1. Advanced Radar Systems

2. Objectives
Describe how a Pulsed Doppler radar works and how it’s able to determine target velocity.
Describe how a Moving Target Indicator (MTI) determines target velocity.
Describe how a pulse compression can provide a high-resolution radar return at the receiver.

3. Velocity Determination for Pulse Radars
Basic Pulse Radar system cannot determine velocity directly from a single pulse.
However, radar modifications enable us to determine movement

4. Advanced Radar Systems
Combine the ranging feature of Pulse radars with the range rate feature of CW radars
Range, bearing, elevation to target
Detects target’s velocity
Uses radial velocity to eliminate stationary returns (clutter) or compute their velocity and future location
Two types: Moving Target Indicator (MTI) and Pulse Doppler (PD)
MTI uses phase measurements to ID moving targets
PD uses frequency measurements to ID moving targets and compute velocity in LOS

5. Moving Target Indicator (MTI)
Compare transmitted phase to return phase to detect motion (i.e., phase shift)
uses phase comparator
One full cycle of phase shift (360 degs) equals a change in range of ½ l
pulse-to-pulse comparisons; resulting phase measurements are averaged to discriminate between moving targets and stationary ones
cancellation circuit eliminates non-zero phase average (Clutter Rejection)
Bearing, Range, and Motion
Primarily used in Search Radar systems
a)       MOVING TARGET INDICATOR (MTI)
i)         measures changes in the phase of the returned signal to determine target motion
ii)       samples from transmitter pulse and return signal fed to a Phase Comparator
iii)      returns that are in phase = largest positive value
iv)     returns that are out of phase = largest negative value

6. Δ phase
Pulse 1
time
Pulse 2
Pulse 3
Pulse 4
Pulse 5
Next Pulse
Avg Phase
Difference
Subtracted
Displayed
Signal
Only moving
Displayed

7. Moving Target Indicator (MTI)
Delay line circuit saves previous phase evaluation for comparison.
Cancellation circuit subtracts previous phase comparison from current phase comparison.
Return from Stationary targets will have same phase comparison and be cancelled out.
Return from Moving targets will have different phase comparison and will be retained / displayed.
v)       also uses Delay line and cancellation circuit to eliminate stationary targets from display; the phase comparator output is delayed and subtracted from the next pulse, thus, stationary targets are removed (since phase difference of returning signal is the same for all pulses)
vi)     commonly used in Search Radar
characterized by low PRF and DC < 10%

8. MTI

9. Pulse Doppler Attributes of pulse radar / technology of CW radar.
Why is a “mixer” added to Pulse Radar?
Sample of transmitted and received signal compared at mixer.
Mixer output is Doppler shift (velocity).
Doppler sorted into velocity categories.
In some applications velocities
are coded by color.
Standard Weather Radar.
More rain / higher wind – higher Doppler.

10. Hurricane Isabel- Photo

11. Hurricane Isabel – Pulse Doppler

12. Pulse Doppler Radar Principle military use as Fire Control Radar
High PRF
Many pulses / high frequency gives large amount of range and range rate data
High degree of accuracy
(range, bearing, elevation, velocity)
Target position now includes velocity
Large receiver Bandwidth (BW) to capture larger fD
   VELOCITY DETERMINATION FOR PULSE RADARS
a)       PULSE DOPPLER
          common to color code signal return information displayed on PPI
v)       standard weather radar; also used in Fire Control/Weapons systems
vi)     only one antenna (like Pulsed)
vii)    characterized by high PRF and DC > 10%
viii)  Having a high PRF allows for several samples to be taken which allows for several range and range rate computations to be made which give you the high degree of accuracy.

13. Pulse Doppler System ft Transmitter Mixer fr Receiver Df Antenna
Display
Doppler
Filter
Mixer ft fr Df
Antenna
   a “mixer” is added to basic Pulse Radar System
ii)       sample of transmitted signal and received signal compared at mixer
iii)      output of mixer is a Doppler (frequency) shift, which is passed to a Bandpass Filter, which sorts it into categories (distinguishes targets by velocity characteristics / accurate to 3 m/s).

14. High Resolution Radar Pulse Compression Synthetic Aperture Radar (SAR)
Inverse Synthetic Aperture Radar (ISAR)

15. Pulse Compression Signal Processing Technique. Same idea as FMCW
Except in a pulse
Combines:
High energy of Long Pulse.
High resolution of Short Pulse.
Pulse is Frequency Modulated.
Increased in frequency over duration of pulse.
1)       HIGH RESOLUTION RADAR
a)       Pulse Compression
i)         Pulse Compression (revisited)
ii)       combines high energy of long pulse with high resolution of short pulse
iii)      pulse is frequency modulated (transmitted pulse increased in frequency over duration of pulse width)
iv)     since each part of pulse has a unique frequency, multiple returns can be completely separated
v)       the ability of a receiver to improve the range resolution over that of a conventional system is called the Pulse Compression Ratio (PCR)

16. Distance between Leading Edge Remains Same
Pulse Compression
Distance between Leading Edge Remains Same
Return pulse passed thru Pulse Compression Circuit.
Lower frequencies pass through slower.
Pulse “piles up” on itself.
Can distinguish multiple returns within PW
Filter Output is a signal with:
greater power for longer range.
narrower pulse width

17. Pulse Compression
Transmitted Pulse
Received Pulse

18. Pulse Compression Filtering

19. Objectives
Describe how a Pulsed Doppler radar works and how it’s able to determine target velocity.
Describe how a Moving Target Indicator (MTI) determines target velocity.
Describe how a pulse compression can provide a high-resolution radar return at the receiver.

20. Assignment Reading Assignment pp 4-16 to 4-18. Do Guided Reading
Problem Set 2