article_snippet: Aircraft flying more than about 200 miles from land over oceans are not tracked by air traffic control radar screens.
article_snippet: If an aircraft's transponders are turned off, an aircraft can be seen only through so-called primary radar.
article_snippet: In these cases, air traffic controllers see only a blip on radar screens.
article_snippet: The blip isn't identified and the plane's altitude is unknown.
article_snippet: But air traffic controllers in Ho Chi Minh City never heard from the crew.
article_snippet: Three minutes after the radio call, the plane's two transponders mysteriously cease transmitting.
article_snippet: But  primary radar operated by the military in Malaysia and Thailand is able to track the 777 for limited periods without Thailand transponder being on.
article_snippet: But still, when an aircraft flies over the ocean, an aircraft will be out of range.
highlights_snippet: Most airliners use ground radar instead of satellites; tracking over oceans is spotty, at best .