Explanation: A ground station transmitting a Mode S interrogation signal, including its PPM pulses, operates on a specific frequency. Air traffic control radar beacon interrogations (which include Mode S) are always transmitted on **1030 MHz**. Aircraft replies, conversely, are sent on 1090 MHz. Furthermore, the question specifies a "phased-array directional antenna." This type of antenna is designed to create a focused, directional beam that can be electronically steered. Ground-based secondary surveillance radars (SSR), including those implementing Mode S, use directional antennas to determine the bearing of aircraft and scan the airspace efficiently. Therefore, the interrogation signal will be transmitted on 1030 MHz with a directional pattern. * A) 1090 MHz directional pattern: Incorrect frequency for interrogation. * B) 1030 MHz omnidirectional pattern: Incorrect pattern for a phased-array directional antenna and typical ground station interrogation. * C) 1090 MHz omnidirectional pattern: Incorrect frequency for interrogation.