What frequency does the glideslope operate at the top?

The correct frequency for a glideslope signal is 150 Hz. This frequency is essential for the precision approach systems used in instrument landing. The glideslope provides vertical guidance to aircraft during the approach phase of landing, operating in conjunction with the localizer, which provides horizontal guidance.

At 150 Hz, the glideslope signal is able to transmit information that pilots can use to maintain the proper descent path as they approach a runway. The use of this specific frequency allows for accurate and reliable performance of the Instrument Landing System (ILS) ensuring that aircraft can land safely even in low visibility conditions.

The other frequencies do not align with the established standards for glideslope operations. For instance, 90 Hz, 120 Hz, and 180 Hz are not used for glideslope signals and do not correspond with the fixed parameters that define the ILS system used in aviation. Thus, the significance of 150 Hz is underlined by its specific application in glideslope functionality, making it the clearly established answer.