08 Basic Radar Theory

Question 1

What frequency do radars operate in (basic radar theory)?

Answer 1

5 to 15 GHz

Question 2

What determines the Max Unambiguous Range?

Answer 2

Pulse Repetition Frequency (PRF)

Question 3

What factors effect the range of a radar?

Answer 3

• Transmitter Power: Range is proportional to Power
• PRF: Range is inversely proportional to the PRF
• Dish RPM: Range is inversely proportional to the RPM

Question 4

What happens to the width of a radar beam as the dish size increases?

Answer 4

As dish size increases, the beam becomes narrower

Question 5

What is the equation for the Range of a target from the Radar?

Answer 5

Range(m) = (Time of travel of pulse x C) / 2

• Time is measured from the leading edge of the sent pulse to the leading edge of the return pulse

Question 6

What is the equation for the Maximum Unambiguous Range of a Radar?

Answer 6

Range(m) = C / (2 x PRF)

Question 7

What is the equation for the maximum theoretical range of a Radar?

Answer 7

Range (nm) = 1.23 x (sqrt(Tx) + sqrt(Hx))

• Tx is the height of the antenna
• Hx is the height of the object/AC

Question 8

What is the Pulse Recurrence Interval for a Radar?

Answer 8

The time interval between the start of one pulse and the start of the next

Question 9

What is the Pulse Repetition Frequency for a Radar?

Answer 9

The number of pulses in one second

Question 10

What is the Pulse Transit time for a Radar?

Answer 10

The time interval between the sending of the pulse, it reaching the target and returning to the Radar

Question 11

For a Radar display, what does a Moving Target indicator do differently from a standard display?

Answer 11

• Only displays moving targets on the indicator

- Stationary objects (such as buildings) are removed

Question 12

Describe atmospheric attenuation of a Radar

Answer 12

• Water in the atmosphere (rain, vapour etc) degrade Radar signals
• Energy from the radar signal is absorbed by the water and converted to heat energy
• More pronounced over longer distances and at higher frequencies

Question 13

Describe the errors associated with Radars

Answer 13

Super-refraction (Ducting):

• Radar signal is bent towards the Earth’s surface
• Usually caused by strong, low level inversion
• Increases Radar range

Sub-refraction:

• Radar signal is bent away from the Earth’s surface
• Caused by warm, moist air moving over a cool surface
• Decreases Radar range

Question 14

What are the different types of ATC radar and is their;

• Range
• PRF
• Antenna RPM

Answer 14

Area (Terminal Area) Radar:

• Long Range
• Low PRF
• Low RPM

Surveillance Radar:

• Shorter Range
• Higher PRF
• Higher RPM

Ground Radar:

• Very short range
• Very high PRF
• Very High RPM

Question 15

What is the approximate range for an ATC Long Range Surveillance Radar?

Answer 15

200 to 300nm

Question 16

What is the Pulse Recurrence Rate for a Radar?

Answer 16

The number of pulses per second

The same as the PRF

Question 17

What is the equation for the minimum range of a radar?

Answer 17

Minimum range (m) = (C x Pulse Length) / 2

Question 18

In order to double the effective range of a Primary Radar the power output must be increased by a factor of what?

Answer 18

16

Question 19

What determines a Primary Radars ability to discriminate between targets in:

• Azimuth
• Depth

Answer 19

• Azimuth: Beam width

- Depth: Pulse length

Question 20

Compared to a Parabolic Reflector, what is one of the advantages of a Flat Plate or ‘Phase Array’ antenna?

Answer 20

• Generates less side lobes than a Parabolic Reflector

- Wastes less energy

Question 21

Describe the phenomenon known as Fading?

Answer 21

• Caused by sky waves and ground waves interfering with each other
• Can occur at night
• When sky waves, refracted from the ionosphere, interfere with ground waves
• When ground waves are reflected up into sky waves by obstacles on the ground

Question 22

What can increase the Skip Distance of HF transmissions?

Answer 22

• Higher frequency transmission: allow the signal to penetrate the ionosphere to a higher level and so will increase the skip distance
• Higher position of the reflecting ionosphere: higher level reflection will increases the skip distance