Which facility can substitute for the middle marker during a Category I ILS approach?

The middle marker in a Category I Instrument Landing System (ILS) approach serves as a precise indication of the aircraft's position relative to the runway for the final approach. When it comes to substituting for the middle marker, a compass locator is the appropriate facility.

A compass locator is a low-frequency marker beacon that provides an audible signal on the airplane's approach equipment, indicating that the aircraft is nearing the decision height during the approach. This allows pilots to determine their position relative to the runway more accurately, especially when the middle marker is not available.

In contrast, a VOR (VHF Omnidirectional Range) is used primarily for navigation and does not provide the specialized approach guidance necessary as a substitute for the middle marker. The localizer is part of the ILS itself and provides lateral guidance but does not function to indicate vertical position like the middle marker does. The glideslope is another component of the ILS that provides vertical guidance, but it also does not serve as a substitute for the middle marker.

Thus, the compass locator’s role in providing relevant positional information specifically for the approach makes it the suitable substitute in this case.