Revision Week 2 Flashcards
Explain the Doppler effect
An apparent change in frequency caused by a change of motion of an object relative to an observer
What are the considerations of a continuous wave radar?
Only gives velocity
Cannot measure range
Needs 2 aerials (one to receive)
Not usually fitted in aircraft
What is coherence radar?
Consistency or continuity in phase of a signal from one pulse to the next
What is FMCW?
Frequency modulating continuous wave
Frequency is changed to allow range to be calculated
What do we need for a Doppler radar?
Coherent radar
Reciever mist be linked to transmitter to know phase
What is a ICW radar?
Interrupted continuous wave radar Aka Pulse Doppler Coherent wave Can calculate range Only requires one areial
What is are the types of intrapulse modulation?
Types of pulse compression
Frequency modulating on pulse (FMOP)
Phase modulation on pulse (PMOP)
What is the purpose of pulse compression?
Get the range and detection capability of a long pulse with the target discrimination of a short pulse
What are the 2 ways to achieve multi frequency?
Frequency diversity
Frequency agility
What is frequency diversity?
Transmitting on multiple frequencies at the same time
What is frequency agility?
Ability for a radar system to switch between multiple frequencies in a controlled manner
What are blind speeds?
When the Doppler shift of a returning pulse is equal to the PRF or it’s harmonics
What is a Doppler notch?
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)
What are tangential fade?
The ability to distinguish a target becomes harder as you approach the tangent due to the decrease in Doppler effect
What are the 2 types of jitter?
Discrete - pre set PRIs transmitted randomly. Spaces between PRI are usually equal
Random - randomly generated PRIs transmitted between an upper and lower set limit
Examples of interpulse modulation
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
Explain MTI (moving target indicator) diagram
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
What is blind range?
Echo that is returning at the same time as another pulse is sent out. Equal to MUR or other factors of it
What are the benefits of intrapulse modulation (pulse compression)
Range resolution
What are interpulse and intrapulse modulation?
Interpulse - transmitted at intervals that are varied
Intrapulse - occurs across the pulse duration
What are the benefits of interpulse modulation?
Blind speeds and ranges as PRF and PRI changes
Second time round returns (as radar will see jitter from false target when MUR changes)
What is an Element?
Describes an individual PRI.
Number of different PRIs = number of elements
What is a Position?
Describes number of PRIs in cycle
Can use same element more than once
What is firing order?
Describes the order in which PRIs are transmitted
Starts with shortest PRI in sequence
What is Cyclic length?
Time taken for transmission of complete PRI stagger
What is multi- frequency? And what does it help?
Multi frequency radars use two or more illumination frequencies
Can improve detection ranges by minimizing weather effects, jamming and mutual interference.
What is dwell and switch?
The radar transmits a series of pulses on one PRF before switching to another PRF for a number of pulses
What is multi-pulse?
Changes the position, order or other characteristics of pulses within a frame to convey data
Draw out the components of a radar system
Oscillator – mixer – high pass filter — amplifier-
oscillator duplexer
Processer - low pass filter- mixer - amplifier bandpass filter - attenuator
What is the purpose of the oscillator?
To generate and provide the sinusoidal signal
What is the function of a mixer?
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
What is a filter?
Removes the unwanted frequencies
Lowpass - filters out high
Highpass - filters out low
Bandpass - filters out above and below
What is the function of an attenuator?
To attenuate any high power signals to prevent them from overloading the processing circuit
Why do we need SATCOM?
Developed for long range communications
Recent conflicts have been in remote areas with poor comms networks
What are the different types of orbit associated with SATCOM?
Circular
Elliptical
What is apogee?
Point of maximum range and minimum speed
What is perigee?
Point of minimum range and maximum speed
Draw an eliptical orbit
Earth eliptical orbit Apogee Perigee Equitoral plane
What is the advantage and disadvantages of Geo stationary Orbit?
Infinite Dwell time/doesn’t require following antenna
Expensive /impossible to repair
Advantages and disadvantages of sub synchronous orbit
Faster coverage of large areas
Need lots for full coverage
Advantages/ disadvantages of low earth orbit
Can be fixed/decreased lag time
Need lots for full coverage
What is the difference between Geosynchronous and geostationary?
Geosynchronous is angled from equatorial line
Slightly eliptical
Describe geostationary orbit
Same angular velocity as earth
Must be above equator
Expand SEAD and briefly explain
Suppression of enemy air defense
Designed to neutralize or temporarily degrade enemy air defense
Achieved by destructive or desruptive means
Eg Jamming
Expand and briefly explain DEAD
Destruction of enemy air defense
Designed to permanently suppress enemy air defense
Dropping a HARM missile on it and blow it up.
Expand and Explain TST
Time sensitive Targeting
Targets require immediate response as the pose a danger to friendly forces or are highly lucrative, fleeting targets of interest
Give 6 components of integrated air defense systems (IADS)
Fighter aircraft Naval vessels Airborne early warning aircraft Missile guidance radars Target tracking radars Target acquisition radars C2 nodes Data links
What are the stages of the engagement process?
Search Acquire Track Launch Intercept
What radars are used in the Search phase of engagement process?
EW
HF
What radars are used in the Acquire phase of engagement process?
TA
What radars are used in the track phase of engagement process?
TT
What radars are used in the launch phase of engagement process?
AI
FC
MG
What radars can operate acquire, track and launch phases
MF
What are the features of a multi function radar?
Electronically scanned phased array
Rapid scanning over sectors of 90 degrees
Can use pulse compression, frequency agility, Pulsed Doppler and Multiple PRF
What are the three methods of missile homing?
Active
Semi active
Passive
How does a passive missile homing work?
Locks onto emissions from the target (Radar or IR)
Covert
Fire and forget
HARM
How does Semi Active missile homing work?
Still homes passively
Locks onto reflected signal from an external illuminating emitter
How does an active homing missiles work?
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
Compare active and semi active missile guidance
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.
What are the three types of command line of sight (CLOS)
Manual - tracking missile and target done manually
Semi automatic - tracking target manually, missile automatically
Automatic - both automatic
Explain track via missile systems
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
What features must a RWR have?
Sensitive Accurate and immediate direction finding 100% intercept probability Measure parametric data Identify emitter and prioritise Display results clearly
What are the 3 types of receivers?
Crystal video receiver (CVR)
Instantaneous Frequency measurement reciever (IFM)
Scanning Superheterodyne Reciever (SHR)
Advantages and disadvantages of CVR
Advantages - Wide bandwidth, cover large parts of frequency spectrum at once, cheap, simple.
Disadvantages - unable to measure Frequency, poor sensitivity. cannot detect CW.
Advantages and Disadvantages of IFM
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
Advantages and disadvantages of Scanning Superheterodyne Reciever
Advantages - very sensitive, very selective in frequency,
Disadvantages - heavily reliant on software, very complex, requires knowledge of target emitters
What is Dwell?
Dwell is how long the superheterodyne receiver will look at or revisit a given frequency
What is range advantage?
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
What are the two types of RF counter measures?
Electronic
Mechanical
What are the 2 types of electronic RF countermeasures?
Jamming
Deception
What are the three types of jamming?
Spot
Barrage
Sweep
What is sweep jamming?
A moving spot jammer
Can jam multiple frequencies in quick succession but not all at same time which can reduce effectiveness
What is spot jamming?
Jamming one frequency
All it’s power on one frequency
Makes it Vunerable to frequency hopping
What is barrage jamming?
Jams multiple frequencies at a lower power
Spreads power out among lots of frequencies
Makes it more Vunerable to burn through
What is burn through?
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
What are your types of Deception under ‘electronic repeater Jamming’ ?
Range/velocity gate pull off
Angle deception
False target generation
What are the different false targets can be generated and how?
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
What are the mechanical countermeasures?
Chaff
Decoys
What is chaff cut to for a specific wavelength?
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
What are the 2 types of decoy?
Active (transmits a jamming signal)
Passive (resembles the aircraft purely on radar cross section/reflectiveness and materials)
How do we make an aircraft stealthy?
Low RCS
Radar absorbent material
Shape (no big reflective areas)
Size (smaller better than massive)
Definition of countermeasures
A countermeasure is defined as an action taken by a platform to negate the effect of a threat
What are the IR sources on an aircraft?
Exhaust plume Engine nozzles Engine inlets Leading edge Nose
What 3 characteristics must flares have?
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
What are the 2 flare countermeasures?
Dual band detectors
Kinematic flare rejection
What is Duel band detector?
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
What is kinematic flare detection?
IR seeker predicts aircraft path, when flares come out the back it can disregard them as it knows the limits of that aircrafts Performance.
What is the IRCCCM for duel band detector?
Spectral flares
Replicate a similar IR profile as the aircraft at a higher intensity
What are the IRCCCM for kinematic flare rejection?
Aerodynamic (forward firing) flares
What are MTV flares?
Effective against earlier missiles but show at night and give off UV
What Are some requirements of a MWS?
Spherical coverage High probability of detection Long range Low false alarm rate Speed
What are the stages of a typical missile flight?
Eject Boost Sustain Coast Fuse
What is the MWR process?
Detection
Track
declaration
What are the types MWS?
IR
UV
Radar
Laser
Advantages and disadvantages of radar MWS?
Doesn’t rely on emissions
Calculates range and velocity
Accurate range and velocity information
Limited detection range
Overt
advantages and disadvantages of UV MWS
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
IR MWS advantages and disadvantages
Covert
Can detect during entire fight
High false alarm rate
Lower range resolution
What is laser scattering?
Atmosphere diffuses the laser beam away from designator spot
What are the laser countermeasures?
Absorbtion
Reflection
Ablation
Jamming
What are the disadvantages of IRCM?
Beacon
Lots of wasted energy as omnidirectional
Less effective range
Advantages of DIRCM
Focus energy, better range for same power
Very effective
How is DIRCM operated?
UV MWS detects the threat, this is passed over to DIRCM
DIRCM acquires and uses MDD data to demodulate the seeker head
Closed loop IRCM
Uses reflected energy from the missile to determine the modulation and thus figure out the appropriate jamming signal
What are the 3 counter measures used in IR missiles?
IRCM
Flares
Active metal decoy (AMD)
What are the different types of guidance employed by a weapons system?
Passive homing
Active
Semi active
Command guidance - uses an uplink from a tracking radar
What is the basic principle of LPI?
Low probability of intercept.
Detect a target without being detected
State the three levels of LPI?
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
4 aspects for effective implementation of the LPI
Beam control
Waveform management
Power optimisation
Low observability
How may you cover beam control?
Narrow mainlobe
Low sidelobe level
Bistatic radar
Examples of waveform management
Spread spectrum technique
Pulse compression
Frequency agility
Examples of power management
Only transmit when operationally necessary
Decreases power as target comes closer
Examples of low observability
Lower RCS Lower ARP to reduce Doppler returns Physical observability (camo)
What is FRUITING?
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
What is Garbling?
Replies from 2 aircraft overlap If their range separation is low
Makes the replies unable to be determined
How are modes distinguished between?
Changing the time period between the pulses
Mode 1
Aircraft type/mission
2 digit code
Mode 2
4 digit octal code
Unique identifier
Assigned by command
Mode 3/A
4 digit octal code assigned by ATC
Mode C can give barometric altitude
Says who is controlling you
Mode S
Used for TCAS
24 bit unique aircraft address
Gives ICAO reporting information
What are the advantages of SSR?
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
SSR disadvantages
Needs the AC to be fitted with a transponder
Ground antenna are highly directional but cannot be designed without sidelobes
Applications of SSR
Identify possible hostile targets
Communications of emergencies etc
Safety of flight via TCAS/ATC
