求翻译：All of the ground-based navigation aids are being supplemented by satellite-based aids like Global Positioning System (GPS), which make it possible for aircrews to know their position with great precision anywhere in the world. With the arrival of Wide Area Augmentation System (WAAS), GPS navigation has become accurate enough for vertical (altitude) as well as horizontal use, and is being used increasingly for instrument approaches as well as en-route navigation. However, since the GPS constellation is a single point of failure that can be switched off by the U.S. military in time of crisis, onboard Inertial Navigation System (INS) or ground-based navigation aids are still required for backup.是什么意思？

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