Radars detect aircraft by emitting radio waves from an antenna and listening for echoes that bounce back from the aircraft in flight. When a radar wave encounters an aircraft, part of the wave reflects off the plane’s metal surfaces, such as its wings and fuselage. The radar receiver then captures these reflected signals, known as echoes, and processes them to determine the position, distance (range, and speed) of the aircraft. Radars are designed to detect targets of varying sizes and can distinguish between different types of aircraft based on their radar cross section and movement characteristics.
Radar systems can identify the type of plan to some extent based on the radar return signals they receive. Different types of aircraft have varying sizes, shapes and radar cross-sections, which influence how they reflect radar waves. By analyzing the resistance and pattern of radar returns, as well as through the use of databases of known aircraft characteristics, radar operators can often identify the general type of aircraft, such as a commercial airliner, military fighter jet or smaller private plane. However, identifying specific patterns or configurations usually requires additional information or more advanced radar systems.
The phrase “flying under the radar” metaphorically refers to actions or events that go unnoticed or evade detection, which often involves avoiding scrutiny or attention. In literal terms, flying under the radar can mean operating at altitudes or in ways that minimize detection by radar systems. Aircraft flying at very low altitudes, below the coverage area of ground radar systems, or using terrain features to shield from radar waves, can reduce their radar visibility. This technique is often used by military aircraft flying low-level missions or by aircraft trying to avoid detection for various reasons.