Tutorial Questions

Question 1

Which factors define a long range early warning, target indicator and tracking radar

Answer 1

LREW: maximise probability of detection and range, range accuracy and bearing resolution not important. Low frequency (D band) high power output low PRF.

TI: Improved accuracy and range resolution. High refresh rate. Vertical coverage. Resistant to jamming. Higher frequency (E/F band) and PRF.

Tracking: Accurate range, bearing and elevation. High data rate. Control system to keep locked on target. I-K band. High PRF and short pulses

Question 2

What is the difference between primary and secondary radar?

Answer 2

Primary: transmits RF energy and receives reflections from target

Secondary: transmits RF energy. Target transponder transmits response.

Question 3

How does operating frequency affect radar performance?

Answer 3

Higher frequencies are more accurate but have lower range. Lower frequencies, less affected by weather and atmospheric conditions

Question 4

What are the components of the radar block diagram?

Answer 4

Trigger
Modulator
Oscillator
Tx/Rx Switch
Antenna

RF Amplifier
Mixer
IF Amplifier
Detector
Video amplifier
Display

Between Oscillator and Mixer:
Automatic Frequency Control
Local oscillator

Question 5

What is the equation for average power?

Answer 5

P(avg)=P(pk) * Tau * PRF

Where tau is pulse duration

Question 6

Why would you use a long pulse duration?

Answer 6

A longer pulse duration (tau) will give the radar a longer range by increasing average power but increase the range resolution (BAD!)

Question 7

What effect does a short pulse have on range resolution and minimum range?

Answer 7

A Short pulse will give a lower range resolution, making it easier to distinguish targets close to one another.
The short pulse, a short pulse will also give a lower minimum range

R(min) = R(res) = C*tau/2

Question 8

Explain the operation of a Magnatron?

Answer 8

Electrons emitted from cathode
Magnetic field causes electrons to curve as they make their way from cathode to anode.
As electrons enter the resonant cavities they begin to oscillate in a function defined by the inductance and the capacitance.
The RF output is taken from a singe resonant cavity.

F(res) = 1/2pi*root(LC)

Where L is inductance and C is capacitance

Question 9

What are the principle requirements of a radar receiver?

Answer 9

High gain - to amplify reflected signal to a level suitable for feeding to a PPI

Good signal to noise ratio

Dynamic range - Sufficient to detect weak long range signals without becoming saturated by stronger short range signals

Wide bandwidth - to prevent distortion of signal

Advanced :
ECM resistance
Moving target indicator/pulse Doppler

Question 10

Explained how bandwidth affects the shape of our radar pulse and what effect this has on performance

Answer 10

To produce an accurate approximation of a square wave a number of odd harmonics of the fundamental frequency are used. This increases bandwidth. However a wider bandwidth will introduce more noise into the system.

Question 11

Explain the difference between linear and longer rhythmic amplifiers and how they affect the output of the IF amp chain

Answer 11

In a linear amplifier, the input voltage is multiplied by a constant gain to produce the output voltage. beyond a certain level of input the amplifier will reach its saturation level after which point output will remain constant. This gives maximum detection ranges in good weather conditions

In a logarithmic amplifier the output voltage is proportional to the log of the input signal. The amplitude of strong signals, especially clutter is greatly reduces in relation to weaker signals independent of range. The disadvantage is a loss of maximum detection range. Used in navigation radars

Question 12

Why are antenna sidelobes undesirable?

Answer 12

1. Provide a path for electromagnetic interference and jamming signals.
2. Take power from the main lobe

Question 13

What happens to a radar beams mainlobe and sidelobes? If shading/weighting is applied?

Answer 13

1. Reduce sidelobe levels
2. Widen main lobe
3. Produced amplitude of main lobe

Question 14

What effect do we need to compensate for in an end-Fed slotted array?

Answer 14

Squint angle- maximum radiation from main lobe is not at right angles to the array face due to size of wave guide and slot spacing

Question 15

What is three factors affect propagation of a radar wave?

Answer 15

Reflection - from earth surface (Land/Sea)
Refraction - caused by atmospheric conditions
Diffraction - caused by constrained dimensions because reflection

Question 16

Explain why the radar horizon is greater than the optical horizon

Answer 16

Changes in atmospheric conditions (temp, humidity, Pressure) which all decrease with height cause refractive index to fall.

As microwaves propagate in a straight line their altitude increases as the horizon falls away.

Microwaves bend down towards horizon.

Microwave/radar horizon is 15% farther than optical horizon

Question 17

What effects do super refraction and sub refraction have on a plan position indicator (PPI) display?

Answer 17

Super-refraction:
Increased horizontal range and decreased vertical range
Suspect range information
Produces second trace echos if PRF is to high

Sub-reflection;
Reduced horizontal range
Targets appear larger on PPI display

Question 18

What effect creates the vertical lobe structure in surveillance radar?

Answer 18

Surface reflections and multi-path interference

Question 19

What factors affect the number of lobes on a vertical coverage diagram (VCD)?

Answer 19

Number of lobes=Height of aerial(m)/Half wavelength of transmission (m)

Therefore a higher frequency give a greater number of lobes giving vitualy gapless coverage

Question 20

What are the causes of attenuation for a transmitted radio wave?

Answer 20

Absorption of energy by gas molecules
Scattering and absorption of the energy by precipitation (rain, fog, snow etc.)

Question 21

What is four characteristics of an object determine its radar cross-section?

Answer 21

M.A.S.S

Material
Aspect
Size
Shape

Question 22

How can the radar cross-section of a target be reduced?

Answer 22

Shaping
Radar absorbing materials (RAM)
Passive cancellation
Active cancellation

Question 23

How does the RCS of a sphere vary with wavelength

Answer 23

When the sphere is small relative to the wavelength there will be more scattering and less specular reflection, decreasing RCS

Question 24

Explain what clutter is and why it causes problems in radar systems?

Answer 24

Plus a consists of unwanted echoes that arrived at the same time through either the main lobe or side lobes. These echoes can come from land, sea, cloud and rain.
Clutter can make it more difficult to differentiate targets

Question 25

What techniques can use to reduce clutter in radar?

Answer 25

Small radar resolution cell Careful siting of the antenna Tilting beam upwards to avoid sea clutter Using more than one antenna beam Polarisation discrimination Swept gain (Sensitivity time constant) Differentiation (Fast time constant) Automatic gain control

Question 26

Which radar parameters affect the amount of clutter experienced?

Answer 26

Polarisation (horizontal polarisation reduces sea clutter, circular polarisation reduces rain)

Question 27

Explain the operation of a klystron

Answer 27

Multi cavity klystrons are used as amplifiers 1. Electrons generated by electron ‘gun’ 2. At the buncher/input cavity resonant frequency RF is used to speed up slower electrons 3. As electrons travel down the tube their relative speeds cause them to bunch together 4. At the catcher/output cavity the microwave energy is extracted using a standing wave. This results in a high gain output 5. Electrons are caught and cooled in the collector 6. Focusing magnets maintain the electron beam

Question 28

Explain the operation of a travelling wave tube (TWT)?

Answer 28

TWT acts as an RF amplifier 1. Electron gun releases electrons 2. Low power microwave introduced to a helexical wave guide. The input signal therefore travels a longer distance than the electrons in the tube. 3. Continuous velocity modulation of the electron beam amplifies input signal. 4. RF energy extracted through wave guide, electrons absorbed in collector

Question 29

Compare and contrast a magnetron klystron and TWT

Answer 29

Compared to a magnetron the klystron and TWT: Can make use of a low power modulator instead of high level Offer good frequency stability Separate low power frequency generator and amplifier Frequency agility Greater size and cost Cooling required The klystron uses cavities where the TWT does not so bandwidth can be large

Question 30

Explain the operation of a moving target indicator (MTI) radar

Answer 30

MTI uses a single delay line canceller. A subtractor is fed one normal video input and another that is delayed by 1 PRF. This causes stationary targets to be removed leaving behind moving targets. Multiple delay line cancellers can also be used to improve the output for slow moving clutter

Question 31

What are the two main problems with MTI radar and how are they overcome?

Answer 31

Blind speed- when target Doppler is equal to the PRF of the radar or multiples of the PRF then the delay line counsellor will treat the return as if from a stationary target Using staggered PRF moves the first blind speed to a much higher value Blind phase - Blind phase is a problem as occurs in phase processed MTI radar. In a single delay line counsellor it occurs when two successive echoes from a moving target have the same amplitude and are therefore cancelled to leave a null. Vex processing uses two channels for detection the direct Chanel (I-chanel) and the quadrature Chanel (Q-Chanel) therefore if one Chanel suffers from blind phase the other will not

Question 32

Why do we use pulse compression and what are the benefits?

Answer 32

To obtain long range and low PRF would normally require a high peak power. This can be mitigated by using pulse compression. Compressing the pulses also gives better (lower) range resolution

Question 33

Explain the operation and coding process of linear frequency modulation on the pulse (FMOP)

Answer 33

Also known as chirp the frequency of each transmissive pulse is increased as a constant rate throughout its duration. This stretches the pulse in time prior to transmission. Received echoes are passed through a filter. It’s introduces a timelag that decreases lineally with frequency at the same rate as the frequency of the echoes increases. A surface acoustic wave (SAW) expander device is used to sweep the frequency up or down through the centre frequency

Question 34

Explain the operation and coding process of phase modulation on the pulse (PMOP)

Answer 34

In p.m. OP the modulation output two coded states one and zero as represented by the wave being shifted by 180°. The receipt echoes are passed through a delay line.. that one time the signal is an output terminal corresponds to the sum of whatever segments of a received pulse currently occupied the individual segments of the line.

Question 35

Explain the operation of pulse Doppler radar

Answer 35

As opposed to MTI radar post Doppler can measure target velocity. Range ambiguities are avoided by using a low PRF and Doppler frequency (velocity) ambiguities are avoided with a high PRF. The coherent phase detector in a postop radar receiver convert the received signal to baseband and is censored as a frequency of zero. if the transmitted signal has been subject to shift and all spectral lines as previously described will be shifted by the doctor frequency of the target.

Question 36

Compare postop and moving target indicator using relative pulse repetition frequencies and their inherent ambiguities with regards to range and velocity measurements

Answer 36

MTI uses low PRS to avoid range ambiguity. The consequence is that velocity (frequency) measurements is ambiguous and results in blind speeds which must be resolved. Pulse Doppler uses a higher PRF to avoid frequency ambiguities. This gives postop larger range ambiguity.

Question 37

Which factors apart from target movement will produce a Doppler shift in a pulse Doppler radar

Answer 37

1. Own movement 2. Antenna rotation 3. Transmitter instability - Drift in frequency of coherent oscillator (COHO) 4. Internal motion - wind affecting rain, sea etc

Question 38

Explain how we can determine the velocity range of a pulse Doppler and how this affects velocity ambiguity

Answer 38

A medium PRF radar has the additional problem that the Doppler shift may be ambiguous as it is higher than the PRF. Multiple PRFs enable the velocity ambiguity to be resolved using correlation.

Question 39

How does a pulse stopper radar calculate if a target is incoming or outgoing?

Answer 39

Doppler frq = (2(transmit frq)/C) * Vcos Therefore a positive doplar shift indicates a closing target and a negative shift indicates an opening target

Question 40

How does pulse Doppler reject radar clutter

Answer 40

My filtering out low frequency Doppler shifts and using a high enough PRF that’s high levels of Doppler do not fall within the clutter of adjacent pulses

Question 41

Which is superior in a high class environment MTI or PD?

Answer 41

MTI

Question 42

Explain how blind speed occurs in PD radar and how it is overcome

Answer 42

Or medium PRF will suffer from velocity blindness when the Doppler generated by the target is equal to the PRF of the radar. Using multiple PRS and correlating resolves this problem.