No mechanical devices are used in passive heating systems. They operate with little or no mechanical assistance.

被动加热系统中不使用机械设备。它们在很少或没有机械辅助的情况下运行。

The growth in radio and television broadcasts—especially with digital and high-definition TV—now provides an enormous amount of high-frequency radio waves which are ideally suitable for passive radar systems.

现在无线电和电视广播的增长（尤其是数字电视和高清电视出现以后）提供了海量的高频电波，这些非常适合用于被动雷达系统。

The aviation industry is cautious about adopting new technologies wholesale, so there is a long way to go before conventional radar is turned off in favour of passive systems.

航空业在大规模采用新技术方面很是谨慎，因此传统雷达完全让位于被动雷达还尚有时日。