How to have an Aircraft Height Monitored

Height-keeping performance monitoring is carried out using one of three main methods: ADS-B monitoring, ground based monitoring and GPS monitoring. Each of these methods is briefly described below with links to the relevant Regional Monitoring Agency’s (RMA’s) webpage responsible for each specific monitoring system. At the specific link you will find details of the exact location of the monitoring system and the procedure for having your aircraft monitored over that height monitoring system.

ADS-B Height Monitoring Systems 

ADS-B Height Monitoring Systems (AHMS) use ADS-B messages to obtain geometric height data from ADS-B equipped aircraft. To use this method requires the aircraft to be ADS-B equipped and for the aircraft to fly in a region where ADS-B monitoring is performed. The following RMA’s have ADS-B Height Monitoring Systems: 

Ground-Based Height Monitoring Systems

Ground-based monitoring units are stationary stations installed at fixed locations across the world. These Ground-based systems are known as Height Monitoring Units (HMUs) or Aircraft Geometric Height Measuring Elements (AGHMEs). These systems can compute Altimetry System Error (ASE) for all aircraft flying over their operational coverage areas in straight and level flight. The following ground-based monitoring systems are available:


  • The EurRMA is responsible for 3 HMUs which are located at:
    • Linz (Austria)
    • Nattenheim (Germany)
    • Geneva (Switzerland)
  • JASMA is responsible for 3 HMUs which are located at:
    • Setouchi
    • Niigata
    • Sendai


  • NAARMO is responsible for 2 AGHMEs which are located at:
    • Atlantic City (US)
    • Ottawa (Canada)

Portable GPS-based Monitoring Systems

A portable GPS Monitoring Unit (GMU), or an Enhanced GMU (EGMU), is a mobile system used to record the geometric height of an aircraft in flight using Global Positioning System (GPS) signals. The GMU consists of two GPS antennas to attach to the left and right cockpit windows, a GMU unit and a laptop computer as a recording device. The GMU records continuous three dimensional position data in the level flight portions of a flight. The measured height data is processed after flight by calibration with meteorological data to determine the actual height of the assigned Flight Level at the time. The GMU needs an engineer in the cockpit to record in-flight data and a longer period of level flight than HMUs.

Please contact the NAT CMA for further details of GMU monitoring.