Tel: +973 17 329054

Height Monitoring Unit (HMU)

A HMU (Height Monitoring Unit) is a ground based system consisting of two main components - HME (Height Monitoring Element) and TMU (Total vertical error Monitoring Unit).

The HME captures SSR transponder signals from aircraft replying to interrogations from radar stations. The signals contain the information from modes S, C and A transmissions. The HME determines the geometric height and position of an aircraft by comparing the time of reception of the SSR transponder signals at different receiver locations. This information is transmitted to the TMU as one plot per second. The data is collated by the TMU to create a track history of the aircraft passing through the area of coverage. The track information is then combined with meteorological data to evaluate the overall value for Total Vertical Error (TVE).

When completed, the monitoring process produces TVE (Total Vertical Error), AAD (Assigned Altitude Deviation) and ASE (Altimeter System Error) readings for each aircraft measured. In the European context, Height Monitoring results are automatically transmitted to the EUR RMA at EUROCONTROL Headquarters, Brussels where they are verified and credited to approved aircraft and operators.

Note : European RMA Height Monitoring Results usually transmitted to the concerend RMA within 4 weeks from Date of Flight

EUR Region HMU Locations

  • Linz in Austria - 48°12’08”N....014°17’35”E (near LNZ VOR)
  • Nattenheim in Germany - 49°56’45”N….006°33’25”E (near NTM VOR)
  • Geneva in Switzerland - 46°21’49”N….005°55’34”E (near GVA VOR)

The coverage of each HMU has a radius of 45 NM.

You can find the operational status of the HMU's and more information like the HMU's position and the requirements for a successful monitoring by the HMU's on web pageThe European agency for RVSM monitoring.


The Strumble Height Monitoring Unit (HMU) is located near Strumble (STU) VOR below the centreline of UL9. The horizontal coverage area for the HMU is a 13.8 NM radius circle centred on position N51 56 00’ W004 40’ 00. In the vertical dimension, the coverage is from FL 290 to FL 410 inclusive. The site is unmanned and designed to operate continuously.

NATCMA Website

Aircraft Geometric Height Measurement Element (AGHME)

The NAARMO has developed the AGHME system to monitor all aircraft passing through its coverage volume, with the primary aim of supporting estimation of height-keeping performance parameters for the bulk of operations in North American RVSM airspace. The AGHME system does not require that any special monitoring devices be installed on an aircraft in order that it be monitored. It is necessary, however, that the aircraft have an operational Mode S transponder.

All AGHME systems are functionally identical. Hence, an operator wishing to use an AGHME constellation to satisfy monitoring requirements should follow the instructions provided under AGHME monitoring procedures. This will suffice at any AGHME constellation location.

AGHME constellations will be located at seven sites in the United States and Canada. AGHME locations provides information about these locations and associated availability dates.

AGHME monitoring procedures

AGHME locations






Tel: +973 17 329054

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