1. MTI RADAR

2. More on Pulse Doppler Radar
Outline
More on Pulse Doppler Radar
Finding Doppler Frequency Shift
Determination of Moving/Stationary Objects on A-Scope and PPI
Use of Single and Double Delay Line Cancellers
Blind Speeds
Received Signals from Precipitation
Signal Sampling and Power Spectrum
Doppler Spectrum
Mean Doppler Velocity and Doppler Spectrum Variance
Radar Noise

3. MTI vs. Pulse Doppler Radars
Both distinguish moving objects from stationary objects by looking at the Doppler Frequency shift.
MTI (moving target indication) radars
Typically operate with ambiguous velocity measurements (blind speeds), but with unambiguous range measurements
Pulsed Doppler Radars
PRF usually high enough to operate with unambiguous Doppler measurements but with ambiguous range measurements

4. Operation
Reference signal is sent
Signal echo is measured
Difference between signals is calculated to find Doppler frequency shift

5. Finding Doppler Frequency Shift

6. Determining Moving Objects From A-scope
A-scope is a display of echo amplitude vs. time
Superposition of echoes can be helpful in separating moving objects from stationary object
“Butterfly Effect”

7. Oscillogram of an output signal of a phase-detector

8. Echoes signals of fixed clutter have got the same amplitude pulse to pulse and can be cancelled: magenta: output of the phase-detector (actually period) green: output of the memory (delayed period) blue: cancelled video

9. Determination of Moving Objects on PPI
PPI (plan position indicator)
Angle vs. Range display
Different method must be utilized on PPI – Delay line canceller

10. Single Delay Line Canceller
Signal delay T=1/PRF
Output of canceller is a cosine wave at the Doppler frequency
Amplitude of output is a function of the Doppler frequency and T

11. Frequency Response of Single Delay Line Canceller

12. Frequency Response of Single Delay Line Canceller
Response is zero when
Target velocities that result in zero MTI response are called “blind speeds”
Somewhat effective removal of clutter
Double/Multiple cancellation more effective

13. Frequency Response of Delay Line Cancellers

14. Frequency Response of Delay Line Cancellers
Avoid blind speeds by making first blind speed greater than maximum radial velocity
Increase wavelength of signal propagated
Increase PRF
Low radar frequencies (large wavelength) require larger antenna size
High PRF results in Range Ambiguity!
Example: First blind speed 600 knots
Range (without ambiguity) = 130 nautical miles at 300 MHz or 13 nautical miles at 30 MHz
Trade off between range and velocity ambiguities
One solution is “Staggered PRF MTI”

15. Signal Sampling and Power Spectrum
Conversion from continuous time to discrete samples
Power Spectrum Density (PSD)
Describes power as a function of frequency
Fourier transform of the autocorrelation function, if it can be treated as a stationary process.

16. Doppler Spectrum Spread
Backscattered power received as a function of Doppler frequency, or velocity
Describes the echo of a contributing region of signal
Function given as S(f), S(V), or S(ω);
Doppler Spectrum Spread
Large difference of size means large spread
Turbulence
Air motion across beam

17. Average Power
Average Power can be given in terms of Doppler velocity or frequency

18. Mean Doppler Velocity and Doppler Spectrum Variance
A more convenient measure of the Doppler spectrum spread can be given by the variance, σ2
Found from mean Doppler velocity

19. Radar Noise Thermal Noise Quantization Noise Coherent Noise
Thermal excitation of electrons in electrical components
Always exists in any electrical system
Totally random, but has a normal distribution
Coherent Averaging or Stacking
Quantization Noise
Quantization noise due to rounding errors
Dithering
Coherent Noise
Coherent radar systems use a master oscillator to derive frequencies and timing signals
Leakage from these signals into the receiver causes noise
0/π Phase Modulation