Why 1030 MHz and 1090 MHz only?

The frequencies of 1030 MHz and 1090 MHz are specifically allocated for air traffic control and aircraft transponder communication due to their optimal propagation characteristics, which allow for reliable long-range communication with minimal interference. The 1030 MHz frequency is used for interrogating transponders, while 1090 MHz is used for transmitting replies. These frequencies are part of the L-band spectrum, which is well-suited for radar and surveillance applications, ensuring effective tracking and identification of aircraft.