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EM spec > Revision > Flashcards

Revision Flashcards

1
Q

Explain the military’s main uses of the EM spectrum

A

Radio communications
Primary and secondary radar
IR detection
UV seekers

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2
Q

What does RADAR stand for?

A

Radio detection and ranging

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3
Q

What is the speed of light in all three units?

A

3x10^8 m/s
162,000 nm/s
186,000 miles/s

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4
Q

What is the relationship between wavelength and frequency?

A

The longer the wavelength the lower the frequency

They are inversely proportional

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5
Q

Polarisation is determined by the orientation if which field?

A

Electric field

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6
Q

What are the three types of polarisation?

A

Random (light)
Linear (horizontal/vertical)
Circular (right hand and left hand)

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7
Q

What is the equation that links the speed of light, frequency and wavelength?

A

C = wavelength x frequency

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8
Q

State the relationship between power density and range

A

Power density will decrease as range from the source increases

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9
Q

State the 2 forms of atmospheric attenuation

A

Absorbtion

Scattering

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10
Q

Explain atmospheric absorbtion

A

Occurs when EM energy interacts with gas molecules, the molecules absorb the radiation and energy is lost as heat

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11
Q

Explain atmospheric scattering

A

When EM waves hit molecules they change direction as a result and may scatter in many directions

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12
Q

How much scatter if the circumference of the sphere is smaller than the wavelength?

A

Little energy is scattered

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13
Q

How much scatter if the circumference of the sphere is a similar size to the wavelength?

A

Scatter Large amounts of energy that varies over large values

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14
Q

How much scatter if the circumference of the sphere is bigger than the wavelength?

A

Scatters a constant high level of energy

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15
Q

Explain diffraction in relation to propagation

A

Diffraction is the term used for bending waves around edges of objects

Longer wavelengths are diffracted more than shorter wavelength

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16
Q

State the different wave types

A 
Ground wave (space/surface) 
Sky wave
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17
Q

State the factors affecting a space wave

A

Attenuation (absorbtion), scatter, reflection

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18
Q

What is the average path length for a space wave? How may you increase this?

A

25-40km

Have relay stations to increase distance

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19
Q

Which layers of the Earth’s atmosphere have a major effect on the performance of EM waves and why?

A

Troposphere (all your weather in this region)

Ionosphere (charged ionised particles allow waves to reflect back to surface)

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20
Q

What are the 2 types of transmitters?

A

Occilator (DC into high power radio frequency output)

Amplifier (produces high power replica of the input signal)

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21
Q

What are the 3 main transmitter devices?

A

Magnetron (occilator)
Traveling wave tube (amplifier)
Field effect transistor (FET). (amplifier)

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22
Q

What are the 3 main types of modulation?

A

Amplitude
Frequency
Phase

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23
Q

What are the 8 factors affecting radar?

A 
Frequency
PRF
PW
ARP 
Power 
Beamwidth 
Antenna 
Weight
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24
Q

What are the main components of a radar?

A 
MTG (synchronizer) 
Modulator
Transmitter 
Duplexer (switch and protects reciever) 
Reciever
Display
25
Q

What are the three types of radar display?

A 
A scope (gives shape, outline etc by analysing return amplitude)
B scope (range on y axis, azimuth on x, closest to you is bottom of screen, acts as zoom function) 
PPI (plan position indicator)
26
Q

What target information can we get?

A 
Speed 
Height
Bearing 
Range
Size/shape
Course
27
Q

What are the processes of the radar reciever?

A 
Protection circuit
Amplification of signal
Filter out clutter 
Demodulate 
Process
Convert to video pulses
28
Q

How does demodulation occur?

A

High frequency pulse comes in and is mixed with localized oscillator frequency in the mixer which gives a lower, more manageable intermediate frequency

29
Q

What 4 things should an ES reciever do?

A

Intercept
Measure
Identify
Display

30
Q

Describe amplitude comparison

A

Invokes the measurement of the relative amplitudes of a signal intercepted by the gain patterns of antennas that are orientated at different angles with respect to the target

31
Q

Describe phase comparison

A

Uses the difference in phase from the same signal to work out direction. A time delay is used to out the signals in phase, this delay is used to calculate the relative DoA

32
Q

Describe unintentional pulse modulation

A

Caused by system noise and/or different operation/installation of components and helps with specific emitter identification (SEI)

33
Q

What is SEI

A

Specific emitter identification

34
Q

What are the components of an ES reciever?

A 
Antennae
Amplifier
Reciever 
Processer ------- DoA resolver 
Outputs
35
Q

How can height information be obtained from a circular scanning radar?

A

With a circular scan radar use 2 beams, 1 vertical and one at 45 degrees

36
Q

How does a search radar calculate aircraft height using a V beam?

A

Time difference between illuminations in each beam.

The longer the time interval, the higher the target

37
Q

What are the 2 types of sector scans?

A

Bi directional

Uni directional

38
Q

What are the advantages of a pencil beam?

A

High power
Small beamwidth
Good tracking information (accurate)

39
Q

Describe a raster Scan

A

Bar - a line of search
Frame - complete cycle of bars within the search pattern

Sweeps one bar, moves down to next etc

40
Q

Describe a monopulse radar

A

Uses one radar beam split up until parts

Compare the signal strength of the parts when they return to work out where target is in relation to borepoint

41
Q

What is a phased array scan?

A

Active or passive
Electronically steered
+/- 60 degree of boresight is search area

42
Q

What is the radar range equation?

A

Pr = (Pt x G^2 x sigma x Wavelength ^2) / ((4pi)^3 x r^4 )

Pt power transmitted
G = gain
Sigma = reflection value from the cross sectional area of target

43
Q

How do you convert 400 into dB?

A

10log(400)

44
Q

How do you convert 52 into real numbers?

A

52/10 = 5.2

10^5.2

45
Q

What is SNR?

A

Signal noise ratio

The ability for a radar to see targets over noise

46
Q

Define bandwidth

A

Bandwidth is the difference between upper and lower frequencies and is typically measured in Hz

47
Q

What factors will effect signal quality SNR?

A

Distance from the source
Atmospheric interference (sun spot)
Multipath interference
Jamming

48
Q

What are the 4 types of different scans

A

Circular
Sector
Raster
Complex (phased array)

49
Q

What are the 4 types of tracking scans?

A

Lobe switching
Monopulse
Conical
Unidirectional sector

50
Q

What is the function of a cosecant squared radar?

A

More power at a longer range to give similar size returns for the same size target at different ranges

51
Q

What does a cassegrain antenna achieve and how?

A

It avoids the problem of a centre null by a sub reflector which changes polarisation

52
Q

Define beamwidth

A

The width of the beam in degrees measured at the -3dB power points

53
Q

What are the components of a antenna system?

A

Feed
Reflector
Transmitting medium

54
Q

What are the two types of array?

A

Linear - slots in a line (2D)

Planar - stacked on top of each other (3D)

55
Q

What are the types of beam steering?

A

Electrical (60 degrees left or right of boresight)

Mechanical (if you want to go further)

56
Q

State the relationship between antenna size and wave length

A

Smaller antenna = smaller wavelength

57
Q

What is an ellipse and what is it described by?

A

Made from the triangulated ESM bearings

Described by:

Length of major axis
Length of minor axis
Orientation off north
Coordinates off center

58
Q

What is the main advantage with electronic scanning ?

A

Electronic scanning is much quicker and agile

59
Q

What are the two types of electrical beam steering?

A

Phase

Frequency

EM spec (9 decks)

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