What is the long-base-line (LBL) system?
A repeatedly used area can be equiped with beacons, to determine the position of the vessel.
What is roll, pitch and yaw in this picture?
Roll: tilting rotation around x-axis
Pitch: rotation around y-axis
Yaw: rotation around z-axis
Which possible movements can the vessel make in x-,y- and z-direction?
Surge: move on x-axis
Sway: move on y-axis
Heave: move on z-axis
What types of sonar-mounts are there?
Hull-mounted:
+known position
- noise coupling from vessel
Towed:
+little noise from vessel
- position difficult to determine
autonomous:
+less risk for people
+get to hard to reach areas
-expensive
-difficult to know precise location
What does Sonar stand for?
Sound wave navigation and ranging
How does Sonar work?
Sound wave is transmitted
gets reflected by objects
receives echo and can translate that to a picture
When was the first active Sonar build?
1918
What types of Sonar are there (5)
Submarine detection
Fish finding
Visual aids
Echo/depth sounding
Imaging
What are the 3 steps in the movement of waves?
What are practical assumtions made for simplicity reasons regarding the wave equation?
the system is linear
only first-order terms
supperposition principle applies
the fluid is inviscid
no viscosity
reasonable for water ( low viscosity, high Reynolds number[ratio between inertial and viscous forces])
the fluid is irrational
doesn’t rotate as it moves
significant for highly viscous fluids, turbulences and fluid boundaries
Click the right properties of Sonar in comparison with Radar
What does the linear continuity equation tell us?
defines the change in density due to movement of the fluid
What does the linear equation of state express?
Relates change in density to a change in pressure.
What does the linear Euler’s equation express?
describes fluid motion caused by change in pressure.
What does the Kirchhoff-Helmholtz integral relate to?
The value of a field at some point within a region to the values of the field.
Name 2 assumptions regarding transducers.
end-caps are sufficiently small to be neglected
back surface is fixed
What is the near field region?
Is the region a few wavelengths from the aperature
What is the advantage of the Fresnel approximation over the Fraunhofer approximation?
What is the Rayleigh distance?
Is the distance at which the error in the approximation is a t most one-sixteenth of the wavelength.
What is a Transducer?
Is a device which converts energy from one form to another.
What is a piezoelectric transducer? (Script: 6.1)
Is a transducer with the ability to develop voltage in response to mechanical force and vice versa.
Name a piezoelectric material used for Sonars. (Script: 6.1.1)
PZT (ceramic, cheap, must be polarized)
PVDF (polymer, expensive, requires a higher drive signal)
Name the Components of the transducer
Why to we use transducer coordinates and what do they look like?
Per Transducer we use a coordinate system in which the y-z-plane is the transducer and thus the x-axis being the normal to the generated wave.
By this it is easier to calculate.
What does the sonar equation state?
The received energy has to be larger than the reception threshold of our system (minimal energy required to detect an input)
Received Energy > RT
What are components of the Received enrgy?
Source Level (what do we send out) +
Transmission loss (twice, due to travell) -
Noise Level (different interferences) -
Directivity index (how focussed is my receiver) +
Processing gain +
Target strength (refelectivity of target) +
Which components are lost by using passive sonar?
Transmission loss (only one time, becouse we only listen)
Target strength (because we only listen)
3 possible ways to differentiate image resolution
Image and System resolution are equal
sufficient space to display all your data
Image- < System resolution
we have to pack more information in the Image than needed
reduction of noise possible
Image- > System resolution
insufficient data set
have to interpolate to give enough data to the image
How do we reiceive an echo?
Incident pressure wave generates a mechanical force on the piezoelectric receiver, the receiver produces a voltage which can be transformed into data and can be stored.
What is a Matched filter
Impulse response of the reiceive filter is the same as the one in the transmitt filter, but time is reversed and shifted by T.
Used to increase signal to noise ratio (SNR)
What is the minimal required resolution if we use a Sinusoidal wave?
[sigma]t ~ T
or
[sigma]r = (cT)/2
What is an advantage and a disadvantage of Pulse compression?
+shorter pulse leads to better resolution
-lesser energy leads to shorter distance
What is the differnece linear frequency modulated (LFM) chirp and a hyperolic frequency modulated (HFM) chirp?
LFM the frequency increases/decreases linearly.
HFM the frequency increases/decreases logarithmically.
-slightly worse resolution
-insensitive to Doppler Effect
What is the spatial Doppler wavenumber?
Ku
What is the difference between spatial Doppler effect and temporal Doppler effect?
spatial: caused by rate of change of the distance between target and transmitter
temporal: caused by the velocity of the platform
What is the difference between range-Doppler algorithm and wavenumber algorithm?
RDA: one dimensional Fourier transform
WA: two dimensional Fourier transform
Disadvantages of RDA?
doesn’t work well for wide beamwidths, due to necessity of secondary processing
What do we need for both RDA and WA?
reliance on perfectly linear trajectory of sonar (due to Fourier Transform)
Post-processing algorithm (e.g. autofocusing) to reduce artefacts caused by platform motion
What is windowing used for and how does it work?
What is the phase center approximation used for?
How do we convert our incomming data into the groundplane?
What is the effect of multiple element arrays used in a transducer?
Describe the Mill’s cross setup
What is Ergodicity?
Is the fact that you can’t split parts of a system to smaller levels.
What is the mathematical limit for the Target Strength equation?
5cm Ball —> TS = -32
2m Ball —> TS = 0
—> not possible, bc energy can’t be created
therefore not possible, bc we define the equation at a distance of 1m to the center of the object —> not valid for the case of a ball r>1m, bc otherwise our transmitter would be in the object
Which effect do air bubbles have on scattering?
They have resonance frequency (depend on size, water depth, thermodynamic properties)
bigger bubbles have a lower resonance frequency
In which case does the critical angle effect the scattering on the sea floor?
Below the critical angle, the reflection of the wave at the sediment is transferred allong the sediment surface and is not directed back towards the receiver.
Which order concerning the reflection coefficient is right?
What is the stop and hop approximation?
We assume that the transmitter stops when sending a ping, remains stationary (stop) until echo is received and intantaneously moves to the location the next ping is transmitted (hop)
Why is the stop and hop approx. valid?
We ping in the order of 100ms, thus the error due to canceled platform movement is not significant to other errors in the system.
Last changed2 years ago
