9. Antenna Scanning: How Radars Look Around // Radar Systems // bhaswanth

9.1 Mechanical scanning

The original radar antennas rotated. A motor turned the dish at a few RPM, sweeping the beam through 360° of azimuth. Elevation was either fixed (2D radar) or set separately (3D radar).

Mechanical scan patterns:

Continuous rotation: airport surveillance, marine, weather. Simple, robust, slow (4 to 15 seconds per scan).
Sector scan: oscillate between limits. Coastal surveillance pointing seaward.
Conical scan: the feed is offset and rotates at, say, 30 Hz, scribing a cone in space. Off-axis targets modulate the return at 30 Hz; the modulation phase indicates direction. Old fire-control.
Raster scan: TV-style back-and-forth in azimuth at multiple elevation steps. Airborne search.

The fundamental drawback is inertia. You cannot redirect a multi-ton antenna in milliseconds. New targets are invisible until the next scan cycle.

9.2 Phased arrays: steering with phase, not motion

We met phased arrays in Chapter 13. To recap the key idea: an array of antenna elements, each fed with the same signal but a different phase, produces a beam pointed in a direction determined by the phase gradient across the array. Change the phases electronically, and the beam moves. No mechanical motion required. The beam can move from one direction to another in microseconds.

For a linear array of $N$ elements spaced $d$ apart, with progressive phase shift $\phi$ between adjacent elements, the beam points at angle $\theta$ from broadside satisfying

$\sin\theta = \frac{\phi \lambda}{2 \pi d}$

so $\phi$ controls $\theta$ with electronic precision. A 2D array steers the beam in both azimuth and elevation simultaneously. Beam dwell can be milliseconds; entire-sky search and confirmed-target tracking can be interleaved in real time.

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   Phased array elements (top view):
       
       │  │  │  │  │  │  │   ← N elements, spacing d
       └──┴──┴──┴──┴──┴──┘
        φ_0 φ_1 φ_2 ...   ← progressive phase shifts (phase shifters)
                          
   Resulting beam:
       beam tilts off broadside by angle θ where sin θ = φλ/(2πd)
       
       no movement of elements at all; the steering is in the phases.

9.3 PESA vs AESA

PESA (Passive Electronically Scanned Array) has a single central transmitter feeding a network of phase shifters that distribute power to the elements. Cheaper but inflexible, and one transmitter failure kills the radar.

AESA (Active Electronically Scanned Array) has a complete TX/RX module behind every element, each with its own amplifier, phase shifter, and LNA. Modern AESAs use GaN or GaAs MMIC modules. They offer graceful degradation (one bad element loses 1/N of the power, not the whole radar), simultaneous multiple beams, and very low side-lobe levels.

Operational AESAs include the AN/APG-77 (F-22, ~2000 T/R modules), AN/APG-81 (F-35, ~1700 modules), AN/SPY-6 (U.S. Navy destroyers, an enormous L/S-band array), and Israel's EL/M-2084 Iron Dome tracker. 5G base stations use the same MMIC technology at 28-39 GHz with dozens to hundreds of elements per panel.

9.4 Why beam-steering matters operationally

Mechanically-scanned radars revisit each direction every scan cycle, perhaps every 5 to 12 seconds. Phased arrays can revisit a confirmed target every 100 milliseconds while still scanning the rest of the volume. The track update rate matters: a fast-moving missile can change its trajectory significantly in 5 seconds, but barely at all in 0.1 second. Phased arrays make precision tracking practical.

They also make multi-target tracking practical. The radar dwells on each of dozens of targets in turn, allocating dwell time according to threat priority. Aegis, the U.S. Navy's air defense system, can simultaneously track around a hundred airborne targets and engage a dozen with missiles, all from one set of phased-array faces on each ship.