Radar Theory

Question 1

What is the definition of a radar

Answer 1

Radar is an electrical system that transmits RF EM wave in a region of interest then receives and detects the reflections

Question 2

What does the radar transmitter do?

Answer 2

Generates EM waves

Question 3

What does the antenna do?

Answer 3

Couples the EM generated by the transmitter to the propagation medium.

Question 4

What does the T/R switch do?

Answer 4

1) Connects the antenna to the Tx and Rx
2) Protects the Rx from high power Tx transmissions.

Question 5

How is energy reflected from a target?

Answer 5

The EM wave induces a current onto the target which is reflected into the atmosphere.

Question 6

What is clutter?

Answer 6

Clutter are reflections from things other than the target. They are unwanted by legitimate.

Question 7

What does the receiver do?

Answer 7

1) Amplifies the signal
2) Converts to an IF
3) Detector removes the carrier so that the data can be analyzed by the signal processor
4) Sends signal to an A to D converter
5) Passes signal to a signal/data processor

Question 8

Radar Performance Characteristics
1) Peak/Average Power

Answer 8

More power equates to more range and more detections.

Question 9

Radar Performance Characteristics
2) Frequency

Answer 9

Frequency determines the size of the antenna due to wavelength.

Question 10

Radar Performance Characteristics
3) Pulse Repetition Frequency and Pulse Repetition Interval

Answer 10

These are the rate at which the radar transmits pulses. This will affect range and resolution.

Question 11

Radar Performance Characteristics
4) Pulse Width/Bandwidth

Answer 11

Bandwidth is inversely proportional to pulse width. Increased bandwidth means more noise and increased pulse width means more average power.

Question 12

Radar Performance Characteristics
5) Dwell Time

Answer 12

More dwell time means more pulses on a given bearing which increases detection chances. However, this will slow the search time.

Question 13

Radar Performance Characteristics
6) Beam Width

Answer 13

Wider beam width means more search area covered in less time. This could lead to position accuracy issues.

Question 14

Radar Performance Characteristics
7) Sensitivity

Answer 14

How sensitive is the receiver. More sensitivity could lead to less ability to handle a wide range of signals.

Question 15

Radar Performance Characteristics
8) Range

Answer 15

Is determined by a number of factors.

Question 16

Radar Performance Characteristics
9) Electronic Protective Measures

Answer 16

How well can the radar resist Electronic Attack techniques.

Question 17

How is range determined using a CW radar?

Answer 17

Frequency modulation

Question 18

How does doppler work with RADAR?

Answer 18

1)Relative motion between radar and target
2) Frequency of reflections will be different from the transmitted wave.

Question 19

What is the doppler shift equation

Answer 19

f_d =2V_tgt/wavelength

f_d = difference btw Rx and Tx wave
v_tgt = velocity of tgt
wavelenth = c/Tx radar frequency

Question 20

If target is moving toward us, how do we know?

Answer 20

If F_d is positive

Question 21

What is phased array?

Answer 21

Antenna made up of a number of individual antennas. They don’t have to move.

Question 22

What is radar performance influences by?

Answer 22

1) Strength of the signal coming to the radar from the target
2) The strength of interference

Question 23

What are the different types of interference?

Answer 23

1) Thermal Noise - Signal to noise ratio (SNR)
2) Clutter Noise - Signal to clutter ratio (SCR)
3) Total interfering signal - Signal to interference ratio (SIR)

Question 24

What does the radar range equation compute?

Answer 24

1) Signal to noise ratio (SNR)
2) Predicts the received power of radar waves reflected from the target and the interference

Question 25

What is an isotropic antenna?

Answer 25

It is a theoretical antenna that radiates equally in all directions creating a perfect spherical pattern.

Question 26

What is RRE step 1?

Answer 26

Power density at a distance R

Question 27

What is RRE Step 2

Answer 27

Power density with directivity (gain)

Question 28

What is RRE step 3?

Answer 28

Power reflected by the target

Question 29

What is RRE step 4?

Answer 29

Propagation of reflected signal (similar to isotropic power)

Question 30

What is RRE step 5?

Answer 30

Reflected wave is received by an antenna with area A

Question 31

What is RRE step 6?

Answer 31

Received thermal noise

P_n = kT_oF*B

k = Boltzman constant = 1.38e-23
T_o = STD temp = 290K
F = Noise figure of Rx’er
B = Receiver bandwidth

Question 32

What is RRE step 7

Answer 32

Signal to noise ratio

P_r/P_n

Question 33

What is RRE step 8?

Answer 33

Account for the number of pulses generates with each antenna rotation.

n_p

Question 34

What is RRE step 9

Answer 34

Account for losses (L_s)

Ls = system loss

Ls = LtLaLr*Lsp

Lt = transmit loss
La = atmosphere loss
Lr = Receiver loss
Lsp = signal processor loss

Question 35

What is RRE Step 10?

Answer 35

Rearrange to solve for R which is the max range of the radar with given/calculated parameters.

Question 36

What is a track while scan system?

Answer 36

When a target is detected during a radar volume search a track file is established.

Subsequent passes of the radar will update measurements for that track.

Question 37

What is the difference between coherent and non-coherent systems?

Answer 37

Non-coherent systems only measure amplitude of a received signal.

Coherent systems process a signal as a vector with phase and amplitude. The vector is comprised of I (In phase) and Q (quadrature (out of phase) components.

Question 38

What is the doppler shift?

Answer 38

The difference between transmitted and received frequencies.