What system provides exact alignment and descent path for aircraft?

The Instrument Landing System (ILS) is a critical system used in aviation to provide precise guidance to aircraft during the approach and landing phases. It specifically allows pilots to align with the runway centerline and follow the correct descent path. The ILS comprises two main components: the localizer, which guides the aircraft laterally to align with the runway, and the glideslope, which provides vertical guidance to ensure a safe descent angle.

The ILS is essential for operations in low visibility conditions, as it enables aircraft to land accurately without relying solely on visual references. Its effectiveness lies in the combination of both lateral and vertical guidance, which is crucial for making safe landings, particularly at busy airports like O'Hare.

While other systems like DME, localizer, and glideslope contribute to navigation and approach, the ILS encompasses both alignment and descent, making it the comprehensive answer to the question regarding exact control during aircraft landing.