preparatory signal

例句：

1. 1.The military has been using this preparatory signal to warn of an impending attack.
   军队一直使用这个预备信号来警告即将到来的攻击。
2. 2. The pilot used a preparatory signal to communicate with the ground crew before takeoff.
   飞行员在起飞前使用了预备信号与地面机组进行通信。
3. 3.The company issued a preparatory signal to all employees to ensure they were prepared for the sudden shift in operations.
   公司向所有员工发布了一个预备信号，以确保他们为突然的业务转变做好准备。