阅读段落
Rudimentary air traffic control (ATC) existed well before the Grand Canyon disaster. As early as the 1920s, the earliest air traffic controllers manually guided aircraft in the vicinity of the airports, using lights and flags, while beacons and flashing lights were placed along across-country routes to establish the earliest airways. However, this purely visual system was useless in bad weather, and, by the 1930s, radio communication was coming into use for ATC. The first region to have something approximating today's ATC was New York City, with other major metropolitan areas following soon after.
段落大意
本段介绍了在大峡谷空难前就已经存在的基础性的航空交通管制（采用目视导航，恶劣天气下无效，随后使用无线电通信）。
阅读笔记
Rudimentary air traffic control (ATC) existed before:
1920s, the EARLIEST ATC: PURELY visual (useless in bad weather)
By 1930s, radio communication
逻辑关系
对比(逻辑连接词: HOWEVER)
The earliest air traffic controllers manually guided aircraft in the vicinity of the airports, using lights and flags, while beacons and flashing lights were placed along across-country routes to establish the earliest airways.
This purely visual system was useless in bad weather.
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