Revision Flashcards
Explain the military’s main uses of the EM spectrum
Radio communications
Primary and secondary radar
IR detection
UV seekers
What does RADAR stand for?
Radio detection and ranging
What is the speed of light in all three units?
3x10^8 m/s
162,000 nm/s
186,000 miles/s
What is the relationship between wavelength and frequency?
The longer the wavelength the lower the frequency
They are inversely proportional
Polarisation is determined by the orientation if which field?
Electric field
What are the three types of polarisation?
Random (light)
Linear (horizontal/vertical)
Circular (right hand and left hand)
What is the equation that links the speed of light, frequency and wavelength?
C = wavelength x frequency
State the relationship between power density and range
Power density will decrease as range from the source increases
State the 2 forms of atmospheric attenuation
Absorbtion
Scattering
Explain atmospheric absorbtion
Occurs when EM energy interacts with gas molecules, the molecules absorb the radiation and energy is lost as heat
Explain atmospheric scattering
When EM waves hit molecules they change direction as a result and may scatter in many directions
How much scatter if the circumference of the sphere is smaller than the wavelength?
Little energy is scattered
How much scatter if the circumference of the sphere is a similar size to the wavelength?
Scatter Large amounts of energy that varies over large values
How much scatter if the circumference of the sphere is bigger than the wavelength?
Scatters a constant high level of energy
Explain diffraction in relation to propagation
Diffraction is the term used for bending waves around edges of objects
Longer wavelengths are diffracted more than shorter wavelength
State the different wave types
Ground wave (space/surface) Sky wave
State the factors affecting a space wave
Attenuation (absorbtion), scatter, reflection
What is the average path length for a space wave? How may you increase this?
25-40km
Have relay stations to increase distance
Which layers of the Earth’s atmosphere have a major effect on the performance of EM waves and why?
Troposphere (all your weather in this region)
Ionosphere (charged ionised particles allow waves to reflect back to surface)
What are the 2 types of transmitters?
Occilator (DC into high power radio frequency output)
Amplifier (produces high power replica of the input signal)
What are the 3 main transmitter devices?
Magnetron (occilator)
Traveling wave tube (amplifier)
Field effect transistor (FET). (amplifier)
What are the 3 main types of modulation?
Amplitude
Frequency
Phase
What are the 8 factors affecting radar?
Frequency PRF PW ARP Power Beamwidth Antenna Weight
What are the main components of a radar?
MTG (synchronizer) Modulator Transmitter Duplexer (switch and protects reciever) Reciever Display
What are the three types of radar display?
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)
What target information can we get?
Speed Height Bearing Range Size/shape Course
What are the processes of the radar reciever?
Protection circuit Amplification of signal Filter out clutter Demodulate Process Convert to video pulses
How does demodulation occur?
High frequency pulse comes in and is mixed with localized oscillator frequency in the mixer which gives a lower, more manageable intermediate frequency
What 4 things should an ES reciever do?
Intercept
Measure
Identify
Display
Describe amplitude comparison
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
Describe phase comparison
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
Describe unintentional pulse modulation
Caused by system noise and/or different operation/installation of components and helps with specific emitter identification (SEI)
What is SEI
Specific emitter identification
What are the components of an ES reciever?
Antennae Amplifier Reciever Processer ------- DoA resolver Outputs
How can height information be obtained from a circular scanning radar?
With a circular scan radar use 2 beams, 1 vertical and one at 45 degrees
How does a search radar calculate aircraft height using a V beam?
Time difference between illuminations in each beam.
The longer the time interval, the higher the target
What are the 2 types of sector scans?
Bi directional
Uni directional
What are the advantages of a pencil beam?
High power
Small beamwidth
Good tracking information (accurate)
Describe a raster Scan
Bar - a line of search
Frame - complete cycle of bars within the search pattern
Sweeps one bar, moves down to next etc
Describe a monopulse radar
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
What is a phased array scan?
Active or passive
Electronically steered
+/- 60 degree of boresight is search area
What is the radar range equation?
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
How do you convert 400 into dB?
10log(400)
How do you convert 52 into real numbers?
52/10 = 5.2
10^5.2
What is SNR?
Signal noise ratio
The ability for a radar to see targets over noise
Define bandwidth
Bandwidth is the difference between upper and lower frequencies and is typically measured in Hz
What factors will effect signal quality SNR?
Distance from the source
Atmospheric interference (sun spot)
Multipath interference
Jamming
What are the 4 types of different scans
Circular
Sector
Raster
Complex (phased array)
What are the 4 types of tracking scans?
Lobe switching
Monopulse
Conical
Unidirectional sector
What is the function of a cosecant squared radar?
More power at a longer range to give similar size returns for the same size target at different ranges
What does a cassegrain antenna achieve and how?
It avoids the problem of a centre null by a sub reflector which changes polarisation
Define beamwidth
The width of the beam in degrees measured at the -3dB power points
What are the components of a antenna system?
Feed
Reflector
Transmitting medium
What are the two types of array?
Linear - slots in a line (2D)
Planar - stacked on top of each other (3D)
What are the types of beam steering?
Electrical (60 degrees left or right of boresight)
Mechanical (if you want to go further)
State the relationship between antenna size and wave length
Smaller antenna = smaller wavelength
What is an ellipse and what is it described by?
Made from the triangulated ESM bearings
Described by:
Length of major axis
Length of minor axis
Orientation off north
Coordinates off center
What is the main advantage with electronic scanning ?
Electronic scanning is much quicker and agile
What are the two types of electrical beam steering?
Phase
Frequency
