

A

SL-RL-DT

89
Q

Why does this look like this

A

In the surface layer, the speed of sound is increasing with depth. Pressure is dominant. Sound travels faster at higher pressure. But temperature is still a factor so it is curved.

In the Main Thermocline, temperature becomes dominant and the speed of sound decreases as the temperature decreases as it gets deeper.

In the Deep Isothermal Layer, the temperature is constant and the Pressure dominates. Speed of sound increases with pressure.

90
Q

List 3 military applications of sonar

A

Depth measurement
Submarine hunting
Mine detection

91
Q

What does SONAR stand for?

A

Sound Navigation and Ranging

92
Q

Describe the piezoelectric effect

A

The generation of mechanical stress in response to an electric charge to a material.

Can be used to create pressure waves in a medium.

93
Q

Detect to Engage Sequence

A

Detect
Localize
Classify
Identify
Track
Engage

94
Q

What causes a shaddow zone layer?

A

The seasonal layer is warm from the summer.

On a cold day the surface layer becomes cold.

Now there is a warm layer between two cold layers.

Sound refracts towards the slower medium, which is OUT of the warm medium.

Subs will want to hang out in this warmer layer.

95
Q

What are two mechanisms that limit the maximum power that can be transmitted by an acoustic array?

A

Cavitation Threshold
Interaction effects

96
Q

How do you improve cavitation effects?

Interaction effects?

A

Cavitation effects: Deeper

Interaction effects: larger transducers or more spaced apart.

97
Q

Write put the passive sonar eqn and draw it out

A

Forgor to put it in

Sonar (15 decks)

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