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EM spec > Revision Week 2 > Flashcards

Revision Week 2 Flashcards

1
Q

Explain the Doppler effect

A

An apparent change in frequency caused by a change of motion of an object relative to an observer

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2
Q

What are the considerations of a continuous wave radar?

A

Only gives velocity
Cannot measure range
Needs 2 aerials (one to receive)
Not usually fitted in aircraft

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3
Q

What is coherence radar?

A

Consistency or continuity in phase of a signal from one pulse to the next

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4
Q

What is FMCW?

A

Frequency modulating continuous wave

Frequency is changed to allow range to be calculated

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5
Q

What do we need for a Doppler radar?

A

Coherent radar

Reciever mist be linked to transmitter to know phase

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6
Q

What is a ICW radar?

A 
Interrupted continuous wave radar
Aka Pulse Doppler 
Coherent wave 
Can calculate range 
Only requires one areial
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7
Q

What is are the types of intrapulse modulation?

A

Types of pulse compression
Frequency modulating on pulse (FMOP)
Phase modulation on pulse (PMOP)

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8
Q

What is the purpose of pulse compression?

A

Get the range and detection capability of a long pulse with the target discrimination of a short pulse

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9
Q

What are the 2 ways to achieve multi frequency?

A

Frequency diversity

Frequency agility

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10
Q

What is frequency diversity?

A

Transmitting on multiple frequencies at the same time

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11
Q

What is frequency agility?

A

Ability for a radar system to switch between multiple frequencies in a controlled manner

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12
Q

What are blind speeds?

A

When the Doppler shift of a returning pulse is equal to the PRF or it’s harmonics

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13
Q

What is a Doppler notch?

A

Refers to a velocity of contacts below which radar classes them as clutter and does not display them

Tangential fade +/- a little bit (whatever the cut off is)

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14
Q

What are tangential fade?

A

The ability to distinguish a target becomes harder as you approach the tangent due to the decrease in Doppler effect

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15
Q

What are the 2 types of jitter?

A

Discrete - pre set PRIs transmitted randomly. Spaces between PRI are usually equal
Random - randomly generated PRIs transmitted between an upper and lower set limit

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16
Q

Examples of interpulse modulation

A

Changes in PRF/PRI

PRI steady - Same PRI used
Stagger - small or large discrete PRI changes in recognisable pattern
Dwell and switch - PRI changes in recognisable pattern after each set of pulses
Jitter - apparently random small PRI changes (discrete/random)
PRI agility - apparently random large PRI changes

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17
Q

Explain MTI (moving target indicator) diagram

A

Negative — 0 (altitude return) —- Positive

Mainlobe clutter

Moving away from you - negative return
Moving towards you - positive return

In mainlobe clutter - similar to your velocity
Right of mainlobe clutter - faster than your velocity
Left of mainlobe clutter - slower than your velocity

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18
Q

What is blind range?

A

Echo that is returning at the same time as another pulse is sent out. Equal to MUR or other factors of it

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19
Q

What are the benefits of intrapulse modulation (pulse compression)

A

Range resolution

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20
Q

What are interpulse and intrapulse modulation?

A

Interpulse - transmitted at intervals that are varied

Intrapulse - occurs across the pulse duration

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21
Q

What are the benefits of interpulse modulation?

A

Blind speeds and ranges as PRF and PRI changes

Second time round returns (as radar will see jitter from false target when MUR changes)

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22
Q

What is an Element?

A

Describes an individual PRI.

Number of different PRIs = number of elements

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23
Q

What is a Position?

A

Describes number of PRIs in cycle

Can use same element more than once

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24
Q

What is firing order?

A

Describes the order in which PRIs are transmitted

Starts with shortest PRI in sequence

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25
Q

What is Cyclic length?

A

Time taken for transmission of complete PRI stagger

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26
Q

What is multi- frequency? And what does it help?

A

Multi frequency radars use two or more illumination frequencies

Can improve detection ranges by minimizing weather effects, jamming and mutual interference.

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27
Q

What is dwell and switch?

A

The radar transmits a series of pulses on one PRF before switching to another PRF for a number of pulses

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28
Q

What is multi-pulse?

A

Changes the position, order or other characteristics of pulses within a frame to convey data

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29
Q

Draw out the components of a radar system

A

Oscillator – mixer – high pass filter — amplifier-

                           oscillator                                  duplexer

Processer - low pass filter- mixer - amplifier bandpass filter - attenuator

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30
Q

What is the purpose of the oscillator?

A

To generate and provide the sinusoidal signal

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31
Q

What is the function of a mixer?

A

Mixes a modulated frequency and local oscillator frequency to give 2 intermediate frequencies which are the sum of and the difference between the two inputs

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32
Q

What is a filter?

A

Removes the unwanted frequencies

Lowpass - filters out high
Highpass - filters out low
Bandpass - filters out above and below

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33
Q

What is the function of an attenuator?

A

To attenuate any high power signals to prevent them from overloading the processing circuit

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34
Q

Why do we need SATCOM?

A

Developed for long range communications

Recent conflicts have been in remote areas with poor comms networks

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35
Q

What are the different types of orbit associated with SATCOM?

A

Circular

Elliptical

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36
Q

What is apogee?

A

Point of maximum range and minimum speed

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37
Q

What is perigee?

A

Point of minimum range and maximum speed

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38
Q

Draw an eliptical orbit

A 
Earth 
 eliptical orbit 
Apogee
Perigee 
Equitoral plane
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39
Q

What is the advantage and disadvantages of Geo stationary Orbit?

A

Infinite Dwell time/doesn’t require following antenna

Expensive /impossible to repair

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40
Q

Advantages and disadvantages of sub synchronous orbit

A

Faster coverage of large areas

Need lots for full coverage

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41
Q

Advantages/ disadvantages of low earth orbit

A

Can be fixed/decreased lag time

Need lots for full coverage

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42
Q

What is the difference between Geosynchronous and geostationary?

A

Geosynchronous is angled from equatorial line

Slightly eliptical

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43
Q

Describe geostationary orbit

A

Same angular velocity as earth

Must be above equator

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44
Q

Expand SEAD and briefly explain

A

Suppression of enemy air defense

Designed to neutralize or temporarily degrade enemy air defense

Achieved by destructive or desruptive means

Eg Jamming

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45
Q

Expand and briefly explain DEAD

A

Destruction of enemy air defense

Designed to permanently suppress enemy air defense

Dropping a HARM missile on it and blow it up.

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46
Q

Expand and Explain TST

A

Time sensitive Targeting

Targets require immediate response as the pose a danger to friendly forces or are highly lucrative, fleeting targets of interest

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47
Q

Give 6 components of integrated air defense systems (IADS)

A 
Fighter aircraft
Naval vessels
Airborne early warning aircraft 
Missile guidance radars
Target tracking radars
Target acquisition radars
C2 nodes 
Data links
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48
Q

What are the stages of the engagement process?

A 
Search 
Acquire
Track
Launch
Intercept
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49
Q

What radars are used in the Search phase of engagement process?

A

EW

HF

50
Q

What radars are used in the Acquire phase of engagement process?

A

TA

51
Q

What radars are used in the track phase of engagement process?

A

TT

52
Q

What radars are used in the launch phase of engagement process?

A

AI
FC
MG

53
Q

What radars can operate acquire, track and launch phases

A

MF

54
Q

What are the features of a multi function radar?

A

Electronically scanned phased array
Rapid scanning over sectors of 90 degrees

Can use pulse compression, frequency agility, Pulsed Doppler and Multiple PRF

55
Q

What are the three methods of missile homing?

A

Active
Semi active
Passive

56
Q

How does a passive missile homing work?

A

Locks onto emissions from the target (Radar or IR)
Covert
Fire and forget

HARM

57
Q

How does Semi Active missile homing work?

A

Still homes passively

Locks onto reflected signal from an external illuminating emitter

58
Q

How does an active homing missiles work?

A

Has both a transmitter and receiver in the missile

Tracks the missile from reflected energy from it’s own transmissions

Overt

Fire and forget

AMRAAM

59
Q

Compare active and semi active missile guidance

A

Autonomy - active is autonomous, semi active requires constant illumination

Launcher Vunerability - Semi active constantly illuminate target so launcher is a target

Complexity and cost - active is more expensive as has transmitter and receiver

ESM - Semi active constantly illuminate so ESM systems can detect the whole time. Active may only turn on late in engagement.

60
Q

What are the three types of command line of sight (CLOS)

A

Manual - tracking missile and target done manually
Semi automatic - tracking target manually, missile automatically
Automatic - both automatic

61
Q

Explain track via missile systems

A

Missile sends information to ground station via downlink.

Guidance corrections completed on ground and sent to missile on uplink.

Missile is recieving signals reflected from ground target

62
Q

What features must a RWR have?

A 
Sensitive
Accurate and immediate direction finding 
100% intercept probability
Measure parametric data
Identify emitter and prioritise
Display results clearly
63
Q

What are the 3 types of receivers?

A

Crystal video receiver (CVR)
Instantaneous Frequency measurement reciever (IFM)
Scanning Superheterodyne Reciever (SHR)

64
Q

Advantages and disadvantages of CVR

A

Advantages - Wide bandwidth, cover large parts of frequency spectrum at once, cheap, simple.

Disadvantages - unable to measure Frequency, poor sensitivity. cannot detect CW.

65
Q

Advantages and Disadvantages of IFM

A

Advantages - measures frequency, wide open (high bandwidth), cheap, simple

Disadvantages - can only process 1 signal at a time,
Some equipment doesn’t provide PRF or PW

66
Q

Advantages and disadvantages of Scanning Superheterodyne Reciever

A

Advantages - very sensitive, very selective in frequency,

Disadvantages - heavily reliant on software, very complex, requires knowledge of target emitters

67
Q

What is Dwell?

A

Dwell is how long the superheterodyne receiver will look at or revisit a given frequency

68
Q

What is range advantage?

A

The RWR has the advantage of detecting an emitter at a greater range before the emitter can detect the return pulse from the RWR platform

69
Q

What are the two types of RF counter measures?

A

Electronic

Mechanical

70
Q

What are the 2 types of electronic RF countermeasures?

A

Jamming

Deception

71
Q

What are the three types of jamming?

A

Spot
Barrage
Sweep

72
Q

What is sweep jamming?

A

A moving spot jammer

Can jam multiple frequencies in quick succession but not all at same time which can reduce effectiveness

73
Q

What is spot jamming?

A

Jamming one frequency

All it’s power on one frequency

Makes it Vunerable to frequency hopping

74
Q

What is barrage jamming?

A

Jams multiple frequencies at a lower power

Spreads power out among lots of frequencies

Makes it more Vunerable to burn through

75
Q

What is burn through?

A

Jammer power outputs are limited and eventually, as target moves closer to radar, it’s reflected energy is greater than it’s power jammer and can be seen

76
Q

What are your types of Deception under ‘electronic repeater Jamming’ ?

A

Range/velocity gate pull off
Angle deception
False target generation

77
Q

What are the different false targets can be generated and how?

A

False target by range - delay and re transmit replicas of radar pulse

False target by velocity - transmit jamming signals at different frequencies (falsifying Doppler shift)

False target by angle - jamming signal into sidelobe can create false targets

78
Q

What are the mechanical countermeasures?

A

Chaff

Decoys

79
Q

What is chaff cut to for a specific wavelength?

A

Chaff is cut to 1/2 wavelength to be effective at that frequency.

Metallic strips of varying lengths

Create cloud of chaff which confuses radar

80
Q

What are the 2 types of decoy?

A

Active (transmits a jamming signal)

Passive (resembles the aircraft purely on radar cross section/reflectiveness and materials)

81
Q

How do we make an aircraft stealthy?

A

Low RCS
Radar absorbent material
Shape (no big reflective areas)
Size (smaller better than massive)

82
Q

Definition of countermeasures

A

A countermeasure is defined as an action taken by a platform to negate the effect of a threat

83
Q

What are the IR sources on an aircraft?

A 
Exhaust plume 
Engine nozzles 
Engine inlets 
Leading edge
Nose
84
Q

What 3 characteristics must flares have?

A

Peak intensity - must radiate with sufficient intensity to be both credible and more attractive

Fast rise time - must reach effective level of intensity before leaving missile FOV

Burn time - must maintain credible signature until target is no longer in missile FOV

85
Q

What are the 2 flare countermeasures?

A

Dual band detectors

Kinematic flare rejection

86
Q

What is Duel band detector?

A

Flare counter counter measure

Compares the IR signature between bands 1 and 4 to create an aircraft profile. It can then work out if it’s a flare or not based on this profile

MTV shows high peak but then burns down so not realistic target

87
Q

What is kinematic flare detection?

A

IR seeker predicts aircraft path, when flares come out the back it can disregard them as it knows the limits of that aircrafts Performance.

88
Q

What is the IRCCCM for duel band detector?

A

Spectral flares

Replicate a similar IR profile as the aircraft at a higher intensity

89
Q

What are the IRCCCM for kinematic flare rejection?

A

Aerodynamic (forward firing) flares

90
Q

What are MTV flares?

A

Effective against earlier missiles but show at night and give off UV

91
Q

What Are some requirements of a MWS?

A 
Spherical coverage
High probability of detection
Long range
Low false alarm rate 
Speed
92
Q

What are the stages of a typical missile flight?

A 
Eject 
Boost
Sustain
Coast 
Fuse
93
Q

What is the MWR process?

A

Detection
Track
declaration

94
Q

What are the types MWS?

A

IR
UV
Radar
Laser

95
Q

Advantages and disadvantages of radar MWS?

A

Doesn’t rely on emissions
Calculates range and velocity
Accurate range and velocity information

Limited detection range
Overt

96
Q

advantages and disadvantages of UV MWS

A

Can detect firing and in flight emissions
Covert
Low false alarm rate

Doesn’t detect post burn out phase
Atmospheric absorbtion limits detectable signal strength

97
Q

IR MWS advantages and disadvantages

A

Covert
Can detect during entire fight

High false alarm rate
Lower range resolution

98
Q

What is laser scattering?

A

Atmosphere diffuses the laser beam away from designator spot

99
Q

What are the laser countermeasures?

A

Absorbtion
Reflection
Ablation
Jamming

100
Q

What are the disadvantages of IRCM?

A

Beacon
Lots of wasted energy as omnidirectional
Less effective range

101
Q

Advantages of DIRCM

A

Focus energy, better range for same power

Very effective

102
Q

How is DIRCM operated?

A

UV MWS detects the threat, this is passed over to DIRCM

DIRCM acquires and uses MDD data to demodulate the seeker head

103
Q

Closed loop IRCM

A

Uses reflected energy from the missile to determine the modulation and thus figure out the appropriate jamming signal

104
Q

What are the 3 counter measures used in IR missiles?

A

IRCM
Flares
Active metal decoy (AMD)

105
Q

What are the different types of guidance employed by a weapons system?

A

Passive homing
Active
Semi active

Command guidance - uses an uplink from a tracking radar

106
Q

What is the basic principle of LPI?

A

Low probability of intercept.

Detect a target without being detected

107
Q

State the three levels of LPI?

A

Low probability of intercept -detect a target but not be detected itself, at the same range but outside the main beam

Low probability of identification - can be detected but not easily identified

Quiet radar - can detect a target and yet not be detected by an ESM reciever located on the target

108
Q

4 aspects for effective implementation of the LPI

A

Beam control
Waveform management
Power optimisation
Low observability

109
Q

How may you cover beam control?

A

Narrow mainlobe
Low sidelobe level
Bistatic radar

110
Q

Examples of waveform management

A

Spread spectrum technique
Pulse compression
Frequency agility

111
Q

Examples of power management

A

Only transmit when operationally necessary

Decreases power as target comes closer

112
Q

Examples of low observability

A 
Lower RCS 
Lower ARP to reduce Doppler returns
Physical observability (camo)
113
Q

What is FRUITING?

A

Multiple replies to the same ground station
Multiple fruits can generate a false target on the ATC screen
Because they all transmit on 1090MHz
False replies unsynchronized in time

114
Q

What is Garbling?

A

Replies from 2 aircraft overlap If their range separation is low
Makes the replies unable to be determined

115
Q

How are modes distinguished between?

A

Changing the time period between the pulses

116
Q

Mode 1

A

Aircraft type/mission

2 digit code

117
Q

Mode 2

A

4 digit octal code
Unique identifier
Assigned by command

118
Q

Mode 3/A

A

4 digit octal code assigned by ATC

Mode C can give barometric altitude

Says who is controlling you

119
Q

Mode S

A

Used for TCAS
24 bit unique aircraft address
Gives ICAO reporting information

120
Q

What are the advantages of SSR?

A

As the pulses are coded they can send additional information between the two stations

Less power is needed at the ground station for a given range

The signal strength is stronger when received at the ground station

121
Q

SSR disadvantages

A

Needs the AC to be fitted with a transponder

Ground antenna are highly directional but cannot be designed without sidelobes

122
Q

Applications of SSR

A

Identify possible hostile targets

Communications of emergencies etc

Safety of flight via TCAS/ATC

EM spec (9 decks)

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