What mode of TCAS only interrogates Mode S transponders?

The mode of TCAS that only interrogates Mode S transponders is known as 'Mode S All Call.' This mode is specifically designed to interact with aircraft equipped with Mode S transponders, which provide more sophisticated data transmission compared to other modes. Mode S allows for selective interrogation of transponders and better handling of aircraft information with enhanced data exchange capabilities, leading to improved situational awareness and traffic management.

In this mode, the TCAS system sends a request to all aircraft in the vicinity that are equipped with Mode S transponders, which can respond with detailed information such as altitude and identification. This capability is essential for accurate tracking and collision avoidance since it relies on the advanced features of Mode S transponder communications.

In contrast, other modes mentioned do not have the same functionality and level of specificity as 'Mode S All Call.' Basic Mode often refers to older systems that may not have the advantages provided by Mode S, and Mode A does not support the selective interrogation of specific transponders that Mode S entails. General Mode typically encompasses operations that may involve various transponder types without the dedicated focus of interrogating Mode S only.