In aerial combat movies and TV dramas, the line "Enemy Tracking!" is often accompanied by a rapid and piercing "beep" sound. The pilot then pulls the control stick, releases the jamming bomb, and performs extreme maneuvering avoidance. This sound is the warning signal issued by the aircraft's onboard radar warning device. As the core equipment of modern fighter situational awareness, this seemingly inconspicuous device constantly guards the safety of fighter jets. A radar alarm is essentially an 'airborne signal reconnaissance soldier'. To detect and track fighter jets, enemy radar must continuously emit electromagnetic wave signals. The task of the alarm is to capture these "radio wave traces" that propagate in the air. It receives radar signals through antennas located throughout the aircraft, analyzes the characteristics of the signals, and determines whether they are enemy search radars, tracking radars, or missile guidance radars that have already been launched, thereby distinguishing threat levels and issuing warnings to pilots. The radar warning system was born on the Vietnam battlefield, when American fighter jets were frequently hit by Soviet S-75 air defense missiles. The first generation of radar warning systems emerged, specifically designed to capture the tracking radar signals of these air defense missiles and gain valuable evasion time for pilots. During the Cold War, airborne fire control radar entered a period of rapid development, and crystal video receivers became the core components of the warning system. The SPO-15 warning system widely used by the Soviet Union is a representative of this. It can recognize low repetition rate and medium repetition rate radar signals, but once it encounters pulse Doppler radar, it can only issue rough warnings and cannot distinguish specific tracking modes. In the Gulf War, the MiG-29 fighter jet was shot down by the US Sparrow missile, most likely because the warning system failed to recognize the enemy radar's single pulse tracking mode, and the pilot did not detect the danger in a timely manner. With the upgrading of air combat technology, scanning superheterodyne receivers have replaced crystal video receivers. This receiver can accurately analyze the frequency of radar signals and determine the type of radar. If signals in the same direction and frequency are continuously received during multiple sweep cycles, it can be determined that the fighter jet is being continuously illuminated or tracked. The alarm will then switch to a rapid continuous sound, and the threat sign on the display will continue to flash, reminding the pilot that danger is imminent. In the 1990s, instantaneous frequency measurement receivers became the "new heart" of alarm devices. Its response speed has been greatly improved, it can capture radar pulse signals one by one, and even recognize special signal patterns for missile targeting. When the enemy radar switches attack mode, the alarm captures this signal and determines that the fighter has entered the effective range of the missile, immediately issuing a high threat warning. In the 21st century, in order to cope with the constantly evolving missile radar, digital signal processing warning devices have emerged. The superheterodyne digital receiver can accurately calculate the signal frequency, and its performance far exceeds that of traditional analog alarms. After 2020, RF direct acquisition receivers have entered the stage. They do not require signal conversion and can directly collect and record the raw radio signals emitted by radar and other equipment. Moreover, the system is simpler and the performance is more stable, becoming the development direction of future alarm devices. It is worth mentioning that fighter jet fire control radar and missile guidance radar have also played the game of "camouflage". The operating frequency of some missile guidance radars overlaps with that of airborne fire control radars, making it difficult for the alarm to distinguish whether it is aimed or locked. With the application of new technologies such as millimeter wave radar, the challenges faced by alarm systems are becoming increasingly significant. In the future, with the integration of intelligent and information technology, the "airborne life guard" of this fighter jet will continue to evolve. (New Society)