The G-222 aircraft, which were built between 1977 and 1985, are receiving avionics upgrades, while two will also be given sound-proof, VIP transport modules.

G-222型飞机是在1977年至1985年间建造的，目前正在接受航空电子设备升级，其中两架还将配备隔音的贵宾运输模块。

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Unlike high altitude aircraft, helicopters aren't so vulnerable to being seen by radar, as they fly low and are often hidden by the terrain. The big issue is the sound.

与在高空飞行的飞机不同，直升机不容易被雷达发现，因为它的飞行高度较低，且常常被地形所隐藏，因此最大的问题是噪音。

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An eerie drone fills the air, falling somewhere between the sound of a buzzing wasp and a light aircraft taking off.

一个神秘的嗡嗡声弥漫在空中，落在了某个地方，那声音间于嗡嗡作响的蜜蜂和轻型飞机起飞的声音之间。

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Voters have no reason to trust government pledges that flights on the new runway will be limited to environmentally sound aircraft.

ECONOMIST: Airport expansion

His SpaceShipOne has flown to the top of the atmosphere and last year became the first privately built aircraft to break the sound barrier.

BBC: NEWS | Science/Nature | Space future: Race for the X-Prize

Meanwhile, Boeing's Sonic Cruiser (a futuristic aircraft designed to fly at almost the speed of sound) has not been well received by airlines.

ECONOMIST: EasyJet