Objectives

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

3. Mechanically Scanned Array (MSA) Radar A Typical Fighter Radar • Limitations • Beam Positioning • Low Bandwidth • Small Number of Tracked Targets • Single Point of Failure • RF Energy Losses • Low Signal to Noise Ratio

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 – 10-100 x better MTBF • Can have 10% failure of TR modules and suffer no antenna loss ESA phase shifters 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)

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.

8. t = d sine c 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 e

9. Frequency Shifting f fo • 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.. Beam below the boresight axis - elements lag in phase by Δ, with the lowermost element receiving the greatest phase shift..

11. Electronic Steering www.explorelearining.com

12. e d d sin e Phase Beam Steering • 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. 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

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

14. (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) Phase Difference Computations Looking ‘at’ the radar, vice ‘from’ the radar Dfa,e (radians) = (2p/l) [a da sin a + e de sin e]

15. 0 1 2 3 0 1 2 3 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!].

16. 0 1 2 3 0 0 1 .545 -.631 2 1.09 3 1.64 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 UPlooking out of the array. [warning: the signs are important!]. d = .03 m l = .06 m Radians

17. 0 1 2 3 0 1 2 3 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!].

18. 0 1 2 3 0 000 072 144 216 1 335 047 119 191 2 310 022 094 166 3 285 357 069 141 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 UPlooking out of the array. [warning: the signs are important!]. d = 9 cm l = .025 m Degrees

19. Fleet Uses of Electronic Scanning SPS-48 Air Search TACAN • SPY-1 • Ticonderoga • Arleigh Burke 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. Assignment... Radar Tracking Systems Chapter 5 Complete Guided Reading