Which of the following is NOT a mandatory parameter for transponders in aviation?

In aviation, transponders are vital for communication between an aircraft and air traffic control, and they have several key functionalities. The mandatory parameters for transponders generally include a transponder code, altitude reporting, and a mode of operation.

The transponder code (also known as the squawk code) is essential for identifying aircraft on radar. Each aircraft is assigned a unique code, which helps air traffic controllers track and manage airspace traffic effectively.

Altitude reporting is another critical function, as it provides air traffic control with the aircraft's altitude, ensuring safe vertical separation between aircraft.

The mode of operation dictates how the transponder behaves in different situations and is necessary for the transponder to communicate the aircraft's position and intentions adequately.

Operating frequency, on the other hand, while important for communication systems, is generally standardized across transponder types and does not vary from one aircraft to another. For transponders, the typical operating frequency is set by international regulations, meaning that it is not a customizable or mandatory parameter for each individual transponder installation. This is why it is not considered a mandatory parameter in the same way as the other options mentioned.