How Radar Detects Aircraft

01 The Principle: Echo Location with Radio

Radar — an acronym coined by the United States Navy in 1940 for radio detection and ranging — works on a principle that is physically identical to how bats and dolphins navigate: emit a pulse of energy, listen for the echo, and measure the time it takes to return. Radio waves travel at the speed of light, approximately 3 × 10⁸ meters per second. An aircraft at a range of 150 kilometers reflects a pulse that returns in one millisecond. The round-trip time, halved and multiplied by the speed of light, gives the distance. The direction the antenna was pointing when the echo returned gives the azimuth. Together, range and azimuth place the target on the controller's scope.

A radar system consists of a transmitter producing electromagnetic waves in the radio or microwave domain, a transmitting antenna that focuses the energy into a beam, a receiving antenna — often the same antenna, switched between transmit and receive modes — and a receiver and processor that extracts target information from the returned signal. The antenna rotates, sweeping the beam through 360 degrees, and each sweep updates the position of every target within range. The result, on a classic plan position indicator (PPI) display, is a circular map of the surrounding sky with blips that move sweep by sweep.

02 Primary Radar: The Brute-Force Approach

The simplest form of radar, called primary surveillance radar, transmits a short, high-power pulse and then listens for echoes from any object in the beam's path. Every conductive surface — the aluminum fuselage of an airliner, the steel hull of a ship, even a dense column of rain — reflects some fraction of the radio energy back toward the antenna. The fraction reflected depends on the target's radar cross-section (RCS), a measure of how detectable it is. A large commercial airliner has an RCS on the order of 100 square meters. A small drone might have an RCS of 0.01 square meters — four orders of magnitude less detectable.

Primary radar's great strength is that it detects everything: it does not require cooperation from the target. A lost aircraft with a failed transponder, a hostile military jet, or a piece of debris will all produce a return. Its weakness is that it cannot tell the controller what the blip is. Primary radar sees a Boeing 737 and a flock of birds with the same indifference, and weather clutter can mask genuine targets. Modern primary radar systems use moving target indication (MTI) processing, which compares successive sweeps and suppresses returns from stationary or slow-moving objects, filtering out terrain and most weather while highlighting moving aircraft.

Chart: Radar frequency bands and approximate maximum detection ranges. Higher frequency offers better resolution but shorter range due to atmospheric attenuation. Values are illustrative for typical systems.

03 Secondary Radar: Interrogation and Reply

To solve the identification problem, ATC adopted secondary surveillance radar (SSR). Unlike primary radar, SSR is an active cooperative system: the ground station transmits an interrogation pulse on 1030 MHz, and any aircraft equipped with a transponder replies on 1090 MHz with a coded message. The reply contains the aircraft's Mode A squawk code — a four-digit identifier assigned by ATC — and, in Mode C or Mode S, the aircraft's pressure altitude. Mode S, the current standard, assigns each aircraft a unique 24-bit address and supports extended squitter messages that carry additional data including callsign, velocity, and position.

Secondary radar transformed the controller's display from anonymous blips into a labeled, identified traffic picture. It also solved the altitude problem: primary radar measures azimuth and range, but determining altitude from the ground requires a separate elevation beam or a height-finding radar. With Mode C or Mode S, the aircraft's own altimeter reports altitude in the transponder reply, giving the controller three-dimensional position without additional hardware. The limitation of SSR is that it only sees cooperating aircraft. A failed transponder, a military aircraft with its transponder off, or a non-equipped drone will be invisible to secondary radar — which is why primary and secondary radar are operated together, the one identifying, the other catching what the other misses.

04 The Doppler Effect: Measuring Velocity from Frequency Shift

A target that is moving toward or away from the radar produces a Doppler shift in the returned signal. The echo from an approaching aircraft is at a slightly higher frequency than the transmitted pulse; the echo from a receding aircraft is at a slightly lower frequency. The magnitude of the shift is proportional to the radial velocity of the target — the component of its velocity along the line of sight to the radar. An aircraft closing at 300 meters per second on an S-band radar (3 GHz) produces a Doppler shift of approximately 6 kilohertz, easily measurable with modern digital signal processing.

Doppler processing is the foundation of pulse-Doppler radar, which filters targets by their velocity as well as their range. This is what allows airborne fighter radars to detect moving aircraft against the overwhelming clutter of ground returns. A stationary building and a moving jet are at the same range and azimuth, but their Doppler signatures are completely different: the building has zero Doppler shift; the jet, closing at hundreds of meters per second, has a large and characteristic shift. By filtering out zero-Doppler returns, pulse-Doppler radar suppresses ground clutter and reveals only moving targets — the same principle, applied in reverse, that allows police radar guns to measure vehicle speed.

Chart: The radar range equation's fourth-root relationship between RCS and detection range. This is why stealth aircraft cannot achieve full invisibility — only reduced detection distance.

05 Phased Arrays: The End of the Rotating Antenna

For seventy years, the defining mechanical feature of a radar was the rotating antenna. It swept the beam mechanically, completing a rotation every 4–12 seconds, and each sweep gave one positional update. The limitation is obvious: between sweeps, the target moves, and fast-moving or maneuvering targets can be lost if the update rate is too slow. The solution is the phased array antenna, a flat panel containing hundreds or thousands of individual radiating elements, each with an electronically controlled phase shifter. By adjusting the relative phase of each element, the beam can be steered electronically — in microseconds, without any mechanical motion.

Phased arrays, originally developed for missile defense and naval air-defense systems, allow a radar to track multiple targets simultaneously while spending more dwell time on each one. A modern active electronically scanned array (AESA) can form multiple beams, search in one direction while tracking in another, and adapt its waveform to the target type. The PATRIOT missile system's radar, the Aegis combat system's SPY arrays, and the latest fighter radars (the F-35's APG-81, the F-22's APG-77) are all AESAs. The technology is migrating to civilian applications as well — the FAA's NextGen surveillance systems use phased-array principles, and modern weather radars increasingly employ electronic scanning to produce volumetric scans in the time a mechanical dish would take for a single planar sweep.

06 Stealth: The Contest Between Detection and Evasion

Radar's capability created its own countermeasure: stealth, the engineering of aircraft shapes and materials to reduce the radar cross-section and thus the detection range. The physics that makes stealth possible is the radar range equation, which states that detection range is proportional to the fourth root of the radar cross-section. This fourth-root relationship is critical: to halve detection range, one must reduce RCS by a factor of sixteen — not by a factor of two. A conventional fighter has an RCS of roughly 3–5 square meters. The F-117 Nighthawk, the first operational stealth aircraft, was designed to have an RCS of approximately 0.003 square meters — roughly the size of a marble — reducing detection range by roughly a factor of seven.

Stealth is achieved through two mechanisms. First, shape: facets and angled surfaces deflect reflected energy away from the receiver, rather than back toward it. The F-117's faceted design, the B-2's flying-wing planform, and the F-35's chined fuselage all direct radar energy into the sky or the ground rather than back to the radar. Second, radar-absorbent materials (RAM) convert incident electromagnetic energy into heat rather than reflecting it. The contest is ongoing. Low-frequency radars (VHF and UHF bands, with wavelengths comparable to the aircraft's dimensions) can detect stealth aircraft at reduced but useful ranges, because the shaping that defeats X-band radar is less effective against meter-scale wavelengths. Multistatic radar — where the transmitter and receiver are in different locations — can catch the energy that a stealth shape deflects sideways rather than back toward the source. No aircraft is truly invisible to radar; stealth buys time and distance, reducing the effective engagement envelope, not eliminating detection.

07 From Tube to Chip: The Signal Processing Revolution

The radar systems of the 1940s were analog devices built around magnetron tubes, and their processing consisted of a human operator watching a flickering phosphor screen. The transformation since then is not in the fundamental physics — radio waves still reflect, and the speed of light has not changed — but in the signal processing that extracts information from the returned signal. A modern radar digitizes every return and subjects it to fast Fourier transforms, matched filtering, constant false alarm rate processing, and target-tracking algorithms that would have been computationally impossible a generation ago.

The result is that a radar that once produced a single blip per target per sweep now produces a track with a velocity vector, a projected future position, and a classification. The operator's role has shifted from detecting targets to managing tracks that the machine has already established. This is true across the field: the air traffic controller's display is populated by automated correlation software that matches primary and secondary returns into a single track; the fighter pilot's radar categorizes each detected target and prioritizes the threats; the weather radar's processor separates precipitation from biological clutter and estimates rainfall rates in real time. The echo has not changed. What we do with it has.