What is the minimum ceiling reported that restricts aircraft from operating in the ILS critical area?

The minimum ceiling that restricts aircraft from operating in the ILS critical area is 800 feet. This regulation is in place to ensure that aircraft do not adversely affect the precision of the Instrument Landing System (ILS), which relies on accurate signal transmission and reception for safe landings. The ILS critical area is defined to protect the system from interference caused by aircraft movements, particularly when visibility is limited and aircraft are using the approach procedures that depend on this system.

When the ceiling is at or below 800 feet, aircraft are prohibited from entering the ILS critical area to maintain the precision and reliability of the landing guidance provided to aircraft. This ensures that only those aircraft that are cleared to land under the appropriate conditions can operate in the critical area, thus enhancing safety and minimizing interference with the ILS signals.

The other options suggest higher minimum ceiling values, which would not provide the necessary protection for the ILS critical area during critical phases of flight when low visibility conditions are a factor.