HONG KONG — If the missing Malaysia Airlines jetliner flew north over the Asian mainland after it lost contact with ground controllers on March 8, it would have had a difficult time avoiding detection by Chinese, Indian or American radar, current and former military officers say.

Investigators say that automated signals from the jet, a Boeing 777, that were picked up by a satellite more than seven hours after the plane took off indicate that by then it must have been somewhere near one of two broad arcs on the map. One arc stretches from northern Thailand and Laos through China to Kazakhstan in Central Asia; the other extends from western Indonesia into the Indian Ocean west of Australia.

The northern arc crosses some of the most closely defended borders in the world. Experts on the radar systems in use in the area say that a Boeing 777, which has a large radar profile, would more likely than not have been detected by Chinese and Indian air defense forces and by American forces in Afghanistan.

China, the original destination for the flight, is guarded by military radar systems at high elevations in its border areas and even in its interior, where the People’s Liberation Army controls most of the country’s airspace. The border between India and Pakistan is highly militarized, and radar experts and Indian military officials discounted the possibility that a jetliner could pass through the area undetected. Farther northwest, the United States Air Force has its own radar installations in Afghanistan to protect air bases there from intruders.