Radar

Radar is a sensing technology that uses electromagnetic radiation, usually radio waves or microwaves, to detect objects and determine their range, direction, and motion. A conventional system transmits a signal and analyzes echoes returned by aircraft, ships, terrain, precipitation, or other targets. The name derives from “radio detection and ranging.” Radar supports navigation, surveillance, weather observation, and imaging without depending on visible illumination. Its capabilities vary with wavelength, transmitted waveform, antenna design, and processing methods. (weather.gov)

Historical development

Radar emerged from research into radio propagation and reflection rather than from a single invention. In September 1922, U.S. Navy researchers Albert Hoyt Taylor and Leo Clifford Young observed changes in a radio signal caused by a passing vessel. Subsequent Naval Research Laboratory work developed aircraft detection and pulsed ranging. In Britain, Robert Watson-Watt used radio receiving equipment in February 1935 to demonstrate echoes from aircraft. These experiments helped establish radio detection as a practical surveillance method. (nrl.navy.mil)

In December 1938, the experimental XAF radar was installed aboard USS New York. During World War II, radar became widely used for aircraft warning and naval operations. Weather echoes, initially an unwanted complication in military surveillance, also revealed its value for observing precipitation. Subsequent development extended radar into specialized meteorological networks, electronically steered systems, and airborne and spaceborne imaging instruments. (nrl.navy.mil)

Measurement principles

A transmitted wave interacts with a target through scattering; only a fraction of its energy returns to the receiver. In a monostatic radar, whose transmitter and receiver are effectively colocated, the target range $R$ follows from the round-trip delay $\Delta t$:

where $c$ is approximately the speed of light in air. The factor of two accounts for travel to the target and back. Direction is determined from the antenna beam’s orientation or from comparisons between signals received by different antenna elements. (weather.gov)

Relative motion produces a Doppler effect. For a monostatic radar observing a target moving much more slowly than light, the magnitude of the Doppler shift is approximately $2|v_r|/\lambda$, where $v_r$ is radial velocity and $\lambda$ is wavelength. This measures motion toward or away from the radar, not the complete velocity vector. Weather radars can determine radial velocity from changes in echo phase between successive pulses. (ll.mit.edu)

Echo strength depends on distance and the target’s radar cross section, an effective measure of its scattering response rather than simply its physical area. Shape, material, viewing direction, wavelength, and polarization affect this quantity. Under ideal free-space conditions, received power from an isolated target in a monostatic system decreases approximately with the fourth power of range. Receiver sensitivity and propagation losses therefore strongly influence detection distance. (ll.mit.edu)

Hardware and waveforms

A typical radar contains a transmitter, antenna, receiver, timing circuitry, and signal-processing equipment. A duplexer can allow a pulsed system to share one antenna between transmission and reception while protecting the receiver from the powerful transmitted signal. Mechanically scanned antennas rotate or tilt; a phased array instead controls relative phases across multiple radiating elements to steer its beam electronically. (nrl.navy.mil)

Pulsed radar separates echoes by arrival time. Continuous-wave radar can measure Doppler velocity, but an unmodulated continuous signal does not independently establish absolute range. Frequency-modulated continuous-wave radar changes its transmitted frequency, commonly through linear sweeps called chirps. Comparing received and transmitted signals produces beat frequencies from which range and motion can be estimated. (ll.mit.edu)

Range resolution is the ability to distinguish targets at nearby distances, not the accuracy of an individual distance estimate. For an ideal waveform with effective bandwidth $B$, it is approximately

Pulse compression combines a longer coded or frequency-modulated transmission with processing that achieves finer range resolution. Angular resolution depends principally on wavelength and effective aperture size. (e2e.ti.com)

Detection and limitations

Radar receivers must distinguish weak target returns from noise and clutter—unwanted echoes from terrain, sea surfaces, precipitation, or other objects. Processing may integrate successive returns, filter stationary echoes, and apply detection thresholds. Fourier transforms help separate signals by frequency or Doppler velocity, while tracking algorithms associate detections over time. Threshold selection involves a trade-off between missed targets and false alarms. (ll.mit.edu)

Radar is not universally “all-seeing.” Terrain can obstruct coverage; atmospheric attenuation, refraction, and multipath propagation can weaken or displace apparent returns. Finite beam width limits target separation. Repeated pulses can create range ambiguity, and discrete sampling can create Doppler ambiguity. Longer observation and suitable waveform design improve some measurements but impose trade-offs involving coverage, update rate, and processing requirements. (ll.mit.edu)

Applications and imaging

In air traffic control, primary radar detects reflected signals without requiring aircraft cooperation. Secondary surveillance radar instead interrogates an onboard transponder, whose replies provide identification and altitude information. Meteorological radar measures precipitation echoes and radial motion, enabling observation of precipitation distribution and storm circulation. These measurements require interpretation: echo intensity is not a direct, universally applicable measure of rainfall at the ground. (faa.gov)

Synthetic aperture radar combines echoes collected as an aircraft or spacecraft moves, creating an effective aperture larger than its physical antenna. It improves along-track resolution and produces images sensitive to surface roughness and electrical properties. As a form of remote sensing, it can image in darkness and through cloud cover. Repeated observations and interferometry also allow measurements of surface displacement, including changes associated with earthquakes and moving ice. (science.nasa.gov)