中文摘要：

A／C模式的二次雷达在目标数量增加到一定程度时暴露出许多问题，S模式是欧美国家开发的先进二次雷达协议，正在逐步取代传统的A／C模式。文中全面而简略地阐述了S模式的协议内容和工作机制，包括：S模式的起源和特点；S模式询问信号和应答信号的格式、含义、分类及询问应答机制；S模式的上下行数据格式、含义及涉及的重要问题；S模式的兼容性、锁定技术、Squitter模式、数据校验和干扰问题。最后，介绍了S模式在多个领域的应用情况。

英文摘要：

Mode A/C limits the fast growth of aircraft number in busy airports. America and Europe have developed mode S protocol for secondary surveillance radar（ SSR）, which is replacing mode A/C in many countries. This paper gives a general introduction to and overview of mode S, including its origin and features, formats, meanings, sorts and operation principles of interrogation and transponder signals, the uplink and downlink data and related important questions, compatibility, lockout technique, squirter and extended squirter, data check, and interferences. Finally, it introduces some applications of mode S in many fields.