What altitude measurement systems must aircraft have when in RVSM airspace?

In Reduced Vertical Separation Minimum (RVSM) airspace, aircraft are required to have two independent altitude measurement systems. This requirement is critical because RVSM airspace operates with reduced vertical separation between aircraft, allowing for just 1,000 feet of vertical space instead of the traditional 2,000 feet. To ensure safety and maintain accurate altitude assignments, two altitude measurement systems are necessary. This redundancy helps to provide cross-verification of altitude readings, thereby minimizing the risk of altitude deviations that could lead to potential in-flight conflicts.

The presence of two independent systems enhances the reliability of altitude data and ensures that both systems are functioning correctly. If one system fails or provides erroneous data, the second system can still ensure the pilot and air traffic control have accurate information regarding the aircraft's altitude, which is vital for maintaining safe distances between flights in such closely spaced airspace.