Phased Array Radars Naval Weapons Systems

Limitations of Mechanical Scanning Radars Positioning Antenna is SLOW Reduced reaction times Blind Sided! Mechanical error 1. 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. If only we could do scanning electronically and eliminate the mechanical movement of the antenna.

Electronic Scanning Increased Data Rates Instantaneous Beam Positioning Elimination of Mechanical Errors Multi-mode Operation Multi-target capability 1. Increased data rates: a. There is a reduction of system reaction times (i.e.. don’t have to wait for the antenna to come back around. 2. Virtually instantaneous radar beam positioning. a. Can change the beams and the beam axis almost instantaneously. Don’t have to mechanically move/tilt the antenna. 3. Elimination of mechanical errors and failures associated with mechanically scanned systems. 4. Increased flexibility: a. Can develop more complex radar systems including - 3D Radar with only one radar - Multi mode operations (Search, Tracking, and Fire Control!) - Missile guidance, air traffic control can be done all at the same time. 5. Sounds great but how can we do that?

Phased Array / SPY – 1D p. 135 in book

Phase Concept Think about this: The electromagnetic energy received at a point in space from two or more closely spaced radiating elements is a maximum when the energy from each radiating element arrives at a point in phase.

Phase Relationships Constructive Interference Destructive Interference 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. 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.

Generating a Directional Main Lobe in a Phased Array Radar Radiating in Phase Boresight Axis Wave Fronts of Maximum Energy Altering the Phase to Change the Axis Boresight Axis Beam Axis 1. Left graphic - Shows how multiple radiators put out sinusoidal waves in phase which combine to produce a much higher amplitude wave in a direction perpendicular to the antenna boresight. [If students have trouble relating to multiple sources producing a single beam use the example of tying several flash lights together and shining it out. At a distance, the group of flash lights would look like a single source] REMEMBER: We will represent the maximum beam (summation) as a wave front. 2. Right graphic: - Shows how if we shift the emission of the individual transmitter slightly then we can essential steer the axis of the beam off the antenna bore- sight. Note that the wave lengths of each transmission must be the same to get a repeatable wave.

Beam Positioning P Point “P” where all waves arrive in phase 1. Graphic shows the next step - How the wave fronts combine and converge in a direction and form the axis of the beam. - Point “P” is very far from the antenna, path lengths are the same as P approaches infinity, range is large compared to element separation - Energy from different elements will maintain the same phase relationship Note: Just like we talked about before. The strongest signals are down the axis 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? Point “P” where all waves arrive in phase

Methods of Beam Steering 3 Kinds Time Delay Scanning Frequency Scanning Phase Scanning

Methods of Beam Steering Time Delay Scanning Use of time delay to achieve the desired phase relationship Time delay networks installed in front of each radiating element Expensive, Complex & Heavy 1. Discuss Graphic on next slide. In all three methods of Electronic Beam Forming: 1. The energy received the same way it is transmitted. 2. The beam can be shifted in both azimuth and elevation without mechanical movement.

Time Delay Scanning d Time delay networks Impractical at high power levels

Methods of Beam Steering Frequency Scanning The physical length of the wave guide to delay the frequency interval Vary the frequency about a base frequency Very simple and relatively inexpensive. In all three methods of Electronic Beam Forming: 1. The energy receive the dame way it is transmitted. 2. The beam can be shifted in both azimuth and elevation without mechanical movement.

Frequency Scanning Case #1 All Waves transmitted are in phase Case #2 Wave guide feed horns Case #1 All Waves transmitted are in phase 1. Use graphic to show: a. How the wave guide feed horns are equally spaced and allow some portion of the main wave to escape (transmitted). b. Those escaping waves will be like individual antenna and have the same wave length as the original wave. 2. In case #1, a. All transmitted waves will be in phase so the beam axis will be perpendicular to the antenna (wave guide feed horns). b. This frequency is the base frequency of the radar (wave guide is designed based on this frequency.) 3. In case #2, a. The frequency of the main wave was slightly lengthened. b. Since the feed horns are still at the same spacing, the transmitted waves will be “phase shifted” at a constant amount with each successive feed horn. c. This has the effect of moving the axis of the beam summations like in the other graphics. d. How much the frequency is changed above or below the base frequency determines how much the beam axis will be deflected? 4. Note: that feed horns could have been located every other wave peak. ANY STAR TREK FANS? NEXT SLIDE! Wave guide Case #2 Waves transmitted are phase shifted

Methods of Beam Steering Phase Scanning Radiating elements fed by phase-shifting networks (Phasers) Phasers are electronically adjustable More expensive, than frequency scanning but cheaper than time delay Most widely used – SPY! SPY BABY! 1. Third type is phase shifting. a. Uses an electronic device to shift the phase call a phaser b. Phase shifting is more expensive and complex to frequency scanning. c. But is considerably less expensive than time delay in dollars, power and weight. In all three methods of Electronic Beam Forming: 1. The energy receive the dame way it is transmitted. 2. The beam can be shifted in both azimuth and elevation without mechanical movement.

Phase Scanning Phasers!!

Fleet Uses of Electronic Scanning 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’ - 4080 phasers in the arrays - 1/3 redundancy so can accept heavy losses to array without impacting performance.

AN/SPS – 48

Synthetic Aperture Array Radar * Uses the same theory of electronic scanning. * Motion of the target used as the antenna aperture * Sequential instead of simultaneous * High resolution imaging from aircraft

ISAR ISAR Gee, looks just like Fig 7-14, phased array!!

Electronic Scanning Summary Electronic scanning is GROOVY! Yeah Baby, Yeah!!! Multiple functions Search Track (so, TWS) Beam Steering Time delay, Frequency, and Phase scanning VERY BASIC LOOK AT ELECTRONIC SCANNING!

QUESTIONS?