traffic control uses primary and secondary radars. Primary radars are a "classical" radar which reflects all kind of echoes, including aircraft and clouds. Secondary radar emits pulses and listens for special answer of digital data emitted by an Aircraft Transponder as an answer. Transponders emit different kind of data like a 4 octal ID (mode A), the onboard calculated altitude (mode C) or the Callsign (not the flight number) (mode S). Military use transponders to establish the nationality and intention of an aircraft, so that air defenses can identify possibly hostile radar returns. This military system is called IFF (Identification Friend or Foe). Air Traffic Control (ATC) Radars Secondary Surveillance Radar (SSR) (Airport Surveillance Radar) Ground Control Approach (GCA) Radars Precision Approach Radar (PAR) Systems Distance Measuring Equipment (DME) Radio Beacons Radar Altimeter (RA) Systems Terrain-Following Radar (TFR) Systems Radar altimeters measure an aircraft's true height above ground. Space and range instrumentation radar systems Space (SP) Tracking Systems Range Instrumentation (RI) Systems Video Relay/Downlink Systems Space-based radar Incoherent scatter Mapping radars Mapping radars are used to scan a large region for remote sensing and geography applications. They generally use synthetic aperture radar, which limits them to relatively static targets, normally terrain. Specific radar systems can sense a human behind walls. This is possible since the reflective characteristics of humans are generally more diverse than those of the materials typically used in construction. However, since humans reflect far less radar energy than metal does, these systems require sophisticated technology to isolate human targets and moreover to process any sort of detailed image. Through-the-wall radars can be made with Ultra Wideband impulse radar, micro-Doppler radar, and synthetic aperture radar (SAR). Imaging radar 3D radar Speed radar Radar gun, for traffic policing and as used in some sports Radars for biological research Radar range and wavelength can be adapted for different surveys of bird and migration and daily habits. They can have other uses too in the biological field. Insect radar Surveillance radar (mostly X and S band, i.e. primary ATC Radars) Tracking radar (mostly X band, i.e. Fire Control Systems) Wearable radar and miniature radar systems are used as electric seeing aids for the visually impaired, as well as early warning collision detection and situational awareness. See also Radar engineering details Automatic Radar Plotting Aid Low probability of intercept Radar tracker Notes Radar