KC05 - Basic Radar Theory

Question 1

Describe how a radar determines target; Presence, Range, Position, Closing Speed and Identity

Answer 1

Depending on the type of radar employed, a radar system is capable of detecting the following target characteristics:

Target Presence (Echo)
Position (Scan Pattern and Beam Shaping)
Range (Time)
Velocity (Doppler)
Identity (IFF/NCTR)

Question 2

Define Radar Cross Section (RCS)

Answer 2

RCS determines how ‘big’ of a target or how detectable you are to the threat radar, it’s dependent on the target’s size and shape, and also the polarisation of the transmitted EM wave.

Question 3

List factors affecting RCS

Answer 3

Is dependent on the following factors:
Power Density and shape (aspect) - design and manoeuvres
Reflectivity (material)
Frequency or wavelength
Polarisation (interaction of polarised wave on the shape of the target)

Question 4

Define the following; Radar Frequency (RF)

Question 5

Define the following; Coherency

Question 6

Define the following; Pulse Repetition Interval (PRI)

Question 7

Define the following; Pulse Repetition Frequency (PRF)

Question 8

Define the following; Pulse Width (PW)

Question 9

Define the following; Pulse Energy and Duty Cycle Radar Frequency (RF)

Question 10

Define the following; Coherency

Question 11

Define the following; Pulse Repetition Interval (PRI)

Answer 11

Ideal RF is represented as a digital pulse
ON and OFF time of the transmitter
A pulse exists when TX is ON

Question 12

Define the following; Pulse Repetition Frequency (PRF)

Answer 12

The number of times per second that a pulse is transmitted. (inverse of PRI)
Example: 9 pulses in one second therefore 9Hz PRF

Question 13

Define the following; Pulse Width (PW)

Answer 13

It’s the duration of the pulse, it’s the actual transmitting RF energy. Typical operating figures:
Search Radar PW = 2.5 micro sec
Nav Radar PW = 1 micro sec
High Res Radar PW = 0.1 micro sec

Question 14

Define the following; Pulse Energy

Answer 14

• Pulse energy is the amount of peak energy (Peak Power) measured over a period of time (Pulse Width)
• The energy of the pulse directly affects the radar’s maximum detection range of a target
• Pulse energy determines the maximum detection range and not the peak power

Question 15

Define the following; Duty Cycle

Answer 15

• Duty cycle is the ratio of the actual pulse transmission time to total time
• A pulsed radar system can only receive echoes when it is not transmitting
• A large duty cycle means more transmitting and less listening and can therefore create a
• phenomenon known as eclipsing.

Question 16

State the relationship between pulse width and bandwidth

Answer 16

• Pulse width and bandwidth are inversely proportional
• Short duration pulses have wide bandwidths and vice versa
• Both of these parameters will have an effect on the Range Resolution which will be discussed in upcoming slides

Question 17

List different Radar Types

Answer 17

Continuous Wave (CW) Radars
- Classic CW
- Frequency Modulated CW (FMCW)

Pulsed Radar
- Non-Doppler
- Doppler

Question 18

Describe Radar angular resolution

Answer 18

Determined by the characteristics of the beamwidth of the transmitted signal

• 2 targets that are side by side in the beamwidth will be identified as one target

Question 19

Describe Radar Range Resolution

Answer 19

Determined by the pulse width of the transmit signal

• When there are two targets, one in front and one behind, that are in the same pulse width the radar system will represent the targets as one on the display

Question 20

Define the Radar Resolution Cell (RRC)

Answer 20

RRC - smallest volume of space that can be resolved by a radar, of set parameters. It is NOT the smallest object that a radar can see but the smallest volume of space.

Question 21

Define Minimum Detection Range

Answer 21

• The radar receiver cannot receive any echoes whilst the transmitter is still radiating
• Minimum Detection Range is the minimum distance at which an echo can be received as soon as the radar stops transmitting
• Pulse width directly impacts the minimum detection range
  Longer pulse width = increased minimum detection range

Question 22

Define Maximum Detection Range

Answer 22

• Maximum detection range is the maximum calculated distance that the radar system can detect a target
• This is dependent upon the parameters of the radar system (reference the radar range equation) and is greatly affected by pulse energy
• The trade-offs will be explained on the next slide.

Question 23

Define Maximum Unambiguous Range

Answer 23

• MUR is the maximum range that a target can be away from the radar (echo received) before the next pulse is transmitted
• Any target within the MUR will have its range displayed correctly anything outside can result in an incorrect display
• Increasing PRI will increase MUR however the trade-off will be decreased Maximum Ambiguous Doppler (covered in the Doppler module).

Question 24

Define Radar Horizon

Answer 24

• Radar Horizon is defined by the distance at which the radar beam rises enough above the Earth’s surface to make detection of a target at low level impossible
• Keep in mind radar horizon is a limiting factor for ground or surface based radars
• Typically the radar horizon will be shorter than the maximum detection range

Question 25

Define Eclipsing

Answer 25

• Due to using the same antenna for transmit and receive
• Eclipsing occurs when the echo returns at exact the same time when the next pulse is due out
• Higher duty cycles will increase the chance of eclipsing.