12.1 Air traffic control
- Primary surveillance radar (PSR): L- or S-band rotating radar that detects aircraft purely by reflection. Range 60 to 250 nm. Sees non-cooperative targets, including a crashed plane or a hijacker who has turned off the transponder.
- Secondary surveillance radar (SSR): interrogates at 1030 MHz, transponders reply on 1090 MHz with a four-digit "squawk" (Mode A), altitude (Mode C), or ICAO ID and other data (Mode S). Not purely radar; relies on cooperative transponders.
- ADS-B: aircraft broadcast position and velocity from GPS every second. Not radar, but integrates with radar data in the controller's display.
The ASR-9, ASR-11, and ATCR-22 are typical airport surveillance radars: 2D rotating L or S-band, MTI processing, integrated SSR decoder.
12.2 Weather radar
NEXRAD (U.S. WSR-88D) operates at S-band (~2.7 GHz), mechanically scans, and transmits pulse-Doppler with full I/Q. Modern NEXRADs use dual polarization: alternating horizontal and vertical polarization between pulses. Differential reflectivity () and differential phase () reveal whether scatterers are roughly spherical (small drizzle, hail), oblate (large flattened rain drops), or elongated (tornado debris, ice needles), distinguishing rain from hail from biological echo from tornado debris in real time. A hurricane shows up as a rotating signature in the Doppler velocity field. Smaller airport TDWR (Terminal Doppler Weather Radar) units operate at C-band with finer range resolution, detecting low-altitude wind shear and microbursts that have caused commercial accidents.
12.3 Military radar
- Surveillance and early warning: AN/SPY-6 (Aegis), Russian Voronezh, Chinese JY-26. Multi-megawatts, multi-thousand-kilometer range.
- Fire control: AN/APG-77, AN/APG-81 (F-22 and F-35 nose radars). Track and engage simultaneously, AESAs.
- Missile guidance: many missiles carry small monopulse radars in the nose (Patriot, AMRAAM, S-400).
- Terrain following: F-111 and Tornado used radar to map terrain ahead and fly automatically below 100 feet AGL.
- Synthetic-aperture and inverse SAR for reconnaissance, see below.
12.4 Automotive radar: 24, 77, 79 GHz
Modern cars carry 5 to 12 radars: short-range 24 GHz for blind-spot and parking, medium-range 77 GHz for adaptive cruise control and front-collision warning, and long-range 77 GHz for highway-distance ACC. A few models use 79 GHz for high-resolution imaging. The waveform is almost universally fast-chirp FMCW. Antennas are MMIC patch arrays integrated with the chip; the whole radar is the size of a deck of cards.
The challenges are unique to cars: targets are 30 to 300 m away; Doppler ambiguity matters because cars move at 0 to 60 m/s; angular resolution is critical because the car needs to know whether a stopped vehicle is in its lane or the next one over. Modern automotive radars achieve 0.1 m range resolution and roughly 1° angular resolution with MIMO arrays of around 12 TX and 16 RX elements. By volume, automotive is now the most common radar deployment ever, with hundreds of millions on the road.
12.5 Ground-penetrating radar (GPR)
Specialized radar for looking down into the ground or into walls. Frequencies are 100 MHz to 2 GHz; lower frequencies penetrate farther but with worse resolution. Applications: archaeology, locating buried pipes and rebar, forensics, ice-thickness measurement. The transmitter is hand-pushed across the ground; the receiver records B-scans, and operators read hyperbolic signatures from buried point reflectors.
12.6 Synthetic aperture radar (SAR)
A satellite or aircraft moves while observing the same patch of ground; combining the returns coherently across positions produces an image with resolution far better than the physical antenna size would suggest. The "aperture" is synthesized from many small physical apertures. Resolution is set by the aperture length flown during integration, not the antenna's width. SAR works at any time of day, in any weather, through cloud cover. Operational examples: TerraSAR-X, RADARSAT, Sentinel-1, ALOS-2, ICEYE's small commercial SAR satellites. Inverse SAR (ISAR) does the dual: a stationary radar images a moving target like a ship; the target's motion is the synthetic aperture. ISAR signatures help identify ship classes.
12.7 Over-the-horizon (OTH) radar
HF radars at 3 to 30 MHz reflect off the ionospheric F-layer, seeing targets 500 to 4000 km beyond the radio horizon. Targets are aircraft and ships. The U.S. once operated AN/FPS-118; Russia operates Container; Australia operates JORN (Jindalee). Resolution is poor by line-of-sight standards (kilometers, not meters), and the ionospheric path varies with time of day, season, and solar activity. Useful for very-early warning, less so for fire control.
12.8 Police, sports, gesture
- Police radar: CW or FMCW Doppler at 24 or 33 GHz (the Stalker family). Lidar has largely replaced radar for fixed roadside enforcement, but Doppler radar remains common in patrol cars.
- Sports radar: pitch speed (baseball, cricket), serve speed (tennis), club speed (golf). Same physics, 24 GHz.
- Gesture radar: Google's Project Soli put a 60-GHz radar in Pixel phones to detect tiny finger gestures hands-free. The chip recognizes thumb-rubbing, swiping, and taps from the Doppler signature alone, without any camera.