Radar Theory Flashcards
What is the definition of a radar
Radar is an electrical system that transmits RF EM wave in a region of interest then receives and detects the reflections
What does the radar transmitter do?
Generates EM waves
What does the antenna do?
Couples the EM generated by the transmitter to the propagation medium.
What does the T/R switch do?
1) Connects the antenna to the Tx and Rx
2) Protects the Rx from high power Tx transmissions.
How is energy reflected from a target?
The EM wave induces a current onto the target which is reflected into the atmosphere.
What is clutter?
Clutter are reflections from things other than the target. They are unwanted by legitimate.
What does the receiver do?
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
Radar Performance Characteristics
1) Peak/Average Power
More power equates to more range and more detections.
Radar Performance Characteristics
2) Frequency
Frequency determines the size of the antenna due to wavelength.
Radar Performance Characteristics
3) Pulse Repetition Frequency and Pulse Repetition Interval
These are the rate at which the radar transmits pulses. This will affect range and resolution.
Radar Performance Characteristics
4) Pulse Width/Bandwidth
Bandwidth is inversely proportional to pulse width. Increased bandwidth means more noise and increased pulse width means more average power.
Radar Performance Characteristics
5) Dwell Time
More dwell time means more pulses on a given bearing which increases detection chances. However, this will slow the search time.
Radar Performance Characteristics
6) Beam Width
Wider beam width means more search area covered in less time. This could lead to position accuracy issues.
Radar Performance Characteristics
7) Sensitivity
How sensitive is the receiver. More sensitivity could lead to less ability to handle a wide range of signals.
Radar Performance Characteristics
8) Range
Is determined by a number of factors.
Radar Performance Characteristics
9) Electronic Protective Measures
How well can the radar resist Electronic Attack techniques.
How is range determined using a CW radar?
Frequency modulation
How does doppler work with RADAR?
1)Relative motion between radar and target
2) Frequency of reflections will be different from the transmitted wave.
What is the doppler shift equation
f_d =2V_tgt/wavelength
f_d = difference btw Rx and Tx wave
v_tgt = velocity of tgt
wavelenth = c/Tx radar frequency
If target is moving toward us, how do we know?
If F_d is positive
What is phased array?
Antenna made up of a number of individual antennas. They don’t have to move.
What is radar performance influences by?
1) Strength of the signal coming to the radar from the target
2) The strength of interference
What are the different types of interference?
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)
What does the radar range equation compute?
1) Signal to noise ratio (SNR)
2) Predicts the received power of radar waves reflected from the target and the interference
What is an isotropic antenna?
It is a theoretical antenna that radiates equally in all directions creating a perfect spherical pattern.
What is RRE step 1?
Power density at a distance R
What is RRE Step 2
Power density with directivity (gain)
What is RRE step 3?
Power reflected by the target
What is RRE step 4?
Propagation of reflected signal (similar to isotropic power)
What is RRE step 5?
Reflected wave is received by an antenna with area A
What is RRE step 6?
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
What is RRE step 7
Signal to noise ratio
P_r/P_n
What is RRE step 8?
Account for the number of pulses generates with each antenna rotation.
n_p
What is RRE step 9
Account for losses (L_s)
Ls = system loss
Ls = LtLaLr*Lsp
Lt = transmit loss
La = atmosphere loss
Lr = Receiver loss
Lsp = signal processor loss
What is RRE Step 10?
Rearrange to solve for R which is the max range of the radar with given/calculated parameters.
What is a track while scan system?
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.
What is the difference between coherent and non-coherent systems?
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.
What is the doppler shift?
The difference between transmitted and received frequencies.
