1. Objectives
Distinguish advantages of electronic scanning over the traditional scanning methods.
Explain the principle of electronic beam steering and how phase shift is employed.
Distinguish between active and passive electronic steering
Calculate for an array the required phase shift (element to element) to steer a beam at any arbitrary angle in combined azimuth and elevation.

2. Current Generation Fighter Radars Use Mechanically Scanned Arrays
Most of the Fighter aircraft fielded around the world today use mechanically scanned array Radars, where the antenna physically moves inside the fighter’s nose. 6

3. Mechanically Scanned Array (MSA) Radar A Typical Fighter Radar
The typical fighter radar has a Traveling Wave Tube (TWT) transmitter matched to the mechanical antenna with a bandwidth of about 2%. The limitations of this radar include the relatively low accuracy of positioning the beam, the minimal available bandwidth, RF energy losses, noise, and the ability of a single failure in the transmitter to render the entire Radar inoperative.
Radar requires mechanical components to steer the beam
Limitations
Reduced reaction times.
Limited # of Targets
Antenna positioning is relatively slow due to antenna inflexibility and inertia.
Introduction:
a. We have discussed a radar system which works and is fairly efficient.
b. The weak link in our system is the mechanical nature of the system.
2. Our system can handle multiple targets but it is still slow for the modern high speed (greater than the speed of sound) weapons.
a. Still must mechanically train the radar antenna.
b. Mechanical system tend to break down more frequently than electronics
c. There are vibrations inherent in mechanical systems.
3. Could make better mechanical systems but requires greater expense and more power.
a. Can never get around the antenna inertia and inflexibility of a mechanical scanning system.
Limitations
Beam Positioning
Low Bandwidth
Small Number of Tracked Targets
Single Point of Failure
RF Energy Losses
Low Signal to Noise Ratio 7

4. Scanning the Array: ESA
Electronically Scanned Array (ESA) Advantages:
Beam can switch direction electronically, and therefore very fast
Eliminates the gimbal – mechanical point of failure
Enables Agile Beam Radar operation
Improved range resolution
High reliability – x better MTBF
Can have 10% failure of TR modules and suffer no antenna loss
ESA
phase
shifters
Advantages -
High Data Rates
Instantaneous Beam Positioning
Elimination of Mechanical Breakdown
Increased Flexibility
Multi-mode Operation
Multi-target capability
Radar main beam
points in the direction
of the phase front

5. Active Electronic Scanning Array
2000 finger-size Tx/Rcvr elements
Capable of changing direction/shape/power instantaneously
120o FOV either side of nose
Each module can be assigned separate function
RWR/Jammer/Track/Search

6. How does this work? Recall Constructive/Destructive Interference
Elements excited in phase – constructive – Broadside Array
Elements excited 180º out of phase – destructive - Endfire Array
Can control the beam position by controlling the phase difference between elements.
Specific phase shift ()  specific azimuth/elevation angle (α, ε)
ELECTRONIC BEAM STEERING - The major question you are probably asking is “How does this thing work?”! Quite simply - by electronically using a combination of Constructive and Destructive interference while stimulating individual elements or groups of elements within the array. Positioning the antenna beam to a specific angle ( q ) off the antenna boresight axis is accomplished by applying a phase shift ( f ) to the specific set of array elements which shift the angle of constructive interference (recall our discussion about putting dipole antennas next to each other and how we were able to steer the beam - Lesson 2).

7. Beam Positioning Array Beam “On” the antenna boresight
All participating elements are in phase on same freq and xmit at same time to maximize energy at point P.
1. Graphic shows the next step
- How the wave fronts combine and converge in a direction and form the axis of the beam.
Note: Just like we talked about before. The strongest signals are down the access of the beam.
2. Can see that by just changing the phase of the individual waves slightly then we can “steer” the beams.
How can we change the phase of the individual waves?
A.     When the phased array beam is “ON” the antenna boresight: all participating elements are in phase to maximize energy at point P (recall Broadside array).

8. Time Delay
Use of time delay to achieve the desired phase relationship
Time delay networks installed in front of each radiating element
Expensive, Complex and Heavy
t = d sine c e

9. Frequency Shifting fo f e = sin-1[1(f-fo)] fd
Decreasing frequency causes phase shift at element opening
Design of waveguide creates delay
Vary the frequency about base frequency
Very simple and relatively inexpensive
e = sin-1[1(f-fo)] fd

10. Phase Relationships
Beam along the boresight axis - all elements are radiated in phase.
Beam above the boresight axis - elements lag in phase by Δ , with the uppermost element receiving the greatest phase shift..
1. Constructive and Destructive interference
a. Waves can be added together.
b. If the summation of the waves causes the amplitude to be more than the
individual waves then the summation is said to be constructive.
c. If the summation of the waves causes the amplitude to be less than
either of the waves then the summation is said to be destructive.
[If students are confused can show graphic 4 from lesson 2 (Energy Fundamentals) which shows how two waves are added together to double or eliminate the wave]
2. IF there are two or more sources radiating in phase then, the summed wave front will move out into space perpendicular to the antenna bore sight.
Beam below the boresight axis - elements lag in phase by Δ, with the lowermost element receiving the greatest phase shift..

11. Electronic Steering

12. Phase Beam Steering Dfadj = (2p/l) d sinε
Phased Array beam “Off” the antenna boresight: all participating elements must be fired out of phase to maximize energy at point P.
Interference is based on the difference in path lengths. e
d sin e d
Dfadj = (2p/l) d sinε
Dfadj = phase shift between adjacent elements in radians
= wavelength in meters
d = distance between radiating elements in meters
ε = desired angular offset
B.      When the phased array beam is “OFF” the antenna boresight: all participating elements must be fired out of phase to maximize energy at point P (recall Endfire array).
Note: The distance r2 > r1, therefore, energy from element 2 must travel farther to reach point P (i.e. positive phase shift).
Note: To achieve maximum constructive interference at point P, energy from all elements must arrive in phase and at same time.

13. Phased Array (Sign Conventions)
Left Azimuth and Down from Boresight = ( - ) sign
Right Azimuth and Up from Boresight = ( + ) sign a da e de e a
Dfa,e (radians) = (2p/l) [a da sin a + e de sin e]

14. Phase Difference Computations
Looking ‘at’ the radar, vice ‘from’ the radar
(0, 0)
(1, 0)
(2, 0)
(3, 0)
(0, 1)
(1, 1)
(2, 1)
(3, 1)
(0, 2)
(1, 2)
(2, 2)
(3, 2)
(0, 3)
(1, 3)
(2, 3)
(3, 3)
A. ELEVATION if q is above boresight { + }
if q is below boresight { - }
B. AZIMUTH { + } if counterclockwise (left)
{ - } if clockwise (right)
C.      Use boresight as origin and think of yourself as looking at the array.
D.      Picture only showing 16 elements; in reality there are thousands! EX: SPY - 1A on CG, DDG; each of the 4 arrays has 4100 elements, any pair of which may be combined to create a single beam. Example: An array split into groups of 40 elements can generate 100 different beams (100 targets to track!)
E.       To calculate the total phase signal sent to an element ( f e, a ) use the following equation:
- de and da refer to the element spacing in vertical and horizontal directions respectively.
-  is elevation angle (up is positive)
-  is azimuth angle (right is positive)
Dfa,e (radians) = (2p/l) [a da sin a + e de sin e]

15. Example Problem 1
A phased array radar has a four by four array with equal vertical and horizontal spacing at l/2 m. The radar operates at 5000 MHz. Find all the phase shifts relative to the reference element to steer the beam 220 LEFT and 100 UP. [warning: the signs are important!]. 3 2 1

16. Example Problem 1 Radians
A phased array radar has a four by four array with equal vertical and horizontal spacing at l/2 m. The radar operates at 5000 MHz. Find all the phase shifts relative to the reference element to steer the beam 220 LEFT and 100 UP looking out of the array. [warning: the signs are important!]. 3 2 1
-.631
.545
1.09
1.64
d = .03 m
l = .06 m
Radians

17. Example Problem 2
A phased array radar has a four by four array with equal vertical and horizontal spacing at 9 cm. The radar operates at 12 GHz. Find all the phase shifts (in degrees) relative to the reference element to steer the beam 300 LEFT and 150 UP. [warning: the signs are important!]. 3 2 1

18. Example Problem 2 Degrees
A phased array radar has a four by four array with equal vertical and horizontal spacing at 9 cm. The radar operates at 12 GHz. Find all the phase shifts (in degrees) relative to the reference element to steer the beam 300 LEFT and 150 UP looking out of the array. [warning: the signs are important!]. 1 2 3
d = 9 cm
l = .025 m
000
072
144
216 1
335
047
119
191
Degrees 2
310
022
094
166 3
285
357
069
141

19. Fleet Uses of Electronic Scanning
TACAN
SPY-1
Ticonderoga
Arleigh Burke
SPS-48 Air Search
1. Electronic Scanning is on the newest classes of ships.
2. SPY-1A Characteristics
- Each array covers 92 degrees so there is overlap.
- Array is 12’ by 12’
phasers in the arrays
- 1/3 redundancy so can accept heavy losses to array without impacting
performance.
AN/APG-81

20. Objectives
Distinguish advantages of electronic scanning over the traditional scanning methods.
Explain the principle of electronic beam steering and how phase shift is employed.
Distinguish between active and passive electronic steering
Calculate for an array the required phase shift (element to element) to steer a beam at any arbitrary angle in combined azimuth and elevation.

21. Radar Tracking Systems Chapter 5 Complete Guided Reading
Assignment...
Radar Tracking Systems Chapter 5 Complete Guided Reading