Can a small group of drones（无人机）guarantee the safety and reliability of railways and,at the same time, help railway operators save billions of euros each year? That is the very likely future of applying today's "eyes in the sky" technology to making sure that the millions of
kilometres of rail tracks and infrastructure（基础设施）worldwide are safe for trains on a 24/7 basis.
Drones are already being used to examine high-tension electrical lines. They could do precisely the same thing to inspect railway lines and other vital aspects of rail infrastructure such as the correct position of railway tracks and switching points. The more regularly they can be
inspected, the more railway safety, reliability and on-time performance will be improved. Costs would be cut and operations would be more efficient（高效）across the board.
That includes huge savings in maintenance costs and better protection of railway personnel safety. It is calculated that European railways alone spend approximately 20 billion euros a year on maintenance, including sending maintenance staff, often at night, to inspect and repair the rail
infrastructure. That can be dangerous work that could be avoided with drones assisting the crews'efforts.
By using the latest technologies, drones could also start providing higher-value services for railways, detecting faults in the rail or switches, before they can cause any safety problems. To perform these tasks, drones for rail don't need to be flying overhead. Engineers are now working
on a new concept: the rail drones of the future. They will be moving on the track ahead of the train,
and programmed to run autonomously. Very small drones with advanced sensors and AI and travelling ahead of the train could guide it like a co-pilot. With their ability to see ahead, they could signal any problem, so that fast-moving trains would be able to react in time.
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安装适当的软件后，利用智能手机中的磁传感器可以测量磁感应强度 B。如图，在手机上建立直角坐标系，手机显示屏所在平面为 xOy 面。某同学在某地对地磁场进行了四次测量，每次测量时 y 轴指向不同方向而 z 轴正向保持竖直向上。根据表中测量结果可推知（）
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2022 年 3 月，中国航天员翟志刚、王亚平、叶光富在离地球表面约400km 的“天宫二号”空间站上通过天地连线，为同学们上了一堂精彩的科学课。通过直播画面可以看到，在近地圆轨道上飞行的“天宫二号”中，航天员可以自由地漂浮，这表明他们（ ）
