Why do planes disappear from radar when crossing the ocean?

According to Unilad Tech, radar, a radio detection and positioning system, was invented in 1935 by two British inventors Robert Watson-Watt and Arnold Wilkins. Thanks to radar, air traffic control can track a plane's position in the sky. The system is also used for navigation and collision avoidance between aircraft.

Radar depends on straight-line transmission. The curvature of the Earth and geographical barriers limit radar observations. The majority of flights over oceans are beyond the coverage of ground-based radar stations (within 240 - 320 km from the coast). The Earth is spherical, so radar waves traveling straight will be directed out into space, creating a giant "blind spot" in the middle of the ocean. However, installing hundreds of radar stations at sea is impossible because there is no solid ground.

Additionally, high terrain or isolated areas of airspace away from busy flight routes can cause radar signals to be disrupted or create gaps in coverage. While satellite-based surveillance systems can improve coverage, blind spots still exist globally. Radar signals can also be disrupted by environmental conditions. For example, heavy rain can disrupt or weaken radar signals, leading to temporary loss of communications.

Slashgear said commercial aviation uses two types of radar systems. Primary surveillance radar (PSR) works by emitting radio waves, then receiving signals reflected back from the surface of an object. The PSR can show an object's position, but cannot determine its identity (flight name) or altitude itself. Secondary surveillance radar (SSR) sends interrogation signals and receives response data codes from the aircraft's transponder, thereby collecting important information including flight identification code, cruise altitude, speed and direction. SSR plays a key role when aircraft fly where radar coverage is limited.

"If the transponder is broken or someone intentionally turns it off, the SSR will basically no longer see the plane. The PSR can still detect the plane as a dot, but the accuracy is limited, especially in remote areas or at low altitudes," flightradar24 quoted professor Igor Petrov, an aviation systems expert at the Moscow Institute of Civil Aviation.

A famous case where air traffic control lost signal from SSR was Malaysia Airlines flight MH370 in 2014. The Boeing 777-200ER plane carrying 239 people took off from Kuala Lumpur, Malaysia to Beijing, China and disappeared from civil radar. Military radar showed that the plane was off course, and the transponder signal was also lost. Although authorities conducted a large-scale search, they did not discover the plane's wreckage, although some small pieces of debris were collected on various islands. The report from the Malaysian government ruled out mechanical or computer failure, and pointed to the transponder being intentionally turned off.