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Advanced EW Week 1 > Day 3 > Flashcards

Day 3 Flashcards

1
Q

Characterstics of a search radar e.g LREW

A 
Big antenna
Low freq
High power
1 deg beamwidth
Fan shaped
Long ARP
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2
Q

Characteristics of height finding radar?

A

Vertical scan
Could use 2 antennae - one for azimuth one for height
Can be but doesn’t need to be rotated

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3
Q

Characteristics of surface search radar?

A

Same as search but height is known
Circular scan
Size of platform limits power and size of radar
Height of radar limits max range and ARP

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4
Q

Characterstics of airborne radar?

A

Generally small radar as airborne
Fighter aircraft - small forward facing pencil beam
Airliner - forward looking simple sector scan (weather)
AWACS - Long range, low ARP
MPA - variable

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5
Q

Explain circular scan

A

360 degree search arc at a constant scan rate

Early warning
Nav
Target acquisition
surface search

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6
Q

Explain how a circular scan can obtain height finding information

A

2 beams, 1 at 45 degree to vertical beam. Measures time difference between illuminations in each beam.

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7
Q

What are the 2 types of sector scan?

A 
Bi directional (up and down or left and right)
Uni direcional (single direction and resets to original position)
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8
Q

Characterstics of a raster scan

A

Pencil beam = high power, small beamwidth, small area coverage
BAR is a line of search which make up a FRAME which is a complete cycle of bars within the search pattern.
Searches a pre-determined matrix

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9
Q

What can raster scan be used for?

A

Airborne interception
Artillery shell trackers
Target acquisition

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10
Q

Limitations of Raster scan

A

Paints a small area of the sky
Can be time consuming
Solution is PALMER RASTER SCAN where beam is CONICALLY scanned around the raster site.

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11
Q

Tracking scan functions and the 4 main types

A

Primary functions : Targeting data & weapons systems to hit targets

SAM systems & AAA systems

Lobe switching
Monopulse
Conical scan

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12
Q

Explain lobe switching

A

Compares signal strength between lobes and aims antenna until returns are equal meaning target is within boresight.

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13
Q

Explain monopulse scan types

A

Radar transmits a single pulse and splits the beam into parts, compares signal strength of parts when they return

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14
Q

Explain conical scan

A

A conical rotation around the boresight to achieve coverage over a selected area. When target is in the middle, all returns will be the same.

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15
Q

Explain passive scanning

A

Lobe switching and conical scanning set to receive only

LORO lobe on receive only
COSRO Conical scan receive only

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16
Q

Explain phased arrays

A 
Passive or active
Complex functions and scan patterns
Electronic or mechanical scan
Electronic scanning enables multi function
Overcome 60 degree steering limitation
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17
Q

Components of an antenna?

A

Feed
Reflector
Transmitting medium e.g waveguide, CoAx

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18
Q

What is the purpose of an antenna?

A

Focus RF energy in a certain direction

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19
Q

What is the purpose of the Feed?

A

To match the transmission line to free space & illuminate only the reflector

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20
Q

What is the function of an reflector?

A

To focus RF in a certain direction

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21
Q

What is the difference between waveguide and coax?

A

Waveguide - heavy, inflexible but suffers no loss

Coax - Flexible but not suitable for high freq or power over 1ghz

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22
Q

Explain and identify antenna shapes

A

The larger the antenna the tighter the possibe beamwidth

Parabolic
Truncated parabolic

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23
Q

What is the function of a cosecant squared radar?

A

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

24
Q

Explain tapered illumination

A

Reduces spillover and sidelobes by changing amplitude through the feed horn.

25
Q

What are the different feed shapes and what do they produce?

A

Circular aperture: Pencil beam
Rectangular aperture: Fan beam

You can use multiple feeds to find height.

26
Q

Explain a cassegrain

A

Twists polarisation to avoid centre null by using a sub reflector.

27
Q

What is gain?

A

Measure of antennas efficiency to concentrate the energy in 1 direction.

Takes into account actual losses

Measures in decibels dBi

28
Q

What is beamwidth?

A

The half power point on the main lobe. The shape of the beam in either plane measured in degrees.

29
Q

Why do we need varying beamwidth?

A

Small for target discrimination

Large for early warning

30
Q

What is the formula for calculating beamwidth?

A

Beamwidth = (70 x lambda) / Diameter of antenna

31
Q

Name the different lobes

A

Main lobe
Side lobe
Back lobe

32
Q

Explain the term sidelobe in relation to the main beam

A

Energy reflected near to the parabola edges produce extra lobes of RF energy radiating out from the antenna

33
Q

What is a back lobe?

A

Energy spills over the edge of the reflector creating a lobe pointing in opposite direction to main lobe

34
Q

What are the 5 types of arrays?

A 
Linear
Planar
Broad wall slot arrays
Edge wall arrays
Beam steering arrays
35
Q

What is a linear array?

A

It is a single row of radiating elements 1/2 wavelength apart

36
Q

What is a planar array?

A

A number of arrays stacked vertically above each other 1/2 wavelength apart

37
Q

What is a broad wall slot array?

A

Slots are cut in the wide surface of the wave guard 1/2 wavelength apart.

(Slots allow wavelengths to escape)

Doesnt care about sidelobes
light in weight
compact
produces good beam shape

38
Q

What is tapering?

A

Vary the spacing between the slots to produce and illumination taper

39
Q

What are edge wall arrays?

A

Used when minimal side lobes are required. Slots are diagonal on edge of wave guard.

Wave guides need to be longer
Less compact

40
Q

What are the types of array/beam steering?

A 
Mechanical
Electrical (Freq + phase)
Time delay
41
Q

Explain mechanical beam steering?

A

Beams are steering by the mechanical rotation of the antenna.

Aircraft radar will tilt the main array to increase radar FOV.

42
Q

Explain electronic beam steering

A

Freq - Change the half wavelength relationship of the slots causing the beam to distort in azimuth.

Phase - Beam is steered by controlling the phase of illumination.

Time delay - Pulses emitted at different times on the same phase

43
Q

Why is DOA important?

A

Fixing emitters
Deinterleaving
Location of threats

44
Q

Explain baseline?

A

The larger the baseline, the better the crosscut and smaller the area of probability known as the Ellipse. This is where the intercept bearings intersect.

45
Q

How is an ellipse described? 4 points

A

Length of major axis
Length of minor axis
Orientation (from N 0-179)
Position to centre point (Lat Long)

46
Q

Considerations for baseline? 7 points

A 
Intel 
Strike ToT
Blind when turning
Too close / On top
High terrain
Threats
Cueing other assets - e.g use other ships ESM
47
Q

What are the 4 ways of measuring DOA?

A

Directional antennae
Amplitude comparison
Monopulse
Phase comparison

48
Q

Explain directional antennae

A

360 degree
When antenna points at radar, it provides direction
More accurate with smaller beamwidth

Disadvantage
Mechanical scanning antennae requires large radomes
High cost & needs to withstand g-forces

Advantage
Highly accurate
Only 1 antennae needed

49
Q

Explain amplitude comparison

A

Most common form
4 point 360 degree spiralling antennae
Detects direction based on different phase and amplitude of signal on different antennae

Advantages
Simple and low cost
Quick and easy

Disadvantages
Not very accurate

50
Q

Explain Monopulse

A

Radar system that compares the received signal from a single radar pulse via 4 receiver horns.
Works on just amplitude.
Equal amp in each horn means the target is central.

51
Q

Explain phase comparison

A

Used when more accurate DOA is required. Calculates a difference in phase angle based on a signals wave front. This angle is proportional to DOA.

Accurate to 1.5 degrees

Requires considerable processing time
High accuracy only achievable with specific antennae setup

52
Q

3 types of Elint aerial

A

Typical spinner system
Inferferometry (Phase comparison)
Polarisation sensing

53
Q

Explain a Typical Spinner System

A

Arc of instant coverage = beamwidth
Can spin at high speed (20Hz) for all round coverage
Frequency 1 GHz upwards
Gain > 20dB

54
Q

Explain a Interferometry System

A 
Arc of coverage = 360 degrees
Elevation coverage = lower hemisphere
Frequency 0.1-18GHz
Direction finding accuracy in the order of 1 degrees
Gain 0 dB
55
Q

Explain Polarisation Sensing

A

Rotating horn
Rotating aerial
Switching between aerials with fixed polarisation

56
Q

How does a RWR locate threats?

A

Use power measurement, displaying a threat if power is above a threshold

Accuaracy is proportional to the accuaracy of MDD

Must known:
power orgininally transmitted
Atmospheric effects
Gain/losses of the RWR’s antenna

Relies on:
ELINT gathering
Ability of MA (EW) Operators

57
Q

What is a RWR detection and declaration sequence?

A 
Identifies
Measures the received power
Compares to the known transmitter power
Range calculated
Warning given

Advanced EW Week 1 (4 decks)

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