Revision

Question 1

Explain the military’s main uses of the EM spectrum

Answer 1

Radio communications
Primary and secondary radar
IR detection
UV seekers

Question 2

What does RADAR stand for?

Answer 2

Radio detection and ranging

Question 3

What is the speed of light in all three units?

Answer 3

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

Question 4

What is the relationship between wavelength and frequency?

Answer 4

The longer the wavelength the lower the frequency

They are inversely proportional

Question 5

Polarisation is determined by the orientation if which field?

Answer 5

Electric field

Question 6

What are the three types of polarisation?

Answer 6

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

Question 7

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

Answer 7

C = wavelength x frequency

Question 8

State the relationship between power density and range

Answer 8

Power density will decrease as range from the source increases

Question 9

State the 2 forms of atmospheric attenuation

Answer 9

Absorbtion

Scattering

Question 10

Explain atmospheric absorbtion

Answer 10

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

Question 11

Explain atmospheric scattering

Answer 11

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

Question 12

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

Answer 12

Little energy is scattered

Question 13

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

Answer 13

Scatter Large amounts of energy that varies over large values

Question 14

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

Answer 14

Scatters a constant high level of energy

Question 15

Explain diffraction in relation to propagation

Answer 15

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

Longer wavelengths are diffracted more than shorter wavelength

Question 16

State the different wave types

Answer 16

Ground wave (space/surface) 
Sky wave

Question 17

State the factors affecting a space wave

Answer 17

Attenuation (absorbtion), scatter, reflection

Question 18

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

Answer 18

25-40km

Have relay stations to increase distance

Question 19

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

Answer 19

Troposphere (all your weather in this region)

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

Question 20

What are the 2 types of transmitters?

Answer 20

Occilator (DC into high power radio frequency output)

Amplifier (produces high power replica of the input signal)

Question 21

What are the 3 main transmitter devices?

Answer 21

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

Question 22

What are the 3 main types of modulation?

Answer 22

Amplitude
Frequency
Phase

Question 23

What are the 8 factors affecting radar?

Answer 23

Frequency
PRF
PW
ARP 
Power 
Beamwidth 
Antenna 
Weight

Question 24

What are the main components of a radar?

Answer 24

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

Question 25

What are the three types of radar display?

Answer 25

``` 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) ```

Question 26

What target information can we get?

Answer 26

``` Speed Height Bearing Range Size/shape Course ```

Question 27

What are the processes of the radar reciever?

Answer 27

``` Protection circuit Amplification of signal Filter out clutter Demodulate Process Convert to video pulses ```

Question 28

How does demodulation occur?

Answer 28

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

Question 29

What 4 things should an ES reciever do?

Answer 29

Intercept Measure Identify Display

Question 30

Describe amplitude comparison

Answer 30

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

Question 31

Describe phase comparison

Answer 31

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

Question 32

Describe unintentional pulse modulation

Answer 32

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

Question 33

What is SEI

Answer 33

Specific emitter identification

Question 34

What are the components of an ES reciever?

Answer 34

``` Antennae Amplifier Reciever Processer ------- DoA resolver Outputs ```

Question 35

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

Answer 35

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

Question 36

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

Answer 36

Time difference between illuminations in each beam. | The longer the time interval, the higher the target

Question 37

What are the 2 types of sector scans?

Answer 37

Bi directional | Uni directional

Question 38

What are the advantages of a pencil beam?

Answer 38

High power Small beamwidth Good tracking information (accurate)

Question 39

Describe a raster Scan

Answer 39

Bar - a line of search Frame - complete cycle of bars within the search pattern Sweeps one bar, moves down to next etc

Question 40

Describe a monopulse radar

Answer 40

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

Question 41

What is a phased array scan?

Answer 41

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

Question 42

What is the radar range equation?

Answer 42

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

Question 43

How do you convert 400 into dB?

Answer 43

10log(400)

Question 44

How do you convert 52 into real numbers?

Answer 44

52/10 = 5.2 10^5.2

Question 45

What is SNR?

Answer 45

Signal noise ratio The ability for a radar to see targets over noise

Question 46

Define bandwidth

Answer 46

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

Question 47

What factors will effect signal quality SNR?

Answer 47

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

Question 48

What are the 4 types of different scans

Answer 48

Circular Sector Raster Complex (phased array)

Question 49

What are the 4 types of tracking scans?

Answer 49

Lobe switching Monopulse Conical Unidirectional sector

Question 50

What is the function of a cosecant squared radar?

Answer 50

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

Question 51

What does a cassegrain antenna achieve and how?

Answer 51

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

Question 52

Define beamwidth

Answer 52

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

Question 53

What are the components of a antenna system?

Answer 53

Feed Reflector Transmitting medium

Question 54

What are the two types of array?

Answer 54

Linear - slots in a line (2D) | Planar - stacked on top of each other (3D)

Question 55

What are the types of beam steering?

Answer 55

Electrical (60 degrees left or right of boresight) | Mechanical (if you want to go further)

Question 56

State the relationship between antenna size and wave length

Answer 56

Smaller antenna = smaller wavelength

Question 57

What is an ellipse and what is it described by?

Answer 57

Made from the triangulated ESM bearings Described by: Length of major axis Length of minor axis Orientation off north Coordinates off center

Question 58

What is the main advantage with electronic scanning ?

Answer 58

Electronic scanning is much quicker and agile

Question 59

What are the two types of electrical beam steering?

Answer 59

Phase | Frequency