RVSM Height Monitoring

In accordance with the Isle of Man Aircraft Registry’s Registry Publication 30 'Operating Guidance and Information for Reduced Vertical Separation Minima (RVSM) Operations', all  ‘M’ registered operators that have an RVSM approval and that operate or intend to operate in airspace where RVSM is applied are required to participate in the regional RVSM monitoring program.

Following approval to operate in RVSM airspace, the operator must ensure that height-keeping performance is monitored:

a) at least once every 2 years; or

b) within intervals of 1000 flight hours per aircraft, whichever is the longer period.

Note: Where an operator has 2 or more aircraft of the same type, the monitoring must be conducted on a minimum of 2 aircraft of each aircraft type grouping.

For further information on how to have an aircraft height monitored as part of the ongoing height-keeping performance monitoring program, can be found on the relevant Regional Monitoring Agency, see links below.

For any further clarification or to request data from an HMU overflight, please contact the Registry in the first instance.


Reduced Vertical Separation Minima (RVSM) was introduced to reduce the vertical separation above flight level (FL) 290 from the traditional 2000-ft minimum to 1000-ft minimum. This allows aircraft to fly more fuel efficient profiles, increases airspace capacity and reduces in-flight delays by providing six additional cruising levels between FL290 and FL410. The North Atlantic (NAT) was the first region to introduce RVSM, in 1997. Since then RVSM has been introduced across the world.

When an aircraft is height monitored, the Altimetry System Error (ASE) of the aircraft is estimated to establish the accuracy of the aircraft’s altimeter equipment. The diagram below illustrates how the accuracy of the aircraft’s altimeter is calculated. The actual altitude of the aircraft is compared to the assigned altitude of the aircraft and the altitude displayed in the cockpit to calculate the Total Vertical Error (TVE), Altimetry System Error (ASE) and the Assigned Altitude Deviation (AAD) where:

  • TVE = The vertical geometric difference between the actual pressure altitude flown by an aircraft and its assigned pressure altitude (flight level).
  • ASE = The difference between the altitude indicated by the altimeter display (assuming a correct altimeter barometric setting) and the pressure altitude corresponding to the undisturbed ambient pressure.
  • AAD = The difference between the transponded Mode C altitude and the assigned altitude/flight level.
Height Monitoring planes

To find out more about the ASE, its potential causes and the importance of identifying aircraft that are exhibiting ASE, visit Skybrary.

Height-keeping performance monitoring is carried out using one of three main methods: ground based monitoring; ADS-B 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.

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 NATCMA is responsible for 1 HMU which is located at Strumble (Wales)
  • 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 6 AGHMEs which are located at:
    • Atlantic City (US)
    • Wichita (US)
    • Cleveland (US)
    • Phoenix (US)
    • Ottawa (Canada)
    • Lethbridge (Canada)

ADS-B Height Monitoring Systems

ADS-B Height Monitoring Systems (AHMS) use ADS-B receivers 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:

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.