What type of antenna pattern is radiated from a ground station's phased-array directional antenna in a Mode S interrogation signal?

A phased-array directional antenna utilized in Mode S interrogation signals emits a specific 1030 MHz directional pattern. This type of antenna is designed to focus radio frequency energy into a narrow beam, allowing for precise targeting of aircraft transponders. As a result, the interrogation signals can be sent out in a manner that enables better reception and reduces interference from unwanted signals.

The choice of a 1030 MHz frequency is significant because it is the designated frequency for aircraft transponder interrogations in the Mode S system. This operational frequency enhances the effectiveness of the interrogations by allowing them to differentiate between signals and improve communication clarity in air traffic control environments.

While other patterns may be relevant in different contexts, such as a long-range omnidirectional pattern or a short-range directional pattern, the Mode S system specifically relies on the 1030 MHz signal to communicate effectively with transponders, making that choice the most accurate in this scenario.