回叫信号

例句：

1. 1. The police used a radio to send out a "broadcast" signal.
   警方使用无线电发送了一个“广播”信号。
2. 2. The military has developed a system for sending back-channel signals during battle.
   军队开发了一种在战斗中发送反向信道信号的系统。
3. 3. The engineers were able to receive the signal and relay it back to headquarters.
   工程师们能够接收到信号并将其转发回总部。