What does the approach controller mainly rely on in vertical navigation?

The primary reliance of an approach controller in vertical navigation is on vertical speed indicators. These indicators provide real-time information about the aircraft's rate of climb or descent, which is crucial during the approach phase of flight. By monitoring vertical speed, controllers can ensure that the aircraft is maintaining an appropriate and safe profile for descent, allowing for accurate adjustment of altitude as the aircraft approaches the runway.

Vertical speed information allows the controller to assess whether an aircraft is descending too rapidly or not fast enough, providing a basis for giving instructions to modify the descent rate. This information is vital for maintaining safe separation from other air traffic and for ensuring that the aircraft reaches the correct altitude at the correct point in the approach.

In contrast, while flight altitude refers to the height of the aircraft above sea level or ground level, it does not convey information about how quickly the aircraft is changing altitude. Time intervals may be useful for spacing between aircraft but do not directly influence vertical navigation. Ground distance reflects the horizontal distance to the runway but is not pertinent to managing vertical navigation during an approach. Thus, vertical speed indicators are the most important tool for approach controllers in managing the safe vertical movement of aircraft.